Showing papers in "Ecology Letters in 2005"
TL;DR: An overview of recent advances in species distribution models, and new avenues for incorporating species migration, population dynamics, biotic interactions and community ecology into SDMs at multiple spatial scales are suggested.
Abstract: In the last two decades, interest in species distribution models (SDMs) of plants and animals has grown dramatically. Recent advances in SDMs allow us to potentially forecast anthropogenic effects on patterns of biodiversity at different spatial scales. However, some limitations still preclude the use of SDMs in many theoretical and practical applications. Here, we provide an overview of recent advances in this field, discuss the ecological principles and assumptions underpinning SDMs, and highlight critical limitations and decisions inherent in the construction and evaluation of SDMs. Particular emphasis is given to the use of SDMs for the assessment of climate change impacts and conservation management issues. We suggest new avenues for incorporating species migration, population dynamics, biotic interactions and community ecology into SDMs at multiple spatial scales. Addressing all these issues requires a better integration of SDMs with ecological theory.
5,620 citations
TL;DR: In this article, the negative and positive effects of agricultural land use for the conservation of biodiversity, and its relation to ecosystem services, need a landscape perspective, which is difficult to be found in the literature.
Abstract: Understanding the negative and positive effects of agricultural land use for the conservation of biodiversity, and its relation to ecosystem services, needs a landscape perspective. Agriculture can contribute to the conservation of high-diversity systems, which may provide important ecosystem services such as pollination and biological control via complementarity and sampling effects. Land-use management is often focused on few species and local processes, but in dynamic, agricultural landscapes, only a diversity of insurance species may guarantee resilience (the capacity to reorganize after disturbance). Interacting species experience their surrounding landscape at different spatial scales, which influences trophic interactions. Structurally complex landscapes enhance local diversity in agroecosystems, which may compensate for local high-intensity management. Organisms with high-dispersal abilities appear to drive these biodiversity patterns and ecosystem services, because of their recolonization ability and larger resources experienced. Agri-environment schemes (incentives for farmers to benefit the environment) need to broaden their perspective and to take the different responses to schemes in simple (high impact) and complex (low impact) agricultural landscapes into account. In simple landscapes, local allocation of habitat is more important than in complex landscapes, which are in total at risk. However, little knowledge of the relative importance of local and landscape management for biodiversity and its relation to ecosystem services make reliable recommendations difficult.
3,460 citations
TL;DR: In contrast to the expanding edge, the low-latitude limit (rear edge) of species ranges remains understudied, and the critical importance of rear edge populations as long-term stores of species' genetic diversity and foci of speciation has been little acknowledged.
Abstract: Modern climate change is producing poleward range shifts of numerous taxa, communities and ecosystems worldwide. The response of species to changing environments is likely to be determined largely by population responses at range margins. In contrast to the expanding edge, the low-latitude limit (rear edge) of species ranges remains understudied, and the critical importance of rear edge populations as long-term stores of species' genetic diversity and foci of speciation has been little acknowledged. We review recent findings from the fossil record, phylogeography and ecology to illustrate that rear edge populations are often disproportionately important for the survival and evolution of biota. Their ecological features, dynamics and conservation requirements differ from those of populations in other parts of the range, and some commonly recommended conservation practices might therefore be of little use or even counterproductive for rear edge populations.
1,908 citations
1,545 citations
TL;DR: The importance of adaptation in the response of plant species to past climate change has been questioned as mentioned in this paper, and the authors of this paper review recent work that has documented climate-related genetic diversity within populations or on the microgeographical scale.
Abstract: Climate is a potent selective force in natural populations, yet the importance of adaptation in the response of plant species to past climate change has been questioned. As many species are unlikely to migrate fast enough to track the rapidly changing climate of the future, adaptation must play an increasingly important role in their response. In this paper we review recent work that has documented climate-related genetic diversity within populations or on the microgeographical scale. We then describe studies that have looked at the potential evolutionary responses of plant populations to future climate change. We argue that in fragmented landscapes, rapid climate change has the potential to overwhelm the capacity for adaptation in many plant populations and dramatically alter their genetic composition. The consequences are likely to include unpredictable changes in the presence and abundance of species within communities and a reduction in their ability to resist and recover from further environmental perturbations, such as pest and disease outbreaks and extreme climatic events. Overall, a range-wide increase in extinction risk is likely to result. We call for further research into understanding the causes and consequences of the maintenance and loss of climate-related genetic diversity within populations.
