The nature and extent of microbial biodiversity remain controversial with persistent debates over... more The nature and extent of microbial biodiversity remain controversial with persistent debates over patterns of distributions (i.e. cosmopolitanism versus endemism) and the processes that structure these patterns (neutrality versus selection). We used culture-independent approaches to address these issues focusing on two groups of ciliates, the Oligotrichia (Spirotrichea) and Choreotrichia (Spirotrichea) across an environmental gradient. We assessed SSU rDNA diversity in ciliate communities at six stations in Long Island Sound spanning the frontal region that separates the fresher Connecticut River outflow plume from the open Sound. As in previous studies, we find one abundant cosmopolitan species (Strombidium biarmatum), a few moderately abundant sequences, and a long list of rare sequences. Furthermore, neither ciliate diversity nor species composition showed any clear relationship to measured environmental parameters (temperature, salinity, accessory pigment composition and chorophyll). Overall, we observed that diversity decreased moving from nearshore to offshore. We also conducted analyses to detect clustering among the sampled communities using the software Unifrac. This approach revealed three significant clusters grouping samples from nearshore, surface and deep/well mixed stations. We find no strong fit of our communities to log series, geometric or log normal distributions, though one of the 3 clusters is most consistent with a log series distribution. However, when we remove the abundant cosmopolitan species S. biarmatum, all three clusters fit to a log series distribution. These analyses suggest that, with the exception of one cosmopolitan species, the oligotrich and choreotrich communities at these stations may be distributed in a neutral manner.
Arbuscular mycorrhizal fungi (AMF) are more widely distributed and can associate with a wide rang... more Arbuscular mycorrhizal fungi (AMF) are more widely distributed and can associate with a wide range of plant species. AMF are keystone organisms that form an interface between soils and plant roots. They are also sensitive to environmental changes. AMF are important microbial symbioses for plants under conditions of P-limitation. The AMF are crucial for the functioning of terrestrial ecosystems as they form symbiotic interactions with plants. Mycorrhizal fungi are known to influence plant diversity patterns in a variety of ecosystems globally. AMF hyphae form an extensive network in the soil. The length is a common parameter used to quantifying fungal hyphae. The mycelial network of AM fungi extends into the soil volume and greatly increases the surface area for the uptake of immobile nutrients. Also, AM symbioses improve plants tolerance to drought and enhance plants' tolerance of or resistance to root pathogens. Also, the networks of AM hyphae play a crucial role in the formation of stable soil aggregates and in the building up of a macroporous structure of soil that allows penetration of water and air and thereby prevents erosion. The functioning of AMF symbiosis is mediated by direct and indirect effects of biotic and abiotic factors of the surrounding rhizosphere, the community, and the ecosystem. AMF have great potential in the restoration of disturbed land and low fertility soil. However, despite the importance of AMF to terrestrial ecosystems, little is known about the effects of environmental changes on AMF abundance, activity and the impact of these changes on the ecosystem services. Therefore, it is important to gain a clearer understanding of the effects of environmental changes on the AM fungal species to guide conservation and restoration efforts.
An optimal foraging model is presented which combines simultaneous and nonsimultaneous modes of s... more An optimal foraging model is presented which combines simultaneous and nonsimultaneous modes of search for food. This model is more complex than those in the literature which are constructed using one search mode exclusively. Solutions to this foraging model lead to diets which are seldom composed of a single food or of foods in proportion to their abundance in the environment. Therefore, the most commonly cited qualitative conclusions of optimal foraging theory are not adequate to explain foraging choices under the range of probable search modes.
We experimentally separated the effects of two components of plant diversity-plant species richne... more We experimentally separated the effects of two components of plant diversity-plant species richness and plant functional group richness-on insect communities. Plant species richness and plant functional group richness had contrasting effects on insect abundances, a result we attributed to three factors. First, lower insect abundances at higher plant functional group richness were explained by a sampling effect, which was caused by the increasing likelihood that one low-quality group, C 4 grasses, would be present and reduce average insect abundances by 25%. Second, plant biomass, which was positively related to plant functional group richness, had a strong, positive effect on insect abundances. Third, a positive effect of plant species richness on insect abundances may have been caused by greater availability of alternate plant resources or greater vegetational structure. In addition, a greater diversity of insect species, whose individual abundances were often unaffected by changes in plant species richness, may have generated higher total community abundances. After controlling for the strong, positive influence of insect abundance on insect diversity through rarefaction, insect species richness increased as plant species richness and plant functional group richness increased. Although these variables did not explain a high proportion of variation individually, plant species richness and plant functional group richness had similar effects on insect diversity and opposing effects on insect abundances, and both factors may explain how the loss of plant diversity influences higher trophic levels.
