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Scatterhoarders drive long- and short-term population dynamics of a nut-producing tree, while pre-dispersal seed predators and herbivores have little effect
Journal of Ecology
Abstract
Both seed predators and herbivores can have profound effects on individual plant growth, reproduction and survival, but their population‐level effects are less well understood. While most plants interact with a suite of seed predators and herbivores over their life cycle, few studies incorporate the effects of multiple interacting partners and multiple life stages on plant population growth.
We constructed a matrix model using 6 years of data from a rare, seed‐producing population of American chestnut ( Castanea dentata ). We combined field demographic data with published experimental results on the effects of pre‐dispersal seed predators (weevils) and post‐dispersal seed predators (scatter‐hoarding vertebrates) and incorporated the effect of vertebrate herbivores estimated from the field data. We explored the impact of these three different animal interactions for short‐term (transient) and long‐term (asymptotic) tree population growth. In addition, we used the model to explore the conditions under which scatter hoarding would function as a mutualism.
Seed predators had greater effect on both short‐ and long‐term population growth than herbivores. Although weevil infestation can greatly reduce the probability of germination, pre‐dispersal seed predators had smaller effects on both short‐ or long‐term population growth than post‐dispersal predators. The elasticities of weevil‐related parameters were also small. The effect of browsers on both the short‐ and long‐term population growth rate were the smallest of the effects studied. Post‐dispersal seed predation affected population growth the most in the interactions studied. The probability of seed removal was among the largest elasticities, similar in magnitude to survival of large trees.
Synthesis . Our results indicate that neither weevils nor the intensity of browse damage observed at our study site are likely to hinder tree regeneration or reintroduction, although both reduced population growth. Although researchers and forest managers often assume that seeds are unimportant for long‐lived tree populations, our test of this assumption shows that scatterhoarders and other post‐dispersal seed consumers can significantly limit natural regeneration. Forest management that alters scatterhoarder behaviour could have significant effects on tree population dynamics that are largely unexplored.
... Most studies have been carried out on herbs, which are expected to be more sensitive to changes in fecundity than trees (Franco and Silvertown, 2004). Nonetheless, seed predation has been shown to reduce population growth rates of a long-lived tree (Elwood et al., 2018). ...
... To simulate reduced seed predation by rodents, we added the observed levels of seed removal in our experiments back to our fecundity estimates (Elwood et al., 2018;Maron and Kauffman, 2006). For olopua, our experiment focused on predation of green seeds on the ground but a significant portion of seed predation appeared to occur in the canopy. ...
... If and how seed predation can affect the population dynamics of long-lived tree species has rarely been evaluated. Our results are consistent with the few studies that have done so (Elwood et al., 2018;Miller-ter Kuile et al., 2021), and indicate that high levels of seed predation can indeed scale up to impact population persistence for the longlived trees in our study. ...
... Thus the former group of rodents perhaps function more effectively as seed dispersers, whereas the latter group behaves mostly as seed predators. It is important to acknowledge that the net effect of any hoarding animal on its host plant can be beneficial or detrimental not only as a consequence of its position along this continuum but also as a function of the difference in population recruitment of that plant with and without the animal (Jansen & Forget, 2001;Zwolak & Crone, 2012;Elwood et al., 2018). Consequently, any synzoochorous interaction will be located along a positive-negative continuum irrespective of its net outcome on the host-plant population. ...
... Due to the dual nature of this interaction, synzoochory should be considered not only by ecologists interested in mutualistic interactions and ecological networks but also by those working on antagonistic interactions and food-web dynamics. Considering synzoochorous interactions between granivorous animals and plants will provide a more accurate perspective on the functioning of natural systems (Elwood et al., 2018). ...
... In addition, very few studies collect data on seed fate past the seed-caching stage, a very early stage in recruitment for evaluating the true effectiveness of dispersal. Consequently, we know little about the impact of synzoochory relative to other dispersal modes (endozoochory, myrmecochory, anemochory, etc.) on plant fitness and population dynamics (Hulme, 1998(Hulme, , 2002Jansen & Forget, 2001;Vander Wall & Longland, 2004;Pesendorfer et al., 2016;Elwood et al., 2018), as well as on community structure and mutualistic network topology and architecture (Donatti et al., 2011). Filling this gap will not only improve our understanding of natural systems but will also have enormous value for developing appropriate conservation and restoration strategies (Pons & Pausas, 2008;Puerta-Piñero et al., 2012a;Pesendorfer et al., 2016). ...
Synzoochory is the dispersal of seeds by seed‐caching animals. The animal partner in this interaction plays a dual role, acting both as seed disperser and seed predator. We propose that this duality gives to synzoochory two distinctive features that have crucial ecological and evolutionary consequences. First, because plants attract animals that have not only positive (seed dispersal) but also negative (seed predation) impacts on their fitness, the evolution of adaptations to synzoochory is strongly constrained. Consequently, it is not easy to identify traits that define a synzoochorous dispersal syndrome. The absence of clear adaptations entails the extra difficulty of identifying synzoochorous plants by relying on dispersal traits, limiting our ability to explore the full geographic, taxonomic and phylogenetic extent of synzoochory. Second, the positive and negative outcomes of interactions with synzoochorous animals are expressed simultaneously. Consequently, synzoochorous interactions are not exclusively mutualistic or antagonistic, but are located at some point along a mutualism–antagonism continuum. What makes synzoochory interesting and unique is that the position of each partner along the continuum can be evaluated for every plant–animal interaction, and thus the continuum can be precisely described by assessing the relative frequency of positive and negative interaction events in each pairwise interaction. Herein we explore these two main features of synzoochory with a comprehensive quantitative survey of published studies on synzoochory. Synzoochory has been recorded for at least 1339 plant species differing in life forms, from annual and short‐lived herbs to long‐lived trees, belonging to 641 genera and 157 families widely distributed across the globe and across the seed plant phylogeny. Over 30 animal families belonging to five disparate taxonomic groups (rodents, marsupials, birds, insects, and land crabs) potentially act as synzoochorous dispersers. Although synzoochory appears to be fundamentally a secondary dispersal mode, many abundant and dominant trees are primarily synzoochorous. In addition, we found evidence of the existence of diplosynzoochory (caching animals acting both as primary and secondary dispersers of the same individual seed), mostly in nut‐bearing trees. Finally, we found that synzoochorous interactions are widely spread across the mutualism–antagonism continuum. Nevertheless, there were some differences among disperser species and functional groups. Corvids and some rodents (cricetids, nesomyids, sciurids) were located in the positive‐effects region of the continuum and presumably behave mostly as dispersers, whereas land crabs and insects were located in the negative‐effects extreme and behave mostly as seed predators. Our review demonstrates that synzoochory is not an anecdotal ecological interaction. Rather, it is pivotal to the functioning of many ecosystems where the natural regeneration of keystone plant species depends on the activity of granivorous animals that play a dual role. This distinctive interaction should not be ignored if we wish to have an accurate understanding of the functioning of natural systems.
... However, these survivability estimates fall within the reported values of 80-93% seedling survival after 1 year in nursery settings (Wang et al. 2013). Our findings are consistent with studies suggesting that browse pressure seems to have relatively little effect on survivability due to the American Chestnut's remarkable ability to resprout (Elwood et al. 2018). Therefore, the greatest factors contributing to recruitment are likely related to seed predation (Elwood et al. 2018) or adequate site conditions at cached locations where germination can occur (Wang et al. 2013). ...
... Our findings are consistent with studies suggesting that browse pressure seems to have relatively little effect on survivability due to the American Chestnut's remarkable ability to resprout (Elwood et al. 2018). Therefore, the greatest factors contributing to recruitment are likely related to seed predation (Elwood et al. 2018) or adequate site conditions at cached locations where germination can occur (Wang et al. 2013). ...
We monitored the reproduction, dispersal, and regeneration of a wild population of Castanea dentata (American Chestnut), established from 4 seed-bearing trees planted in a western Maine forest in 1982. The 40-year-old parent trees, sourced from wild stock of a relict population in northern Michigan, show no obvious signs of blight and have been producing viable seeds now for >20 years. Over the course of 2 surveys conducted in 2019 and 2020, we mapped and measured 1348 offspring, varying in size from seedlings to nearly mature trees. As of October 2020, the natural spread of this population had expanded to at least 370 m from the parent trees, with an average dispersal distance of 124 m. While previous publications have focused on the scatter-hoarding behavior that gave rise to this expanding wild population, we report on possible factors affecting their spread, their fate, and prognosis for the future. Given the absence of other reproductive populations of American Chestnut in the immediate vicinity, our data provide rare insights into natural seed dispersal from a known point of origin while documenting the return of a functionally extinct species to a northern hardwood forest ecosystem.
... Predace semen ale ne vždy způsobuje velké očekávané škody na populacích i přes velké relativní početní ztráty (Andersen, 1989;Fröborg & Eriksson, 2003). Důležitá je i postdispersní predace, která může někdy být na populační úrovni významnější než predispersní (v případě velkých semen jako u kaštanovníku C. dentata) (Elwood et al., 2018). ...
... Rozdíly mezi produkovanými výsledky a jejich realističnost oproti maticovým modelům lze demonstrovat na výše zmíněných analýzách. Růstová rychlost populace se liší pouze zanedbatelně (Easterling et al., 2000) a to i v případech, kdy bylo v rámci populační studie s interakcí s herbivory testováno, zda malé množství velikostních kategorií pro maticový model ovlivní výsledky projekce (oproti IPM, kde se kategorie nedefinují a data jsou kontinuálního charakteru) (Elwood et al., 2018). V porovnání s reálnými daty stabilní velikostní struktury byly společně maticové i IPM modely poměrně odpovídající. ...
Pathogens and herbivores are two important groups of organisms with predominantly negative impact on their plant hosts. Although relationships at the individual level are relatively well understood, the data acquired by those studies cannot be easily translated into effects on the population. Various types of population models, also used in the case of the interactions, give us valuable insight into population dynamics. This bachelor thesis at first describes mechanisms and effects of plant pathogens (with emphasis on pathosystems with anther smut Microbotryum violaceum) and herbivores at the individual and population level for the demonstration of diversity of the systems and to stress the discrepancy between the two hierarchical levels. Furthermore, matrix and integral projection models are introduced as invaluable tools for population dynamics projections and understanding the underlying results of the interactions on plant populations. These models use the entire plant life cycle and require long-term censuses of demographic data. The role of these models in plant-pathogen interactions seems heavily underappreciated, however, and thus there is only a small number of studies with their application. The thesis describes mechanisms of the models, modifications in specific life-cycles and their application. Lastly, SIR models are introduced concisely.
