[Show abstract][Hide abstract] ABSTRACT: Despite their value, coastal ecosystems are globally threatened by anthropogenic impacts, yet how these impacts are driven by economic development is not well understood. We compiled a multifaceted dataset to quantify coastal trends and examine the role of economic growth in China's coastal degradation since the 1950s. Although China's coastal population growth did not change following the 1978 economic reforms, its coastal economy increased by orders of magnitude. All 15 coastal human impacts examined increased over time, especially after the reforms. Econometric analysis revealed positive relationships between most impacts and GDP across temporal and spatial scales, often lacking dropping thresholds. These relationships generally held when influences of population growth were addressed by analyzing per capita impacts, and when population density was included as explanatory variables. Historical trends in physical and biotic indicators showed that China's coastal ecosystems changed little or slowly between the 1950s and 1978, but have degraded at accelerated rates since 1978. Thus economic growth has been the cause of accelerating human damage to China's coastal ecosystems. China's GDP per capita remains very low. Without strict conservation efforts, continuing economic growth will further degrade China's coastal ecosystems.
[Show abstract][Hide abstract] ABSTRACT: Interactions between plants and herbivores often vary on a geographic scale. Although theory about plant defenses and tolerance is predicated on temporal or spatial variation in herbivore damage, no single study has compared the pattern of herbivory, plant defenses and tolerance to herbivory of a single species across a latitudinal gradient. In 2002–2005 we surveyed replicate salt marshes along the Atlantic coast of the United States from Florida to Maine. At each field site we scored leaves of Iva frutescens for herbivore damage. In laboratory experiments we measured constitutive resistance and induced resistance in I. frutescens from high and low latitude sites along the Atlantic Coast. In another common garden experiment we studied tolerance to herbivory of I. frutescens from various sites. Theory predicts that constitutive resistance should matter more when damage is high, and induced resistance when herbivory is high but variable. In the field, average levels of herbivore damage, and spatial and temporal variation in herbivore damage were all greater at low versus high latitudes, indicating that constitutive as well as induced resistance should be stronger at low latitudes. Consistent with this prediction, constitutive resistance to herbivory was stronger at low latitudes. Induced resistance to herbivores was also stronger at low latitudes: it was deployed faster and lasted longer. Theory also predicts that tolerance to herbivory should be greater where average herbivory damage is greater; however, tolerance to herbivory in Iva did not depend on geographic origin. Our results emphasize the value of considering multiple ways in which plants respond to herbivores when examining geographic variation in plant–herbivore interactions.
[Show abstract][Hide abstract] ABSTRACT: Understanding the processes determining community structure is one of the major goals of ecological research. Both deterministic and stochastic processes may shape community structure. The challenge is to understand the relative influence of each type of process across different environmental conditions. We investigated the influence of deterministic and stochastic processes on plant community assembly in tidal marshes across a strong abiotic (salinity) gradient in three estuaries in Georgia, USA using probabilistic Raup–Crick community dissimilarity. Our results indicated that deterministic processes had an increasingly important influence on structuring plant communities in salt and brackish marshes, probably due to high heterogeneity of microhabitats produced by the interplay between abiotic stress and biotic interactions. In contrast, the influence of deterministic processes on plant community assembly decreased in tidal freshwater marshes, suggesting an increasingly important role of stochastic processes in plant community assembly in tidal freshwater marshes, probably due to the higher species richness, higher recruitment from seed, and lower levels of abiotic stress in these habitats. At the estuarine scale (across tidal freshwater, brackish and salt marshes in each estuary), our results suggested that deterministic processes also had a relatively important influence on shaping plant community structure. Our results illustrated that plant community structure in tidal marshes is influenced by both deterministic and stochastic processes, but that the relative influence of these two types of processes varies across estuarine landscapes.