Humans are modifying both the identities and numbers of species in ecosystems, but the impacts of... more Humans are modifying both the identities and numbers of species in ecosystems, but the impacts of such changes on ecosystem processes are controversial. Plant species diversity, functional diversity, and functional composition were experimentally varied in grassland plots. Each factor by itself had significant effects on many ecosystem processes, but functional composition and functional diversity were the principal factors explaining plant productivity, plant percent nitrogen, plant total nitrogen, and light penetration. Thus, habitat modifications and management practices that change functional diversity and functional composition are likely to have large impacts on ecosystem processes.
This article was published in an Elsevier journal. The attached copy is furnished to the author f... more This article was published in an Elsevier journal. The attached copy is furnished to the author for non-commercial research and education use, including for instruction at the author's institution, sharing with colleagues and providing to institution administration. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright
Species coexistence involving trophic interactions has been investigated under two theoretical fr... more Species coexistence involving trophic interactions has been investigated under two theoretical frameworks-partitioning shared resources and accessing exclusive resources. The influence of body size on coexistence is well studied under the exclusive resources framework, but has received less attention under the sharedresources framework. We investigate body-size-dependent allometric extensions of a classical MacArthur-type model where two consumers compete for two shared resources. The equilibrium coexistence criteria are compared against the general predictions of the alternative framework over exclusive resources. From the asymmetry in body size allometry of resource encounter versus demand our model shows, counterintuitively, and contrary to the exclusive resource framework, that a smaller consumer should be competitively superior across a wide range of supplies of the two resource types. Experimental studies are reviewed to resolve this difference among the two frameworks that arise from their respective assumptions over resource distribution. Another prediction is that the smaller consumer may have relatively stronger control over equilibrium resource abundance, and the loss of smaller consumers from a community may induce relatively stronger trophic cascades. Finally, from satiating consumers' functional response, our model predicts that greater difference among resource sizes can allow a broader range of consumer body sizes to coexist, and this is consistent with the predictions of the alternative framework over exclusive resources. Overall, this analysis provides an objective comparison of the two alternative approaches to understand species coexistence that have heretofore developed in relative isolation. It advances classical consumer-resource theory to show how body size can be an important factor in resource competition and coexistence.
Grazing occurs over a third of the earthÕs land surface and may potentially influence the storage... more Grazing occurs over a third of the earthÕs land surface and may potentially influence the storage of 10 9 Mg year)1 of greenhouse gases as soil C. Displacement of native herbivores by high densities of livestock has often led to overgrazing and soil C loss. However, it remains unknown whether matching livestock densities to those of native herbivores can yield equivalent soil C sequestration. In the Trans-Himalayas we found that, despite comparable grazing intensities, watersheds converted to pastoralism had 49% lower soil C than watersheds which retain native herbivores. Experimental grazerexclusion within each watershed type, show that this difference appears to be driven by indirect effects of livestock diet selection, leading to vegetation shifts that lower plant production and reduce likely soil C inputs from vegetation by c. 25 gC m)2 year)1. Our results suggest that while accounting for direct impacts (stocking density) is a major step, managing indirect impacts on vegetation composition are equally important in influencing soil C sequestration in grazing ecosystems.
The high-altitude rangelands of the Trans-Himalaya represent a grazing ecosystem which has suppor... more The high-altitude rangelands of the Trans-Himalaya represent a grazing ecosystem which has supported an indigenous pastoral community for millennia alongside a diverse assemblage of wild herbivores including burrowing mammals (pikas and voles). Pastoralists consider the small mammals to cause rangeland degradation and as competitors for their livestock, and actively eradicate them at many places. We present data on the ways in which small herbivores like pikas and voles mediate plant community dynamics. Vegetation cover and plant species richness were compared on and off both active and abandoned small mammal colonies. Plant species richness was higher inside colonies (about 4-5 species/plot) than outside (about 3 species/plot) whereas vegetation cover was only marginally lower (52% compared to 60%). Soil disturbance due to small mammals is seen to be associated with higher plant diversity without causing dramatic decline in overall vegetation cover. Such disturbance-mediated dynamics and vegetation mosaics produce a rich array of testable hypotheses that can highlight how small mammals influence assembly processes, succession, and dominance hierarchies in plant communities in this arid ecosystem. So, eradicating small mammals may lead to declining levels of diversity in this ecosystem, and compromise ecosystem-functioning. Changes in traditional pastoral practices and overstocking are more likely to be responsible for degradation. We emphasize that eradicating small mammals can lead to loss of diversity in this ecosystem and it is not a solution for the degradation problems.
