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Contrasting impacts of different-sized herbivores on species richness of Mediterranean annual pastures differing in primary productivity
Abstract
Vertebrate herbivores can be key determinants of grassland plant species richness, although the magnitude of their effects can largely depend on ecosystem and herbivore characteristics. It has been demonstrated that the combined effect of primary productivity and body size is critical when assessing the impact of herbivores on plant richness of perennial-dominated grasslands; however, the interaction of site productivity and herbivore size as determinants of plant richness in annual-dominated pastures remains unknown. We experimentally partitioned primary productivity and herbivore body size (sheep and wild rabbits) to study the effect of herbivores on the plant species richness of a Mediterranean semiarid annual plant community in central Spain over six years. We also analyzed the effect of grazing and productivity on the evenness and species composition of the plant community, and green cover, litter, and plant height. We found that plant richness was higher where the large herbivore was present at high-productivity sites but barely changed at low productivity. The small herbivore did not affect species richness at either productivity site despite its large effects on species composition. We propose that adaptations to resource scarcity and herbivory prevented plant richness changes at low-productivity sites, whereas litter accumulation in the absence of herbivores decreased plant richness at high productivity. Our results are consistent with predictions arising from a long history of grazing and highlight the importance of both large and small herbivores to the maintenance of plant diversity of Mediterranean annual-dominated pastures.
... Note that systems C and D have the same effectiveness although they differ in the magnitude of competitive exclusion, while system E shows lower effectiveness, although the magnitude of competitive release is the same as in system D. This distinction is important, and implies that variation in the effect of a releasing factor on species diversity along an environmental gradient (e.g., the commonly observed increase in the positive effect of grazing on species diversity with increasing productivity, [28,29]) can be generated by two different mechanisms: changes in the magnitude of competitive release, and/or changes in the effectiveness of the releasing factor. We are not aware of any previous attempt to theoretically or experimentally separate these two components. ...
... Third, a previous study has shown that species richness in this system is strongly limited by a small number of resident grass species, which exclude a large number of forb species from the community [30]. Fourth, the system has a long history (>20 years) of cattle grazing, and previous studies have shown that grazing significantly promotes the diversity of similar systems [13,28,29,31,32]. Experiments focusing on Mediterranean grasslands also show that grazing often reduces the abundance of grasses but increases the abundance of forbs [33]. ...
... As expected, cattle grazing significantly increased species richness in our study system (Fig 4B, Figure A in S1 File). This finding is consistent with previous studies focusing on Mediterranean grasslands [28,29,45]. Several lines of evidence suggest that the mechanism underlying this effect was competitive release of forb species from the negative effect of grasses. ...
A fundamental notion in community ecology is that local species diversity reflects some balance between the contrasting forces of competitive exclusion and competitive release. Quantifying this balance is not trivial, and requires data on the magnitude of both processes in the same system, as well as appropriate methodology to integrate and interpret such data. Here we present a novel framework for empirical studies of the balance between competitive exclusion and competitive release and demonstrate its applicability using data from a Mediterranean annual grassland where grazing is a major mechanism of competitive release. Empirical data on the balance between competitive exclusion and competitive release are crucial for understanding observed patterns of variation in local species diversity and the proposed approach provides a simple framework for the collection, interpretation, and synthesis of such data.
... In contrast, small herbivores are more selective grazers that may decrease species richness by feeding on high-nutrient content plant parts (Brown and Heske 1990) and selectively foraging for less abundant subordinate plants. Studies comparing the eff ects of the two body sizes on vegetation structure and species diversity are rare (but see Bakker et al. 2006, Rueda et al. 2013. ...
... A positive relationship has been observed between precipitation or ANPP and the eff ects of grazing by large herbivores on structure and diversity across plant communities at a global scale (Milchunas et al. 1988, Milchunas and Lauenroth 1993, Bakker et al. 2006) and within local regions at topographic scales , Osem et al. 2004). In contrast, Bakker et al. (2006) in a cross-site study and Rueda et al. (2013) at a topographic scale did not fi nd a relationship between ANPP and the eff ects of small herbivores on plant species richness. Do structural and diversity responses vary with annual fl uctuations in precipitation and ANPP on a temporal scale? ...
... Th erefore, we rejected our hypothesis that grazing eff ects on species richness would be greater in more productive years. For small herbivores, our fi nding is consistent with Bakker et al. (2006) and Rueda et al. (2013). Productivity is considered Litter increased by 4% with exclusion of large herbivores and 7% with additional exclusion of small herbivores. ...
The response of semiarid grasslands to small, non‐colonial herbivores has received little attention, focusing primarily on the effects of granivore assemblages on annual plant communities. We studied the long‐term effects of both small and large herbivores on vegetation structure and species diversity of shortgrass steppe, a perennial semiarid grassland considered marginal habitat for small mammalian herbivores. We hypothesized that 1) large generalist herbivores would affect more abundant species and proportions of litter‐bare ground‐vegetation cover through non‐selective herbivory, 2) small herbivores would affect less common species through selective but limited consumption, and 3) herbivore effects on plant richness would increase with increasing aboveground net primary production (ANPP). Plant community composition was assessed over a 14‐year period in pastures grazed at moderate intensities by cattle and in exclosures for large (cattle) and large‐plus‐small herbivores (additional exclusion of rabbits and rodents). Exclusion of large herbivores affected litter and bare ground and basal cover of abundant, common and uncommon species. Additional exclusion of small herbivores did not affect uncommon components of the plant community, but had indirect effects on abundant species, decreased the cover of the dominant grass Bouteloua gracilis and total vegetation, and increased litter and species diversity. There was no relationship between ANPP and the intensity of effects of either herbivore body size on richness. Exclusion of herbivores of both body sizes had complementary and additive effects which promoted changes in vegetation composition and physiognomy that were linked to increased abundance of tall and decreased abundance of short species. Our findings show that small mammalian herbivores had disproportionately large effects on plant communities relative to their small consumption of biomass. Even in small‐seeded perennial grasslands with a long history of intensive grazing by large herbivores, non‐colonial small mammalian herbivores should be recognized as an important driver of grassland structure and diversity.
... The combined effects of two herbivorous species on vegetation can be highly context-dependent (Olff and Ritchie 1998), and therefore understanding these interactions is essential to the conservation or management of both wildlife and domestic grazing animals (du Toit 2011; Odadi et al. 2011). However, most previous research has focused on the grazing effects of herbivore species from widely disparate animal taxa, such as invertebrates versus vertebrates (Alba-Lynn and Detling 2008) or mega-herbivores versus small mammals (Retzer 2007;Davidson et al. 2010;Rueda et al. 2013); few studies have mentioned instances in which two mammals share similar ecological characteristics in temporalspatial niches, such as behavior or dietary overlap (Davidson and Lightfoot 2006). ...
... However, the intensive activity of herbivores at high densities creates widespread soil disturbances and detrimental erosion, reduces the vegetation cover of palatable species, and leaves tolerant plant species, thus reducing plant diversity (Olff and Ritchie 1998). Rueda et al. (2013) confirmed that small herbivores did not influence the species richness, but largely affected the species composition. When abundances of plateau pikas are high, their grazing and digging cause drastic reductions in plant cover and diversity (Guo et al. 2012a) and have been cited as one of the main factors leading to grassland degradation (Xin 2008). ...
Small herbivores play keystone functional roles in grassland ecosystems. Recognising the combined effects where herbivores co-exist is important for guiding grassland restoration and biodiversity conservation. On the Tibetan Plateau, both plateau pikas and Himalayan marmots are regarded as pests by Tibetan people and local government, but little is known about their combined effects. We conducted a field study to determine the combined effects of pikas and marmots on plant biodiversity and vegetation structure. Plateau pikas alone consistently reduced the plant height and diversity and increased the vegetation cover of physically unpalatable plants. However, the co-existence of marmots with pikas decreased the vegetation cover of physically unpalatable plants, while increasing the cover of palatable plants and plant diversity, ultimately changing the impact of pikas alone and modulating aspects of the plant community. These results illustrate that increasing the abundance and richness of small herbivores in grassland ecosystems may promote plant diversity and benefit vegetation restoration rather than aggravate the degradation of grasslands.
... Additionally, the composition and traits of plants and herbivores could influence how herbivores affect plant communities (Olff & Ritchie 1998;Nolan, Connolly & Wachendorf 2001). Previous studies documented that the impact of herbivores depends on habitat primary productivity (Bakker et al. 2006;Rueda, Rebollo & Garc ıa-Salgado 2013). Yet, there has been comparatively little consideration of how different herbivores affect plant communities that, in turn, differ in plant diversity. ...
... Understanding the interactive effects of plant and herbivore characteristics on plant community dynamics is of fundamental and practical importance. Although the effects of different herbivores and plant species on plant community characteristics have been analysed separately (Duffy 2002;Rueda, Rebollo & Garc ıa-Salgado 2013), their effects in nature may be interactive. Whether plant diversity alters the effects of different-sized herbivores has never been tested. ...
1.It is well documented that large herbivores have pronounced effects on plant communities in grassland ecosystems, and the extent and course of their effects can largely depend on both plant and herbivore characteristics. Previous studies highlighted the importance of plant productivity in predicting the impact of herbivores on grasslands. Yet, there has been little consideration of how different herbivores affect plant communities that, in turn, differ in plant diversity.2.In a two-year grazing experiment, we tested the effects of large herbivores (cattle or sheep, or both together) on plant communities under high and low plant diversity levels in eastern Eurasian steppe.3.We found that, at high plant diversity grassland, mixed grazing by cattle and sheep significantly increase plant diversity, but we found no effect of grazing by cattle or sheep alone. Grazing by cattle or sheep alone or mixed grazing by cattle and sheep did not significantly affect plant biomass in the high diversity grassland. However, at low plant diversity grassland, grazing by cattle alone and mixed grazing by cattle and sheep significantly increased plant diversity, but significantly decreased plant biomass. There was no significant impact on both plant diversity and biomass from sheep grazing.4.Synthesis and applications. We conclude that the effects of grazing in grassland strongly depend on herbivore assemblages and pre-grazing plant diversity. Herbivore grazing might contribute more to the maintenance of grassland structure and ecosystem functioning under high plant diversity compared to low plant diversity. Furthermore, our data suggest that multiple-species mixed grazing regimes in grassland systems with high plant diversity could represent the optimal protocol for grazing management. This study emphasizes the importance of maintaining both plant and herbivore diversity to optimize ecosystem functioning.This article is protected by copyright. All rights reserved.