[Show abstract][Hide abstract] ABSTRACT: An important recent advance in food web ecology has been the application of theory regarding spatial gradients to studies of the factors that affect animal population dynamics. Building on extensive studies of the Spartina alterniflora food web at the local scale, we hypothesized that geographic variation in S. alterniflora quality is an important bottom‐up control on food web structure and that geographic variation in S. alterniflora quality would interact with the presence of predators and top omnivores to mediate herbivore densities.We employed a four‐factor fully crossed experiment in which we (i) collected plants from high‐ and low‐latitude locations and grew them in a common garden and varied (ii) plant fertilization status (mimicking the plant quality differences due to marsh elevation), (iii) mesopredator density and (iv) omnivore density.Our results suggest that the single most important factor mediating insect herbivore densities is local variation in plant quality – induced in our experiment by fertilization and demonstrated repeatedly as a consequence of marsh elevation.Top‐down effects were generally weak and in those cases where predators did exert a significant suppressing effect on herbivores, that impact was itself mediated by host‐plant characteristics.Finally, despite observed variation in plant quality with latitude, and the separately measurable effects of this variation on herbivores, geographic‐scale variation in plant quality was overwhelmed by local conditions in our experiments.Synthesis. We suggest that a first‐order understanding of variation across large latitudinal ranges in the Spartina alterniflora arthropod food web must begin with local variation in plant quality, which provides strong bottom‐up forcing to herbivore populations. A second‐order understanding of the arthropod food web should consider the role of predation in controlling herbivores feeding on low‐quality plants. Finally, while latitudinal variation in plant quality probably explains some variation in herbivore densities, it is probably more of a response to herbivore pressure than a driver of the herbivore dynamics. Although extrapolating from local to geographic scales presents multiple challenges, it is an essential task in order for us to develop an understanding that is general rather than site‐specific.
Journal of Ecology 09/2013; 101(5). · 5.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Many species are expanding their distributions to higher latitudes due to global warming. Understanding the mechanisms underlying these distribution shifts is critical for better understanding the impacts of climate changes. The climate envelope approach is widely used to model and predict species distribution shifts with changing climates. Biotic interactions between species, however, may also influence species distributions, and a better understanding of biotic interactions could improve predictions based solely on climate envelope models. Along the northern Gulf of Mexico coast, USA, sub-tropical black mangrove (Avicennia germinans) at the northern limit of its distribution grows sympatrically with temperate salt marsh plants in Florida, Louisiana, and Texas. In recent decades, freeze-free winters have led to an expansion of black mangrove into salt marshes. We examined how biotic interactions between black mangrove and salt marsh vegetation along the Texas coast varied across 1) a latitudinal gradient (associated with a winter-temperature gradient); 2) the elevational gradient within each marsh (which creates different marsh habitats); and 3) different life history stages of black mangroves (seedlings versus juvenile trees). Each of these variables affected the strength or nature of biotic interactions between black mangrove and salt marsh vegetation: 1) Salt marsh vegetation facilitated black mangrove seedlings at their high-latitude distribution limit, but inhibited black mangrove seedlings at lower latitudes; 2) mangroves performed well at intermediate elevations, but grew and survived poorly in high and low marsh habitats; 3) the effect of salt marsh vegetation on black mangroves switched from negative to neutral as black mangroves grew from seedlings into juvenile trees. These results indicate that the expansion of black mangroves is mediated by complex biotic interactions. A better understanding of the impacts of climate change on ecological communities requires incorporating context-dependent biotic interactions into species range models. This article is protected by copyright. All rights reserved.