1,255 citations
TL;DR: It is shown that approximately 50% of the variation in community composition is explained by both environmental and spatial variables, and metacommunity characteristics such as dispersal type, habitat type and spatial scale predicted part of the detected variation in metacomunity structure.
Abstract: The processes controlling the abundances of species across multiple sites form the cornerstone of modern ecology. In these metacommunities, the relative importance of local environmental and regional spatial processes is currently hotly debated, especially in terms of the validity of neutral model. I collected 158 published data sets with information on community structure, environmental and spatial variables. I showed that approximately 50% of the variation in community composition is explained by both environmental and spatial variables. The majority of the data sets were structured by species-sorting dynamics (SS), followed by a combination of SS and mass-effect dynamics. While neutral processes were the only structuring process in 8% of the collected natural communities, disregarding neutral dispersal processes would result in missing important patterns in 37% of the studied communities. Moreover, metacommunity characteristics such as dispersal type, habitat type and spatial scale predicted part of the detected variation in metacommunity structure.
1,192 citations
TL;DR: This research agenda discusses critical questions and key approaches in determining the various aspects of community structure that influence function in real landscapes, especially compensatory community responses that stabilize function, or non-random extinction sequences that rapidly erode it.
Abstract: Human domination of the biosphere has greatly altered ecosystems, often overwhelming their capacity to provide ecosystem services critical to our survival. Yet ecological understanding of ecosystem services is quite limited. Previous work maps the supply and demand for services, assesses threats to them, and estimates economic values, but does not measure the underlying role of biodiversity in providing services. In contrast, experimental studies of biodiversity-function examine communities whose structures often differ markedly from those providing services in real landscapes. A bridge is needed between these two approaches. To develop this research agenda, I discuss critical questions and key approaches in four areas: (1) identifying the important 'ecosystem service providers'; (2) determining the various aspects of community structure that influence function in real landscapes, especially compensatory community responses that stabilize function, or non-random extinction sequences that rapidly erode it; (3) assessing key environmental factors influencing provision of services, and (4) measuring the spatio-temporal scale over which providers and services operate. I show how this research agenda can assist in developing environmental policy and natural resource management plans.
1,190 citations
TL;DR: This work proposes that rapid evolution be defined as a genetic change occurring rapidly enough to have a measurable impact on simultaneous ecological change, and proposes a framework for decomposing rates of ecological change into components driven by simultaneous evolutionary change and by change in a non-evolutionary factor.
Abstract: Recent studies have documented rates of evolution of ecologically important phenotypes sufficiently fast that they have the potential to impact the outcome of ecological interactions while they are underway. Observations of this type go against accepted wisdom that ecological and evolutionary dynamics occur at very different time scales. While some authors have evaluated the rapidity of a measured evolutionary rate by comparing it to the overall distribution of measured evolutionary rates, we believe that ecologists are mainly interested in rapid evolution because of its potential to impinge on ecological processes. We therefore propose that rapid evolution be defined as a genetic change occurring rapidly enough to have a measurable impact on simultaneous ecological change. Using this definition we propose a framework for decomposing rates of ecological change into components driven by simultaneous evolutionary change and by change in a non-evolutionary factor (e.g. density dependent population dynamics, abiotic environmental change). Evolution is judged to be rapid in this ecological context if its contribution to ecological change is large relative to the contribution of other factors. We provide a worked example of this approach based on a theoretical predator–prey interaction [Abrams, P. & Matsuda, H. (1997). Evolution, 51, 1740], and find that in this system the impact of prey evolution on predator per capita growth rate is 63% that of internal ecological dynamics. We then propose analytical methods for measuring these contributions in field situations, and apply them to two long-term data sets for which suitable ecological and evolutionary data exist. For both data sets relatively high rates of evolutionary change have been found when measured as character change in standard deviations per generation (haldanes). For Darwin's finches evolving in response to fluctuating rainfall [Grant, P.R. & Grant, B.R. (2002). Science, 296, 707], we estimate that evolutionary change has been more rapid than ecological change by a factor of 2.2. For a population of freshwater copepods whose life history evolves in response to fluctuating fish predation [Hairston, N.G. Jr & Dillon, T.A. (1990). Evolution, 44, 1796], we find that evolutionary change has been about one quarter the rate of ecological change – less than in the finch example, but nevertheless substantial. These analyses support the view that in order to understand temporal dynamics in ecological processes it is critical to consider the extent to which the attributes of the system under investigation are simultaneously changing as a result of rapid evolution.