... The invasion and dominance of exotic plant species has reduced native species diversity of plant communities in many ecosystems [3,4,5,6]. The ability of invasive plants to become dominant depends on their interactions with native vegetation composition, grazing intensity, herbivore body size, plant productivity and climatic variability [7,8,9]. Herbivore selectivity of grazing patches, associated with other disturbances (e.g. ...
... Diet selection is one of the main mechanisms through which grazing animals influence plant communities [58]. Grazing impact on grassland plant species richness differs between sheep and cattle [59,60,17,61,9]. Sheep grazing may promote the invasion of E. plana in grasslands, since the preferred species-inter-tussock vegetation-must compete for growth resources against tall dominant species that are not incurring the same defoliation costs [62]. ...
Invasion by exotic grasses is a severe threat to the integrity of grassland ecosystems all over the world. Because grasslands are typically grazed by livestock and wildlife, the invasion is a community process modulated by herbivory. We hypothesized that the invasion of native South American grasslands by Eragrostis plana Nees, an exotic tussock-forming grass from Africa, could be deterred by grazing if grazers switched dietary preferences and included the invasive grass as a large proportion of their diets. Bos taurus (heifers) and Ovis aries (ewes) grazed plots with varying degrees of invasion by E. plana in a replicated manipulative experiment. Animal positions and species grazed were observed every minute in 45-min grazing session. Proportion of bites and steps in and out of E. plana tussocks were measured and used to calculate several indices of selectivity. Both heifers and ewes exhibited increasing probability of grazing E. plana as the proportion of area covered by tussocks increased, but they behaved differently. In agreement with expectations based on the allometry of dietary preferences and morphology, ewes consumed a low proportion of E. plana, except in areas that had more than 90% E. plana cover. Heifers consumed proportionally more E. plana than ewes. Contrary to our hypothesis, herbivores did not exhibit dietary switching towards the invasive grass. Moreover, they exhibited avoidance of the invasive grass and preference for short-statured native species, both of which should tend to enhance invasion. Unless invasive plants are highly palatable to livestock, the effect of grazing to deter the invasion is limited, due to the inherent avoidance of the invasive grass by the main grazers in the ecosystem, particularly sheep.
... While both north american and southern african mesic savanna grasslands were historically grazed by multiple large herbivore species, currently grazing by a diversity of species occurs only in southern african protected savanna grasslands. studies suggest that different herbivore assemblages affect plant community structure in varying ways (Olff and ritchie 1998;Bakker and Olff 2003;Olofsson et al. 2004;smet and Ward 2005;Bakker et al. 2006;rueda et al. 2013). thus, when comparing the impacts of large herbivore loss on mesic savanna grasslands, responses are likely to be contingent upon the types and diversity of large herbivores removed. ...
... a more general understanding of the impacts of large herbivore loss on mesic savanna grassland plant communities Asterisks denote significant differences between mean plot dominance (±1 se) in grazed and ungrazed treatments within a year at a site (P ≤ 0.05) has been hindered by the fact that large herbivore assemblages can differ quite dramatically on different continents and these different assemblages appear to affect plant community structure in different ways (Olff and ritchie 1998; Bakker and Olff 2003;Olofsson et al. 2004;smet and Ward 2005;Bakker et al. 2006;rueda et al. 2013). thus, when comparing the impacts of large herbivore loss, herbaceous community responses will likely be contingent upon the types and diversity of large herbivores removed, as well as other factors, such as evolutionary history, soil fertility, and plant composition and diversity, that can differ between savanna grasslands on different continents. ...
Large herbivore grazing is a widespread disturbance in mesic savanna grasslands which increases herbaceous plant community richness and diversity. However, humans are modifying the impacts of grazing on these ecosystems by removing grazers. A more general understanding of how grazer loss will impact these ecosystems is hampered by differences in the diversity of large herbivore assemblages among savanna grasslands, which can affect the way that grazing influences plant communities. To avoid this we used two unique enclosures each containing a single, functionally similar large herbivore species. Specifically, we studied a bison (Bos bison) enclosure at Konza Prairie Biological Station, USA and an African buffalo (Syncerus caffer) enclosure in Kruger National Park, South Africa. Within these enclosures we erected exclosures in annually burned and unburned sites to determine how grazer loss would impact herbaceous plant communities, while controlling for potential fire-grazing interactions. At both sites, removal of the only grazer decreased grass and forb richness, evenness and diversity, over time. However, in Kruger these changes only occurred with burning. At both sites, changes in plant communities were driven by increased dominance with herbivore exclusion. At Konza, this was caused by increased abundance of one grass species, Andropogon gerardii, while at Kruger, three grasses, Themeda triandra, Panicum coloratum, and Digitaria eriantha increased in abundance.
... However, the biotic community present in a given environment can also deeply affect a plant community. Several organisms, from microscopic [2] to large herbivores [3], can affect, both directly and/or indirectly, plant occurrence, evenness and productivity. Such organisms may affect plants indirectly by causing changes to environmental conditions [4,5], or directly through disease, parasitism, competition and herbivory [6]. ...
Ant-aphid interactions may affect host plants in several ways, however, most studies measure only the amount of fruit and seed produced, and do not test seed viability. Therefore, the aim of this study was to assess the effects of the presence of ant-aphid interactions upon host plant productivity and seed viability in two different contexts: isolated and within an arthropod community. For this purpose we tested the hypothesis that in both isolated and community contexts, the presence of an ant-aphid interaction will have a positive effect on fruit and seed production, seed biomass and rate of seed germination, and a negative effect on abnormal seedling rates, in comparison to plants without ants. We performed a field mesocosm experiment containing five treatments: Ant-aphid, Aphid, Community, Ant-free community and Control. We counted fruits and seeds produced by each treatment, and conducted experiments for seed biomass and germinability. We found that in the community context the presence of an ant-aphid interaction negatively affected fruit and seed production. We think this may be because aphid attendance by tending-ants promotes aphid damage to the host plant, but without an affect on seed weight and viability. On the other hand, when isolated, the presence of an ant-aphid interaction positively affected fruit and seed production. These positive effects are related to the cleaning services offered to aphids by tending ants , which prevent the development of saprophytic fungi on the surface of leaves, which would cause a decrease in photosynthetic rates. Our study is important because we evaluated some parameters of plant fitness that have not been addressed very well by other PLOS ONE |
... Herbivory by small mammals has been recognized as an important ecological process regulating plant populations and communities (Van der Wal et al. 2000, Howe et al. 2006, Smit et al. 2010, Gough et al. 2012, McLaughlin and Zavaleta 2013, Rebollo and García-Salgado 2013, although most mammalian-focused herbivory research has focused on large-bodied species with some suggesting that larger mammalian herbivores are stronger regulators than smaller species (Bakker et al. 2006). Our findings and those by others (e.g., Goheen et al. 2013, McLaughlin and Zavaleta 2013, Rueda et al. 2013, Lyly et al. 2014 suggest that the strength of effect of mammalian herbivores in regulating vegetation is not related to body size. Rather, the entire herbivore community must be considered when evaluating herbivore impacts. ...
Mammalian herbivory can have profound impacts on plant population and community dynamics. However, our understanding of specific herbivore effects remains limited, even in regions with high densities of domestic and wild herbivores, such as the semiarid conifer forests of western North America. We conducted a seven-year manipulative experiment to evaluate the effects of herbivory by two common ungulates, Cervus elaphus (Rocky Mountain elk) and cattle Bos taurus (domestic cattle) on growth and survival of two woody deciduous species, Populus trichocarpa (cottonwood) and Salix scouleriana (Scouler's willow) in postfire early-successional forest stands. Additionally, we monitored belowground herbivory by Thomomys talpoides (pocket gopher) and explored effects of both aboveground and belowground herbivory on plant vital rates. Three, approximately 7 ha exclosures were constructed, and each was divided into 1-ha plots. Seven herbivory treatments were then randomly assigned to the plots: three levels of herbivory (low, moderate, and high) for both cattle and elk, and one complete ungulate exclusion treatment. Treatments were implemented for seven years. Results showed that cattle and elk substantially reduced height and growth of both cottonwood and willow. Elk had a larger effect on growth and subsequent plant height than cattle, especially for cottonwood, and elk effects occurred even at low herbivore densities. Pocket gophers had a strong effect on survival of both plant species while herbivory by ungulates did not. However, we documented significant interaction effects of aboveground and belowground herbivory on survival. Our study is one of the first to evaluate top-down regulation by multiple herbivore species at varying densities. Results suggest that traditional exclosure studies that treat herbivory as a binary factor (either present or absent) may not be sufficient to characterize top-down regulation on plant demography. Rather, the strength of top-down regulation varies depending on a number of factors including herbivore species, herbivore density, interactions among multiple herbivore species, and varying tolerance levels of different plant species to herbivory.