Competitive interactions among species lie at the foundation of our understanding of the structur... more Competitive interactions among species lie at the foundation of our understanding of the structure and diversity of ecological communities. For the past century, various theoretical, laboratory and field studies have sought to understand how species that compete can coexist. Almost two decades ago, two influential reviews (Connell 1983; Schoener 1983) demonstrated that, at least in published studies, interspecific competition appears to be frequent. Just as importantly, these reviews found little evidence for competitive exclusion, that is, when one species completely eliminates another species when they occur together. In these reviews, competitive exclusion appeared especially rare for mobile animal species, as opposed to sedentary species such as inter-tidal organisms and vascular plants. Since 1983, competition theory has focused on identifying mechanisms to explain this “unexpected” prevalence of coexistence. In this chapter, I show that mobile animal species are highly likely ...
Herbivory by both grazing and browsing ungulates shapes the structure and functioning of terrestr... more Herbivory by both grazing and browsing ungulates shapes the structure and functioning of terrestrial ecosystems worldwide, and both types of herbivory have been implicated in major ecosystem state changes. Despite the ecological consequences of differences in diets and feeding habits among herbivores, studies that experimentally distinguish effects of grazing from spatially co-occurring, but temporally segregated browsing are extremely rare. Here we use a set of long-term exclosures in northern Utah, USA, to determine how domestic grazers vs. wild ungulate herbivores (including browsers and mixed feeders) affect sagebrush-dominated plant communities that historically covered ~62 million ha in North America. We sampled plant community properties and found that after 22 years grazing and browsing elicited perceptible changes in overall plant community composition and distinct responses by individual plant species. In the woody layer of the plant community, release from winter and spri...
A rid and semiarid ecosystems (drylands) often contain a higher diversity of animals and plants t... more A rid and semiarid ecosystems (drylands) often contain a higher diversity of animals and plants than would be expected from their low productivity. High spatial heterogeneity of resources and physical habitats, exhibited at a wide range of spatial scales (Rundel 1996, Holling 1992, Peterson et al. 1998), may be a major factor explaining such high diversity. For example, at extremely small scales (<10 cm), branched plant material and various soil physical processes can create spatial niches for invertebrates, cyanobacteria, and other cryptogamic organisms (Lightfoot and Whitford 1991). At somewhat larger scales (<10 m), desert shrubs may aggregate water and organic material in ''islands of fertility,'' yielding a highly patchy heterogeneous distribution of resources (e.g., seeds, water) for other plants and animals (Gibbens and Beck 1988, Halvorson et al. 1997, chapter 13 this volume, chapter 11 this volume). At even larger scales (>100 m), soil erosion patterns create topographic variation that locally concentrates available water and nutrients, yielding a marked heterogeneity in the distribution of productivity across the landscape (Milne 1992). These heterogeneous distributions of physical environments, biotic material, and resources are likely to have strong effects on biodiversity. Ecologists have long associated greater spatial heterogeneity with higher species diversity (MacArthur 1964; Brown 1981; May 1988). Within a particular physical environment (habitat), this association exists presumably because collections of species that use similar resources, or ''guilds,'' can coexist whenever they can more finely divide up space and different-sized resource ''packages''
Because a diversity of resources should support a diversity of consumers, most models predict tha... more Because a diversity of resources should support a diversity of consumers, most models predict that increasing plant diversity increases animal diversity. We report results of a direct experimental test of the dependence of animal diversity on plant diversity. We sampled arthropods in a well-replicated grassland experiment in which plant species richness and plant f~~nctional richness were directly manipulated. In simple regressions, both the number of species planted (l o g transformed) and the number of functional groups planted significantly increased arthropod species richness but not arthropod abundance. However, the number of species planted was the only significant predictor of arthropod species richness when both predictor variables were included in ANOVhs or a MANOVA. Although highly significant, arthropod species richness regressions had low R2 values, high intercepts (24 arthropod species in monocultures), and shallow slopes. Analyses of relations among plants and arthropod trophic groups indicated that herbivore diversity was influenced by plant, parasite, and predator diversity. Furthermore, herbivore diversity was more strongly correlated with parasite and predator diversity than with plant diversity. Together with regression results, this suggests that, although increasing plant diversity significantly increased arthropod diversity, local herbivore diversity is also maintained by, and in turn maintains, a diversity of parasites and predators.