Global Change Biology 04/2013; · 8.22 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: High-latitude plants are often more palatable to herbivores than low-latitude conspecifics. Does increased plant palatability lead to better herbivore performance? Our field and laboratory work investigated (A) whether high-latitude plants have traits indicating that they should be higher-quality foods for herbivores; (B) whether geographic differences in plant quality are more important than local adaptation of herbivores. We studied 3 plant species and 6 invertebrate herbivores in U.S. Atlantic Coast. Past studies had shown high-latitude individuals of these plants are more palatable than low-latitude conspecifics. We documented plant traits and herbivore performance (body size) in the field across latitude. We collected individuals from different latitudes for factorial (plant region x herbivore region) laboratory experiments, examining how herbivore performance was affected by plant region, herbivore region, and their interaction (i.e., local adaptation). Field surveys suggested high-latitude plants were likely of higher quality to herbivores. Leaf nitrogen content in all plant species increased toward high latitudes, consistent with lower leaf C/N and higher leaf chlorophyll content at high latitudes. Furthermore, leaf toughness decreased toward higher latitudes in 1 species. The body size of 4 herbivore species increased with latitude, consistent with high-latitude leaves being of higher quality, while 2 grasshopper species showed the opposite pattern, likely due to life-history constraints. In the laboratory, high-latitude plants supported better performance in 4 herbivore species (marginal in the 5th). The geographic region where herbivores were collected affected herbivore performance in all 6 species; however, the pattern was mixed, indicating a lack of local adaptation by herbivores to plants from their own geographic region. Our results suggest that more-palatable plants at high latitudes support better herbivore growth. Given that geographic origin of either plants or herbivores can affect herbivore performance, the nature of plant-herbivore interactions is likely to change if climate change "reshuffles" plant and herbivore populations across latitude.
PLoS ONE 03/2013; 8(3):e59829. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To add to our understanding of species richness-effects on ecosystem processes, we studied the importance of species complementarity in driving decomposition in a saltmarsh in Georgia, USA. We studied pair-wise interactions of both detritivores and plant litter species and how they affect decomposition rates in an experiment located on the mid-marsh platform. Needle rush, Juncus roemerianus, had 2-3 times higher decomposition rates than cordgrass, Spartina alterniflora, or live oak, Quercus virginiana. Mixing litter types did not promote decomposition rates. Cordgrass decomposition was 1.5-times higher when periwinkles, Littoraria irrorata, were present than in detritivore-free controls. In contrast, neither coffee-bean snails, Melampus bidentatus, nor wharf crabs, Armases cinereum, increased cordgrass decomposition rates. Mixing detritivore species did not increase cordgrass mass loss beyond expected rates from an additive model. We conclude that in this system, species do not act complementarily with each other, but that decomposition rates are controlled by the dominant species of angiosperms and invertebrate detritivores.
[Show abstract][Hide abstract] ABSTRACT: A landscape-scale experiment shows that excessive nutrient levels can
cause the loss of salt marshes -- a result that was not seen in smaller
studies. This illustrates the value of large-scale, long-term studies in
ecology. See Letter p.388
[Show abstract][Hide abstract] ABSTRACT: Background/Question/Methods
Global climate changes are altering functioning of the earth’s ecosystems. Tidal saltmarshes are threatened by sea level rise and changes in weather patterns and river discharge. In order to predict the consequences of climate change on coastal communities, we need to understand how climate drivers mediate ecosystem production. In case of the tidal marshes this understanding is currently ambiguous and geographically limited to the central and northern part of the Atlantic coast.
In 2000-2011 we conducted annual surveys of Spartina alterniflora biomass in tidal marshes of the coast of Georgia. Plant length was measured at permanent plots established near the creekbank and in the midmarsh zone at 9 sites located over a salinity gradient of the Altamaha river estuary. Data on plant length was converted into biomass using site specific regressions. Spartina alterniflora biomass, air temperature, precipitation, sea level and Altamaha river discharge were analyzed using multilevel modeling.
Spartina ANPP strongly varied between years: in the creekbank zone biomass was up to 8 times greater in El Nino years as in La Nina years, in the midmarsh zone the difference was up to 4.8 fold. Biomass fluctuations resulted from changes in plant density as well as plant size. River discharge had stronger effects on creekbank than mid-marsh production and local precipitation conversely had stronger effects on mid-marsh than creekbank production. The effects of river discharge were correlated with average salinity at the site, with sites having higher average salinities (closer to the river mouth) being more strongly affected. No effect of sea level on ANPP had been detected. The decrease in salinity in high discharge years was most likely the proximate driver of the increased production. Our results differ from analyses from tidal marshes of the central and northern part of the Atlantic coast which can be due to differences in climate or hydrology and/or reflect differences in statistical and sampling methodology.