... Herbivory by small mammals has been recognized as an important ecological process regulating plant populations and communities (Van der Wal et al. 2000, Howe et al. 2006, Smit et al. 2010, Gough et al. 2012, McLaughlin and Zavaleta 2013, Rebollo and García-Salgado 2013, although most mammalian-focused herbivory research has focused on large-bodied species with some suggesting that larger mammalian herbivores are stronger regulators than smaller species (Bakker et al. 2006). Our findings and those by others (e.g., Goheen et al. 2013, McLaughlin and Zavaleta 2013, Rueda et al. 2013, Lyly et al. 2014 suggest that the strength of effect of mammalian herbivores in regulating vegetation is not related to body size. Rather, the entire herbivore community must be considered when evaluating herbivore impacts. ...
Citation: Endress, B. A., B. J. Naylor, B. K. Pekin, and M. J. Wisdom. 2016. Aboveground and belowground mammalian herbivores regulate the demography of deciduous woody species in conifer forests. Ecosphere 7(10): Abstract. Mammalian herbivory can have profound impacts on plant population and community dynamics. However, our understanding of specific herbivore effects remains limited, even in regions with high densities of domestic and wild herbivores, such as the semiarid conifer forests of western North America. We conducted a seven-year manipulative experiment to evaluate the effects of herbivory by two common ungulates, Cervus elaphus (Rocky Mountain elk) and cattle Bos taurus (domestic cattle) on growth and survival of two woody deciduous species, Populus trichocarpa (cottonwood) and Salix scoule-riana (Scouler's willow) in postfire early-successional forest stands. Additionally, we monitored below-ground herbivory by Thomomys talpoides (pocket gopher) and explored effects of both aboveground and belowground herbivory on plant vital rates. Three, approximately 7 ha exclosures were constructed, and each was divided into 1-ha plots. Seven herbivory treatments were then randomly assigned to the plots: three levels of herbivory (low, moderate, and high) for both cattle and elk, and one complete ungulate exclusion treatment. Treatments were implemented for seven years. Results showed that cattle and elk substantially reduced height and growth of both cottonwood and willow. Elk had a larger effect on growth and subsequent plant height than cattle, especially for cottonwood, and elk effects occurred even at low herbivore densities. Pocket gophers had a strong effect on survival of both plant species while herbivory by ungulates did not. However, we documented significant interaction effects of aboveground and below-ground herbivory on survival. Our study is one of the first to evaluate top-down regulation by multiple herbivore species at varying densities. Results suggest that traditional exclosure studies that treat herbivo-ry as a binary factor (either present or absent) may not be sufficient to characterize top-down regulation on plant demography. Rather, the strength of top-down regulation varies depending on a number of factors including herbivore species, herbivore density, interactions among multiple herbivore species, and varying tolerance levels of different plant species to herbivory.
... [11,70,83] Plant-vertebrate Interactive effects between abiotic stress, tolerance to herbivory, and herbivore body size determine plant abundances and richness. [84,85] Plant-fungi Fungal pathogens mediate coexistence through trade-offs between competitive ability and resistance to pathogens and through pathogen specialization. ...
The quest for understanding how species interactions modulate diversity has progressed by theoretical and empirical advances following niche and network theories. Yet, niche studies have been limited to describe coexistence within tropic levels despite incorporating information about multi-trophic interactions. Network approaches could address this limitation, but they have ignored the structure of species interactions within trophic levels. Here we call for the integration of niche and network theories to reach new frontiers of knowledge exploring how interactions within and across trophic levels promote species coexistence. This integration is possible due to the strong parallelisms in the historical development, ecological concepts, and associated mathematical tools of both theories. We provide a guideline to integrate this framework with observational and experimental studies.
... For example, the effects of heavy livestock grazing on plant species richness can be anywhere from slightly positive to negligible or distinctly negative 12 for a variety of reasons. [13][14][15][16] The Need for Usable Science ...
On the Ground
• Animals are critical components of rangeland ecosystems, and domestic livestock provide an extremely important management tool on rangelands.
• Decades of research have yielded much valuable information to support sustainable and effective grazing management, but increased complexity resulting in part from expanding environmental, economic, and societal pressures demands future investments in usable science focused on rangeland animals.
• Three priorities for usable science are recommended:
• Proactive drought planning
• Better matching livestock production systems to rangeland resources
• Comprehensive synthesis of and effective communication concerning environmental impacts (positive, negative, and neutral) of livestock on rangelands.
... One hypothesis that warrants testing is that systems using multiple grazers might be more sustainable in maintaining plant community structure. This could occur if diet complementarities among different herbivores mitigate the directional disturbances induced by selective grazing of a single species (Augustine and McNaughton 1998;Odadi et al. 2011;Rueda et al. 2013;Zhong et al. 2014;Liu et al. 2015). Timing and duration of grazing via grazing rotation could also be used, for instance, to restrict the growth of dominant early growing species to enlarge the population of later growing species (Ash and McIvor 1998;Holechek et al. 2004). ...
Context
Many studies have examined how intensity of grazing and patterns of precipitation individually and interactively influence the spatial and temporal dynamics of grassland vegetation, such as dominance, succession, coexistence, and spatial heterogeneity. However existing models have rarely considered the diet preferences of grazers and how they interact with variation in precipitation amount and timing.
Objective and methods
We examined how plant community structure responds to the individual and combined effects of grazing intensity, selective grazing, and patterns of precipitation, based on a six-year grazing experiment with seven levels of field-manipulated grazing intensity in a typical steppe of Inner Mongolia.
Results
The palatable species, mainly forbs, were most severely damaged at intermediate levels of grazing intensity; given that these species are the major contributors to plant community diversity, a U-shaped diversity-grazing intensity relationship resulted. In contrast, spatial heterogeneity of aboveground biomass and species composition peaked at intermediate levels of grazing intensity. Cold season precipitation positively correlated with the abundance of the dominant C3 grasses and correlated negatively with the subdominant forbs and C4 plants. Thus, when cold season precipitation increased, plant community species diversity decreased. Grazing intensity and precipitation did not interact in their effects on species richness.
Conclusions
These findings contrast with the predictions from current disturbance–diversity models and indicate that diet selection of grazing animals is an important factor shaping the diversity-grazing intensity relationship in semi-arid grasslands. Future grassland biodiversity conservation and management practices should take diet preference of grazing animals into account.
... Comprender los efectos interactivos de las características de las plantas y los herbívoros en la dinámica de la comunidad vegetal es de importancia fundamental y práctica. Aunque los efectos de diferentes herbívoros y especies de plantas sobre las características de la comunidad vegetal se han analizado por separado (Duffy, 2002;Rueda et al., 2013), sus efectos en la naturaleza pueden ser interactivos. ...
Las hormigas cortadoras de hojas (HCH) cumplen un rol importante denominándolas ‘ingenieras de los ecosistemas’ dado que modifican la estructura y función de los mismos, principalmente sobre las propiedades del suelo y la estructura florística de la vegetación. En particular las del género Atta, movilizan grandes cantidades de suelo para construir cavidades y túneles subterráneos que albergan a los individuos de la colonia garantizando condiciones ambientales adecuadas para el hongo que cultivan. Los cambios sobre las propiedades del suelo y la vegetación son motivo de investigación. Teniendo en cuenta lo descripto, el objetivo general consiste en establecer el grado de interacción existente entre la herbivoría y las modificaciones de suelo que provocan las colonias de Atta vollenweideri, y su efecto sobre las especies herbáceas de un pastizal natural bajo bosques nativos en un gradiente espacial.
Los estudios se desarrollaron en cinco establecimientos agropecuarios del centro-norte de la Provincia de Entre Ríos, Argentina. Se evaluaron los cambios en las propiedades físicas de suelo a través de la resistencia mecánica a la penetración, como también se recolectaron muestras de suelo de horizontes superficiales para obtener variables físicas como porcentaje de arena, limo y arcilla; y químicas (pH, carbono orgánico, fósforo extraíble, capacidad de intercambio catiónico, cationes Ca++, Mg++, Na+, y la conductividad eléctrica). Ambas mediciones se realizaron en un gradiente espacial de 60 metros de distancia respecto al nido tomando 10 nidos de referencia. Respecto a la vegetación se evaluó los cambios en la cobertura vegetal de especies herbáceas y arbustivas utilizando el método de línea de intersección y la composición florística de herbáceas utilizando la escala Braun-Blanquet en el gradiente ambiental antes mencionado. Por otra parte, se seleccionaron cinco nidos en dos establecimientos agropecuarios, y se cuantificó la actividad forrajera según el número de hormigas que ingresan al nido cargadas con fragmentos vegetales. Por último, para establecer el grado de interacción entre los herbívoros presentes en el sistema (HCH y ganado bovino) y los cambios en el suelo se localizaron cinco nidos al azar distribuidos en tres establecimientos agropecuarios. En cada nido se colocaron jaulas de exclusión para los diferentes herbívoros (ganado bovino y HCH) en cuatro posiciones (5, 15, 45 y 85 metros de la base de los nidos), evaluando tres tratamientos: exclusión completa de herbívoros (mayores y menores), exclusión de herbívoros mayores, y sin exclusión. Se evaluó la biomasa vegetal y cobertura-abundancia de las especies herbáceas utilizando la escala Braun-Blanquet.
La construcción y el mantenimiento de los nidos asociados a las actividades de corte de las colonias alteran la estructura del suelo, y la distribución espacial de los nutrientes, principalmente por la traslocación y depósito del material de los horizontes subsuperficiales del perfil del suelo hacia la superficie. Durante los primeros 30 metros aledaños a los nidos, se registraron cambios significativos sobre la resistencia mecánica a la penetración atribuibles a las modificaciones de los componentes estructurales del suelo. Por otro lado, desde los 15 m hasta la cresta del nido, la actividad de las HCH provocó una disminución del contenido de CO y un incremento de pH, CE y Ca++, mientras que, en la parte central del nido se registró un aumento significativo del contenido de arcilla, Na+ y PSI. En este sentido, es apreciable la alteración en la distribución espacial de las características del suelo evaluadas, que provocan alta heterogeneidad edáfica pudiendo afectar otras variables de pequeña escala del paisaje dentro del ecosistema.