896 citations
TL;DR: A framework for understanding how zero-inflated data sets originate and deciding how best to model them is proposed and the different kinds of zeros that occur in ecological data are defined and classified.
Abstract: A common feature of ecological data sets is their tendency to contain many zero values. Statistical inference based on such data are likely to be inefficient or wrong unless careful thought is given to how these zeros arose and how best to model them. In this paper, we propose a framework for understanding how zero-inflated data sets originate and deciding how best to model them. We define and classify the different kinds of zeros that occur in ecological data and describe how they arise: either from 'true zero' or 'false zero' observations. After reviewing recent developments in modelling zero-inflated data sets, we use practical examples to demonstrate how failing to account for the source of zero inflation can reduce our ability to detect relationships in ecological data and at worst lead to incorrect inference. The adoption of methods that explicitly model the sources of zero observations will sharpen insights and improve the robustness of ecological analyses.
870 citations
704 citations
TL;DR: It is shown that lower elevational limits for 16 butterfly species in central Spain have risen on average by 212‚m in 30 years, accompanying a 1.3‚°C rise in mean annual temperature, which represents an average reduction in habitable area by one-third.
Abstract: The first expected symptoms of a climate change-generated biodiversity crisis are range contractions and extinctions at lower elevational and latitudinal limits to species distributions. However, whilst range expansions at high elevations and latitudes have been widely documented, there has been surprisingly little evidence for contractions at warm margins. We show that lower elevational limits for 16 butterfly species in central Spain have risen on average by 212 m (± SE 60) in 30 years, accompanying a 1.3 � C rise (equivalent to c. 225 m) in mean annual temperature. These elevational shifts signify an average reduction in habitable area by one-third, with losses of 50‐80% projected for the coming century, given maintenance of the species thermal associations. The results suggest that many species have already suffered climate-mediated habitat losses that may threaten their long-term chances of survival.
TL;DR: In this paper, the authors review models of cooperation that are based on two simple games: the Prisoner's Dilemma, and the Snowdrift game, and point out a number of promising natural systems in which the theory can be tested experimentally.
Abstract: Understanding the mechanisms that can lead to the evolution of cooperation through natural selection is a core problem in biology. Among the various attempts at constructing a theory of cooperation, game theory has played a central role. Here, we review models of cooperation that are based on two simple games: the Prisoner's Dilemma, and the Snowdrift game. Both games are two-person games with two strategies, to cooperate and to defect, and both games are social dilemmas. In social dilemmas, cooperation is prone to exploitation by defectors, and the average payoff in populations at evolutionary equilibrium is lower than it would be in populations consisting of only cooperators. The difference between the games is that cooperation is not maintained in the Prisoner's Dilemma, but persists in the Snowdrift game at an intermediate frequency. As a consequence, insights gained from studying extensions of the two games differ substantially. We review the most salient results obtained from extensions such as iteration, spatial structure, continuously variable cooperative investments, and multi-person interactions. Bridging the gap between theoretical and empirical research is one of the main challenges for future studies of cooperation, and we conclude by pointing out a number of promising natural systems in which the theory can be tested experimentally.
TL;DR: Differences in litter calcium concentrations among tree species resulted in profound changes in soil acidity and fertility that were similar within and among tree groups, and were associated with increased native earthworm abundance and diversity.
Abstract: Tree species can influence biogeochemistry through variation in the quantity and chemistry of their litter, and associated impacts on the soil heterotrophic community. However, the role that different plant traits play in these processes is not well understood, nor is it clear whether species effects on soils largely reflect a gymnosperm vs. angiosperm contrast. Using a replicated, long-term monoculture plot experiment, we examined variation in soils among 14 gymnosperm and angiosperm tree species 30 years after plot establishment, and assessed the role of litter chemistry vis-a`-vis such variation. Differences in litter calcium concentrations among tree species resulted in profound changes in soil acidity and fertility that were similar within and among tree groups. Tree species rich in calcium were associated with increased native earthworm abundance and diversity, as well as increased soil pH, exchangeable calcium, per cent base saturation and forest floor turnover rate.