[Show abstract][Hide abstract] ABSTRACT: Background/Question/Methods
Efforts to manage grasslands in general, and coastal tallgrass prairie in particular, often focus on managing for native plant diversity, assuming that a diverse plant community will support higher trophic levels that are often conservation targets. This assumption is based upon a prevailing hypothesis in ecology that suggests herbivores should be more diverse and abundant where plants are more diverse and abundant; however, this prevailing hypothesis has led ecologists to overlook other factors that are potentially important to herbivore communities, such as micronutrient concentrations. We used a natural experiment to examine the relative importance of factors affecting herbivore community structure by measuring plant and herbivore biomass and diversity and plant, litter and soil macro- and micro-nutrients across a range of human influence in a coastal tallgrass prairie south of Houston.
These data show that plant community attributes alone do not adequately predict density or richness of herbivores, but that plant micro-nutrients (specifically calcium) are important in mediating herbivore community structure. Most strikingly, areas with low plant richness and biomass due to high amounts of calcium in soil have grasshopper diversity equal to areas with high plant species richness and biomass. Surprisingly, these areas with low plant richness and biomass had higher grasshopper abundance than areas with high plant richness and biomass. These results suggest that herbivore communities are not regulated solely by plant diversity and abundance, and therefore, managing for grassland plant diversity may not be sufficient to achieve high diversity and density of higher prairie trophic levels.
[Show abstract][Hide abstract] ABSTRACT: Oysters are ecosystem engineers in marine ecosystems, but the functions of oyster shell deposits in intertidal salt marshes are not well understood. The annual plant Suaeda linearis is associated with oyster shell deposits in Georgia salt marshes. We hypothesized that oyster shell deposits promoted the distribution of Suaeda linearis by engineering soil conditions unfavorable to dominant salt marsh plants of the region (the shrub Borrichia frutescens, the rush Juncus roemerianus, and the grass Spartina alterniflora). We tested this hypothesis using common garden pot experiments and field transplant experiments. Suaeda linearis thrived in Borrichia frutescens stands in the absence of neighbors, but was suppressed by Borrichia frutescens in the with-neighbor treatment, suggesting that Suaeda linearis was excluded from Borrichia frutescens stands by interspecific competition. Suaeda linearis plants all died in Juncus roemerianus and Spartina alterniflora stands, regardless of neighbor treatments, indicating that Suaeda linearis is excluded from these habitats by physical stress (likely water-logging). In contrast, Borrichia frutescens, Juncus roemerianus, and Spartina alterniflora all performed poorly in Suaeda linearis stands regardless of neighbor treatments, probably due to physical stresses such as low soil water content and low organic matter content. Thus, oyster shell deposits play an important ecosystem engineering role in influencing salt marsh plant communities by providing a unique niche for Suaeda linearis, which otherwise would be rare or absent in salt marshes in the southeastern US. Since the success of Suaeda linearis is linked to the success of oysters, efforts to protect and restore oyster reefs may also benefit salt marsh plant communities.
[Show abstract][Hide abstract] ABSTRACT: We examined geographic variation in the structure and function of salt marsh communities along the Atlantic and Gulf coasts of the United States. Focusing on the arthropod community in the dominant salt marsh plant Spartina alterniflora, we tested two hypotheses: first, that marsh community structure varies geographically, and second, that two aspects of marsh function (response to eutrophication and addition of dead plant material) also vary geographically. We worked at eleven sites on the Gulf Coast and eleven sites on the Atlantic Coast, dividing each coast up into two geographic areas. Abiotic conditions (tidal range, soil organic content, and water content, but not soil salinity), plant variables (Spartina nitrogen content, height, cover of dead plant material, but not live Spartina percent cover or light interception), and arthropod variables (proportional abundances of predators, sucking herbivores, stem-boring herbivores, parasitoids, and detritivores, but not total arthropod numbers) varied among the four geographic regions. Latitude and mean tidal range explained much of this geographic variation. Nutrient enrichment increased all arthropod functional groups in the community, consistent with previous experimental results, and had similar effects in all geographic regions, contrary to our hypothesis, suggesting widespread consistency in this aspect of ecosystem function. The addition of dead plant material had surprisingly little effect on the arthropod community. Our results caution against the uncritical extrapolation of work done in one geographic region to another, but indicate that some aspects of marsh function may operate in similar ways in different geographic regions, despite spatial variation in community structure.