Se comprobó que nidos de Atta vollenweideri afectan a la comunidad de especies herbáceas y arbustivas en un bosque nativo degradado de la Ecorregión del Espinal Argentino, realizando una gran selección de especies herbáceas pioneras en ecosistemas boscosos posicionados en etapas iniciales a intermedias de la sucesión vegetal. Además, las colonias cosechan anualmente un 9.5% de la productividad primaria de un pastizal natural en los bosques nativos estudiados. Además, es posible afirmar que en términos estacionales conforman su dieta mostrando gran plasticidad, lo cual implica un comportamiento estrictamente selectivo sobre especies monocotiledóneas herbáceas graminiformes y ciperáceas cuando la abundancia relativa es alta, de lo contrario expresa un oportunismo muy marcado cortando otras especies vegetales. Por otro lado, los cambios sobre la comunidad vegetal de las especies herbáceas en un gradiente ambiental respecto al nido se atribuyen a los efectos de la herbivoría. En este sentido las HCH provocan un impacto local de importancia hasta los 15 metros de distancia en la riqueza, dominancia y diversidad de especies vegetales, disminuyendo su efecto a los 85 metros de distancia. El ganado bovino es el herbívoro dominante en la interacción explicado posiblemente por su comportamiento en la búsqueda de alimento y su menor selectividad. Atta vollenweideri fue informada como una especie de HCH selectiva, pero durante los resultados de esta tesis se ha mostrado su marcado generalismo en el forrajeo de pastizales. Por otra parte, los resultados en la exclusión de la herbivoría permiten afirmar que las modificaciones en las propiedades físicas y químicas del suelo no afectaría directamente la composición florística del pastizal natural en bosques nativos.
Los resultados de este trabajo aportarían al conocimiento de la ecología de HCH dado que la unidad de muestreo se mantuvo en las mismas condiciones ambientales propias de la especie evaluando nidos vivos, y además, es el primer trabajo que evalúa las posibles causas en los cambios de la vegetación herbáceas en colonias adultas y vivas. Por otro lado, no solamente se midió el efecto de los herbívoros menores, sino que además, el impacto que provocarían los herbívoros mayores -en este caso el ganado doméstico- y su interacción con las hormigas cortadoras sobre la variabilidad florística.
El modelo teórico propuesto sobre interacción que existe entre los herbívoros y el suelo sobre la modificación de las comunidades vegetales en un gradiente espacial respecto a los nidos de Atta vollenweideri no podría ser extrapolable en su totalidad al resto de las especies del género dado que presentan marcadas diferencias que se resumen a continuación: a- no realizan nidos con túmulos externos compactos de gran tamaño, b- el ingreso del material cortado no se realiza sobre el túmulo sino, en la mayoría de las restantes especies de Atta aunque no en todas, es alejado de los nidos y por galerías subterráneas, c- las propiedades físicas y químicas de los suelos son marcadamente diferentes a las informadas en la bibliografía, por la cual sus procesos edafogénicos cambian, d- la capacidad y selección de la actividad de corte vegetal es diferente entre las especies por la disponibilidad del material en cada área de distribución geográfica de las mismas, e- la composición florística de las especies vegetales son distintas dado que los nidos se encuentran en otras ecorregiones con condiciones ambientales diferentes, y f- el impacto y la diversidad de la herbivoría, tanto mayores y menores, no es la misma en todo el rango de distribución del género.
En este sentido, los resultados obtenidos permiten afirmar que la vegetación herbácea adyacente de nidos de Atta vollenweideri, se ve modificada en su composición florística a causa -fundamentalmente- de la presencia de los herbívoros mayores. La herbivoría de las hormigas cortadoras de hojas presentan relativa importancia hasta los 15 a 40 metros de distancia respecto al nido, y luego su efecto es menor. Por otra parte, la modificación del suelo provocado por el mantenimiento y construcción de los nidos juega un rol significativamente menor sobre el cambio en la vegetación herbácea. Sin embargo, es posible mencionar que si bien el efecto de la herbivoría de las HCH a los 85 metros de distancia sobre la composición florística y producción de biomasa de especies herbáceas es relativamente menor respecto al bovino, motiva a futuras investigaciones que amplíen el rango de estudio para determinar la distancia en la cuál el efecto es nulo.
... Dynamics of litter decomposition are greatly affected by soil properties and plant characteristics (Song et al. 2017;Naeem et al. 2021). Similarly, herbivore grazing also has pronounced impacts on soil-plant interaction (Liu et al. 2015;Rueda et al. 2013), indicating that herbivore grazing can alter litter decomposition process via different pathways (Sun et al. 2018;Wang et al. 2018). Most previous studies were conducted on the influence of long-term grazing by herbivores with different intensities on litter dynamics (Chuan et al. 2018;Garibaldi et al. 2007;Semmartin et al. 2008;Song et al. 2017), however, the study on the effects of moderate grazing by herbivore species or assemblage on leaf litter decomposition remains obscure. ...
AimsChanges caused by single herbivore species grazing to soil and plant interfaces can significantly affect the leaf litter decomposition rates within ecosystems. However, we intended to study how grazing by multiple herbivore species regulate the litter decomposition process and associated soil or plant properties in Leymus chinensis dominant meadow steppe.Methods
Two consecutive randomized field experiments were conducted between May 2017 and May 2019: 1) a grazing experiment with four (control or no, sheep, cattle, and mixed (sheep + cattle)) grazing treatments to explore the effects of moderate grazing on soil properties and L. chinensis litter quality and 2) the litter decomposition experiment to evaluate the decay rate in grazed or non-grazed litter by using litter bag technique.ResultsLitter decay rate was the highest in mixed grazed litter as compared to litter grazed by individual herbivore species. Both herbivore grazing and incubation time affected decay rate of L. chinensis litter by modifying soil moisture and nitrogen (N) content, as well as, litter N and lignin contents. Plant litter quality has been displayed to be a dominant regulator of litter decay rate than soil physical or chemical properties in mixed grazed litter.Conclusions
Livestock diversification benefited litter decomposition indirectly by modifying plant litter quality with time. Therefore, herbivore assemblages should be given more importance in future research on litter decomposition process to mediate nutrient cycling in grassland.
... Herbivory is a fundamental driver of grassland plant community composition. Nearly all grasslands worldwide have been grazed by populations of wild and domestic herbivores, such as cattle [2], horses [3], sheep [4], deer [5], rabbits [6], and kangaroos [7]. Herbivore grazing is a multiple-component process that includes wounding, defoliation, and saliva deposition [8]. ...
Herbivore grazing is a multiple-component process that includes wounding, defoliation, and saliva deposition. Despite the extensive published research on mechanical wounding and defoliation, no analysis to identify the genes that specify defoliation and mechanical wounding has been performed. Moreover, the influence of the expression of these genes on plant regrowth after defoliation remains poorly understood.
Seven cDNA libraries for RNA samples collected from stubble tissues that had been mechanically wounded or defoliated at 2, 6 and 24 h along with the control were sequenced using the Illumina/Solexa platform. A comparative transcriptomic analysis of the sequencing data was conducted. In total, 1,836 and 3,238 genes were detected with significant differential expression levels after wounding and defoliation, respectively, during one day. GO, KOG and pathway-based enrichment analyses were performed to determine and further understand the biological functions of those differentially expressed genes (DEGs). The results demonstrated that both wounding and defoliation activated the systemic synthesis of jasmonate (JA). However, defoliation specifically reduced the expression levels of ribosomal protein genes, cell division or cell expansion-related genes, and lignin biosynthesis genes and may have negatively affected plant growth. Further analysis revealed that the regrowth of elongating leaves was significantly retarded after defoliation at 6 h through the following 7 days of measurement, suggesting that the gene expression pattern and phenotype are consistent. Fifteen genes were selected, and their expression levels were confirmed by quantitative RT-PCR (qRT-PCR). Thirteen of them exhibited expression patterns consistent with the digital gene expression (DGE) data.
These sequencing datasets allowed us to elucidate the common and distinct mechanisms of plant responses to defoliation and wounding. Additionally, the distinct DEGs represent a valuable resource for novel gene discovery that may improve plant resistance to defoliation from various processes.
Over a quarter of the world's land surface is grazed by cattle and other livestock, which are replacing wild herbivores, potentially impairing ecosystem structure and functions. Previous research suggests that cattle at moderate stocking rates can functionally replace wild herbivores in shaping understory communities, but it is unclear whether this is also true under high stocking rates. It is also unclear whether wild herbivore effects on plant communities moderate, enhance, or are simply additive to the effects of cattle at high stocking rates. To evaluate the influence of cattle stocking rates on the ability of cattle to functionally replace wild herbivores and test for interactive effects between cattle and wild herbivores in shaping understory vegetation, we assessed herbaceous vegetation in a long-term exclosure experiment in a semi-arid savanna in central Kenya that selectively excludes wild mesoherbivores (50-1000 kg) and megaherbivores (elephant and giraffe). We tested the effects of cattle stocking rate (zero/moderate/high) on herbaceous vegetation (diversity, composition, leafiness) and how those effects depend on the presence of wild mesoherbivores and megaherbivores. We found that herbaceous community composition (primary ordination axis) was better explained by the presence/absence of herbivore types than by total herbivory, suggesting that herbivore identity is a more important determinant of community composition than total herbivory at high cattle stocking rates. The combination of wild mesoherbivores and cattle stocked at high rates led to increased bare ground and annual grass cover, reduced perennial grass cover, reduced understory leafiness, and enhanced understory diversity. These shifts were weaker or absent when cattle were stocked at high stocking rates in the absence of wild mesoherbivores. Megaherbivores tempered the effects of cattle stocked at high rates on herbaceous community composition but amplified the effects of high cattle stocking rate on bare ground and understory diversity. Our results show that, contrary to previous findings at moderate stocking rates, cattle at high stocking rates do not functionally replace wild herbivores in shaping savanna herbaceous communities. In mixed-use rangelands, interactions between cattle stocking rate and wild herbivore presence can lead to non-additive vegetation responses with important implications for both wildlife conservation and livestock production.