TL;DR: The results show that the simultaneous operation of trait-based assembly rules and species-level priority effects drives community assembly, making it both deterministic and historically contingent, but at different levels of community organization.
Abstract: Despite decades of research, it remains controversial whether ecological communities converge towards a common structure determined by environmental conditions irrespective of assembly history. Here, we show experimentally that the answer depends on the level of community organization considered. In a 9-year grassland experiment, we manipulated initial plant composition on abandoned arable land and subsequently allowed natural colonization. Initial compositional variation caused plant communities to remain divergent in species identities, even though these same communities converged strongly in species traits. This contrast between species divergence and trait convergence could not be explained by dispersal limitation or community neutrality alone. Our results show that the simultaneous operation of trait-based assembly rules and species-level priority effects drives community assembly, making it both deterministic and historically contingent, but at different levels of community organization.
TL;DR: It is shown that habitat loss appeared to disrupt ecosystem functioning by affecting extinction order, species richness and abundance, and found that large-bodied bee and beetle species tended to be both most extinction-prone and most functionally efficient, contributing to rapid functional loss.
Abstract: By causing extinctions and altering community structure, anthropogenic disturbances can disrupt processes that maintain ecosystem integrity. However, the relationship between community structure and ecosystem functioning in natural systems is poorly understood. Here we show that habitat loss appeared to disrupt ecosystem functioning by affecting extinction order, species richness and abundance. We studied pollination by bees in a mosaic of agricultural and natural habitats in California and dung burial by dung beetles on recently created islands in Venezuela. We found that large-bodied bee and beetle species tended to be both most extinction-prone and most functionally efficient, contributing to rapid functional loss. Simulations confirmed that extinction order led to greater disruption of function than predicted by random species loss. Total abundance declined with richness and also appeared to contribute to loss of function. We demonstrate conceptually and empirically how the non-random response of communities to disturbance can have unexpectedly large functional consequences.
TL;DR: The response of vascular plant species richness to long-term habitat loss and fragmentation of Estonian calcareous grasslands (alvars) and the magnitude of extinction debt in local communities is examined.
Abstract: We examined the response of vascular plant species richness to long-term habitat loss and fragmentation of Estonian calcareous grasslands (alvars). The current number of habitat specialist species in 35 alvars was not explained by their current areas and connectivities but it was explained by their areas and connectivities 70 years ago (R(2) = 0.27). We estimated the magnitude of extinction debt in local communities by assuming an equilibrium species richness in 14 alvars that had lost only a small amount of area and by applying this model to the remaining alvars, in which the average area has declined from 3.64 km(2) in the 1930s to 0.21 km(2) at present. The extinction debt estimated for individual alvars was around 40% of their current species number. Our conclusions are applicable to temperate grasslands in general, which have lost much area because of agricultural intensification and cessation of traditional management.
TL;DR: In the field of restoration ecology, the field has gained a strong academic foothold, addressing problems faced by restoration practitioners, bringing new focus to existing ecological theory and fostering a handful of novel ecological ideas as mentioned in this paper.
Abstract: Restoration ecology is a young academic field, but one with enough history to judge it against past and current expectations of the science's potential. The practice of ecological restoration has been identified as providing ideal experimental settings for tests of ecological theory; restoration was to be the ‘acid test’ of our ecological understanding. Over the past decade, restoration science has gained a strong academic foothold, addressing problems faced by restoration practitioners, bringing new focus to existing ecological theory and fostering a handful of novel ecological ideas. In particular, recent advances in plant community ecology have been strongly linked with issues in ecological restoration. Evolving models of succession, assembly and state-transition are at the heart of both community ecology and ecological restoration. Recent research on seed and recruitment limitation, soil processes, and diversity–function relationships also share strong links to restoration. Further opportunities may lie ahead in the ecology of plant ontogeny, and on the effects of contingency, such as year effects and priority effects. Ecology may inform current restoration practice, but there is considerable room for greater integration between academic scientists and restoration practitioners.
TL;DR: This theory suggests that mobile higher order organisms are potent stabilizers when embedded in a variable, and expansive spatial structure, but when space is compressed and higher order consumers strongly couple local habitats then mobile consumers can have an inordinate destabilizing effect.