[Show abstract][Hide abstract] ABSTRACT: Oil spills represent a major environmental threat to coastal wetlands, which provide a variety of critical ecosystem services to humanity. The U.S. Gulf of Mexico is a hub of oil and gas exploration activities that historically have impacted intertidal habitats such as salt marsh. Following the BP Deepwater Horizon oil spill, we sampled the terrestrial arthropod community and marine invertebrates found in stands of Spartina alterniflora, the most abundant plant in coastal salt marshes. Sampling occurred in 2010 as oil was washing ashore and a year later in 2011. In 2010, intertidal crabs and terrestrial arthropods (insects and spiders) were suppressed by oil exposure even in seemingly unaffected stands of plants; however, Littoraria snails were unaffected. One year later, crab and arthropods had largely recovered. Our work is the first attempt that we know of assessing vulnerability of the salt marsh arthropod community to oil exposure, and it suggests that arthropods are both quite vulnerable to oil exposure and quite resilient, able to recover from exposure within a year if host plants remain healthy.
PLoS ONE 03/2012; 7(3):e32735. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Nutrient addition to grasslands consistently causes species richness declines and productivity increases. Competition, particularly for light, is often assumed to produce this result. Using a long-term dataset from North American herbaceous plant communities, we tested whether height and clonal growth form together predict responses to fertilization because neither trait alone predicted species loss in a previous analysis. Species with a tall-runner growth form commonly increased in relative abundance in response to added nitrogen, while short species and those with a tall-clumped clonal growth form often decreased. The ability to increase in size via vegetative spread across space, while simultaneously occupying the canopy, conferred competitive advantage, although typically only the abundance of a single species within each height-clonal growth form significantly responded to fertilization in each experiment. Classifying species on the basis of two traits (height and clonal growth form) increases our ability to predict species responses to fertilization compared to either trait alone in predominantly herbaceous plant communities. Electronic supplementary material The online version of this article (doi:10.1007/s00442-012-2264-5) contains supplementary material, which is available to authorized users.
[Show abstract][Hide abstract] ABSTRACT: Understanding of how plant communities are organized and will respond to global changes requires an understanding of how plant species respond to multiple environmental gradients. We examined the mechanisms mediating the distribution patterns of tidal marsh plants along an estuarine gradient in Georgia (USA) using a combination of field transplant experiments and monitoring. Our results could not be fully explained by the "competition-to-stress hypothesis" (the current paradigm explaining plant distributions across estuarine landscapes). This hypothesis states that the upstream limits of plant distributions are determined by competition, and the downstream limits by abiotic stress. We found that competition was generally strong in freshwater and brackish marshes, and that conditions in brackish and salt marshes were stressful to freshwater marsh plants, results consistent with the competition-to-stress hypothesis. Four other aspects of our results, however, were not explained by the competition-to-stress hypothesis. First, several halophytes found the freshwater habitat stressful and performed best (in the absence of competition) in brackish or salt marshes. Second, the upstream distribution of one species was determined by the combination of both abiotic and biotic (competition) factors. Third, marsh productivity (estimated by standing biomass) was a better predictor of relative biotic interaction intensity (RII) than was salinity or flooding, suggesting that productivity is a better indicator of plant stress than salinity or flooding gradients. Fourth, facilitation played a role in mediating the distribution patterns of some plants. Our results illustrate that even apparently simple abiotic gradients can encompass surprisingly complex processes mediating plant distributions.