The present study aimed to characterize the dynamics of NDVI and meteorological conditions, relating both to the annual dynamics of biomass accumulation in natural pastures of the Pampa biome as a way of subsidizing growth modeling. Forage accumulation rate data from a long-term experiment, NDVI data from the MODIS images, and meteorological data measured at the surface were used. We verify that the agrometeorological element associated to the accumulation of forage in the natural grasslands is different according to the season, which is typical of the subtropical climate. Winter is the critical season for livestock production due to the lower forage accumulation rate and lower NDVI and meteorological data as indicators of the Pampa biome natural grasslands growth values of NDVI, conditioned by the decrease of solar radiation and air temperature. In the summer, the limiting factor to forage accumulation is the hydric condition. It was also verified that the variability in the growth of grasslands can be associated with the ENSO phenomenon, being the El Niño favorable and the La Niña unfavorable, especially in the spring-summer period. Considering the verified associations, spectral indices combined with agrometeorological elements are recommended to the adjustment of models of forage accumulation in the Pampa biome natural grasslands.
DOI: http://dx.doi.org/10.1590/1678-4499.2017222
Dehesas or montados are man-made systems whose conservation interest as a Mediterranean landscape or agroforestry system has been highlighted in the scientific literature due to their high levels of biodiversity. The understanding of how management practices influence the biodiversity of the dehesa requires a comprehensive approach that considers the broad spectrum of biological components (i.e., fungi, lichens, vascular plants, macroinvertebrates, and vertebrates) that make the dehesa an ecosystem. We have reviewed around seventy publications compiled through Scopus and other specialized bibliographic sources not included in Scopus that study the impact of land use and land cover on the biodiversity of Iberian dehesas. The most significant results have been organized and summarized into a group of factors related to habitat structure and management practices. The conclusion to be drawn from the scientific studies is that not all biological components respond in the same way to land-cover and human practices, and the maintenance of a high overall diversity in dehesas depends on a complex understorey vegetation and habitat heterogeneity.
1. Some plant functional traits evolved with high temporal resource variability and disturbance in ecosystems where these factors are prevalent. Persistence of characteristics of these functional traits in ecosystems may depend on continued resource variability and disturbance, which in turn may promote functional diversity. In Mediterranean ecosystems, experiments that eliminate temporal resource variability and disturbance are needed to detect functional trait dependence on these factors. 2. The purpose of this study was to experimentally assess how interannual rainfall variability, summer drought and seasonal grazing modify the characteristics of functional traits (life span, flowering time, seed size and plant size) in old-field (6—15 years) Mediterranean herbaceous communities. 3. We designed a 9-year factorial field experiment that manipulated Mediterranean rainfall variability in three ways: (i) constant water availability with no summer drought; (ii) autumn and spring water availability but with summer drought; and (iii) no water supplied to rainfall; and grazing regimes: (i) autumn grazing; (ii) spring grazing; and (iii) non-grazing, in each of the three scenarios of water availability. At a community scale, we measured abundance of different categories within four plant functional traits: plant life span (annual and perennial), flowering time of annuals (spring and summer) and seed and plant sizes of spring annuals (small and large). 4. Interannual rainfall variability in autumn and spring (IRVAS), summer drought and grazing reduced perennial cover. IRVAS was necessary for the persistence of small-seeded and small-size spring annuals. IRVAS and summer drought increased spring annuals in grazed treatments. 5. Results suggest that IRVAS, summer drought and grazing favour the coexistence of species, through improved functional diversity in seed and plant sizes and increased abundance of spring annuals, the most species-rich functional group. Both effects may be the reason for the high species richness in grazed Mediterranean herbaceous communities.
Two independent models concerning the effects of grazing on vegetation have gained wide acceptance in the last decade: Westoby et al.'s state- and- transition (ST) model, and Milchunas et al.'s generalized model of the effects of grazing on plant community structure and diversity (MSL model). These two prevailing models, as they stand, are conceptually divergent. The MSL model implicitly assumes that, at a given site, for each grazing intensity there is a single equilibrium situation with a single diversity value. The S-T model suggests that rangeland dynamics include irreversible transitions and alternative equilibria. Here we propose a modification of the original MSL model, to encompass a wider range of real situations and to place it within the context of the S-T model. The-four extreme cases proposed in the original MSL model are revisited, taking into account that (1) the "moisture" gradient can be generalized as a "productivity" gradient; (2) the selective pressure of herbivores on systems with long history of grazing has fluctuated over time, allowing the development of different pools of species adapted to low or high grazing intensities; and (3) systems with long evolutionary history of grazing. have developed resilience mechanisms that allow reversible shifts in floristic composition with changes in grazing intensities. The grazing intensity vs. diversity curves thus postulated for systems with a long evolutionary history of grazing are similar to those proposed by the original MSL model because resilience mechanisms allow for reversible changes associated with grazing intensity. In contrast, the curves postulated for systems with short evolutionary history. of grazing include different alternative branches, indicating irreversible transitions, because resilience mechanisms to grazing were not fully developed. By incorporating these modifications, the divergence between the original MSL and S-T models can be resolved. A set of published examples from real systems is presented and compared with the predictions of the modified model. The modified MSL model is applicable to a wider range of real situations than the MSL model in its original formulation.
Previous studies hair: indicated that the relationship between community biomass and species density can be represented by a multivariate model in which abiotic variables influence species density both through effects oil biomass and through effects on the species pool. In this paper, we use data from grazed and ungrazed coastal meadows in Finland to evaluate and extend this general conceptual model of the Factors controlling species density. Structural equation analysis was used to evaluate a model for all meadows and then to perform a multigroup analysis to determine how grazed and ungrazed meadows differ. By itself, biomass could explain only 12% of the variation in species density while the multivariate model was able to explain 47% using five types of predictor variables: site, soil, flooding, grazing, and biomass. Analyses found that flooding explained the greatest amount of variability in species density, primarily through negative effects on the species pool. Grazing was also found to have ii strong effect on species density and results suggest that its negative influence may be largely through I reductions in the species pool in grazed meadows. The most important difference found between grazed and ungrazed meadows was that species density had a strong negative relationship to biomass in the ungrazed meadows but no significant relationship in the grazed ones. Thus, it appears that the influence of competition on species density was much greater in ungrazed meadows compared to grazed ones.
Mammalian herbivores can have dramatic effects on the vegetation through different mechanisms, and may therefore modify resource distribution or create habitat for other species. Organisms that cause such changes are classified as ecosystem engineers. The European rabbit (Oryctolagus cuniculus) is a semi-fossorial mammal native to the Iberian Peninsula and a keystone species in Mediterranean foodwebs. Rabbits are central-place foragers, but few studies have analysed their ecosystem engineering effects in the vicinity of warrens. Here, we explored the role of rabbits as ecosystem engineers in Mediterranean pastures in central Spain. We studied the spatial patterns and internal heterogeneity in the vegetation community of different spatial subunits created by rabbits (warrens, the warren influence area, and latrines). Within the different subunits, rabbit activities gave rise to patches and gradients with different plant species richness, diversity and height, and a high internal heterogeneity (dissimilarity) in floristic composition. Through different mechanisms, they have a significant effect on species coexistence and competitive interactions, and increase the number of available niches for plants which otherwise would not be able to establish. In conclusion, rabbits can act as allogenic ecosystem engineers in Mediterranean pastures, increasing heterogeneity and contributing to the high biodiversity of Mediterranean annual pastures.
Grazing history alone is not a good predictor of plant-herbivore interactions. Interactions occur along gradients of convergent to divergent selection pressures with increasing environmental moisture and of intolerance to tolerance of grazing with increasingly long evolutionary histories of grazing. Feedback mechanisms between plants and grazing animals are well developed in grasslands with long evolutionary histories of grazing. Feedback mechanisms are manifest in the rapid switching capabilities (of plant species and modes of competition) of subhumid grasslands with long evolutionary histories of grazing and divergent selection pressures. Switching capabilities do not exist in semiarid grasslands with long evolutionary histories of grazing and convergent selection pressures. Rather, for heavily grazed dominant species dominance increases. Feedback mechanisms are not well developed in systems with short evolutionary histories of grazing. Here, differences in response to grazing by semiarid and subhumid situations arise primarily from differences in the grazing tolerance of plants adapted to semiaridity or of plants adapted to competition for light and from the different effects of grazing on canopy structure. -from Authors
Species diversity has declined in ecosystems worldwide as a result of habitat fragmentation, eutrophication, and land-use
change. If such decline is to be halted ecological mechanisms that restore or maintain biodiversity are needed. Two long-term
field experiments were performed in native grassland to assess the effects of fire, nitrogen addition, and grazing or mowing
on plant species diversity. In one experiment, richness declined on burned and fertilized treatments, whereas mowing maintained
diversity under these conditions. In the second experiment, loss of species diversity due to frequent burning was reversed
by bison, a keystone herbivore in North American grasslands. Thus, mowing or the reestablishment of grazing in anthropogenically
stressed grasslands enhanced biodiversity.