We argue for expanding the role of theory in ecology to accelerate scientific progress, enhance t... more We argue for expanding the role of theory in ecology to accelerate scientific progress, enhance the ability to address environmental challenges, foster the development of synthesis and unification, and improve the design of experiments and large-scale environmental-monitoring programs. To achieve these goals, it is essential to foster the development of what we call efficient theories, which have several key attributes. Efficient theories are grounded in first principles, are usually expressed in the language of mathematics, make few assumptions and generate a large number of predictions per free parameter, are approximate, and entail predictions that provide well-understood standards for comparison with empirical data. We contend that the development and successive refinement of efficient theories provide a solid foundation for advancing environmental science in the era of big data.
The effects of grazing on soil organic carbon (SOC) dynamics, particularly in the tropics, are st... more The effects of grazing on soil organic carbon (SOC) dynamics, particularly in the tropics, are still poorly understood. Plant compensation to grazing, whereby plants maintain leaf area (C input capacity) despite consumption (C removal) by grazers, has been demonstrated in tropical grasslands but its influence on SOC is largely unexplored. Here, the effect of grazing on plant leaf area index (LAI) was measured in a field experiment in Serengeti National Park, Tanzania. LAI changed little for grazing intensities up to 70%. The response curve of LAI versus grazing intensity was used in a mass balance model, called SNAP, of SOC dynamics based on previous data from the Serengeti. The model predicted SOC to increase at intermediate grazing intensity, but then to decline rapidly at the highest grazing intensities. The SNAP model predictions were compared with observed SOC stocks in the 24 grazed plots of a 10-year grazing exclosure experiment at eight sites across the park that varied in mean annual rainfall, soil texture, grazing intensity and plant lignin and cellulose. The model predicted current SOC stocks very well (R 2 > 0.75), and suggests that compensatory plant responses to grazing are an important means of how herbivores might maintain or increase SOC in tropical grasslands.
Large mammalian herbivores occupy half of the earth's land surface and are important both ecologi... more Large mammalian herbivores occupy half of the earth's land surface and are important both ecologically and economically 1 , but their diversity is threatened by human activities 2. We investigated how the diversity of large herbivores changes across gradients of global precipitation and soil fertility. Here we show that more plant-available moisture reduces the nutrient content of plants but increases productivity, whereas more plant-available nutrients increase both of these factors. Because larger herbivore species tolerate lower plant nutrient content but require greater plant abundance, the highest potential herbivore diversity should occur in locations with intermediate moisture and high nutrients. These areas are dry enough to yield high quality plants and support smaller herbivores, but productive enough to support larger herbivores. These predictions fit with observed patterns of body size and diversity for large mammalian herbivores in North America, Africa and Australia, and yield a global map of regions with potentially high herbivore diversity. Thus, gradients of precipitation, temperature and soil fertility might explain the global distribution of large herbivore diversity and help to identify crucial areas for conservation and restoration.
Mammalian herbivores can have pronounced effects on plant diversity but are currently declining i... more Mammalian herbivores can have pronounced effects on plant diversity but are currently declining in many productive ecosystems through direct extirpation, habitat loss and fragmentation, while being simultaneously introduced as livestock in other, often unproductive, ecosystems that lacked such species during recent evolutionary times. The biodiversity consequences of these changes are still poorly understood. We experimentally separated the effects of primary productivity and herbivores of different body size on plant species richness across a 10-fold productivity gradient using a 7-year field experiment at seven grassland sites in North America and Europe. We show that assemblages including large herbivores increased plant diversity at higher productivity but decreased diversity at low productivity, while small herbivores did not have consistent effects along the productivity gradient. The recognition of these large-scale, cross-site patterns in herbivore effects is important for the development of appropriate biodiversity conservation strategies.
