Niches versus neutrality: uncovering the drivers of diversity in a species-rich community.
TL;DR: This work provides the first empirical evidence that a niche-neutral model can explain niche space occupancy pattern in a natural species-rich community and suggests this class of model may be a useful hypothesis for the generation and maintenance of species diversity in other size-structured communities.
Abstract: Ecological models suggest that high diversity can be generated by purely niche-based, purely neutral or by a mixture of niche-based and neutral ecological processes. Here, we compare the degree to which four contrasting hypotheses for coexistence, ranging from niche-based to neutral, explain species richness along a body mass niche axis. We derive predictions from these hypotheses and confront them with species body-mass patterns in a highly sampled marine phytoplankton community. We find that these patterns are consistent only with a mechanism that combines niche and neutral processes, such as the emergent neutrality mechanism. In this work, we provide the first empirical evidence that a niche-neutral model can explain niche space occupancy pattern in a natural species-rich community. We suggest this class of model may be a useful hypothesis for the generation and maintenance of species diversity in other size-structured communities.
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TL;DR: A framework for disentangling the relative importance of deterministic and stochastic processes in generating site-to-site variation in species composition along ecological gradients and among biogeographic regions that differ in the size of the regional species pool is developed.
Abstract: Deterministic theories in community ecology suggest that local, niche-based processes, such as environmental filtering, biotic interactions and interspecific trade-offs largely determine patterns of species diversity and composition. In contrast, more stochastic theories emphasize the importance of chance colonization, random extinction and ecological drift. The schisms between deterministic and stochastic perspectives, which date back to the earliest days of ecology, continue to fuel contemporary debates (e.g. niches versus neutrality). As illustrated by the pioneering studies of Robert H. MacArthur and co-workers, resolution to these debates requires consideration of how the importance of local processes changes across scales. Here, we develop a framework for disentangling the relative importance of deterministic and stochastic processes in generating site-to-site variation in species composition (β-diversity) along ecological gradients (disturbance, productivity and biotic interactions) and among biogeographic regions that differ in the size of the regional species pool. We illustrate how to discern the importance of deterministic processes using null-model approaches that explicitly account for local and regional factors that inherently create stochastic turnover. By embracing processes across scales, we can build a more synthetic framework for understanding how niches structure patterns of biodiversity in the face of stochastic processes that emerge from local and biogeographic factors.
1,116 citations
TL;DR: This work redefined the traditional concept of assembly rules in a more general framework where the co‐occurrence of species is a product of chance, historical patterns of speciation and migration, dispersal, abiotic environmental factors, and biotic interactions, with none of these processes being mutually exclusive.
Abstract: Understanding how communities of living organisms assemble has been a central question in ecology since the early days of the discipline. Disentangling the different processes involved in community assembly is not only interesting in itself but also crucial for an understanding of how communities will behave under future environmental scenarios. The traditional concept of assembly rules reflects the notion that species do not co-occur randomly but are restricted in their co-occurrence by interspecific competition. This concept can be redefined in a more general framework where the co-occurrence of species is a product of chance, historical patterns of speciation and migration, dispersal, abiotic environmental factors, and biotic interactions, with none of these processes being mutually exclusive. Here we present a survey and meta-analyses of 59 papers that compare observed patterns in plant communities with null models simulating random patterns of species assembly. According to the type of data under study and the different methods that are applied to detect community assembly, we distinguish four main types of approach in the published literature: species co-occurrence, niche limitation, guild proportionality and limiting similarity. Results from our meta-analyses suggest that non-random co-occurrence of plant species is not a widespread phenomenon. However, whether this finding reflects the individualistic nature of plant communities or is caused by methodological shortcomings associated with the studies considered cannot be discerned from the available metadata. We advocate that more thorough surveys be conducted using a set of standardized methods to test for the existence of assembly rules in data sets spanning larger biological and geographical scales than have been considered until now. We underpin this general advice with guidelines that should be considered in future assembly rules research. This will enable us to draw more accurate and general conclusions about the non-random aspect of assembly in plant communities.