... Up to 90% of tropical tree species and 60% of temperate woody species rely on animal agents to facilitate seed dispersal (Vander Wall & Moore, 2016). Notably, increasing evidence indicates the herbivorous roles of many of these seed dispersal agents, especially rodents, causes high seedling and sapling mortality rates and thus also mediates the recruitment process (Elwood, Lichti, Fitzsimmons, & Dalgleish, 2018;Norghauer, Röder, & Glauser, 2016;Watt, 1919). Therefore, many vertebrates, with their simultaneous roles as enemies to young trees and seed dispersers, likely regulate the natural recruitment dynamics of seed plants (Lichti, Steele, & Swihart, 2017). ...
... Recruitment dynamics are a complicated process and are affected by several factors, including light availability, space limitations and the CNDD effects (Elwood et al., 2018;Eriksson & Ehrlén, 1992). ...
Seed dispersal and subsequent recruitment dynamics play a crucially important role in regulating species coexistence and structuring tree diversity in diverse forests. Wildlife, which can dually shape the tree recruitment process by simultaneously functioning as natural enemies and seed dispersers, are undergoing widespread changes in population and behaviour due to the detrimental effects of expanding global road networks. However, the impact of these changes on recruitment dynamics through the alteration of seed dispersal processes remains understudied.
Here, we assessed how roads affect animal‐mediated recruitment dynamics from the seed to adult stages of trees using a ubiquitous vertebrate‐seed model: rodents and acorns. To quantify the degree to which proximity to a road alters seed dispersal and regulates subsequent recruitment dynamics, we conducted seed dispersal and predation experiments, and investigated the natural recruitment of a dominant tree species (Quercus aliena) from September 2009 to July 2012 in a subtropical forest of central China.
Roads caused a decreased seed dispersal distance and increasing larder hoarding, demonstrating a weakened contribution of rodents to seed dispersal services. These patterns were stronger in the masting year than in the non‐masting year. Correspondingly, seedlings, saplings and adult trees had higher densities near roads than far from roads. Near roads, recruit transition rates (i.e. seedling‐to‐sapling and sapling‐to‐juvenile) were low, and the influential strength of conspecific adult density was weakened over these recruit transitions, indicating that space limitation and lottery competition, not conspecific negative density‐dependent effects, approximately determined the near‐road recruitment process. Furthermore, most adults near roads were young, and their ages matched the road ages.
Synthesis. Roads diminished animal‐mediated seed dispersal services and disrupted subsequent recruitment dynamics in Quercus aliena populations, thus weakening the key process promoting diversity in forest ecosystems. Given that these animal‐regulated roles in recruitment dynamics are widespread stabilizing forces for tree coexistence, our findings suggest that the ubiquity of roads and their continued expansion will contribute to the increase in forests dominated by a relatively low number of species. Consequently, the growing expansion of road networks will ultimately cause profound changes in community composition and structure across diverse forests worldwide.
... Scatterhoarders, primarily rodents and corvids, bury nuts in shallow, dispersed caches within topsoil or litter (Vander Wall, 2001). These animals, interacting with over 1200 plant species (Gómez et al., 2019), play a pivotal role in the population dynamics of dispersed plants (Elwood et al., 2018). Scatterhoarders can either promote or hinder plant reproduction, depending on the balance of costs and benefits (Bogdziewicz et al., 2020;Gómez et al., 2019). ...
Theoretical models suggest that species abundance plays a crucial role in mutualism; high densities can lead to overexploitation. Additionally, mutualistic benefits are expected to increase under abiotic stress.
We investigated the interplay between density dependence and abiotic factors in conditional mutualism, focussing on the interactions between yellow‐necked mice (Apodemus flavicollis) and common oaks (Quercus robur) across 3 years. We controlled for seed abundance, while mouse densities varied. Our approach included monitoring mouse abundance, measuring seed removal, determining the fate of harvested seeds and conducting seedling emergence trials to assess the advantages of seeds caching.
We found that mouse abundance exerted a strong influence on all studied aspects of seed dispersal. Higher densities led to increased seed removal and greater dispersal distances. However, they also decreased the probability of seed caching and increased seed consumption, degrading dispersal quality and shifting the interaction towards antagonism. Furthermore, the reliance of seedling recruitment on burial varied over time, likely becoming more critical during dry conditions. This indicates that plants face the worst conditions when high abundance of mice coincides with abiotic stress.
Synthesis: Our results supports the notion that increased rodent abundance reduces plant recruitment, revealing the conflicting interests of the interacting species. However, the dynamics of rodent–oak interactions are shaped not only by the density‐dependent foraging decisions of rodents, but also by the benefits of seed burial under stress. These findings illustrate how the interplay between population density and abiotic factors jointly dictate the costs and benefits of mutualistic interactions.
... These species are more reliant upon recruitment (and thus higher elasticity values for recruitment; Caswell 2001) relative to long-lived species with lower fecundity Silvertown et al. 1996). However, long-lived species with lower fecundity rates can also experience reduced population growth rates under predation pressure (Bricker and Maron 2012;Elwood et al. 2018). Silvertown et al. (1996) emphasize that a large perturbation to a life-history transition with only a small elasticity value may have a greater impact on population growth rate than a small perturbation to a life-history transition that has a larger elasticity value. ...
Understanding the effects of seed predation, dispersal, and recruitment on the population dynamics of rare plant species is essential for generating effective management strategies. Unfortunately for most rare plants, the parameterization of these processes is limited and generally not included in demographic analyses. This exclusion can lead to biased estimates of vital rates and overall population growth rates, as well as limit inferences about inter-population processes like colonization and demographic rescue that can affect population viability. Based on previous empirical studies from Fort Liberty (formerly Fort Bragg) North Carolina (USA), we constructed a spatially explicit demographic model that accounts for pre-dispersal seed predation, dispersal, and habitat suitability for Lindera subcoriacea (bog spicebush), a rare shrub in the southeastern United States. We demographically modeled three scenarios: S1 did not include any of the three parameters; S2 accounted for seed predation and dispersal; and S3 included all three of the parameters. Results suggested that pre-dispersal seed predation, dispersal, and habitat suitability negatively impact the population growth rates of bog spicebush relative to the naïve demographic model. After 100 annual time steps, scenarios S1, S2, and S3 led to a 96%, 49%, and 1% increase in population size, respectively. In addition, over the course of 100 years, results of scenarios S2 and S3 demonstrated limited increases in site occupancy, with newly occupied areas located < 1 km from previously occupied habitat. Our results suggest additional parameterization of plant demographic models may be an informative endeavor and warranted, even in the absence of empirical data.
... mediated seed dispersal can avoid the death of seeds or seedlings due to density constraints; furthermore, it can help plants exploit habitats and expand their niches (Elwood et al., 2018;Acevedo-Quintero et al., 2020). However, some animals (for example, Sciurotamias davidianus and Nucifraga caryocatactes) disperse and consume nuts (e.g., Pinus koraiensis and Quercus mongolica), drupes, and other seeds, or even cut off the cotyledons and seedlings to prevent seed germination and seedling colonization (Yi et al., 2019). ...
During the restoration of secondary forests in Northeast China, it is critically important to promote seed recruitment of Korean pine (Pinus koraiensis), which is highly dependent on rodents and significantly affected by spatially heterogeneous understory vegetation of forest gaps. Understanding the link between understory microhabitats and rodent-mediated seed dispersal, including post-dispersal seed predation and scatter-hoarding seed escaped predation, is essential to improve seed survival. Previous studies have focused on their qualitative relationships by roughly classifying understory vegetation characteristics, which cannot precisely predict regeneration direction. Quantifying the spatial heterogeneity of understory vegetation parameters (e.g., canopy cover, CC; gap fraction, GF) using terrestrial laser scanning (TSL) provides a vital way to determine quantitative relationships. In this study, we collected point cloud data in forest gaps with three sizes and six within-gap positions through TSL to quantify the understory microhabitats using CC and GF and integrated these parameters with spatial site data of Korean pine seeds scatter-hoarding and post-dispersal predation by rodents. The results showed that the influence of the within-gap position on the understory microhabitats was insignificant, and the spatial site selection of post-dispersal seed predation and seed storage was concentrated in the range of 0–22 m away from the seed station, irrelevant to the gap size. Considering the risk of predation by enemies and seed pilferage by competitors in forest gaps, rodents scatter-hoarded or post-dispersal predated seeds in the CC range of 0.2–0.3, and post-dispersal predated seeds less often in the CC range > 0.8. The ratio of post-dispersal seed predation to scatter-hoarding escaped predation firstly increased and then decreased along with the increase in CC and GF. Therefore, the spatial heterogeneity in understory microhabitats caused by gap treatment had an uncertain effect on rodent-mediated seed dispersal, depending on the relative ratio of post-dispersal seed predation to seed scatter-hoarding.
... Most scatter-hoarding mammals are well suited for remembering their hiding places based on spatial memory and olfactory cues (Vander Wall 2000). The high olfactory capacity of rodents increases their chances of recovering cached seeds, and these post-dispersal interactions are critical in the recruitment effectiveness of nut-producing trees (Haugaasen et al. 2010;Elwood et al. 2017). Estimates of scatter-hoarding retrieval by squirrels and other small mammals often exceed 90% (Steele et al. 2005). ...
Zoochory is fundamental for habitat colonization by plants with large and heavy seeds, such as Araucaria angustifolia. Recruitment by this endangered South American conifer is heavily impacted by animals, but the effect of different zoochoric dispersal modes and deposition sites on successful establishment remains unknown. In this study, we evaluated the effect of dispersal mode (based on seed disperser), distance from adult conspecifics, and disposition site on long-term recruitment success. The experimental design included two environments (forest and open field), in each of which we established 30 microhabitat sampling stations simulating deposition sites (10 under a conspecific; 10 far from a conspecific and next to a landmark; and 10 far from a conspecific and without a landmark). Each microhabitat contained three levels of dispersal type (intact buried seed; intact seed on the ground surface; and partially predated seed on the ground surface), each with five seeds, totaling 900 seeds. In the forest environment, an experiment was also conducted to verify the fate of seeds using a spool-and-line technique. Neither environment nor microhabitat alone significantly affected seed survival, although dispersal type was significant in seed survival, as only buried seeds survived until the last survey. The spool-and-line seed experiment showed that most A. angustifolia seeds were predated after removal (81.5%) and only buried seeds survived, reinforcing the role of scatter-hoarding animals as important agents in A. angustifolia dispersal. Our results suggest that A. angustifolia seeds suffer high predation rates, with only a few seeds escaping from predators and recruiting seedlings into the population (only 1.1% of all seeds used in both experiments). We conclude that seed survival is a critical life cycle step in this plant species and that dispersal mode is critical for its reproductive success.