The nature and extent of microbial biodiversity remain controversial with persistent debates over... more The nature and extent of microbial biodiversity remain controversial with persistent debates over patterns of distributions (i.e. cosmopolitanism versus endemism) and the processes that structure these patterns (neutrality versus selection). We used culture-independent approaches to address these issues focusing on two groups of ciliates, the Oligotrichia (Spirotrichea) and Choreotrichia (Spirotrichea) across an environmental gradient. We assessed SSU rDNA diversity in ciliate communities at six stations in Long Island Sound spanning the frontal region that separates the fresher Connecticut River outflow plume from the open Sound. As in previous studies, we find one abundant cosmopolitan species (Strombidium biarmatum), a few moderately abundant sequences, and a long list of rare sequences. Furthermore, neither ciliate diversity nor species composition showed any clear relationship to measured environmental parameters (temperature, salinity, accessory pigment composition and chorophyll). Overall, we observed that diversity decreased moving from nearshore to offshore. We also conducted analyses to detect clustering among the sampled communities using the software Unifrac. This approach revealed three significant clusters grouping samples from nearshore, surface and deep/well mixed stations. We find no strong fit of our communities to log series, geometric or log normal distributions, though one of the 3 clusters is most consistent with a log series distribution. However, when we remove the abundant cosmopolitan species S. biarmatum, all three clusters fit to a log series distribution. These analyses suggest that, with the exception of one cosmopolitan species, the oligotrich and choreotrich communities at these stations may be distributed in a neutral manner.
Arbuscular mycorrhizal fungi (AMF) are more widely distributed and can associate with a wide rang... more Arbuscular mycorrhizal fungi (AMF) are more widely distributed and can associate with a wide range of plant species. AMF are keystone organisms that form an interface between soils and plant roots. They are also sensitive to environmental changes. AMF are important microbial symbioses for plants under conditions of P-limitation. The AMF are crucial for the functioning of terrestrial ecosystems as they form symbiotic interactions with plants. Mycorrhizal fungi are known to influence plant diversity patterns in a variety of ecosystems globally. AMF hyphae form an extensive network in the soil. The length is a common parameter used to quantifying fungal hyphae. The mycelial network of AM fungi extends into the soil volume and greatly increases the surface area for the uptake of immobile nutrients. Also, AM symbioses improve plants tolerance to drought and enhance plants' tolerance of or resistance to root pathogens. Also, the networks of AM hyphae play a crucial role in the formation of stable soil aggregates and in the building up of a macroporous structure of soil that allows penetration of water and air and thereby prevents erosion. The functioning of AMF symbiosis is mediated by direct and indirect effects of biotic and abiotic factors of the surrounding rhizosphere, the community, and the ecosystem. AMF have great potential in the restoration of disturbed land and low fertility soil. However, despite the importance of AMF to terrestrial ecosystems, little is known about the effects of environmental changes on AMF abundance, activity and the impact of these changes on the ecosystem services. Therefore, it is important to gain a clearer understanding of the effects of environmental changes on the AM fungal species to guide conservation and restoration efforts.
An optimal foraging model is presented which combines simultaneous and nonsimultaneous modes of s... more An optimal foraging model is presented which combines simultaneous and nonsimultaneous modes of search for food. This model is more complex than those in the literature which are constructed using one search mode exclusively. Solutions to this foraging model lead to diets which are seldom composed of a single food or of foods in proportion to their abundance in the environment. Therefore, the most commonly cited qualitative conclusions of optimal foraging theory are not adequate to explain foraging choices under the range of probable search modes.
We experimentally separated the effects of two components of plant diversity-plant species richne... more We experimentally separated the effects of two components of plant diversity-plant species richness and plant functional group richness-on insect communities. Plant species richness and plant functional group richness had contrasting effects on insect abundances, a result we attributed to three factors. First, lower insect abundances at higher plant functional group richness were explained by a sampling effect, which was caused by the increasing likelihood that one low-quality group, C 4 grasses, would be present and reduce average insect abundances by 25%. Second, plant biomass, which was positively related to plant functional group richness, had a strong, positive effect on insect abundances. Third, a positive effect of plant species richness on insect abundances may have been caused by greater availability of alternate plant resources or greater vegetational structure. In addition, a greater diversity of insect species, whose individual abundances were often unaffected by changes in plant species richness, may have generated higher total community abundances. After controlling for the strong, positive influence of insect abundance on insect diversity through rarefaction, insect species richness increased as plant species richness and plant functional group richness increased. Although these variables did not explain a high proportion of variation individually, plant species richness and plant functional group richness had similar effects on insect diversity and opposing effects on insect abundances, and both factors may explain how the loss of plant diversity influences higher trophic levels.