719 citations
TL;DR: This work introduces 12 different forms of functional rarity along gradients of species scarcity and trait distinctiveness and highlights the potential key role offunctional rarity in the long-term and large-scale maintenance of ecosystem processes.
Abstract: Rarity has been a central topic for conservation and evolutionary biologists aiming to determine the species characteristics that cause extinction risk. More recently, beyond the rarity of species, the rarity of functions or functional traits, called functional rarity, has gained momentum in helping to understand the impact of biodiversity decline on ecosystem functioning. However, a conceptual framework for defining and quantifying functional rarity is still lacking. We introduce 12 different forms of functional rarity along gradients of species scarcity and trait distinctiveness. We then highlight the potential key role of functional rarity in the long-term and large-scale maintenance of ecosystem processes, as well as the necessary linkage between functional and evolutionary rarity.
233 citations
TL;DR: The results demonstrate that at high fertility dominant species differ in resource use strategy, but as soil fertility declines over the long-term, dominant species increasingly converge on a resource-retentive strategy, which suggests that differentiation in resourceUse strategy is required for co-existence at highertility but not in low fertility ecosystems.
Abstract: 1. Functional trait diversity can reveal mechanisms of species co-existence in plant communities. Few studies have tested whether functional diversity for foliar traits related to resource use strategy increases or decreases with declining soil phosphorus (P) in forest communities.
2. We quantified tree basal area and four foliar functional traits (i.e. nitrogen (N), phosphorus (P), thickness and tissue density) for all woody species along the c. 120 kyr Franz Josef soil chronosequence in cool temperate rainforest, where strong shifts occur in light and soil nutrient availability (i.e. total soil P declines from 805 to 100 mg g–1). We combined the abundance and trait data in functional diversity indices to quantify trait convergence and divergence, in an effort to determine whether mechanisms of co-existence change with soil fertility.
3. Relationships between species trait means and total soil N and P were examined using multiple regression, with and without weighting of species abundances. We used Rao’s quadratic entropy to quantify functional diversity at the plot scale, then compared this with random expectation, using a null model that randomizes abundances across species within plots. Taxonomic diversity was measured using Simpson’s Diversity. Relationships between functional and taxonomic diversity and total soil P were examined using jackknife linear regression.
4. Leaf N and P declined and leaf thickness and density increased monotonically with declining total soil P along the sequence; these relationships were unaffected by abundance-weighting of species in the analyses. Inclusion of total soil N did not improve predictions of trait means. All measures of diversity calculated from presence/absence data were unrelated to total soil N and P. There was no evidence for a relationship between Rao values using quantitative abundances and total soil P. However, there was a strongly positive relationship between Rao, expressed relative to random expectation, and total soil P, indicating trait convergence of dominant species as soil P declined.
5. Synthesis: Our results demonstrate that at high fertility dominant species differ in resource use strategy, but as soil fertility declines over the long-term, dominant species increasingly converge on a resource-retentive strategy. This suggests that differentiation in resource use strategy is required for co-existence at high fertility but not in low fertility ecosystems.
200 citations
TL;DR: These results suggest that plant population and community dynamics in grassland communities should increase with increasing biomass and decrease with distur-bance ingrassland communities, and emphasize that contrasting community assembly processes mayoccurfordifferentnicheaxes, even withinasinglecommunity.