... It is important to take this into account when planning an experiment. Performing manipulative experiments and using the proper seed-tracking method, which enables us to follow the process until the final stage (either predation or seedling establishment) with the evaluation of costs and benefits, we could get a much more complete picture about the seed dispersal effectiveness of this group which is crucial to get a better insight into mutualistic-antagonistic relationships in seed dispersal systems by small rodents (see also Forget and Wenny, 2005;Yi et al., 2008;Hirsch et al., 2012;Zwolak and Crone, 2012;Rosin and Poulsen, 2017;Elwood et al., 2018;Zhang et al., 2021). ...
Zoochory is an ecologically and evolutionarily important seed dispersal type. The decline and extinction of seed-dispersing large herbivores severely threatens dispersal-driven ecosystem processes in many regions. Hence the relative importance of small rodents and lagomorphs (Glires, Mammalia) as dispersal vectors might increase due to their ubiquity, diversity and abundance. Here we provide a review of rodent- and lagomorph-mediated seed dispersal based on approximately 600 papers found in an extensive literature search. We highlight that small rodents and lagomorphs disperse seeds via various mechanisms. The seldom documented epi- and endozoochory are probably universal in these groups. Due to their small home range, short fur and small body size, these mechanisms generally operate at small scales and mainly for small seeds. Taxon-specific feeding, nesting and behavioural characteristics provide a wide spectrum of other seed dispersal types, such as synzoochory (food caching). The studied taxa generally support seed dispersal within a particular habitat patch, contributing to the persistence of local populations, but in rare cases, long-distance dispersal events might occur. Besides seed dispersal, rodents and lagomorphs can also support plant establishment and provide safe sites for seeds where they can survive stochastic events. Studies reviewed here show a strong bias both in scope and geographical distribution: synzoochorous dispersal of woody plants is known in detail, and most studies were conducted in the same few countries and habitat types. In contrast, other mechanisms such as endozoochory, epizoochory and habitat types like grasslands and anthropogenic habitats have rarely been studied. We show examples on ecosystem services and disservices related to rodent- and lagomorph-mediated seed dispersal as well as the importance of these processes in habitat conservation and restoration.
... Another challenge is to unpack the consequences of changing synchrony for recruitment and wider community dynamics. Declining synchrony has been demonstrated to decrease individual plant fitness as measured by viable seed production [40], but the next step is to link this with tree regeneration and population growth [74,75]. Trophic consequences of changes in synchrony are potentially substantial but remain unexplored. ...
Climate change is reshaping global vegetation through its impacts on plant mortality, but recruitment creates the next generation of plants and will determine the structure and composition of future communities. Recruitment depends on mean seed production, but also on the interannual variability and among-plant synchrony in seed production, the phenomenon known as mast seeding. Thus, predicting the long-term response of global vegetation dynamics to climate change requires understanding the response of masting to changing climate. Recently, data and methods have become available allowing the first assessments of long-term changes in masting. Reviewing the literature, we evaluate evidence for a fingerprint of climate change on mast seeding and discuss the drivers and impacts of these changes. We divide our discussion into the main characteristics of mast seeding: interannual variation, synchrony, temporal autocorrelation and mast frequency. Data indicate that masting patterns are changing but the direction of that change varies, likely reflecting the diversity of proximate factors underlying masting across taxa. Experiments to understand the proximate mechanisms underlying masting, in combination with the analysis of long-term datasets, will enable us to understand this observed variability in the response of masting. This will allow us to predict future shifts in masting patterns, and consequently ecosystem impacts of climate change via its impacts on masting.
This article is part of the theme issue ‘The ecology and evolution of synchronized seed production in plants’.
... Outcomes of hoarder-seed interactions can largely affect growth and persistency of plant populations that have scatter-hoarding as a mode of dispersal. For example, Elwood et al. (2018) showed that results of scatter-hoarding interactions can be more important than pre-dispersal seed predation and herbivory in driving plant population dynamics. Even after seeds are dispersed and buried, scatter-hoarding animals can still later retrieve and eat cached seeds. ...
Seed dispersal and predation are paramount for tropical plant diversity. When encountered by scatter-hoarding frugivores, seeds can be either eaten, dispersed or ignored. But even after dispersal, seed caches are still subjected to predation. Many factors are known to influence these dynamics; however, how frequently hoarders use certain patches has seldom been related to cache predation rates. We used the interaction between agoutis (Dasyprocta leporina), a scatter-hoarding rodent, and Joannesia princeps, a tropical tree, as a model to investigate how the number of visits by hoarders in certain areas influences cache predation and seed fate. Camera-traps were used for 30 days in twenty different locations in Tijuca National Park to assess number of visits by agoutis. Thereafter, we placed seed piles on the same areas and determined their fate using the spool-and-line method to track seeds for over one hundred days. We found a non-linear relationship between how often an area is used by hoarders and the final proportion of dispersed seeds. At areas with a low number of visits, proportion of dispersed seeds was low due to low removal. As frequency of visits by hoarders increased, seed removal and the number of dispersal events increased but so did cache predation. Thus, in areas intensively used by hoarders, high cache predation resulted in a low number of dispersed seeds that remained alive in caches. As a result, dispersal was maximized in areas with intermediate use by scatter-hoarders, where there was a balance between primary seed dispersal and cache predation.
... It would also be useful to know how different life stages are affected by dispersal, competition and seed burial, and to integrate these effects throughout a plant's life span using life cycle models (e.g. Elwood, Lichti, Fitzsimmons, & Dalgleish, 2018). ...
Many species interactions occur along a continuum from mutualistic to antagonistic. Among these interactions, the effects of seed‐dispersing granivores (synzoochory) are especially interesting because the benefits and costs are so striking.
Here, we respond to a commentary by C. M. Moore and J. W. Dittel (‘On mutualism, models, and masting: the effects of seed‐dispersing animals on the plants they disperse’). We focus on the points that were most thought‐provoking to us, specifically the notions that (a) synzoochory is widespread, and therefore ought to be beneficial for plants, (b) dispersal should reduce negative density dependence and (c) plant life spans are at least an order of magnitude longer than typical study lengths.
We argue that (a) widespread occurrence of an interaction cannot serve as an argument that this interaction is mutualistic, (b) dispersal effects on density dependence are variable and need to be tested for every particular species pair and (c) Short‐term studies can be used to gain mechanistic understanding of interactions even without risky long‐term extrapolations.
Synthesis. Evaluating the costs and benefits of synzoochory for plant populations under different ecological contexts will help to understand the ecological principles of the interaction but also allows to move the interaction towards each end of the antagonism–mutualism continuum in nature management and conservation.
... They also point to the importance of studies such as ours that compare experimental results to field patterns. An even stronger approach would be to incorporate the effects of multiple aspects of species interactions and affected life stages into models that calculate their relative contributions to overall invasiveness (e.g., using matrix population models: Elwood et al. 2018). Although it is beyond the scope of this study, this approach would be a valuable direction for future research. ...
The Janzen-Connell effect is often generated by interactions between trees and soil microbes while the enemy release hypothesis states that invasive species are released from regulation by natural enemies. Thus, the strength of the Janzen-Connell effect could differ between native vs. nonnative plants. We tested this hypothesis with congeneric pairs of invasive and native tree species in Europe: boxelder (Acer negundo) vs. Norway maple (A. platanoides), and Northern red oak (Quercus rubra) vs. pedunculate oak (Q. robur). We conducted greenhouse experiments using soil sterilization treatments, field experiments on distance-dependent germination, and field surveys of early life stages of the focal species. Greenhouse and field experiments demonstrated patterns consistent with Janzen-Connell effect that is most likely caused by negative distance-dependence in seed germination and stem growth which was found in both genera of the native, but not the invasive trees. Soil sterilization experiments suggested that these effects are driven by interactions with soil biota. Field surveys revealed the Janzen-Connell pattern in the distribution of seedlings and saplings of the native, but not the invasive Acer species. Our findings indicate that weakened Janzen-Connell effect might contribute to successful invasions of certain nonnative plant species.
Seeds removed by animals have one of two mutually exclusive fates – they are either predated or dispersed and still alive. The quality of seed dispersal by animals and the number of predated seeds will therefore determine net interaction outcomes for plants. Yet, it is poorly understood what proportion of removed seeds animals can predate before benefits of dispersal no longer outweigh costs of seed loss.
Here, we calculated the mutualism-antagonism continuum for seed removal of the fleshy-fruited tree Frangula alnus by the seed-predating bird Coccothraustes coccothraustes in Białowieża Forest. We integrated effects of the bird during seed dispersal (fruit handling, seed predation, and seed deposition) into microhabitat-structured tree population models.
Results of our models showed that the probability of a seed of F. alnus reaching maturity after seed removal by C. coccothraustes decreased from 0.0028% to 0% as seed predation increased from 0% to 100%. Seed removal was beneficial when less than 63.7% of seeds were predated, and antagonistic when more than 72.0% of seeds were predated. Modifying key model parameters (here, the negative effect of fruit pulp on seedling recruitment and the frequency of forest gaps) decreased and increased rates of seed predation, at which costs of seed loss outweighed benefits of seed dispersal (from 37.9% to 80.7%).
Our findings highlight that benefits of animal seed dispersal can largely outweigh costs of seed predation in a fleshy-fruited tree. Yet, the mutualism-antagonism continuum of seed removal depends on intrinsic factors (e.g. variation in interactions among individuals) and extrinsic factors (e.g. the environment) of seed dispersal and plant demography. Because C. coccothraustes was observed predating at least 80% of removed seeds, it appears to be an antagonist of animal-dispersed plants and exploiter of the seed dispersal mutualism.
Species interactions are context dependent, in that their direction and magnitude can vary across ecological conditions. For seed dispersal interactions—especially interactions between plants and seed‐caching animals—the direction of the interactions is often obscured because of seed mortality inherent in seed handling and the delayed effects of fitness benefits received by plants. It is, therefore, an open question in ecology to understand the ecological contexts under which seed dispersal interactions are facilitative, antagonistic or null.
We review the fitness benefits of animal‐mediated seed dispersal, extend a recently published model to include negative density‐dependent effects, and review the feedback between seed production (with a focus on masting) and seed‐caching animal populations.
Negative density‐dependent effects are pervasive and strongly affect the direction of plant‐seed‐disperser interactions, and including them into models will give a more accurate understanding of the direction of the interaction. Including negative density‐dependent effects also makes the interpretation of interaction more mutualistic since seed dispersers decrease seed densities. Additionally, there is substantial interannual variability in seed production in most nut‐producing plant species, and the lags between seed production and seed‐disperser population sizes complicate and limit inferences made about the direction of interactions in any given short‐term study.
Synthesis. If we wish to know the direction of species interactions in real ecological communities, we need models that contain a minimum level of biological realism. For complex and long‐term phenomena such as context‐dependent species interactions we should embrace a multifaceted approach of short‐term field research, long‐term field research, simple models, and complex models to form a more robust understanding of the ecological problem of context dependency.