Humans are modifying both the identities and numbers of species in ecosystems, but the impacts of... more Humans are modifying both the identities and numbers of species in ecosystems, but the impacts of such changes on ecosystem processes are controversial. Plant species diversity, functional diversity, and functional composition were experimentally varied in grassland plots. Each factor by itself had significant effects on many ecosystem processes, but functional composition and functional diversity were the principal factors explaining plant productivity, plant percent nitrogen, plant total nitrogen, and light penetration. Thus, habitat modifications and management practices that change functional diversity and functional composition are likely to have large impacts on ecosystem processes.
This article was published in an Elsevier journal. The attached copy is furnished to the author f... more This article was published in an Elsevier journal. The attached copy is furnished to the author for non-commercial research and education use, including for instruction at the author's institution, sharing with colleagues and providing to institution administration. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright
Species coexistence involving trophic interactions has been investigated under two theoretical fr... more Species coexistence involving trophic interactions has been investigated under two theoretical frameworks-partitioning shared resources and accessing exclusive resources. The influence of body size on coexistence is well studied under the exclusive resources framework, but has received less attention under the sharedresources framework. We investigate body-size-dependent allometric extensions of a classical MacArthur-type model where two consumers compete for two shared resources. The equilibrium coexistence criteria are compared against the general predictions of the alternative framework over exclusive resources. From the asymmetry in body size allometry of resource encounter versus demand our model shows, counterintuitively, and contrary to the exclusive resource framework, that a smaller consumer should be competitively superior across a wide range of supplies of the two resource types. Experimental studies are reviewed to resolve this difference among the two frameworks that arise from their respective assumptions over resource distribution. Another prediction is that the smaller consumer may have relatively stronger control over equilibrium resource abundance, and the loss of smaller consumers from a community may induce relatively stronger trophic cascades. Finally, from satiating consumers' functional response, our model predicts that greater difference among resource sizes can allow a broader range of consumer body sizes to coexist, and this is consistent with the predictions of the alternative framework over exclusive resources. Overall, this analysis provides an objective comparison of the two alternative approaches to understand species coexistence that have heretofore developed in relative isolation. It advances classical consumer-resource theory to show how body size can be an important factor in resource competition and coexistence.
Grazing occurs over a third of the earthÕs land surface and may potentially influence the storage... more Grazing occurs over a third of the earthÕs land surface and may potentially influence the storage of 10 9 Mg year)1 of greenhouse gases as soil C. Displacement of native herbivores by high densities of livestock has often led to overgrazing and soil C loss. However, it remains unknown whether matching livestock densities to those of native herbivores can yield equivalent soil C sequestration. In the Trans-Himalayas we found that, despite comparable grazing intensities, watersheds converted to pastoralism had 49% lower soil C than watersheds which retain native herbivores. Experimental grazerexclusion within each watershed type, show that this difference appears to be driven by indirect effects of livestock diet selection, leading to vegetation shifts that lower plant production and reduce likely soil C inputs from vegetation by c. 25 gC m)2 year)1. Our results suggest that while accounting for direct impacts (stocking density) is a major step, managing indirect impacts on vegetation composition are equally important in influencing soil C sequestration in grazing ecosystems.
The high-altitude rangelands of the Trans-Himalaya represent a grazing ecosystem which has suppor... more The high-altitude rangelands of the Trans-Himalaya represent a grazing ecosystem which has supported an indigenous pastoral community for millennia alongside a diverse assemblage of wild herbivores including burrowing mammals (pikas and voles). Pastoralists consider the small mammals to cause rangeland degradation and as competitors for their livestock, and actively eradicate them at many places. We present data on the ways in which small herbivores like pikas and voles mediate plant community dynamics. Vegetation cover and plant species richness were compared on and off both active and abandoned small mammal colonies. Plant species richness was higher inside colonies (about 4-5 species/plot) than outside (about 3 species/plot) whereas vegetation cover was only marginally lower (52% compared to 60%). Soil disturbance due to small mammals is seen to be associated with higher plant diversity without causing dramatic decline in overall vegetation cover. Such disturbance-mediated dynamics and vegetation mosaics produce a rich array of testable hypotheses that can highlight how small mammals influence assembly processes, succession, and dominance hierarchies in plant communities in this arid ecosystem. So, eradicating small mammals may lead to declining levels of diversity in this ecosystem, and compromise ecosystem-functioning. Changes in traditional pastoral practices and overstocking are more likely to be responsible for degradation. We emphasize that eradicating small mammals can lead to loss of diversity in this ecosystem and it is not a solution for the degradation problems.