Herbivory by domestic and wild ungulates is a major driver of global vegetation dynamics. However, grazing is not considered in dynamic global vegetation models, or more generally in studies of the effects of environmental change on ecosystems at regional to global scale. An obstacle to this is a lack of empirical tests of several hypotheses linking plant traits with grazing. We, therefore, set out to test whether some widely recognized trait responses to grazing are consistent at the global level. We conducted a meta-analysis of plant trait responses to grazing, based on 197 studies from all major regions of the world, and using six major conceptual models of trait response to grazing as a framework. Data were available for seven plant traits: life history, canopy height, habit, architecture, growth form (forb, graminoid, herbaceous legume, woody), palatability, and geographic origin. Covariates were precipitation and evolutionary history of herbivory. Overall, grazing favoured annual over perennial plants, short plants over tall plants, prostrate over erect plants, and stoloniferous and rosette architecture over tussock architecture. There was no consistent effect of grazing on growth form. Some response patterns were modified by particular combinations of precipitation and history of herbivory. Climatic and historical contexts are therefore essential for understanding plant trait responses to grazing. Our study identifies some key traits to be incorporated into plant functional classifications for the explicit consideration of grazing into global vegetation models used in global change research. Importantly, our results suggest that plant functional type classifications and response rules need to be specific to regions with different climate and herbivory history.
A small-scale field study was conducted to explain seedling emergence and recruitment of reproductive individuals in a four-year-old Mediterranean annual plant community. The analyzed levels were populations, functional types, and total number of individuals in the community. We hypothesized that the number of germinable seeds positively affects the number of emerged seedlings, which positively affects the number of reproductive individuals. We hypothesized as well that litter mass, biomass of established adults, and number of non-conspecific emerged seedlings negatively affect the numbers of emerged seedlings and reproductive individuals. The results supported our multivariate causal explanation of plant recruitment, although concordances with the initial hypotheses were more frequent at the community level. The effect of the number of germinable seeds on the number of emerged seedlings was found to be robust only at the community level. At the population level, this relationship did not have a consistent tendency and depended on species identity and range of seed density experienced. Litter had negative effects on total number of individuals in the community, and usually non-significant effects at the population level. Litter effects were found to be inversely related to seed mass, and sharper cotyledons did not improve seedling emergence likely by favoring litter mat penetration. Interactions among plant species appeared not to have any effect on seedling emergence and survival, and had positive effects on certain populations. Biomass of established adult plants exerted no influence on seedling emergence and survival, probably due to their low abundance in this community during the germination period. Results permitted the inference of the main stages in the recruitment process and causal factors. They provided evidence for the greater importance of germination and emergence in comparison with seedling survival to insure reproductive recruitment in this community.
At a site near Jerusalem, the vegetation was initially dominated by Poa bulbosa and prostrate annuals. For most of the 1st growing season following exclosure there was little difference in biomass and species composition. During the dry season, plant litter and seeds remained largely intact on the surface in the exclosure, but were almost entirely removed by grazers outside it. During the 2nd growing season, biomass in the exclosure was usually double that outside, an increase mostly due to large-seeded annual grasses. Near Beer Sheva, the effects on the vegetation of 5-6 years of protection from grazing were recorded at two exclosure fences. Annuals were equally sparse on both sides of the fence, though species composition was different. The biomass of perennials was double inside the exclosure, mainly due to an increase in the biomass of Noaea mucronata and perennial thistles. Most perennial species, except Asphodelus aestivus, were more abundant inside the exclosure than in the grazed area. -from Author
Introduces 6 major phases of human activity in the Mediterranean Basin and outlines the major vegetation types associated with the mediterranean bioclimates. Fire, overgrazing and deforestation have been the main agents of degradation of Mediterranean forest, and are related to vegetational balance and successional sequences. Agriculture also destroys the native vegetation, but old-field succession may lead back to a vegetation more characteristic of the particular bioclimate. Consequences of vegetation degradation include a reduction in productivity, increased erosion and sedimentation, and faunal impoverishment.-P.J.Jarvis
A quick dip into the literature on diversity reveals a bewildering range of indices. Each of these indices seeks to characterize the diversity of a sample or community by a single number. To add yet more confusion an index may be known by more than one name and written in a variety of notations using a range of log bases. This diversity of diversity indices has arisen because, for a number of years, it was standard practice for an author to review existing indices, denounce them as useless, and promptly invent a new index. Southwood (1978) notes an interesting parallel in the proliferation of new designs of light traps and new permutations of diversity measures.
(1) Faecal pellets of rabbits were collected from the Holkham sand dunes, Norfolk. The proportion of leaf epidermis of various plant species in the faecal pellets was compared with the proportions of the species in the surrounding vegetation. (2) Festuca rubra was the major contributor to the diet and was apparently preferred, since the proportion was greater in the faeces than in the vegetation. Ammophila arenaria, Carex arenaria, Poa pratensis and Dicranum scoparium were apparently disliked. (3) Monocotyledonous species formed the major part of the diet of rabbits throughout the year, but the contribution of dicotyledonous species and mosses increased slightly during spring and summer.
(1) Using the results of diet selection studies and two wild and two domestic species, we compare some aspects of ecology in relation to body size, the recent evolutionary history of the species and current forage conditions. (2) Food niche breadth and mter-species diet overlap seemed dependent upon recent evolutionary history as well as upon body size, but values were strongly influenced by forage quantity and quality. (3) Dietary selectivity appears especially sensitive to seasonal changes in forage quality, e.g. large as well as small animals pursued relatively selective strategies when forage conditions permitted, but body size and related nutritional-energetic demands appeared to set the limits where switches from selective to non-selective tactics took place. (4) Sensitivity to diet composition and quality increased with decreasing size except in the domestic sheep. It is likely that anatomical-physiological adaptations, including a relatively large rumeno-reticulum allow domestic sheep to utilize more forage plant species and mhabit a wider variety of niches and ecosystems than most ungulates. Human selection has made the sheep food and habitat generalists despite their relatively small size.
The fate of >13 000 seedlings comprising 17 plant species was studied in a temperate grassland and meadow. Analysis examined seedling encounter (the probability of at least one seedling being damaged) and seedling exploitation (the proportion of seedlings damaged once encountered). Only molluscs and rodents encountered groups of seedlings to any significant extent; arthropods played a minor role in seedling mortality. In spring, seedling herbivory was infrequent, with neither molluscs or rodents encountering >10% of seedlings. In autumn there was an increased abundance of molluscs and rodents. Seedling encounter by molluscs rose in both habitats but the clumped spatial distribution of rodent activity in the meadow led to seedling encounter by rodents increasing only in the grassland. Both molluscs and rodents appeared to encounter large seedlings more frequently than small seedlings. In general, once a group of seedlings was encountered, molluscs and rodents exploited a similar number of seedlings, on average c30%. With increasing seedling size, mollusc seedling exploitation decresed, rodent seedling exploitation increased. Thus rodents exploited a greater proportion of large seedlings than molluscs and molluscs exploited a greater proportion of small seedlings. Molluscs most frequently grazed seedlings, removing <75% of tissue, while rodents consumed seedlings completely. Forb seedlings (mostly legumes) tended to suffer less damage from herbivores than grasses. -from Author
Limitations of linear regression applied on ecological data. - Things are not always linear additive modelling. - Dealing with hetergeneity. - Mixed modelling for nested data. - Violation of independence - temporal data. - Violation of independence spatial data. - Generalised linear modelling and generalised additive modelling. - Generalised estimation equations. - GLMM and GAMM. - Estimating trends for Antarctic birds in relation to climate change. - Large-scale impacts of land-use change in a Scottish farming catchment. - Negative binomial GAM and GAMM to analyse amphibian road killings. - Additive mixed modelling applied on deep-sea plagic bioluminescent organisms. - Additive mixed modelling applied on phyoplankton time series data. - Mixed modelling applied on American Fouldbrood affecting honey bees larvae. - Three-way nested data for age determination techniques applied to small cetaceans. - GLMM applied on the spatial distribution of koalas in a fragmented landscape. - GEE and GLMM applied on binomial Badger activity data.
Context. Economic decision models are seldom used in developing policies for the cost-effective control of invasive species that threaten natural ecosystems. However, their potential value is shown using an example of European rabbits damaging native vegetation in Australia. Aims. To better define the problem of rabbit damage, provide a sound theoretical basis for implementing cost-efficient strategies for rabbit control and show how resources available for ecosystem protection can be most effectively applied. Methods. Adynamic economic decision model was developed, incorporating the costs and effectiveness of three methods for controlling rabbits among native vegetation to consider alternative management strategies. A monetary value on native vegetation was set using the 'avoided' cost of replanting trees on roadsides and from field data we described how capacity of plant communities to regenerate improves if rabbit numbers are reduced. Key results. Model outputs indicated the best combinations of methods for cost-effective rabbit control and showed how the highest benefits could be gained in protecting natural vegetation. Conclusions. The model provided a framework for deciding how limited resources might be used to greatest benefit for protecting native vegetation. Implications. This methodology could apply to other invasive species, provided that natural assets can be given a justifiable monetary value, control costs and effectiveness can be determined and the impact of the pests on assets can be modelled as dynamic population processes.