[Show abstract][Hide abstract] ABSTRACT: Predation on detritivores is expected to decelerate detritivore-mediated decomposition processes. In field mes-ocosms, we studied whether the decomposition of leaf and needle litter of live oak (Quercus virginiana) and loblolly pine (Pinus taeda), respectively, was affected by saltmarsh detritivores (Gastropoda: Littoraria irrorata and Melampus bidentatus) and predacious omnivores (Decapoda: Armases cinereum) and their interactions. Both crabs and snails alone increased decomposition (mass loss) rates of oak litter, while a combination of both resulted in the same mass loss as in animal-free controls, probably due to crabs feeding on snails rather than litter. Neither crabs nor snails alone affect-ed mass loss of pine litter, but a combination of both signif-icantly increased decomposition rates. Irrespective of the litter type, crabs significantly increased mortality of the snails but gained biomass only on pine litter and only when detritivorous snails were present. Our findings suggest that unidirectional facilitation of omnivorous semi-terrestrial crabs by their detritivorous prey (saltmarsh snails) promotes the decomposition of low-quality (pine) litter. On high-quality (oak) litter, by contrast, negative effects of the predator pre-vail, resulting in a drop of decomposition rates when crabs were present, probably owing to predation on detritivorous snails. Thus, the effects of predator/prey-interactions on decomposition processes are context-dependent and are controlled by resource quality.
[Show abstract][Hide abstract] ABSTRACT: Top–down and bottom–up effects interact to structure communities, especially in salt marshes, which contain strong gradients in bottom–up drivers such as salinity and nutrients. How omnivorous consumers respond to variation in prey availability and plant quality is poorly understood. We used a mesocosm experiment to examine how salinity, nutrients, an omnivore (the katydid Orchelimum fidicinium) and an herbivore (the planthopper Prokelisia spp.) interacted to structure a simplified salt marsh food web based on the marsh grass Spartina alterniflora. Bottom–up effects were strong, with both salinity and nutrients decreasing leaf C/N and increasing Prokelisia abundance. Top–down effects on plants were also strong, with both the herbivore and the omnivore affecting S. alterniflora traits and growth, especially when nutrients or salt were added. In contrast, top–down control by Orchelimum of Prokelisia was independent of bottom–up conditions. Orchelimum grew best on a diet containing both Spartina and Prokelisia, and in contrast to a sympatric omnivorous crab, did not shift to an animal-based diet when prey were present, suggesting that it is constrained to consume a mixed diet. These results suggest that the trophic effects of omnivores depend on omnivore behavior, dietary constraints, and ability to suppress lower trophic levels, and that omnivorous katydids may play a previously unrecognized role in salt marsh food webs.
Estuaries and Coasts 01/2012; 35(2). · 2.25 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Summary1. Community assembly theories predict that the success of invading species into a new community should be predictable by functional traits. Environmental filters could constrain the number of successful ecological strategies in a habitat, resulting in similar suites of traits between native and successfully invading species (convergence). Conversely, concepts of limiting similarity and competitive exclusion predict native species will prevent invasion by functionally similar exotic species, resulting in trait divergence between the two species pools. Nutrient availability may further alter the strength of convergent or divergent forces in community assembly, by relaxing environmental constraints and/or influencing competitive interactions.2. To investigate how nutrient availability influences forces of divergence and convergence during the invasion of exotic species into native communities, we conducted multivariate analyses of community composition and functional traits from naturally assembled plant communities in long-term nitrogen (N) addition experiments across North America.3. Relative abundances of key functional traits differed between the native and exotic plant communities, consistent with limiting similarity or a trait bias in the exotic species pool. Environmental context also played an important role in invasion because sites varied in the identity of the traits that predicted dissimilarity between native and exotic communities. Nitrogen enrichment did not alter these patterns.4. Nitrogen enrichment tended to increase exotic abundance, but this result was driven by a dramatic increase in exotics in only a few experiments. When similarity between native and exotic communities was included in the statistical model, N enrichment no longer predicted an increase in exotic relative abundance. Instead, sites with the highest abundance of exotic species were the ones where native and exotic communities had the highest trait similarity.5. Synthesis. Our analysis of natural patterns of invasion across herbaceous communities in North America found evidence of both divergent and convergent forces on community assembly with exotic species. Together, these results suggest that while functionally dissimilar exotic species may be more likely to invade, they are unlikely to become abundant unless they have traits pre-adapting them to environmental conditions in their invaded range. Contrary to prior studies, invasion was not consistently promoted by N enrichment.