Competitive interactions among species lie at the foundation of our understanding of the structur... more Competitive interactions among species lie at the foundation of our understanding of the structure and diversity of ecological communities. For the past century, various theoretical, laboratory and field studies have sought to understand how species that compete can coexist. Almost two decades ago, two influential reviews (Connell 1983; Schoener 1983) demonstrated that, at least in published studies, interspecific competition appears to be frequent. Just as importantly, these reviews found little evidence for competitive exclusion, that is, when one species completely eliminates another species when they occur together. In these reviews, competitive exclusion appeared especially rare for mobile animal species, as opposed to sedentary species such as inter-tidal organisms and vascular plants. Since 1983, competition theory has focused on identifying mechanisms to explain this “unexpected” prevalence of coexistence. In this chapter, I show that mobile animal species are highly likely ...
Herbivory by both grazing and browsing ungulates shapes the structure and functioning of terrestr... more Herbivory by both grazing and browsing ungulates shapes the structure and functioning of terrestrial ecosystems worldwide, and both types of herbivory have been implicated in major ecosystem state changes. Despite the ecological consequences of differences in diets and feeding habits among herbivores, studies that experimentally distinguish effects of grazing from spatially co-occurring, but temporally segregated browsing are extremely rare. Here we use a set of long-term exclosures in northern Utah, USA, to determine how domestic grazers vs. wild ungulate herbivores (including browsers and mixed feeders) affect sagebrush-dominated plant communities that historically covered ~62 million ha in North America. We sampled plant community properties and found that after 22 years grazing and browsing elicited perceptible changes in overall plant community composition and distinct responses by individual plant species. In the woody layer of the plant community, release from winter and spri...
A rid and semiarid ecosystems (drylands) often contain a higher diversity of animals and plants t... more A rid and semiarid ecosystems (drylands) often contain a higher diversity of animals and plants than would be expected from their low productivity. High spatial heterogeneity of resources and physical habitats, exhibited at a wide range of spatial scales (Rundel 1996, Holling 1992, Peterson et al. 1998), may be a major factor explaining such high diversity. For example, at extremely small scales (<10 cm), branched plant material and various soil physical processes can create spatial niches for invertebrates, cyanobacteria, and other cryptogamic organisms (Lightfoot and Whitford 1991). At somewhat larger scales (<10 m), desert shrubs may aggregate water and organic material in ''islands of fertility,'' yielding a highly patchy heterogeneous distribution of resources (e.g., seeds, water) for other plants and animals (Gibbens and Beck 1988, Halvorson et al. 1997, chapter 13 this volume, chapter 11 this volume). At even larger scales (>100 m), soil erosion patterns create topographic variation that locally concentrates available water and nutrients, yielding a marked heterogeneity in the distribution of productivity across the landscape (Milne 1992). These heterogeneous distributions of physical environments, biotic material, and resources are likely to have strong effects on biodiversity. Ecologists have long associated greater spatial heterogeneity with higher species diversity (MacArthur 1964; Brown 1981; May 1988). Within a particular physical environment (habitat), this association exists presumably because collections of species that use similar resources, or ''guilds,'' can coexist whenever they can more finely divide up space and different-sized resource ''packages''
Because a diversity of resources should support a diversity of consumers, most models predict tha... more Because a diversity of resources should support a diversity of consumers, most models predict that increasing plant diversity increases animal diversity. We report results of a direct experimental test of the dependence of animal diversity on plant diversity. We sampled arthropods in a well-replicated grassland experiment in which plant species richness and plant f~~nctional richness were directly manipulated. In simple regressions, both the number of species planted (l o g transformed) and the number of functional groups planted significantly increased arthropod species richness but not arthropod abundance. However, the number of species planted was the only significant predictor of arthropod species richness when both predictor variables were included in ANOVhs or a MANOVA. Although highly significant, arthropod species richness regressions had low R2 values, high intercepts (24 arthropod species in monocultures), and shallow slopes. Analyses of relations among plants and arthropod trophic groups indicated that herbivore diversity was influenced by plant, parasite, and predator diversity. Furthermore, herbivore diversity was more strongly correlated with parasite and predator diversity than with plant diversity. Together with regression results, this suggests that, although increasing plant diversity significantly increased arthropod diversity, local herbivore diversity is also maintained by, and in turn maintains, a diversity of parasites and predators.