Consistent individual tendencies in behaviour, or behavioural types, are likely to impact the dynamics and outcomes of animal‐mediated seed dispersal. We review the extant literature on this issue and outline a conceptual overview to guide this emerging field. We provide an overview of possible ways in which behavioural types can affect animal‐mediated seed dispersal. We summarize theoretical mechanisms linking behavioural types with seed dispersal outcomes and review how behavioural types might affect each stage of seed dispersal, beginning with fruit encounter and harvest, and ending with events that take place after seed deposition.
Since behavioural types involve correlations among different behaviours (i.e. behavioural syndromes), they can generate unexpected associations between different decisions that are involved in seed dispersal, with conflicting (or reinforcing) effects on different stages of seed dispersal. Thus, we draw particular attention to trade‐offs faced by seeds dispersed by individuals with different behavioural tendencies. We also note that since seed dispersal is a multiplicative process with different stages, disperser behavioural types that provide moderately efficient dispersal at each stage will be better for plants than behavioural types that are very efficient at some steps, but inefficient on others. Finally, we provide testable predictions on the links between behavioural types and characteristics of seed dispersal, including, for example, influences on the probability of seed harvest, dispersal distance, deposition sites and condition of dispersed seeds.
We argue that investigating the links between behavioural types and animal‐mediated seed dispersal will provide a better mechanistic understanding of seed dispersal and plant regeneration.
A free plain language summary can be found within the Supporting Information of this article.
Although dispersal is generally viewed as a crucial determinant for the fitness of any organism, our understanding of its role in the persistence and spread of plant populations remains incomplete. Generalizing and predicting dispersal processes are challenging due to context dependence of seed dispersal, environmental heterogeneity, and interdependent processes occurring over multiple spatial and temporal scales. Current population models often use simple phenomenological descriptions of dispersal processes, limiting their ability to examine the role of population persistence and spread, especially under global change. To move seed dispersal ecology forward, we need to evaluate the impact of any single seed dispersal event within the full spatial and temporal context of a plant’s life history and environmental variability that ultimately influences a population’s ability to persist and spread. In this perspective, we provide guidance on integrating empirical and theoretical approaches that account for the context-dependency of seed dispersal to improve our ability to generalize and predict the consequences of dispersal, and its anthropogenic alteration, across systems. We synthesize suitable theoretical frameworks for this work and discuss concepts, approaches, and available data from diverse subdisciplines to help operationalize concepts, highlight recent breakthroughs across research areas, and discuss ongoing challenges and open questions. We address knowledge gaps in the movement ecology of seeds and the integration of dispersal and demography that could benefit from such a synthesis. With an interdisciplinary perspective, we will be able to better understand how global change will impact seed dispersal processes, and potential cascading effects on plant population persistence, spread, and biodiversity.
Overhunting in tropical forests reduces populations of vertebrate seed dispersers.If reduced seed dispersal has a negative impact on tree population viability, overhunting could lead to altered forest structure and dynamics, including decreased biodiversity. However, empirical data showing decreased animal-dispersed tree abundance in overhunted forests contradict demographic models which predict minimal sensitivity of tree population growth rate to early life stages. One resolution to this discrepancy is that seed dispersal determines spatial aggregation, which could have demographic consequences for all life stages. We tested the impact of dispersal loss on population viability of a tropical tree species, Miliusa horsfieldii, currently dispersed by an intact community of large mammals in a Thai forest. We evaluated the effect of spatial aggregation for all tree life stages, from seeds to adult trees, and constructed simulation models to compare population viability with and without animal-mediated seed dispersal. In simulated populations, disperser loss increased spatial aggregation by fourfold, leading to increased negative density dependence across the life cycle and a 10-fold increase in the probability of extinction. Given that the majority of tree species in tropical forests are animal-dispersed, overhunting will potentially result in forests that are fundamentally different from those existing now.
http://rspb.royalsocietypublishing.org/content/282/1798/20142095.short
Both Peromyscus leucopus white-footed mice and Blarina brevicauda short-tailed shrews prey upon the curculionid weevils that attack Quercus alba acorns. Peromyscus leucopus also feeds on Q. alba acorns, but B. brevicauda does not. We conducted feeding experiments using weevil larvae or pupae (1) to determine the relative preference of P. leucopus for infested and noninfested acorns; (2) to examine consumption rates of P. leucopus and B. brevicauda when larvae are freely available, and (3) to determine whether weevils that have exited acorns and burrowed into the soil are differentially vulnerable to these two predators. No preference by P. leucopus was detected in either field or laboratory studies where entire acorns were offered, but P. leucopus showed a significant preference for sound over infested when only nutmeat was presented. No significant changes in the rate of consumption of larvae were detected over a 3-day trial period, nor did consumption rate change as the number of exposed larvae was increased from 30 to 50. Curculio pardalis (89%) and Contrachelus naso (6%) were the most prevalent weevil species encountered. Overwintering weevils burrowed up to 21 cm into the soil. Blarina brevicauda preyed upon pupae at depths to 15 cm, but P. leucopus foraged efficiently to only 3 cm. The null hypothesis that the ratio of sound to infested acorns has no affect on acorn mortality due to granivory by P. leucopus could not be rejected on the basis of field experiments using exclosures. Although neither species clearly regulates weevil abundance, B. brevicauda seems a likely determinant of adult weevil abundance in years following low acorn production.
Few records concerning pre-blight seed production of forest grown American chestnut (Castanea dentata) exist. As blight resistant American chestnut becomes available to use in restoration efforts, many conservation groups are interested in reintroducing the species as a wildlife food resource. As part of a long-term ecological study involving a disjunct population of American chestnut in Wisconsin, we examined seed production of mature forest grown trees in fall 2005 and 2006. Eleven large diameter (> 23 cm DBH), overstory position, fruiting American chestnuts were selected and measured for height, crown spread, crown volume, and dbh. Because seed predation is intense on this species, we indirectly measured seed production by counting the large opened and spiny cupules (i.e., burs) beneath the tree canopy, along with any cupules remaining on the tree. Castanea dentata typically produces three seeds per cupule, allowing for a reasonably accurate estimate of seed production, and cupules are rarely moved by seed predators. One half of the total projected canopy area was sampled plus an additional meter beyond the canopy edge. This method requires no estimation for losses due to predation or trap damage as found in traditional seed trap experiments. Mean seed production per m² of basal area was 69,489 in 2005 and 74,825 in 2006. A significant relationship between total number of seeds produced and DBH was found in 2005 but not in 2006. Using basal area measurements from the stand, we would expect 1.04×10⁶ seeds produced per hectare with 2758.5 kg ha⁻¹ of mast production. Based on a sample of seeds we were able to collect from the forest floor, mean seed weight was 2.65 g (± 0.039 SE). Overall, the results indicate that the decline of American chestnut after infection by the chestnut blight likely had a very significant impact on the availability of important wildlife food resources in stands where the species was dominant.
The influence of native fauna on non-native plant population growth, size, and distribution is not well documented. Previous studies have shown that native insects associated with tall thistle (Cirsium altissimum) also feed on the leaves, stems, and flower heads of the Eurasian congener C. vulgare, thus limiting individual plant performance. In this study, we tested the effects of insect herbivores on the population growth rate of C. vulgare. We experimentally initiated invasions by adding seeds at four unoccupied grassland sites in eastern Nebraska, USA, and recorded plant establishment, survival, and reproduction. Cumulative foliage and floral herbivory reduced C. vulgare seedling density, and prevented almost any reproduction by C. vulgare in half the sites. The matrix model we constructed showed that this herbivory resulted in a reduction of the asymptotic population growth rate (λ), from an 88 % annual increase to a 54 % annual decline. These results provide strong support for the hypothesis that indigenous herbivores limit population invasion of this non-native plant species into otherwise suitable grassland habitat.
This study examines the ecological niche of American chestnut (Castanea dentata (Marsh.) Borkh) and the latest blight resistant American chestnut × Chinese chestnut (Castanea mollissima Blume) hybrids. Planted seedlings of chestnut, tulip poplar (Liriodendron tulipifera L.) and chestnut oak (Quercus prinus L.) were subjected to two levels of light and two soil types in parallel field and greenhouse studies. The field study took place in the Appalachian ridge and valley province of Virginia. Growth and survival were quantified after three growing seasons. The interaction between light levels and topographic position (soil type) was significant for growth rates in the field and greenhouse. Species were significantly different from each other although hybrid varieties were not significantly different from each other or from pure American chestnut. Tulip poplar showed the greatest growth rates under all treatments in the field. Both tulip poplar and chestnut had the greatest growth rates in large gaps within mesic, mid and lower slope (MML) sites in the field. In contrast to growth, optimal conditions for survival differed among species. Tulip poplar had the greatest survival (71%) within large gaps in MML sites while chestnuts and oaks had the greatest overall survival (64%) in small gaps within xeric, upper slope and ridge (XUR) sites. In the greenhouse, tulip poplar did not outperform chestnut. Discrepancies in field and greenhouse studies were accounted for by uncontrolled factors, such as rodent predation. We conclude that optimal sites for planting American chestnut hybrids are in small gaps located within XUR sites.
1. Matrix population models are tools for elucidating the association between demographic processes and population dynamics. A large amount of useful theory pivots on the assumption of equilibrium dynamics. The preceding transient is, however, of genuine conservation concern as it encompasses the short-term impact of natural or anthropogenic disturbance on the population.
2. We review recent theoretical advances in deterministic transient analysis of matrix projection models, considering how disturbance can alter population dynamics by provoking a new population trajectory.
3. We illustrate these impacts using plant and vertebrate systems across contiguous and fragmented landscapes.
4. Short-term responses are of fundamental relevance for applied ecology, because the time-scale of transient effects is often similar to the length of many conservation projects. Investigation of the immediate, post-disturbance phase is vital for understanding how population processes respond to widespread disturbance in the short- and into the long term.
5. Synthesis and applications. Transient analysis is critical for understanding and predicting the consequences of management activities. By considering short-term population responses to perturbations, especially in long-lived species, managers can develop more informed strategies for species harvesting or controlling of invasive species.
Seed predation by weevils (Coleoptera: Curculionidae) has been implicated as a limiting factor in oak recruitment throughout eastern US forests. We examined the effects of weevil seed predation on American chestnut (Castanea dentata (Marsh.) Borkh.). Although an introduced pathogen eradicated sexually reproducing populations of American chestnut in the early 1900s, the recent development of a blight-resistant hybrid makes reintroduction feasible. We nondestructively assessed the amount of weevil damage to seeds using X-ray imagery and traditional float test methods in American and blight-resistant hybrid chestnut. We quantified the effects of weevil damage on seed germination and seedling growth. The float test method misidentified damage for up to 50% of seeds, whereas the X-ray method misidentified only 3% of the sample. Germination declined with damage: the smallest damage level reduced germination from 94% to 32%. No seeds with >50% damage germinated. Weevil damage reduced seedling growth by 50% compared with undamaged seeds. Seedling size increased with seed size, but seed size had no effect on germination. Our results highlight the importance of orchard and seed processing practices that prevent weevil damage to chestnut seeds. Because they drastically reduce germination rates and seedling growth, weevils have the potential to limit seed regeneration and dampen rates of spread in populations following reintroduction.