Abstract: Summary1. Understanding the processes by which species sort themselves into communities remains acentralpuzzleforattemptstomaintainbiodiversity.Itremainsunclearwhetheranysingleassemblyprocess is generally dominant or whether the influence of contrasting processes varies in a predict-able way relative to biotic and abiotic gradients. Abundance-weighted niche overlap betweenspeciesprovidesapowerfulmeansofcontrastingtwomajorassemblyprocesses–nichecomplemen-tarityandenvironmentalfiltering.2. We examined mean overlap for four vegetative functional traits, relative to that expected whenabundances were randomly allocated to species co-occurring in experimental plots in a wet mea-dow. This provided a test of whether any single assembly process prevailed for the meadow as awhole and across all traits. The effects of mowing, fertilization and dominant species removal, andassociated gradients of Simpson’s dominance and biomass on the niche overlap of plots, were alsoexamined.3. Nicheoverlapwashigherthanexpectedatrandomforthreeofthefourtraitsstudied(height,leafand stem dry matter content, leaf C:N ratio). However, niche overlap was lower than expected forspecificleafarea.4. Mowingwasthetreatmentwiththegreatesteffectonbothnicheoverlapandbiomass,withover-lap significantly lower in the absence of mowing for three of the traits, while biomass was lower inmown plots. For three of the traits there was evidence of a significant decrease in overlap withincreasing biomass, but notincreasing dominance. None of the significant mowing effects on over-lapremainedwhentheeffectofbiomasshadbeenremoved.5. Synthesis: Theseresultssuggestthattheimportanceofnichedifferencesbetweenspeciesinstruc-turing grassland communities should increase with increasing biomass and decrease with distur-bance in grassland communities. They also emphasize that contrasting community assemblyprocessesmayoccurfordifferentnicheaxes,evenwithinasinglecommunity.Key-words: coexistence, complementarity, fertilization, functional trait, meadow, mowing,null model, plant population and community dynamics, productivity, removalIntroduction
199 citations
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03 Mar 1997
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TL;DR: A mechanistic neutral model that describes the dynamics of a community of equivalent species under the joint influence of density dependence, environmental forcing, and demographic stochasticity is presented and a new standardized measure of species synchrony in multispecies communities is introduced.
Abstract: Independent species fluctuations are commonly used as a null hypothesis to test the role of competition and niche differences between species in community stability. This hypothesis, however, is unrealistic because it ignores the forces that contribute to synchronization of population dynamics. Here we present a mechanistic neutral model that describes the dynamics of a community of equivalent species under the joint influence of density dependence, environmental forcing, and demographic stochasticity. We also introduce a new standardized measure of species synchrony in multispecies communities. We show that the per capita population growth rates of equivalent species are strongly synchronized, especially when endogenous population dynamics are cyclic or chaotic, while their long‐term fluctuations in population sizes are desynchronized by ecological drift. We then generalize our model to nonneutral dynamics by incorporating temporal and nontemporal forms of niche differentiation. Niche different...
468 citations
TL;DR: The patterns of abundance generated by a simple stochastic birth‐death‐immigration model are described in order to characterize the diversity of neutral communities of ecologically equivalent species and demonstrates the necessity for an appropriate null model when functional hypotheses are being tested.
Abstract: The patterns of abundance generated by a simple stochastic birth‐death‐immigration model are described in order to characterize the diversity of neutral communities of ecologically equivalent species. Diversity is described by species number S and the variance of frequency or log abundance q∼. The frequency distribution of abundance is very generally lognormal, skewed to the left by immigration and resembling descriptions of natural communities. Increased immigration and community size always cause S to increase. Their effect on q∼ is more complicated, but given biologically reasonable assumptions, S and q∼ will be positively correlated in most circumstances. Larger samples contain more species; the graph of log S on log individuals, equivalent to a species‐area curve, is generally convex upward but becomes linear with a slope of about +0.25 when immigration is low and births exceed deaths. When individuals invade a new, vacant environment, both S and q∼ increase through time. Thus, a positive c...
428 citations
"Niches versus neutrality: uncoverin..." refers background in this paper
...Backed up by ecological observations showing that phytoplankton species are affected differently by environmental changes (Reynolds 1989), formal hypothesis testing therefore clearly indicates that phytoplankton communities are not in general neutral in the strong sense (Bell 2000) of the word....