We argue for expanding the role of theory in ecology to accelerate scientific progress, enhance t... more We argue for expanding the role of theory in ecology to accelerate scientific progress, enhance the ability to address environmental challenges, foster the development of synthesis and unification, and improve the design of experiments and large-scale environmental-monitoring programs. To achieve these goals, it is essential to foster the development of what we call efficient theories, which have several key attributes. Efficient theories are grounded in first principles, are usually expressed in the language of mathematics, make few assumptions and generate a large number of predictions per free parameter, are approximate, and entail predictions that provide well-understood standards for comparison with empirical data. We contend that the development and successive refinement of efficient theories provide a solid foundation for advancing environmental science in the era of big data.
The effects of grazing on soil organic carbon (SOC) dynamics, particularly in the tropics, are st... more The effects of grazing on soil organic carbon (SOC) dynamics, particularly in the tropics, are still poorly understood. Plant compensation to grazing, whereby plants maintain leaf area (C input capacity) despite consumption (C removal) by grazers, has been demonstrated in tropical grasslands but its influence on SOC is largely unexplored. Here, the effect of grazing on plant leaf area index (LAI) was measured in a field experiment in Serengeti National Park, Tanzania. LAI changed little for grazing intensities up to 70%. The response curve of LAI versus grazing intensity was used in a mass balance model, called SNAP, of SOC dynamics based on previous data from the Serengeti. The model predicted SOC to increase at intermediate grazing intensity, but then to decline rapidly at the highest grazing intensities. The SNAP model predictions were compared with observed SOC stocks in the 24 grazed plots of a 10-year grazing exclosure experiment at eight sites across the park that varied in mean annual rainfall, soil texture, grazing intensity and plant lignin and cellulose. The model predicted current SOC stocks very well (R 2 > 0.75), and suggests that compensatory plant responses to grazing are an important means of how herbivores might maintain or increase SOC in tropical grasslands.
Large mammalian herbivores occupy half of the earth's land surface and are important both ecologi... more Large mammalian herbivores occupy half of the earth's land surface and are important both ecologically and economically 1 , but their diversity is threatened by human activities 2. We investigated how the diversity of large herbivores changes across gradients of global precipitation and soil fertility. Here we show that more plant-available moisture reduces the nutrient content of plants but increases productivity, whereas more plant-available nutrients increase both of these factors. Because larger herbivore species tolerate lower plant nutrient content but require greater plant abundance, the highest potential herbivore diversity should occur in locations with intermediate moisture and high nutrients. These areas are dry enough to yield high quality plants and support smaller herbivores, but productive enough to support larger herbivores. These predictions fit with observed patterns of body size and diversity for large mammalian herbivores in North America, Africa and Australia, and yield a global map of regions with potentially high herbivore diversity. Thus, gradients of precipitation, temperature and soil fertility might explain the global distribution of large herbivore diversity and help to identify crucial areas for conservation and restoration.
Mammalian herbivores can have pronounced effects on plant diversity but are currently declining i... more Mammalian herbivores can have pronounced effects on plant diversity but are currently declining in many productive ecosystems through direct extirpation, habitat loss and fragmentation, while being simultaneously introduced as livestock in other, often unproductive, ecosystems that lacked such species during recent evolutionary times. The biodiversity consequences of these changes are still poorly understood. We experimentally separated the effects of primary productivity and herbivores of different body size on plant species richness across a 10-fold productivity gradient using a 7-year field experiment at seven grassland sites in North America and Europe. We show that assemblages including large herbivores increased plant diversity at higher productivity but decreased diversity at low productivity, while small herbivores did not have consistent effects along the productivity gradient. The recognition of these large-scale, cross-site patterns in herbivore effects is important for the development of appropriate biodiversity conservation strategies.
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Papers by M. Ritchie