Acorn weevils (Coleoptera: Curculionidae) are well-known predators of oak acorns in the eastern hardwood forest region of the United States. We examined the germination percentage of seeds, as well as the physical characteristics of seedlings, originating from both weevil-damaged and sound acorns of red oak (Quercus rubra L.) to determine if weevil depredation impacts seedling vigor. Seeds were grown under greenhouse conditions for a period of 8 weeks. X-ray imaging was used as a nondestructive method for determining seed condition prior to germination. The combined data set showed a reduction in germination percentage from 86% for sound acorns to 26% for those damaged by weevils. Seven seedling metrics were evaluated for differences between seedlings originating from sound and those from weevil-damaged acorns. Of these metrics, the number of leaf flushes, total number of leaves, root collar diameter, shoot length, and dry mass were significantly (P< 0.05) different. Two variables, root length and shoot diameter, were not significantly different. Early allocation of resources to root establishment likely explains these findings. Because of reduced stem height and leaf number, seedlings from weevil-damaged acorns are likely to be less competitive in the forest understory than those from sound acorns.
Uncertainty associated with ecological forecasts has long been recognized, but forecast accuracy is rarely quantified. We evaluated how well data on 82 populations of 20 species of plants spanning 3 continents explained and predicted plant population dynamics. We parameterized stage‐based matrix models with demographic data from individually marked plants and determined how well these models forecast population sizes observed at least 5 years into the future. Simple demographic models forecasted population dynamics poorly; only 40% of observed population sizes fell within our forecasts’ 95% confidence limits. However, these models explained population dynamics during the years in which data were collected; observed changes in population size during the data‐collection period were strongly positively correlated with population growth rate. Thus, these models are at least a sound way to quantify population status. Poor forecasts were not associated with the number of individual plants or years of data. We tested whether vital rates were density dependent and found both positive and negative density dependence. However, density dependence was not associated with forecast error. Forecast error was significantly associated with environmental differences between the data collection and forecast periods. To forecast population fates, more detailed models, such as those that project how environments are likely to change and how these changes will affect population dynamics, may be needed. Such detailed models are not always feasible. Thus, it may be wiser to make risk‐averse decisions than to expect precise forecasts from models.
Habilidad de los Modelos Matriciales para Explicar el Pasado y Predecir el Futuro de las Poblaciones de Plantas
Stage projection matrices for 21 species of woody plants and 45 herbaceous perennials were compiled. Each matrix was divided into six regions representing: 1) recruitment of seeds to the seed pool; 2) recruitment of seedlings or juveniles from current seed production 3) clonal growth; 4) retrogression, due to plants decreasing in size or reverting in stage; 5) stasis, (survival from one year to the next in the same stage class); 6) progression to later stage classes. Matrix analysis was due to calculate the finite rate of increase λ for each population and to calculate the elasticities of each transition coefficient in the matrices. Elasticities were summed within each of the six regions of the matrix to give measures (E1-E6) of the importance of each component of the life cycle to λ and fitness. Seedling recruitment was more important in herbs than woody plants. Retrogression occurred only in herbs, particularly those with a tuber. Stasis was most important in woody plants. Progression was more important than fecundity in almost all species. Trade-offs among life cycle components were determined. As a whole, r was positively correlated with elasticities for fecundity (E1 +E2 and growth (E3 + E6) and negatively correlated with survival (E4 + E5). In clonal herbs, fecundity and clonal growth were negatively correlated. The division of elasticities into three major components (growth G = E3 + E6; fecundity, F = E1 + E2; and survival, L = E4 + E5) allowed the authors to construct triangular plots in G-L-F space. Iteroparous forest herbs, iteroparous herbs from open habitats, semelparous herbs and woody plants, each occupied a distinct position in G-L-F space. -from Authors
S.B. Vander Wall et al. (Ecology, 86:801806 (2005)) criticized seed dispersal studies that use seed removal as a proxy for seed predation, because secondary dispersal processes following removal are important to seed fates for many plants. We compared seed removal rates with direct estimates of seed mortality and another mortality index, based on a 3-year experiment that included five temperate deciduous tree species and four exclosure treatments designed to identify effects of different seed consumer groups. Patterns of seed removal rates generally did not match patterns of mortality. Removal and mortality rates were both highest in seed-poor years, indicative of response to food limitation, but annual food abundance interacted with seed type differently for removal rates than for mortality rates. The effect of exclosure type (access by different consumers) on removal rates was opposite its effect on mortality rates seeds were removed fastest from exclosures that allowed access to tree squirrels (genus Sciurus L., 1758), but these seeds had the lowest mortality because Sciurus is an important seed disperser. We discuss types of studies in which seed removal may be a reasonable index of seed mortality, and we stress the importance of justifying assumptions concerning links between removal and predation.
A profusion of fruit forms implies that seed dispersal plays a central role in plant ecology, yet the chance that an individual
seed will ultimately produce a reproductive adult is low to infinitesimal. Extremely high variance in survival implies that
variations in fruit production or transitions from seed to seedling will contribute little to population growth. The key issue
is that variance in survival of plant life-history stages, and therefore the importance of dispersal, differs greatly among
and within plant communities. In stable communities of a few species of long-lived plants, variances in seed and seedling
survival are immense, so seed-to-seedling transitions have little influence on overall population dynamics. However, when
seedlings in different circumstances have very different chances of survival—in ecological succession, for example, or when
dispersed seeds escape density-dependent mortality near parent trees—the biased survival of dispersed seeds or seedlings in
some places rather than others results in pervasive demographic impacts.
A high number of tree species, low density of adults of each species, and long distances between conspecific adults are characteristic of many low-land tropical forest habitats. I propose that these three traits, in large part, are the result of the action of predators on seeds and seedlings. A model is presented that allows detailed examination of the effect of different predators, dispersal agents, seed-crop sizes, etc. on these three traits. In short, any event that increases the efficiency of the predators at eating seeds and seedlings of a given tree species may lead to a reduction in population density of the adults of that species and/or to increased distance between new adults and their parents. Either event will lead to more space in the habitat for other species of trees, and therefore higher total number of tree species, provided seed sources are available over evolutionary time. As one moves from the wet lowland tropics to the dry tropics or temperate zones, the seed and seedling predators in a habitat are hypothesized to be progressively less efficient at keeping one or a few tree species from monopolizing the habitat through competitive superiority. This lowered efficiency of the predators is brought about by the increased severity and unpredictability of the physical environment, which in turn leads to regular or erratic escape of large seed or seedling cohorts from the predators.
In many interspecific interactions, the balance of costs and benefits varies with ecological circumstances. As a prominent example, seed-caching granivores may act as seed predators and reduce plant recruitment or as seed dispersers and increase recruitment, making it difficult to interpret whether differences in seed removal by granivores would harm or benefit plant populations. We used a heuristic model to evaluate the outcome of plant-granivore interactions, using commonly measured field data: probability of seedling emergence when granivores are excluded, and emergence of cached and uneaten seeds. Published studies to date suggest that the outcome of plant-rodent interactions tends weakly towards mutualism, but differs among particular plant–granivore pairs and ecological conditions, supporting the notion of context-dependence. A modeling framework also allowed us to distinguish parameters that affect the qualitative outcome of plant–granivore interactions from those that do not. Similar approaches would facilitate more efficient and cost-effective evaluation of complex species interactions.
Three plots with different aspects and slope characteristics were surveyed in 1999 and 2000 to clarify the spatial and temporal variations in insect-infested acorn fall patterns in a Quercus liaotungensis Koidz. forest in the Dongling Mountain region, North China. There was a significant difference in the proportion of infested acorns in the three plots in a low crop year, but not in a mast year. Within oakwoods on the southeast-facing slope, the insect infestation rate on the upper slope was significantly higher than on the lower slope, but not in the northwest-facing plot. Infestation rate in the low crop year in all three plots was significantly higher than infestation in the mast year. Most of the early fallen acorns had a higher proportion of insect infestation, and in the mast year it was much more obvious than in the low crop year. The proportion of infested acorns in seed bank along the topographic gradient showed a similar decreasing trend with acorn fall time, but the proportion on the upper slope was the highest and the proportion on the lower slope was the lowest. Larval emergence from acorns commenced just after acorns fell from the trees and lasted for 40–50 days, with peak emergence occurring from 24 to 32 days after acorn rain began. We conclude that insect-infested acorn distribution in Q. liaotungensis shows spatial and temporal heterogeneity, and an early drop of infested acorns can be a short-term defensive strategy against insect infestation.
Deer have expanded their range and increased dramatically in abundance worldwide in recent decades. They inflict major economic losses in forestry, agriculture, and transportation and contribute to the transmission of several animal and human diseases. Their impact on natural ecosystems is also dramatic but less quantified. By foraging selectively, deer affect the growth and survival of many herb, shrub, and tree species, modifying patterns of relative abundance and vegetation dynamics. Cascading effects on other species extend to insects, birds, and other mammals. In forests, sustained overbrowsing reduces plant cover and diversity, alters nutrient and carbon cycling, and redirects succession to shift future overstory composition. Many of these simplified alternative states appear to be stable and difficult to reverse. Given the influence of deer on other organisms and natural processes, ecologists should actively participate in efforts to understand, monitor, and reduce the impact of deer on ecosystems.
The high diversity of mutualisms has probably limited the development of a general theory about their evolution and their stability. Here we review conflicts of interests occurring in the 13 known nursery pollination systems, where pollinators reproduce within the inflorescence they pollinate. We found three main conflicts of interest between mutualists that correspond to the following evolutionary questions: 1) Why do plants not kill their pollinators’ larvae? 2) Why do pollinators visit deceptive flowers? and 3) Why do pollinators pollinate? We show that the reproductive system of the plant is strongly correlated not only with the set of conflicts of interests that actually occur but also with the proximal resolution of these conflicts. In many cases the evolutionary stability of nursery pollination mutualisms relies on the avoidance of intra-specific competition among pollinator larvae. This stabilizing factor could perhaps also explain the absence of overexploitation in other mutualisms.