Abstract: The dynamics of ecological systems include a bewildering number of biotic interactions that unfold over a vast range of spatial scales. Here, employing simple and general empirical arguments concerning the nature of movement, trophic position and behaviour we outline a general theory concerning the role of space and food web structure on food web stability. We argue that consumers link food webs in space and that this spatial structure combined with relatively rapid behavioural responses by consumers can strongly influence the dynamics of food webs. Employing simple spatially implicit food web models, we show that large mobile consumers are inordinately important in determining the stability, or lack of it, in ecosystems. More specifically, this theory suggests that mobile higher order organisms are potent stabilizers when embedded in a variable, and expansive spatial structure. However, when space is compressed and higher order consumers strongly couple local habitats then mobile consumers can have an inordinate destabilizing effect. Preliminary empirical arguments show consistency with this general theory.
TL;DR: Character displacement research in the past two decades provides sound statistical support for the hypothesis in a wide variety of taxa, albeit with a phylogenetically skewed representation.
Abstract: Ecological character displacement, mostly seen as increased differences of size in sympatry between closely-related or similar species, is a focal hypothesis assuming that species too similar to one another could not coexist without diverging, owing to interspecific competition. Thus, ecological character displacement and community-wide character displacement (overdispersion in size of potential competitors within ecological guilds) were at the heart of the debate regarding the role of competition in structuring ecological communities. The debate has focused on the evidence presented in earlier studies and generated a new generation of rigorous, critical studies of communities. Character displacement research in the past two decades provides sound statistical support for the hypothesis in a wide variety of taxa, albeit with a phylogenetically skewed representation. A growing number of studies are strongly based in functional morphology, and some also demonstrate actual morphologically related resource partitioning. Phylogenetic models and experimental work have added to the scope and depth of earlier research, as have theoretical studies. However, many challenging ecological and evolutionary issues, regarding both selective forces (at the inter- and intraspecific level) and resultant patterns, remain to be addressed. Ecological character displacement and community-wide character displacement are here to stay as the focus of much exciting research.
TL;DR: The results suggest that anthropogenic barriers constitute a severe threat to the persistence of naturally fragmented populations, and link a rapid reduction in genetic diversity to as few as 40 years of anthropogenic isolation.
Abstract: The rapid expansion of road networks has reduced connectivity among populations of flora and fauna. The resulting isolation is assumed to increase population extinction rates, in part because of the loss of genetic diversity. However, there are few cases where loss of genetic diversity has been linked directly to roads or other barriers. We analysed the effects of such barriers on connectivity and genetic diversity of 27 populations of Ovis canadensis nelsoni (desert bighorn sheep). We used partial Mantel tests, multiple linear regression and coalescent simulations to infer changes in gene flow and diversity of nuclear and mitochondrial DNA markers. Our findings link a rapid reduction in genetic diversity (up to 15%) to as few as 40 years of anthropogenic isolation. Interstate highways, canals and developed areas, where present, have apparently eliminated gene flow. These results suggest that anthropogenic barriers constitute a severe threat to the persistence of naturally fragmented populations.
TL;DR: It is shown mathematically that the correlation between interaction frequency and total effect (T) becomes more strongly positive the greater the variation of I relative to the variations of per-interaction effect (P) and the greaterThe correlation between I and P, the more frequent animal mutualists usually contribute the most to plant reproduction.
Abstract: We evaluate whether species interaction frequency can be used as a surrogate for the total effect of a species on another. Because interaction frequency is easier to estimate than per-interaction effect, using interaction frequency as a surrogate of total effect could facilitate the large-scale analysis of quantitative patterns of species-rich interaction networks. We show mathematically that the correlation between interaction frequency (I ) and total effect (T ) becomes more strongly positive the greater the variation of I relative to the variation of per-interaction effect (P ) and the greater the correlation between I and P. A meta-analysis using data on I, P and T for animal pollinators and seed dispersers visiting plants shows a generally strong, positive relationship between T and I, in spite of no general relationship between P and I. Thus, frequent animal mutualists usually contribute the most to plant reproduction, regardless of their effectiveness on a per-interaction basis.
TL;DR: It is hypothesized that belowground changes to the soil microbial community provide a mechanistic link between exotic plant invasion and changes to ecosystem nutrient cycling.