Semiarid rangelands often respond slowly to rest/relaxation of grazing pressure by large herbivores, and the effects of grazing are most often inferred from this direction of study because the imposition of grazing onto previous ungrazed/lightly grazed areas occurred prior to the age of scientific studies. These rangelands host a diversity of small and large herbivores, but grazing studies most often concern effects of the large generalists. Here, the effects of herbivore body size on plant species richness and dominant species, and imposition and relaxation of grazing by large herbivores were studied by opening half of exclosures established in 1939 and building new exclosures to large herbivores, and to small-plus-large herbivores. Plant richness using sensitive species-area sampling was studied in a dry and a wet year, about 62 years after initiating the long-term experiment and about 6-10 years after initiating the altered designs. Convergence of the newly opened to large herbivore grazing treatment to the long-term grazed treatment occurred within 10 years, but convergence of the newly excluded to large herbivore treatment to the long-term excluded treatment had only partly occurred. This indicated that recovery from grazing is slow relative to imposition of grazing by large herbivores, but effects of the additional exclusion of small-plus-large herbivores occurred relatively rapidly. These results were mirrored by trajectories of convergence of the dominant species, and this is discussed with respect to implications for state-and-transition models. Short-term exclusion of small-plus-large herbivores resulted in greater richness than even long-term exclusion of only large herbivores, even though quantities consumed by small herbivores are much less than by large. Grazing effects on plant richness were large in the wet year, but the very dry year suppressed richness on all treatments. When sampling effort and area are the same, the numbers and attributes of species unique to a treatment are indicators of rareness of the richness and traits selected for by the treatment. More unique species were sampled in the small-plus-large herbivore exclosures when comparing body size, and the long-term large herbivore exclosures when comparing time of exclosure. Unique species encountered during sampling the ungrazed treatments were generally forbs, exotic and/or weedy invasive species, and often tall, annual species.
Indicators of rangeland health generally do not include a measure of biodiversity. Increasing attention to maintaining biodiversity in rangelands suggests that this omission should be reconsidered, and plant species richness and diversity are two metrics that may be useful and appropriate. Ideally, their response to a variety of anthropogenic and natural drivers in the ecosystem of interest would be clearly understood, thereby providing a means to diagnose the cause of decline in an ecosystem. Conceptual ecological models based on ecological principles and hypotheses provide a framework for this understanding, but these models must be supported by empirical evidence if they are to be used for decision making. To that end, we synthesize results from published studies regarding the responses of plant species richness and diversity to drivers that are of management concern in Great Plains grasslands, one of North America’s most imperiled ecosystems. In the published literature, moderate grazing generally has a positive effect on these metrics in tallgrass prairie and a neutral to negative effect in shortgrass prairie. The largest published effects on richness and diversity were caused by moderate grazing in tallgrass prairies and nitrogen fertilization in shortgrass prairies. Although weather is often cited as the reason for considerable annual fluctuations in richness and diversity, little information about the responses of these metrics to weather is available. Responses of the two metrics often diverged, reflecting differences in their sensitivity to different types of changes in the plant community. Although sufficient information has not yet been published for these metrics to meet all the criteria of a good indicator in Great Plains Grasslands, augmenting current methods of evaluating rangeland health with a measure of plant species richness would reduce these shortcomings and provide information critical to managing for biodiversity.
Information content may be used as a measure of the diversity of a many-species biological collection. The diversity of small collections, all of whose members can be identified and counted, is defined by Brillouin's measure of information. With larger collections it becomes necessary to estimate diversity; what is estimated is Shannon's measure of information which is a function of the population proportions of the several species. Different methods of estimation are appropriate for different types of collections. If the collection can be randomly sampled and the total number of species is known, Basharin's formula may be used. With a random sample from a population containing an unknown number of species, Good's method is sometimes applicable. With a patchy population of sessile organisms, such as a plant community, random samples are unobtainable since the contents of a randomly placed quadrat are not a random sample of the parent population. To estimate the diversity of such a community a method is proposed whereby the sample size is progressively increased by addition of new quadrats; as this is done the diversity of the pooled sample increases and then levels off. The mean increment in total diversity that results from enlarging the sample still more then provides an estimate of the diversity per individual in the whole population.
Information content may be used as a measure of the diversity of a many-species biological collection. The diversity of small collections, all of whose members can be identified and counted, is defined by Brillouin's measure of information. With larger collections it becomes necessary to estimate diversity; what is estimated is Shannon's measure of information which is a function of the population proportions of the several species. Different methods of estimation are appropriate for different types of collections. If the collection can be randomly sampled and the total number of species is known, Basharin's formula may be used. With a random sample from a population containing an unknown number of species, Good's method is sometimes applicable. With a patchy population of sessile organisms, such as a plant community, random samples are unobtainable since the contents of a randomly placed quadrat are not a random sample of the parent population. To estimate the diversity of such a community a method is proposed whereby the sample size is progressively increased by addition of new quadrats; as this is done the diversity of the pooled sample increases and then levels off. The mean increment in total diversity that results from enlarging the sample still more then provides an estimate of the diversity per individual in the whole population.
To test the hypothesis that the impacts of grazers on plant species richness reverse under contrasting nutrient richness, we analyzed unpublished and published data from lake, stream, marine, grassland, and forest ecosystems. We analyzed data from 30 studies providing 44 comparisons of plant species richness under low vs. high grazing pressure in enriched or nutrient-rich and non-enriched or nutrient-poor ecosystems. All 19 comparisons from non-enriched or nutrient-poor ecosystems exhibited significantly lower species richness under high grazing than under low grazing. In contrast, 14 of 25 comparisons from enriched or nutrient-rich ecosystems showed significantly higher species richness under high grazing than under low grazing. However, nine of these 25 comparisons showed no significant impact of grazers on species richness, while two comparisons showed declines in species richness under high grazing. Based on all the comparisons, plant species richness decreases with high grazing in nutrient-poor ecosystems, while it increases with high grazing in nutrient-rich ecosystems. Although nutrient-rich ecosystems seemed to produce more variable responses to grazers than did nutrient-poor ecosystems, in rare cases high grazing produced a decline in species richness in nutrient-rich environments. We suggest that species richness declines with high grazing in nutrient-poor ecosystems because a limitation of available resources prevents regrowth of species after grazing, which may not be the case in nutrient-rich ecosystems. It is also possible that an increase in species richness under high grazing in nutrient-rich ecosystems may be due to an increase in the dominance of inedible species. Our observation of a grazer reversal of plant species richness under contrasting nutrient richness may have important implications for management of species diversity.
Floristic data collected from permanent plots during 14 consecutive years are used to model the frequency of the 62 most abundant species in relation to post‐ploughing succession, topography and rainfall in annual Mediterranean grasslands located in a Quercus rotundifolia dehesa. The interannual dynamics of species richness are also analysed. From 1980 to 1993, presence/absence data of grassland species were noted in five 20 cm X 20 cm permanent quadrats placed at random in 1980 in 14 permanent plots on a south‐facing slope along the topographic gradient. Weekly autumn rainfall data over the 14 years were analysed using a profile attributes index and Hybrid Multidimensional Scaling to arrange the years according to their autumn rainfall pattern. Generalized Linear Models were used to fit the species richness and species frequency according to topographic position, age since the last ploughing episode, total rainfall in the growing season and autumn rainfall pattern using a forward stepwise procedure. The richness model includes all of these variables, and reveals a relatively high goodness‐of‐fit (71 %). The fact that the meteorological factors play a key role in modelling richness forces us to include them if we wish to use richness as an indicator of the degree of disturbance in these highly fluctuating annual pastures. Models of species dynamics show that although roughly 33 % of the species have a successional behaviour, the majority are more dependent on temporal heterogeneity associated with rainfall or spatial heterogeneity linked to the topographic gradient.
Many areas of heathland in Europe have seen a decline in the area and condition of Calluna vulgaris (heather)-dominated vegetation, with subsequent declines in the associated faunal interest. Grazing, alongside burning, is still the predominant means of managing heathland vegetation, and, therefore, it is by manipulating this management that cost-effective improvements in vegetation condition can be made. This paper investigates the suitability of different grazing treatments for rehabilitating degraded ‘dry heath’. Treatments varied in the intensity (0–1.9 sheep ha−1 year−1) and timing (summer vs. winter) of sheep grazing. These treatments were compared with the behaviour of vegetation outside the fenced area kept under the previous management (open access all year round). As rabbits were common on the site, fences were erected to prevent access to the sheep grazed plots. Vegetation composition remained stable outside the fenced area, whilst all the fenced treatments showed a decrease in heather utilisation and an increase in the relative frequency of heather over the 5 years of the experiment. The increase was in proportion to the reduction in stocking rate, with only slow increases in relative frequency observed in the high grazing treatments (winter and summer). Other species that benefited from reducing grazing included Empetrum nigrum and Vaccinium myrtillus, whilst declines were observed for Agrostis capillaris and total monocotyledonous species. Only small overall differences were observed between the winter low, summer low and no sheep grazing treatments. However, a difference in response was present between the sheep exclosures and the sheep+rabbit exclosures, indicating that rabbits were having a noticeable effect on heather recovery at this site. On this degraded ‘dry heath’ system, imposing a reduction in stocking density improved the condition of the dwarf shrubs present and reduced the grass component of the vegetation. There was little effect of the timing of grazing, such that a reduction in sheep numbers to 0.8/0.9 sheep ha−1 year−1, to give utilisation levels of below 20%, can achieve the desired result of improving vegetation condition whilst still achieving some economic return from grazing. However, the wide range of ‘sustainable’ stocking densities for different heathland systems highlights the need to base effective management on measured utilisation rather than on stock numbers.
Many upland areas of the British Isles have seen declines in the area and condition of heather (Calluna vulgaris)-dominated heathland vegetation. To reverse this decline, management regimes must be designed to rehabilitate areas that have seen this decline. As most of this heathland vegetation is primarily managed by grazing, such management has to determine what stocking levels can maintain the vegetation in a desired state. This paper describes how to reverse this decline through suitable grazing management.
A field experiment involving herbivore exclusion, ploughing, and the combination of both was carried out over a period of 4 - 5 yr in Mediterranean grasslands located along an elevational gradient. The empirical results provide a general hierarchical framework for understanding patterns of plant species diversity in thesegrasslands. In grazed grasslands, plant species density decreased as altitude increased, and this pattern was maintained through time. The reduced seasonality along the climatic gradient is suggested as the extrinsic, indirect control factor. Ploughing caused species loss, but after 4 yr the original diversity was recovered in most grasslands. Our hypothesis is that a negative feedback mechanism regulates species increase towards a characteristic level. A trend of species density reduction was observed in ungrazed grasslands. Plant-herbivore interaction is considered to be essential for maintaining species diversity in grazed grasslands and for the recovery of diversity in mechanically disturbed grasslands.