Many forests of eastern North American are undergoing a species composition shift in which maples (Acer spp.) are increasingly important while oak (Quercus spp.) regeneration and recruitment has become increasingly scarce. This dynamic in species composition occurs across a large and geographically complex region. The elimination of fire has been postulated as the driver of this dynamic; however, some assumptions underlying this postulate have not been completely examined, and alternative hypotheses remain underexplored. Through literature review, and a series of new analyses, we examined underlying assumptions of the “oak and fire” hypothesis and explored a series of alternative hypotheses based on well-known ecosystem drivers: climate change, land-use change, the loss of foundation and keystone species, and dynamics in herbivore populations. We found that the oak–maple dynamic began during a shift in climate regime-from a time of frequent, severe, multi-year droughts to a period of increased moisture availability. Anthropogenic disturbance on the landscape changed markedly during this same time, from an era of Native American utilization, to a time characterized by low population densities, to Euro-American settlement and subsequent land transmogrification. During the initiation of the oak-maple dynamic, a foundation species, the American chestnut, was lost as a canopy tree across a broad range. Several important browsers and acorn predators had substantial population dynamics during this period, e.g. white-tailed deer populations grew substantially concurrent with increasing oak recruitment failure. In conclusion, our analyses suggest that oak forests are reacting to marked changes in a suite of interlocking factors. We propose a “multiple interacting ecosystem drivers hypothesis”, which provides a more encompassing framework for understanding oak forest dynamics.
Aim The character and distribution of American chestnut ( Castanea dentata ) populations in New England are analysed to identify the extent to which the introduced chestnut blight and historic land use practices have affected chestnut distribution and life history.
Location The study focuses on chestnut in Connecticut and Massachusetts but includes analysis of data related to other Castanea species in North America and Europe.
Methods The ecology and palaeoecology of chestnut is investigated using a range of techniques, including examination of the growth form of chestnut trees in plantations located away from blight, mapping of chestnut sprouts and blight‐killed trees at various locations, anatomical examination of chestnut stems, analysis of early forestry practices, identification of changes in the relative abundance of chestnut pollen in Holocene lake sediments and comparison of American chestnut with other Castanea species.
Results Examination of chestnut sprouts surviving in the former range of that species shows that most sprouts originated from suppressed seedlings and that environmental factors severely limited the survival of sprouts from large blight‐killed trees. Palynological data show that chestnut was either present in very low populations or entirely absent from New England and only became abundant after about 2500 years ago. All factors suggest that chestnut abundance is related to the natural disturbance cycle, while human‐induced transformations of the landscape and the introduction of chestnut blight have further transformed the character of chestnut in New England.
Main conclusions Most surviving chestnut sprouts in New England forests represent old seedlings that have continued to re‐sprout since establishment before the introduction of blight nearly 100 years ago. The growth form of these sprouts ensures a minimal substrate of bark tissue for blight establishment so that blight has a relatively minor effect on seedling–sprout survival. The identification of modern chestnut sprouts as old seedlings indicates that the observed distribution of sprouts in New England woodlands is strongly influenced by land use conditions and especially field abandonment at the time when chestnut blight arrived from its point of introduction.
Loss of seeds to consumers is common in plant communities, but the degree to which these losses influence plant abundance or population growth is often unclear. This is particularly the case for postdispersal seed predation by rodents, as most studies of rodent seed predation have focused on the sources of spatiotemporal variation in seed loss but not quantified the population consequences of this loss. In previous work we showed that seed predation by deer mice (Peromyscus maniculatus) substantially reduced seedling recruitment and establishment of Lithospermum ruderale (Boraginaceae), a long-lived perennial forb. To shed light on how rodent seed predation and the near-term effects on plant recruitment might influence longer-term patterns of L. ruderale population growth, we combined experimental results with demographic data in stage-based population models. Model outputs revealed that rodent seed predation had a significant impact on L. ruderale population growth rate (lambda). With the removal of postdispersal seed predation, the projected population growth rates increased between 0.06 and 0.12, depending on site (mean deltalambda across sites = 0.08). Seed predation shifted the projected stable stage distribution of populations from one with a high proportion of young plants to one in which larger adult size classes dominate. Elasticities of vital rates also changed, with germination and growth of seedlings and young plants becoming more important with the removal of seed predation. Simulations varying the magnitude of seed predation pressure while holding other vital rates constant showed that seed predation could lower lambda even if only 40% of available seeds were consumed. These results demonstrate that rodent granivory can be a potent force limiting the abundance of a long-lived perennial forb.
Oak regeneration failure in the hardwood forests of eastern North America has been well documented. Silvicultural treatments of prescribed burning and canopy thinning (fire surrogate) are being studied as possible management tools to promote oak regeneration in oak dominated forests. We examined oak seed production and predation by acorn weevils (Coleoptera: Curculionidae) from two experimental forests in southeastern Ohio, under four different silvicultural treatments. Treatments consisted of replicate stands that received a prescribed burning, thinning, and thinning followed by a prescribed burn, in addition to an untreated control. Initial thinning and prescribed burning treatments were conducted in winter/spring of 2000–2001, with a second prescribed burn in spring 2005. Within each treatment unit, nine black oak and nine chestnut oak trees were selected for study and two 0.25 m2 seed fall traps were placed beneath each tree. Seed collections were completed for five seasons from 2001 to 2005. Data were analyzed using a randomized complete block design, utilizing a mixed-model repeated measures analysis. Overall, stand level treatments resulted in only a slight increase in acorn production (in the burn and thin + burn stands) relative to the control; however, this response was species specific. Masting, the intermittent production of large seed crops, was evident in our study though occurrence of a mast event was not influenced by the treatments. Chestnut oak displayed greater yearly, as well as individual variation in seed production than did black oak. Treatments had no effect on predation by acorn weevils. Predation percentages were lowest during mast years for chestnut oak; however, percentages were mostly stable for black oak across all 5 years. Our data suggest that variation due to climate and genotype are more influential in affecting seed production and masting cycles than are stand level disturbances. Thus, stand level management activities are not likely to have a large influence on oak seed production and oak regeneration must be more strongly influenced at other stages of its life history.
Understanding the role of consumers in plant population dynamics is important, both conceptually and practically. Yet, while the negative effects of herbivory on plant performance have been well documented, we know much less about how individual- level damage translates to impacts on population growth or whether spatial variation in herbivory affects patterns of plant distribution. We studied the role of insect herbivory in the dynamics and distribution of the tree cholla cactus (Opuntia imbricata), a long-lived perennial plant, across an elevational gradient in central New Mexico, USA, from low-elevation grassland (1670 m) to a grassland-mountain transition zone (1720 m) to the rocky slopes of the Los Pinos Mountains (1790 m). Tree cholla density increased significantly with elevation, while abundance of and damage by a suite of native, cactus-feeding insects decreased. We combined field experiments and demographic models to test the hypothesis that systematic spatial variation in chronic insect herbivory limits the tree cholla distribution to a subset of suitable habitat across the gradient. Our results support this hypothesis. We found that key demographic functions (survival, growth, fecundity) and the responses of these functions to experimental reductions in insect herbivory varied across the gradient. The effects of insect exclusion on plant growth and seed production were strongest in the low- elevation grassland and decreased in magnitude with increasing elevation. We used the experimental data to parameterize integral projection models (IPM), which predict the asymptotic rate of population increase (k). The modeling results showed that insect herbivory depressed k and that the magnitude of this effect was context-dependent. The effect of insect herbivory on population growth was strongest at low elevation (Dklow ¼ 0.095), intermediate at mid elevation (Dkmid ¼ 0.046), and weakest at high elevation (Dkhigh ¼� 0.0089). The total effect of insects on k was due to a combination of reductions in growth and in fecundity and their combination; the relative contribution of each of these effects varied spatially. Our results, generated by experimental demography across a heterogeneous landscape, provide new insights into the role of native consumers in the population dynamics and distribution of abundance of long-lived native plants.
The conditional mutualism between scatterhoarders and trees varies on a continuum from mutualism to antagonism and can change across time, space, and among species. We examined four tree species (red oak [Quercus rubra], white oak [Q. alba], American chestnut [Castanea dentata] and hybrid chestnut [C. dentata x C. mollissima) across five sites and three years to quantify the variability in this conditional mutualism. We used a published model to compare the rates of seed emergence with and without burial to the probability that seeds will be cached and left uneaten by scatterhoarders in order to quantify variation in the conditional mutualism that can be explained by environmental variation among sites, years, species, and seed provenance within species. All species tested had increased emergence when buried. However, comparing benefits of burial to the probability of caching by scatterhoarders indicated a mutualism in red oak, while white oak was nearly always antagonistic. Chestnut was variable around the boundary between mutualism and antagonism indicating a high degree of context dependence in the relationship with scatterhoarders. We found that different seed provenances did not vary in their potential for mutualism. Temperature did not explain microsite differences in seed emergence in any of the species tested. In hybrid chestnut only, emergence on the surface declined with soil moisture in the fall. By quantifying the variation in the conditional mutualism that was not caused by changes in scatterhoarder behavior, we show that environmental conditions and seed traits are an important and underappreciated component of the variation in the relationship between trees and scatterhoarders.
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Seed hoarders show different hoarding and eating responses towards insect-infested seeds that can affect the fitness of both the seeds and insects. It remains unclear how seed hoarders adopt different strategies in eating and hoarding infested seeds with and without larvae concealed inside. Here we investigated hoarding and eating responses of Edward's long-tailed rats Leopoldamys edwardsi (scatter hoarders) to weevil infestation of cork oak Quercus variabilis seeds within outdoor enclosures. We provided sound seeds, larvae-emerged seeds, (infested seeds where larvae have emerged) and larvae-concealed seeds (infested seeds with larvae concealed inside) to subjects independently (each seed type presented separately) and in pairwise combinations (sound and larvae-emerged seeds; sound and larvae-concealed seeds). We found that L. edwardsi removed, scatter hoarded and ate fewer larvae-emerged seeds than sound seeds. No difference was found between sound seeds and larvae-concealed seeds. These results suggest that sound and larvae-concealed seeds are more favored by L. edwardsi than larvae-emerged seeds. We posit that not only plants but also insects may benefit from the behavioral responses of hoarders to seed infestation under natural conditions.
A mechanistic understanding of seed movement and survival is important both for the development of theoretical models of plant population dynamics, spatial spread, and community assembly, and for the conservation and management of plant communities under global change. While models of wind-borne seed dispersal have advanced rapidly over the past two decades, models for animal-mediated dispersal have failed to make similar progress due to their dependence on interspecific interactions and complex, context-dependent behaviours. In this review, we synthesize the literature on seed dispersal and consumption by scatter-hoarding, granivorous rodents and outline a strategy for development of a general mechanistic seed-fate model in these systems. Our review decomposes seed dispersal and survival into six distinct sub-processes (exposure, harvest, allocation, preparation, placement, and recovery), and identifies nine intermediate (latent) variables that link physical state variables (e.g. seed and animal traits, habitat structure) to decisions regarding seed allocation to hoarding or consumption, cache placement and management, and deployment of radicle-pruning or embryo excision behaviours. We also highlight specific areas where research on these intermediate relationships is needed to improve our mechanistic understanding of scatter-hoarder behaviour. Finally, we outline a strategy to combine detailed studies on individual functional relationships with seed-tracking experiments in an iterative, hierarchical Bayesian framework to construct, refine, and test mechanistic models for context-dependent, scatter-hoarder-mediated seed fate.