Abstract: Plant invasions have dramatic aboveground effects on plant community composition, but their belowground effects remain largely uncharacterized. Soil microorganisms directly interact with plants and mediate many nutrient transformations in soil. We hypothesized that belowground changes to the soil microbial community provide a mechanistic link between exotic plant invasion and changes to ecosystem nutrient cycling. To examine this possible link, monocultures and mixtures of exotic and native species were maintained for 4 years in a California grassland. Gross rates of nitrogen (N) mineralization and nitrification were quantified with 15N pool dilution and soil microbial communities were characterized with DNA-based methods. Exotic grasses doubled gross nitrification rates, in part by increasing the abundance and changing the composition of ammonia-oxidizing bacteria in soil. These changes may translate into altered ecosystem N budgets after invasion. Altered soil microbial communities and their resulting effects on ecosystem processes may be an invisible legacy of exotic plant invasions.
TL;DR: Focusing on potential connections between species diversity and genetic diversity complements other approaches to synthesis at the ecology–evolution interface, and should contribute to conceptual unification of biodiversity research at the levels of genes and species.
Abstract: Species diversity and genetic diversity remain the nearly exclusive domains of community ecology and population genetics, respectively, despite repeated recognition in the literature over the past 30 years of close parallels between these two levels of diversity. Species diversity within communities and genetic diversity within populations are hypothesized to co-vary in space or time because of locality characteristics that influence the two levels of diversity via parallel processes, or because of direct effects of one level of diversity on the other via several different mechanisms. Here, we draw on a wide range of studies in ecology and evolution to examine the theoretical underpinnings of these hypotheses, review relevant empirical literature, and outline an agenda for future research. The plausibility of species diversity–genetic diversity relationships is supported by a variety of theoretical and empirical studies, and several recent studies provide direct, though preliminary support. Focusing on potential connections between species diversity and genetic diversity complements other approaches to synthesis at the ecology–evolution interface, and should contribute to conceptual unification of biodiversity research at the levels of genes and species.
TL;DR: This review focuses on the ecological interactions operating during invasions by the most prominent group of insect vectors of disease, mosquitoes, focusing on the best-studied invasive mosquito, Aedes albopictus, and develops a model based on observations for effects of resource competition and predation as barriers to invasion.
Abstract: Investigations of biological invasions focus on patterns and processes that are related to introduction, establishment, spread and impacts of introduced species. This review focuses on the ecological interactions operating during invasions by the most prominent group of insect vectors of disease, mosquitoes. First, we review characteristics of non-native mosquito species that have established viable populations, and those invasive species that have spread widely and had major impacts, testing whether biotic characteristics are associated with the transition from established non-native to invasive. Second, we review the roles of interspecific competition, apparent competition, predation, intraguild predation and climatic limitation as causes of impacts on residents or as barriers to invasion. We concentrate on the best-studied invasive mosquito, Aedes albopictus, evaluating the application of basic ecological theory to invasions by Aedes albopictus. We develop a model based on observations of Aedes albopictus for effects of resource competition and predation as barriers to invasion, evaluating which community and ecosystem characteristics favour invasion. Third, we evaluate the ways in which invasive mosquitoes have contributed to outbreaks of human and animal disease, considering specifically whether invasive mosquitoes create novel health threats, or modify disease transmission for existing pathogen–host systems.
TL;DR: This work finds strong positive relationships between species and functional diversity, and increased functional diversity of fish assemblages coinciding with recovery of species diversity in marine reserves, independent of the method used for classifying species in functional groups.
Abstract: The relationship between species and functional diversity remains poorly understood for nearly all ecosystem types, yet determining this relationship is critically important for developing both a mechanistic understanding of community assembly and appropriate expectations and approaches to protecting and restoring biological communities. Here we use two distinct data sets, one from kelp forests in the Channel Islands, California, and one from a global synthesis of marine reserves, to directly test how variation in species diversity translates into changes in functional diversity. We find strong positive relationships between species and functional diversity, and increased functional diversity of fish assemblages coinciding with recovery of species diversity in marine reserves, independent of the method used for classifying species in functional groups. These results indicate that low levels of redundancy in functional species traits exist across a suite of marine systems, and that fishing tends to remove whole functional groups from coastal marine ecosystems.
TL;DR: Simulations showed that nitrogen supply can limit carbon mineralization, microbial growth and enzyme production because of the nitrogen-demanding stoichiometry of enzymes, suggesting that competition from cheaters, slow diffusion and nitrogen limitation may constrain microbial foraging and the enzyme decomposition of complex compounds in natural environments.