[Show abstract][Hide abstract] ABSTRACT: Background/Question/Methods
There is a general consensus that both top-down and bottom-up forces work to structure biological communities. Top-down and bottom-up effects can alter the abundance, biomass or diversity of different trophic levels in a community, and they often interact to impact herbivorous insect populations. Understanding the relative strengths of these forces is an important key to predicting the structure of biological communities.
Salt marshes along the Atlantic and Gulf coasts are similar in many ways, and are inhabited by the same plant and insect species. However, the Atlantic and Gulf coast areas have different tidal regimes, which may result in the two areas not functioning in exactly the same ways. In addition, abiotic factors that vary with latitude may lead to differences between south and north Atlantic sites, and abiotic factors that vary with longitude may lead to differences between west and east Gulf Coast sites.
We hypothesized that the importance of top-down and bottom-up factors in coastal salt marshes might vary geographically. To test this hypothesis, we characterized abiotic conditions and the plant and arthropod communities at 11 sites along each coast in the late summers of 2009 and 2010. We also manipulated wrack (dead plant stems) and nutrient availability in 2 x 6 m plots at each site to evaluate geographic differences in community response. The experiment was established in 2009 and allowed to run to 2010.
Our sampling documented that some abiotic factors (especially soil organic content) varied geographically. Plant height, nitrogen content, thatch cover also varied geographically. Although the total number of arthropods collected did not differ geographically, the trophic composition of samples showed marked variation among geographic regions. Large-scale differences in latitude and in mean tidal range are likely driving much of this variation. Arthropod community structure was little affected by wrack addition, but strongly affected by fertilization; and, the effect of fertilization varied geographically for herbivores and prey items. Although salt marshes are superficially similar from Maine to Texas, they may be structured differently throughout this geographic range. Therefore, extrapolating results from one geographic region to another should be done with caution.
[Show abstract][Hide abstract] ABSTRACT: Background/Question/Methods
Interactions between plants and herbivores often vary on a geographic scale. Although theory about induced defenses and tolerance is predicated on temporal or spatial variation in herbivore damage, few studies have compared induced resistance and tolerance across a latitudinal gradient.
In 2002-2005 we surveyed 15 salt marshes along the Atlantic coast from Florida to Maine on 8 dates. At each field site we visually scored leaves of the saltmarsh shrub Iva frutescens for herbivore damage. In a common garden laboratory experiment we measured induced resistance in Iva frutescens. Potted Iva from high and low latitude sites (9 sites total) were damaged with an Iva specialist beetle Paria atterima. Before the damage and on several dates after the damage, feeding trials with Paria were performed. In each trial 2 leaves, 1 from a damaged and 1 from an undamaged plant, were offered to replicate Paria. As a proxy measure of induced resistance, we calculated a preference index for each trial. In a second common garden experiment we studied tolerance to herbivory of Iva from various sites along the Atlantic Coast. In 2009 we measured a number of plant traits on plants damaged by herbivores in a factorial lab experiment in the previous year. Tolerance was measured using an index of regrowth 1 year following herbivore damage.
Theory predicts that induced resistance should matter more when herbivore damage is variable but sometimes strong. In the field, temporal variation in herbivore damage on Iva was over 3 times greater at low versus high latitudes, indicating that induced resistance should be stronger at low latitudes. Consistent with this prediction, Iva induced resistance to herbivores following damage, and induced resistance lasted longer in low versus high latitude plants (ANOVA, p<0.005). Theory also predicts that tolerance to herbivory should be greater where average herbivory damage is greater (in our case, at low latitudes); however, tolerance to herbivory in Iva only indirectly depended on geographic origin. Tolerance was greater in smaller plants, which were more common at higher latitudes. As a result of this indirect causal pathway, tolerance was greater in high latitude plants, contrary to our expectations. Our results suggest that tolerance to herbivory is a less plastic defense strategy than induced resistance.