A profusion of fruit forms implies that seed dispersal plays a central role in plant ecology, yet the chance that an individual seed will ultimately produce a reproductive adult is low to infinitesimal Extremely high variance in survival implies that variations in fruit production or transitions from seed to seedling will contribute little to population growth. The key issue is that variance in survival of plant life-history stages, and therefore the importance of dispersal, differs greatly among and within plant communities. In stable communities of a few species of long-lived plants, variances in seed and seedling survival are immense, so seed-to-seedling transitions have little influence on overall population dynamics. However, when seedlings in different circumstances have very different chances of survival - in ecological succession, for example, or when dispersed seeds escape density-dependent mortality near parent trees - the biased survival of dispersed seeds or seedlings in some places rather than others results in pervasive demographic impacts.
The successful development of early stages of blight-resistant hybrid stock has increased hopes for restoration of American chestnut (Castanea dentata) to eastern North American forests. However, these forests have undergone substantial ecological change in the century since the functional extirpation of American chestnut, and it remains unknown to what extent American chestnut will be able to recolonize contemporary forests. In particular, high densities of white-tailed deer (Odocoileus virginianus) and competition with mesophytic tree species such as maple (Acer) may impede chestnut regeneration, much as they affect oak (Quercus). We used a split-plot analysis of variance (ANOVA) design to examine the effects of canopy gaps and herbivory on survival and growth of third generation backcrossed (BC3) hybrid chestnut seedlings over two growing seasons in central Indiana, U.S.A. Only 4 of 588 (0.7%) seedlings in closed-canopy plots survived to the end of the study, as opposed to 264 of 589 (45%) seedlings in gap plots. Within the gap treatment, fencing was associated with reduced chestnut survival as well as reduced herbivory and increased cover of non-chestnut vegetation. Our results indicate that herbivory may indirectly benefit chestnut regeneration by suppressing competition. However, this beneficial effect is likely context-dependent and additional work is needed to establish the conditions under which it occurs.
The importance of herbivorous insects for plant demography and fitness is still controversial. The fact that their effect has not be fully evaluated within the context of the complete plant life cycle contributes to this controversy. This study is the first to test directly the influence of flower and seed consumption by a guild of inflorescence-feeding insects on the demography and fitness of a native plant over its whole life cycle within its natural environment. First, we discuss the evidence required to assess the effect of inflorescence-feeding on sequential stages in the plant life cycle. Then, we present the detailed results of two experiments that quantify the effect of inflorescence-feeding insects on seeds, seedlings, juveniles, and subsequent flowering progeny of Cirsium canescens (Platte thistle), a native, monocapic perennial species of Sandhills prairie, Nebraska. Exclusion of inflorescence-feeding insects by insecticide had four main effects. Total seed output increased (P
This book presents current knowledge of seed fate, in both natural and human-disturbed landscapes, from various regions of the world. Habitats considered range from mountains and arid deserts in the temperate zone, to savannas and lowland rain forests in tropical regions. Particular attention is paid to plant diversity conservation when seed removal is affected by factors such as hunting, habitat fragmentation or intensive logging.
Interactions between plants and scatter-hoarding animals may shift from mutualism to predation as a function of the resources available to those animals. Because seed species differ in their nutrient content and defenses to predation, resource selection and cache management by scatter-hoarders, and thus seed fate, may also depend on the relative availability of different seed types. We tracked the fates of tagged Castanea dentata, Quercus alba, and Q. rubra seeds presented to rodents in pairwise combinations and found that C. dentata, which has moderate dormancy prior to germination, survived better in the presence of Q. alba (no dormancy) than with Q. rubra (longer dormancy). Decisions made by scatter-hoarders in response to the composition of available seed resources can alter the relationship between masting and seed dispersal effectiveness in individual tree species and may have influenced the evolution of asynchrony among species-specific masting patterns in temperate forests. In theory, preferential allocation of certain seed species to storage or consumption could also result in indirect apparent predation by one seed species on another.
Invasive diseases can interact with other disturbances and the environment to affect host population decline, realized niche, and interspecific interactions. In my dissertation, I sampled oak-chestnut forests first sampled for vegetation composition 66-89 years ago and conducted a field experiment to evaluate the effects of variance in environment, in invasive disease prevalence, in logging history, and in deer browsing on the abundance, realized niche, and survival of a declining host tree in southwestern Virginia, USA. The disease and host I studied were American chestnut (Castanea dentata) and chestnut blight (Cryphonectria parasitica), a disease introduced in the early 1900’s that received national attention due to chestnut’s timber value and ubiquity. Two sources of mortality studied here, in addition to chestnut blight, had significant effects on chestnut abundance and survival. Current chestnut abundance was significantly lower in areas where preblight chestnuts were logged than in areas where they were not logged. Since chestnut is highly palatable to deer, high levels of deer browsing significantly increased chestnut mortality rate, which affects its interactions with other understory species. Pre-blight chestnut abundance was not predictive of post-blight chestnut abundance. Multiple regressions of principal components representing the environment with pre-blight and current chestnut abundance demonstrated that chestnuts are now more restricted to iii areas with slopes and aspects receiving high light (southern to western facing slopes) and xeric species than pre-blight chestnut was. When assessing a study area with larger variation in elevation, chestnut abundance was also related to elevation and moderately acidic soil pH. Although chestnut blight was predicted to be less prevalent in environments where chestnut abundance was higher, this prediction was not supported. Chestnut blight prevalence was not significantly related to any measured environmental variable, was independent of chestnut density, and infected an average of 150f chestnuts, regardless of chestnut abundance. This work highlights the drastic change in chestnut’s interactions with the environment now that chestnut blight is endemic across its range and the need for a better understanding of chestnut blight’s spatiotemporal dynamics.
This second edition contains ten chapters dealing with oak ( Quercus )-related literature published since the first edition. Included are the relatively new subject areas of forest biomass use for fuel, the importance of carbon sequestration by forests, and how climate change is expected to affect the distribution of oaks and associated tree species. The chapters are grouped into three parts. The first part contains three chapters covering the ecological characteristics and distribution of oak species, the various kinds of oak forests in the USA and how they have been classified, and their history of human use. The second part comprises three chapters covering site productivity and stand development. An understanding of the productive capacity of oak forests is central to a broad spectrum of issues related to their management and potentialities, not only for timber but also for wildlife and other values, including carbon sequestration. The third part comprises four chapters on silvicultural methods and the growth and yield of oak forests.
Cryphonectria parasitica populations in North America are genetically more similar to populations in Japan than in China, consistent with speculations that this fungus was introduced from Japan. Restricted gene flow and genetic drift are probably the dominant evolutionary forces shaping North American populations, with c20% of gene diversity due to differences among populations (GST = 0.20). Two populations of C. parasitica in Michigan and one population in Italy are primarily clonal in structure. Sexual reproduction appears to be common in populations in E North America, although most of these populations deviate significantly from random mating. Deviations from random mating are most likely due to self-fertilization (uniparental inbreeding), restricted dispersal of male gametes, and mating between individuals that are more closely related genetically than would be expected by chance (biparental inbreeding). Aggregations of similar genotypes in space suggest that populations of C. parasitica may be structured into genetic neighborhoods by restricted dispersal. -from Author
Hunting affects a considerably greater area of the tropical forest biome than deforestation and logging combined. Often even large remote protected areas are depleted of a substantial proportion of their vertebrate fauna. However, understanding of the long-term ecological consequences of defaunation in tropical forests remains poor. Using tree census data from a large-scale plot monitored over a 15-year period since the approximate onset of intense hunting, we provide a comprehensive assessment of the immediate consequences of defaunation for a tropical tree community. Our data strongly suggest that over-hunting has engendered pervasive changes in tree population spatial structure and dynamics, leading to a consistent decline in local tree diversity over time. However, we do not find any support for suggestions that over-hunting reduces above-ground biomass or biomass accumulation rate in this forest. To maintain critical ecosystem processes in tropical forests increased efforts are required to protect and restore wildlife populations.
1. Herbivores and granivores represent one of the most influential drivers of plant abundance and population dynamics. Their effect can be, in turn, modulated by biotic or abiotic factors such as community composition, habitat characteristics or space heterogeneity.
2. Recent approaches to the study of herbivore and granivore impacts on plants have considered the combined action of multiple herbivore species, the effect of herbivores on several plant life stages or the effect of environmental gradients on these interactions. However, studies addressing the effect of multiple herbivore species on different plant life stages are still lacking.
3. We estimated the combined effect of multiple exotic herbivores (European rabbits, Oryctolagus cuniculus; black rats, Rattus rattus; and house mouse, Mus musculus) on four different life stages of an endangered plant species (Medicago citrina, Fabaceae). Mortality for seed, seedling and sapling was estimated at three types of plots (open, rat exclusion and rat + rabbit exclusion) replicated at four sites (N = 3 per site and treatment) within Cabrera Island (Balearic Islands, western Mediterranean). Browsing of reproductive adults was simulated under common-garden conditions (Sóller Botanic Garden, Mallorca Island) and its effect on reproductive effort and success measured.
4. European rabbits and black rats had complementary impacts on the different life stages of M. citrina. These included independent effects on different life stages (seed predation by rats, seedling predation by rabbits), which resulted in multiplicative increases in plant mortality, and concurrent effects on the same life stage (sapling predation). In addition, the simulated-herbivory experiment showed that a low rate of canopy removal (25% of initial biomass) already causes a strong decrease in fruit set (from 54% to 30%), but increasing rates of canopy removal do not increase this effect.
5. Synthesis. Our results stress the importance of considering the combined effects of different herbivores on several life stages of the plant’s life cycle and their consecutive effects on population dynamics. From an applied point of view, future reintroduction attempts of M. citrina in Cabrera Island should consider measures to either control the populations of both exotic herbivores or mitigate their impacts on the earlier recruitment stages of the plant (seeds, seedlings and saplings).
We propose a conceptual framework for restoration of threatened plant species that encourages integration of technological, ecological, and social spheres. A sphere encompasses ideas relevant to restoration and the people working within similar areas of influence or expertise. Increased capacity within a sphere and a higher degree of coalescing among spheres predict a greater probability of successful restoration. We illustrate this with C astanea dentata , a foundation forest tree in North America that was annihilated by an introduced pathogen; the species is a model that effectively merges biotechnology, reintroduction biology, and restoration ecology. Because of C . dentata 's ecological and social importance, scientists have aggressively pursued blight resistance through various approaches. We summarize recent advancements in tree breeding and biotechnology that have emerged from C. dentata research, and describe their potential to bring new tools to bear on socio‐ecological restoration problems. Successful reintroduction of C . dentata will also depend upon an enhanced understanding of its ecology within contemporary forests. We identify a critical need for a deeper understanding of societal influences that may affect setting and achieving realistic restoration goals. Castanea dentata may serve as an important model to inform reintroduction of threatened plant species in general and foundation forest trees in particular.