Abstract: Extracellular enzymes allow microbes to acquire carbon and nutrients from complex molecules and catalyse the rate-limiting step in nutrient mineralization. Because the factors regulating enzyme production are poorly understood, I used a simulation model to examine how competition, nutrient availability and spatial structure affect microbial growth and enzyme synthesis. In simulations where enzyme-producing microbes competed with cheaters (who do not synthesize enzymes but take-up product), higher enzyme costs favoured cheaters, while lower rates of enzyme diffusion favoured producers. Cheaters and producers coexisted in highly organized spatial patterns at intermediate enzyme costs and diffusion rates. Simulations with varying nutrient inputs showed that nitrogen supply can limit carbon mineralization, microbial growth and enzyme production because of the nitrogen-demanding stoichiometry of enzymes (C : N ¼ c. 3.5 : 1). These results suggest that competition from cheaters, slow diffusion and nitrogen limitation may constrain microbial foraging and the enzymatic decomposition of complex compounds in natural environments.
TL;DR: The percentage of plots that had lower invader biomass than did the best resident monoculture increased across the species richness gradient, a pattern that cannot be explained by a sampling effect and is a unique signature of complementarity effects.
Abstract: Complementarity and sampling effects may both contribute to increased invasion resistance at higher diversity We measured plant invader biomass across a long-term experimental plant diversity gradient Invader species biomass was inhibited in more diverse plots, largely because of the presence of strongly competitive C4 bunchgrasses, consistent with a sampling effect Invader biomass was negatively correlated with resident root biomass, and positively correlated with soil nitrate concentrations, suggesting that competition for nitrogen limited invader success Resident root biomass increased and soil nitrate concentrations decreased with the presence of C4 grasses and also across the diversity gradient, suggesting that diverse plots are more competitive because of the presence of C4 grasses In addition to this evidence for a sampling effect, we also found evidence for a complementarity effect Specifically, the percentage of plots that had lower invader biomass than did the best resident monoculture (ie that had invader underyielding) increased across the species richness gradient This pattern cannot be explained by a sampling effect and is a unique signature of complementarity effects Our results demonstrate the importance of multiple mechanisms by which diversity can increase invasion resistance
TL;DR: The absence of specialist herbivores in invasive populations resulted in the evolution of lower protection against specialists and increased growth and reproduction, but also allowed a shift towards higher protection against generalist herbivore.
Abstract: The success of invasive plants has been attributed to their escape from natural enemies and subsequent evolutionary change in allocation from defence to growth and reproduction. In common garden experiments with Senecio jacobaea, a noxious invasive weed almost worldwide, the invasive populations from North America, Australia, and New Zealand did indeed allocate more resources to vegetative and reproductive biomass. However, invasive plants did not show a complete change in allocation from defence to growth and reproduction. Protection against generalist herbivores increased in invasive populations and pyrrolizidine alkaloids, their main anti-herbivore compounds, did not decline in invasive populations but were higher overall compared with native populations. In contrast, invasive plants lost additional protection against specialist herbivores adapted to pyrrolizidine alkaloids. Hence, the absence of specialist herbivores in invasive populations resulted in the evolution of lower protection against specialists and increased growth and reproduction, but also allowed a shift towards higher protection against generalist herbivores.
TL;DR: It is found that the trophic composition of the predator assemblage determined the impact of increasing predator species richness on the occurrence of Trophic cascades, and increasing the proportion of intraguild predator species present diminished herbivore suppression and reduced primary productivity.
Abstract: Single trophic-level studies of the relationship between biodiversity and ecosystem functioning highlight the importance of mechanisms such as resource partitioning, facilitation, and sampling effect. In a multi-trophic context, trophic interactions such as intraguild predation may also be an important mediator of this relationship. Using a salt-marsh food web, we investigated the interactive effects of predator species richness (one to three species) and trophic composition (strict predators, intraguild predators, or a mixture of the two) on ecosystem functions such as prey suppression and primary production via trophic cascades. We found that the trophic composition of the predator assemblage determined the impact of increasing predator species richness on the occurrence of trophic cascades. In addition, increasing the proportion of intraguild predator species present diminished herbivore suppression and reduced primary productivity. Therefore, trophic composition of the predator assemblage can play an important role in determining the nature of the relationship between predator diversity and ecosystem function.