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...In contrast, neutral models assume that all species are identical: consequently no single species is at a competitive advantage or disadvantage, and exclusion does not occur (Bell 2000; Hubbell 2001, 2005)....
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TL;DR: In this article, a new approach based on generalized additive models (GAMs) is presented to model trend as a smooth, nonlinear function of time, and they provide a framework for testing the statistical significance of changes in abundance.
Abstract: Knowledge of the direction, magnitude, and timing of changes in bird population abundance is essential to enable species of priority conservation concern to be identified, and reasons for the population changes to be understood. We give a brief review of previous techniques for the analysis of large-scale survey data and present a new approach based on generalized additive models (GAMs). GAMs are used to model trend as a smooth, nonlinear function of time, and they provide a framework for testing the statistical significance of changes in abundance. In addition, the second derivatives of the modeled trend curve may be used to identify key years in which the direction of the population trajectory was seen to change significantly. The inclusion of covariates into models for population abundance is also discussed and illustrated, and tests for the significance of covariate terms are given. We apply the methods to data from the Common Birds Census of the British Trust for Ornithology for 13 species of farmland birds. Seven of the species are shown to have experienced statistically significant declines since the mid-1960s. Two species exhibited a significant increase. The population trajectories of all but three species turned downward in the 1970s, although in most cases the 1980s brought either some recovery or a decrease in the rate of decline. The majority of populations have remained relatively stable in the 1990s. The results are comparable with those from other analysis techniques, although the new approach is shown to have advantages in generality and precision. We suggest extensions of the methods and make recommendations for the design of future surveys.
414 citations
"Niches versus neutrality: uncoverin..." refers methods in this paper
...Prediction 2 We tested for the presence of aggregations of high species richness separated by species-poor gaps along a body mass axis using bootstrapped Generalized Additive Models (GAM) (Fewster et al. 2000)....
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...This is because the detection by GAM fitting of significant species richness aggregations along the body mass axis is sensitive to the number of species present in the distributions....
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...Average sub- sampled distributions are then used for GAM fitting and second derivative analysis....
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...To compensate for this sampling artefact and make sure that any difference between permanent and occasional species is genuine, the GAM technique was applied to subsamples of the permanent distributions (see Appendix S2)....
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...This method is based on sampling the observed species richness–body mass distributions with replacement (n = 400), fitting a GAM to each replicate and calculating the corresponding second-order derivatives, which identify significant turning points (peaks and troughs) in the species richness–body mass distributions....
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TL;DR: It is shown that non-curve-fitting predictions readily derived from neutral theory are easily falsifiable and there is a current overwhelming weight of evidence against neutral theory.
Abstract: We describe a general framework for testing neutral theory. We summarize similarities and differences between ten different versions of neutral theory. Two central predictions of neutral theory are that species abundance distributions will follow a zero-sum multinomial distribution and that community composition will change over space due to dispersal limitation. We review all published empirical tests of neutral theory. With the exception of one type of test, all tests fail to support neutral theory. We identify and perform several new tests. Specifically, we develop a set of best practices for testing the fit of the zero-sum multinomial (ZSM) vs. a lognormal null hypothesis and apply this to a data set, concluding that the lognormal outperforms neutral theory on robust tests. We explore whether a priori parameterization of neutral theory is possible, and we conclude that it is not. We show that non-curve-fitting predictions readily derived from neutral theory are easily falsifiable. In toto, there is a current overwhelming weight of evidence against neutral theory. We suggest some next steps for neutral theory.
356 citations
"Niches versus neutrality: uncoverin..." refers background in this paper
...…many detailed analyses have rejected the assumption of ecological equivalence in a wide range of communities, including phytoplankton, coral reefs, tropical trees, birds, marine invertebrates and mammals (Chave 2004; Dornelas et al. 2006; McGill et al. 2006; Ricklefs 2006; Kelly et al. 2008)....
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