Contents
Summary
378
I.
Castanea dentata as a model at the intersection of reintroduction biology and restoration ecology
379
II.
Technological approaches for recovery of Castanea dentata
381
III.
Ecology of the historically dominant Castanea dentata in contemporary forests
385
IV.
Societal challenges for successful reintroduction of Castanea dentata
388
V.
Conclusions
389
Acknowledgements
389
References
389
Predispersal seed predation by a moth larva, Eupithecia immundata Leinig & Zeller (Geometridae), and its impact on population dynamics of the perennial herb Actaea spicata L. were studied during 7 years in deciduous and mixed coniferous forest populations in southeastern Sweden. Twelve population matrix models were constructed based on transition probabilities among six stage classes in the populations. The fraction of seeds consumed varied between 21% and 80% but was not significantly correlated with seed production. Experimental seed addition compensating for seed losses caused by the seed predator resulted in increased seedling emergence in one of the populations. Population growth rate was positive (λ > 1) in 1 of 6 years in the deciduous forest population and in 2 of 6 years in the mixed coniferous forest population. Survival among reproductive individuals contributed most strongly to λ in both populations. In some years, the projected λ changed from positive to negative values because of seed loss caused by E. immundata. However, results suggest that seed predation did not affect λ to any large extent, despite a considerable seed predation. This study is one of the rare efforts to estimate the impact of seed predation on population dynamics of perennial plants.
Concentrating on material published from 1984-1987, examines 1) the impact of insect feeding on different aspects of plant performance, paying particular attention to flowering, fruit production, post-dispersal seed mortality, seedling mortality, defoliation, growth and reproduction, competitive ability, and death rate of established plants; and 2) reviews the impact of insect feeding on plant population dynamics, focusing on the release of specialist insect herbivores against target weed species in classical biological control projects, and the exclusion of insect herbivores from natural plant communities by repeated application of insecticides. -P.J.Jarvis
Cited By (since 1996): 27, Export Date: 6 May 2012, Source: Scopus
1. Despite a large literature documenting the effects of mutualists and/or antagonists on plant performance, we still have limited insight into the strength of these interactions, as this involves quantifying how one species influences the population dynamics of another.
2. Here, we use data from two example systems, Cynoglossum officinale and Calathea ovandensis, to illustrate how experiments, demographic data and stage-based population models can be combined to estimate interaction strength of insect herbivores on plants. Because many plant populations may not be at equilibrium, we conduct transient analyses and contrast these results to more traditional asymptotic results.
3. We calculate three metrics of interaction strength, Δλasymptotic, the change in asymptotic annual per capita plant population growth rate resulting from herbivore exclusion, Δλ(t), the change in transient λ caused by herbivores at t = M (where M = the time of their maximum effect during the transient phase) and Δλtransient, a time-averaged effect of consumers on λ across the entire transient phase.
4. Fairly strong impacts of insect consumers on plant fecundity do not translate similarly to Δλasymptotic. Results show that Δλ(t) can be larger (or smaller) than Δλasymptotic but in our examples Δλtransient was similar in magnitude to Δλasymptotic.
5. The transient effects of consumers on λ were driven by changes in the elasticity of fecundity across the transient phase. These effects were dynamic even though consumer impacts on demography and vital rates were held constant. The importance of particular stages and transitions to annual population growth vary during the transient phase.
6.Synthesis. We describe three metrics of interaction strength, Δλasymptotic, Δλ(t) and Δλtransient. These metrics have several advantages over more commonly used trait or performance measures that quantify the outcome of interactions. We illustrate how the transient impacts of consumers on λ are dynamic, with the changing stage distribution of a population and transient elasticities driving these effects. More generally, this study shows that the impacts of animals on plant performance do not translate equivalently to plant population growth, thereby underscoring the importance of using population models to extend the inference of individual-level experiments.
Post-dispersal seed predation is only one of many factors underlying plant demography and evolution. Nevertheless, the generalist feeding habits of many post-dispersal seed predators and the limited ability of plants either to compensate for or to respond to post-dispersal seed losses directly suggest that post-dispersal seed predation may have a considerable impact on plant populations. Seed predators probably have little direct influence on the demography of plants that regenerate exclusively by vegetative means or are buffered by a large active seed bank, but such species are only a minority in most plant communities.In general, ants are significant post-dispersal seed predators in arid and semi-arid ecosystems while they act mainly as seed dispersers rather than as predators in temperate ecosystems. Although studies have probably underestimated the importance of invertebrates and birds as seed predators, rodents appear to have greater potential to influence seed dynamics, and are particularly important in temperate ecosystems. For example, production of mast seed crops is more effective at satiating specialist invertebrate seed predators than generalist vertebrates, and recruitment may be limited by post-dispersal seed predation even during mast years.Both spatial variation in post-dispersal seed predation and differences in predation between species are important elements which facilitate the coexistence of different plant species. Where microsites are limiting, selective post-dispersal seed predators can influence pre-emptive competition for these microsites. Seed size determines the extent of density-dependent predation and the exploitation of buried seed. This suggests that post-dispersal seed predators may also play a role in the evolution of seed characteristics. However, conclusions regarding the ecological and evolutionary impact of post-dispersal seed predators will remain speculative without a more substantial empirical base.
Acorn predation by insects and its effects on seedling establishment were investigated among three co-occurring oak species (Quercus variabilis, Q. serrata and Cyclobalanopsis glauca) in a subtropical evergreen broadleaved forest, Southwest China. All oak species had high tannin concentration (over 10%) but differed in acorn mass and germination schedule. We hypothesized that the defensive traits in acorns (e.g. seed mass, tannins and germination schedule) act together to reduce damage from insect seed predators. Q. variabilis had significantly lower predation (51.2%) than either Q. serrata (71.4%) or C. glauca (73.8%). Seedling establishment decreased with increasing injury for all oak species studied. As much as 43% of insect infested acorns germinated and established as viable seedlings despite extensive acorn mortality due to insect seed predators. The larger seed mass of Q. variabilis appears to tolerate insect infestation and retain enough stored reserves for seed germination and early growth of seedlings. Autumn germination also benefited Q. variabilis and Q. serrata, allowing escape from both direct and ancillary effects of insect feeding by fast reserve shifting to immediate germination of mature acorns. Our results indicate that large seed mass, tannin and autumn germination act together to ameliorate effects of insect seed predation, and the joint evolution of resistance and tolerance (as well as escape through autumn germination) in acorns may be the selective consequences driven in part by interactions with insect seed predators.
The joint effects of multiple herbivores on their shared host plant have received increasing interest recently. The influence of herbivores on population dynamics of their host plants, especially the relative roles of different types of damage, is, however, still poorly understood. Here, we present a modelling approach, including both deterministic and stochastic matrix modelling, to be used in estimating fitness effects of multiple herbivores on perennial plants. We examined the effects and relative roles of two specialist herbivores, a pre-dispersal seed predator, Euphranta connexa, and a leaf-feeding moth, Abrostola asclepiadis, on the population dynamics and long-term fitness of their shared host plant, a long-lived perennial herb Vincetoxicum hirundinaria (Asclepiadaceae). We collected demographic data during 3 years and combined these data with the effects of natural levels of herbivory measured from the same individuals. We found that both seed predation and leaf herbivory reduced population growth of V. hirundinaria, but only very high damage levels changed the growth trend of the vigorously growing study populations from positive to negative. Demographic modelling indicated that seed predation had a greater impact on plant population growth than leaf herbivory. The effect of leaf herbivory was weaker and diminished with increasing level of seed predation. Evaluation of individual fitness components, however, suggested that leaf herbivory contributed more strongly to host plant fitness than seed predation. Our results emphasize that understanding the effects of a particular herbivore on plant population dynamics requires also knowledge on other herbivores present in the system, because the effect of a particular type of herbivory on plant population dynamics is likely to vary according to the intensity of other types of herbivory. Furthermore, evaluating herbivore impact from using individual fitness components does not necessarily reflect the long-term effects on total plant fitness.ZusammenfassungDie kombinierten Effekte mehrerer Herbivoren arten auf die gemeinsame Wirtspflanze erfahren derzeit ein zunehmendes Interesse. Der Einfluss der Herbivoren auf die Populationsdynamik ihrer Wirtspflanzen, besonders die relative Rolle unterschiedlicher Typen der Beschädigung, ist jedoch wenig verstanden. Hier präsentieren wir einen Modellansatz, der sowohl deterministische als auch stochastische Matrixmodelle mit einbezieht, und der verwendet wird, um die Fitnesseffekte von vielfachen Herbivoren auf perennierende Pflanzen abzuschätzen. Wir untersuchten die Auswirkungen und die relative Bedeutung von zwei spezialisierten Herbivoren, einem prädispersalen Samenprädator, Euphranta connexa, und einem blattfressenden Falter, Abrostola asclepiadis, auf die Populationsdynamik und Langzeitfitness ihrer gemeinsamen Wirtspflanze, einem langlebigen perennierenden Kraut, Vincetoxicum hirundinaria (Asclepiadaceae). Wir sammelten die demographischen Daten über drei Jahre und kombinierten diese Daten mit den Auswirkungen des natürlichen Levels der Herbivorie, gemessen an den selben Individuen. Wir fanden, dass sowohl die Samenprädation als auch die Blattherbivorie das Populationswachstum von V. hirundinaria verringerte, aber nur äußerst große Beschädigungslevel kehrten den Wachstumstrend der stark wachsenden Untersuchungspopulation vom positiven ins negative. Das demographische Modell zeigte, dass die Samenprädation einen größeren Einfluss auf das Pflanzenpopulationswachstum hatte als die Blattherbivorie. Der Effekt der Blattherbivorie war geringer und wurde mit einem steigenden Level der Samenprädation abgeschwächt. Die Bewertung der individuellen Fitnesskomponenten wies jedoch darauf hin, dass die Blattherbivorie einen größeren Anteil zur Wirtspflanzenfitness beitrug, als die Samenprädation. Unsere Ergebnisse betonen, dass das Verständnis der Auswirkungen eines bestimmten Herbivoren auf die Pflanzenpopulationsdynamik auch Wissen darüber verlangt, welche anderen Herbivoren in dem System vorhanden sind, weil der Effekt eines bestimmten Typs von Herbivorie auf die Pflanzenpopulationsdynamik, wahrscheinlich entsprechend der Intensität anderer Typen von Herbivorie, variiert. Darüber hinaus spiegelt die Bewertung des Einflusses eines Herbivoren auf Basis der Verwendung von individuellen Fitnesskomponenten nicht notwendigerweise die Langzeiteffekte auf die gesamte Pflanzenfitness.