Herbivores are expected to influence grassland ecosystems by modifying root biomass and root spatial distribution of plant communities. Studies in perennial dominated grasslands suggest that grazing intensity and primary productivity may be strong determinants of the vertical distribution of subterranean biomass. However, no studies have addressed this question in annual dominated pastures. In this study we assess the effect of grazing and habitat productivity on the vertical distribution of root mass in an annual dominated Mediterranean pasture grazed by free-ranging sheep and wild rabbits. We evaluate the effects of grazing on total root mass and vertical root distribution (0-4, 4-8 and 8-12 cm depths) in two neighboring topographic sites (uplands and lowlands) with different productivity using a replicated fence experiment which excludes sheep and sheep plus rabbits. We found evidences that grazing affected root biomass and vertical distribution at lowlands (high productivity habitats), where places grazed by sheep plus rabbits exhibit more root mass and a higher concentration of it towards the soil surface than only rabbits and ungrazed places. In contrast, grazing did not affect root biomass and vertical distribution at uplands (low productivity habitats). We suggest that higher nitrogen and organic matter found in lowlands permit a plant adjustment for nitrogen acquisition by increasing biomass allocation to root production which would allow plant regrowth and the quick completion of the annual life cycle. Contrary, soil resources scarcity at uplands do not permit plants modify their root growth patterns in response to grazing. Our study emphasizes the importance of primary productivity in predicting grazing effect on belowground processes in Mediterranean environments dominated by annuals.
Summary 1 Plants often suffer substantial loss of seeds to consumers. However, because the seed- to-seedling transition is frequently ignored, quantitative estimates of the effects of seed consumers on plant population dynamics are rare. 2 We examined how post-dispersal seed predation by rodents affected seedling emer- gence and subsequent adult plant abundance of bush lupine ( Lupinus arboreus ), a large N-fixing shrub common to coastal dunes in California. We monitored patterns of seed- ling emergence and survival over 3 years for seeds sown into exclosed and control plots. 3 We sowed additional cohorts of seeds in the second and third years and compared interannual variation in emergence patterns. 4 Rodent exclusion substantially reduced seedling emergence, with an average of 109 seedlings emerging over 3 years from 476 seeds sown in rodent exclusion plots vs. 26 from control plots. The intensity of granivory, however, varied between years, with rodent exclusion increasing emergence from seeds sown in year one, but not in year two. 5 Winter seedling mortality, due to cutworm herbivory, was similarly high in rodent- free and control plots, and its net impact was to reduce the difference in seedling abund - ance. Thus, by mid-summer in each of the three years, there were only marginally more seedlings in rodent-excluded vs. control plots. 6 The cumulative effect of protecting seeds, was, however, large. After 3 years, an average of four adult lupines were established in rodent-free plots, whereas only 0.5 were found in control plots and lupine biomass was more than 5-fold higher in exclusion plots. 7 Taken together, the results indicate that rodents play a critical role by limiting the abundance and biomass of a large N-fixing shrub in dunes.
Summary • The interactive effect of grazing and small-scale variation in primary productivity on the diversity of an annual plant community was studied in a semiarid Mediterranean rangeland in Israel over 4 years. The response of the community to protection from sheep grazing by fenced exclosures was compared in four neighbouring topographic sites (south- and north-facing slopes, hilltop and wadi (dry stream) shoulders), differing in vegetation, physical characteristics and soil resources. The herbaceous annual vegetation was highly diverse, including 128 species. Average small-scale species richness of annuals ranged between 5 and 16 species within a 20 × 20 cm quadrat, and was strongly affected by year and site. • Above-ground potential productivity at peak season (i.e. in fenced subplots) was typical of semiarid ecosystems (10–200 g m−2), except on wadi shoulders (up to 700 g m−2), where it reached the range of subhumid grassland ecosystems. Grazing increased richness in the high productivity site (i.e. wadi), but did not affect, or reduced, it in the low productivity sites (south- and north-facing slopes, hilltop). Under grazing, species richness was positively and linearly related to potential productivity along the whole range of productivity. Without grazing, this relationship was observed only at low productivity (−2). • The effect of grazing along the productivity gradient on different components of richness was analysed. At low productivity, number of abundant, common and rare species all tended to increase with productivity, both with and without grazing. Rare species increased three times compared with common and abundant species. At high productivity, only rare species continued to increase with productivity under grazing, while in the absence of grazing species number in the different abundance groups was not related to productivity. • In this semiarid Mediterranean rangeland, diversity of the annual plant community is determined by the interaction between grazing and small-scale spatial and temporal variation in primary productivity, operating mainly on the less abundant species in the community.
1. Both top-down and bottom-up influences, such as grazing herbivores and edaphic factors, may maintain species-rich vegetation by preventing dominant plants from reducing diversity. However, the interaction between grazing and other processes maintaining diversity, particularly in ecosystems with multiple herbivores, is poorly understood. We manipulated access by small and large vertebrate herbivores on an edaphically heterogeneous site. We investigated whether: (i) grazing and soil properties interacted in their impact on vegetation, (ii) the effects of herbivores on different plant functional groups depended on soil properties, and (iii) small and large herbivores were functionally equivalent.
1 An experiment was carried out in a species-poor acid grassland to determine the effect of insect, mollusc and rabbit herbivory on the size and composition of the seed bank and on seedling recruitment from the seed bank and seed rain. From 1991 to 1997, insects and molluscs were excluded with pesticides, and rabbits with fences. Seedling recruitment was monitored over 22 months in gaps established in the vegetation in summer 1995.
2 The most common species recorded from the seed bank in early summer 1995 were dicots (17 species), but perennial grasses (five species) were numerically the most abundant (65% of total). There was no relationship between the species composition of the seed bank and the established vegetation.
3 The size of the seed bank of eight species was greater on fenced plots, a result that reflected increased seed rain where rabbits were excluded. Insects and molluscs had no effect on the size of the seed bank of any species. The number of species in the seed bank was not affected by any of the herbivore exclusions.
4 A comparison of seedling emergence in gaps formed over the original soil with gaps where the soil had been sterilized indicated that only Galium saxatile and Cytisus scoparius recruited from the seed bank. Seedling recruitment was almost entirely derived from the recent seed rain, was dominated by the most abundant perennial grasses in the vegetation (Festuca rubra and Holcus lanatus), and had a species composition that resembled the established vegetation. Results highlight that the potential for seedling establishment in gaps to bring about vegetation change in this grassland is low.
5 Six species had higher seedling densities on rabbit-fenced plots, but the significant effect of fencing disappeared by plant maturity for most species. Survival of seedlings was lower on fenced plots where non-grazed biomass accumulated, so that after 22 months Agrostis capillaris was the only species with more plants present where rabbits were excluded. Rumex acetosa and Stellaria graminea showed higher seedling emergence where molluscs were excluded. More seedlings of Rumex acetosa were also found where insects were excluded. These invertebrate effects were still evident at plant maturity.
In the Tunisian arid zone disturbances (e.g. overgrazing and agriculture) and stresses (e.g. aridity, low fertility) drive changes in the structure and functioning of rangelands, with a decrease in perennial plant cover, changes in floristic composition and erosion. Long-term monitoring requires (1) an understanding of the dynamics of vegetation change and associated ecological processes and (2) identification of relevant indicators. Using data from the arid zone of southern Tunisia we tested the hypothesis that plant functional response types could be used to address these two goals. We identified plant functional response types in response to a gradient of soil and vegetation types characterized by changes in perennial plant cover, dominant species and associated soil types. Vegetation samples were stratified by contrasted vegetation patch types with varying perennial plant cover (1.6 to 22%). We focused our analysis of trait responses within dwarf–shrubs, which are the dominants in typical steppe ecosystems of south Tunisia. Available trait data concerned morphology (plant height, leaf type), regeneration (dispersal mode, phenology and regeneration mode) and grazing value. Although we found it difficult to recognize ‘indicator response types’ that could be used directly to monitor changes in community composition, we were able to identify plant response syndromes that are relevant to long-term vegetation changes, and in particular degradation processes, in the region. Two main response types were identified: the decreaser type, made up of small or medium chamaephytes with high grazing palatability and the increaser type with medium to tall chamaephytes and low grazing palatability. These response types are proposed as key elements in a state-and-transition model of vegetation dynamics in the context of agropastoral disturbances and climatic and edaphic stresses.
Meteorological patterns have a decisive influence on the inter-annual dynamics of therophyte pastures under Mediterranean climatic conditions. The germination behaviour of annual pasture species was studied by subjecting two collections of seeds taken from plants and soil-seed banks to two phytotron-simulated weather patterns: early and late autumn rains. Species from these pastures were arranged along a gradient of sensitivity to temperature on the arrival of the first persistent rain. This sensitivity was manifested in both the total germination success of the species and the germination time profile. The different germination patterns of the species can provide competitive advantages depending on the autumn weather conditions.
Both theoretical arguments and empirical evidence suggests that herbivory in general and mammalian winter herbivory in particular is important in arctic–alpine ecosystems. Although knowledge of the effect of herbivores on specific plants and communities is quite extensive, little is known about the relative impact of large and small vertebrate herbivores and how it might vary among different habitats. To address this key issue, we established exclosures with two different mesh sizes in forest and nearby tundra at three different sites in four contrasting locations in the forest–tundra ecotone in northernmost Sweden and Norway. Plant community composition was recorded annually in three permanent plots within each exclosure and an unfenced control. Local densities of vertebrate herbivores were estimated in spring and autumn from 1998 to 2002.