A niche for neutrality.
TL;DR: In this paper, the authors use classical coexistence theory to reframe the debate in terms of stabilizing mechanisms (niches) and fitness equivalence (neutrality) for coexistence.
Abstract: Ecologists now recognize that controversy over the relative importance of niches and neutrality cannot be resolved by analyzing species abundance patterns. Here, we use classical coexistence theory to reframe the debate in terms of stabilizing mechanisms (niches) and fitness equivalence (neutrality). The neutral model is a special case where stabilizing mechanisms are absent and species have equivalent fitness. Instead of asking whether niches or neutral processes structure communities, we advocate determining the degree to which observed diversity reflects strong stabilizing mechanisms overcoming large fitness differences or weak stabilization operating on species of similar fitness. To answer this question, we propose combining data on per capita growth rates with models to: (i) quantify the strength of stabilizing processes; (ii) quantify fitness inequality and compare it with stabilization; and (iii) manipulate frequency dependence in growth to test the consequences of stabilization and fitness equivalence for coexistence.
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TL;DR: The Essay concludes that practitioners theorize, and theorists practice, use these intellectual tools differently because the goals and orientations of theorists and practitioners, and the constraints under which they act, differ.
Abstract: Much has been written about theory and practice in the law, and the tension between practitioners and theorists. Judges do not cite theoretical articles often; they rarely "apply" theories to particular cases. These arguments are not revisited. Instead the Essay explores the working and interaction of theory and practice, practitioners and theorists. The Essay starts with a story about solving a legal issue using our intellectual tools - theory, practice, and their progenies: experience and "gut." Next the Essay elaborates on the nature of theory, practice, experience and "gut." The third part of the Essay discusses theories that are helpful to practitioners and those that are less helpful. The Essay concludes that practitioners theorize, and theorists practice. They use these intellectual tools differently because the goals and orientations of theorists and practitioners, and the constraints under which they act, differ. Theory, practice, experience and "gut" help us think, remember, decide and create. They complement each other like the two sides of the same coin: distinct but inseparable.
2,077 citations
TL;DR: Organizing the material of community ecology according to this framework can clarify the essential similarities and differences among the many conceptual and theoretical approaches to the discipline, and allow for the articulation of a very general theory of community dynamics.
Abstract: Community ecology is often perceived as a mess, given the seemingly vast number of processes that can underlie the many patterns of interest, and the apparent uniqueness of each study system. However, at the most general level, patterns in the composition and diversity of speciesthe subject matter of community ecologyare influenced by only four classes of process: selection, drift, speciation, and dispersal. Selection represents deterministic fitness differences among species, drift represents stochastic changes in species abundance, speciation creates new species, and dispersal is the movement of organisms across space. All theoretical and conceptual models in community ecology can be understood with respect to their emphasis on these four processes. Empirical evidence exists for all of these processes and many of their interactions, with a predominance of studies on selection. Organizing the material of community ecology according to this framework can clarify the essential similarities and dif...
1,792 citations
Cites methods from "A niche for neutrality."
...Adding demographic stochasticity to selectionbased models represents a combination of drift and selection (e.g., Tilman 2004), as do models proposed under the rubric of a niche-neutral reconciliation (e.g., Shipley et al. 2006; Adler et al. 2007)....
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...This framework is reflected in many of the recent efforts at synthesis under the rubric of nicheneutral reconciliation (e.g., Shipley et al. 2006; Adler et al. 2007), and has proven very useful, but its domain is restricted to competitive coexistence and is focused almost entirely on local…...
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TL;DR: It is argued that two types of species differences determine competitive exclusion with opposing effects on relatedness patterns, which means that competition can sometimes eliminate more different and less related taxa, even when the traits underlying the relevant species differences are phylogenetically conserved.
Abstract: Though many processes are involved in determining which species coexist and assemble into communities, competition is among the best studied. One hypothesis about competition's contribution to community assembly is that more closely related species are less likely to coexist. Though empirical evidence for this hypothesis is mixed, it remains a common assumption in certain phylogenetic approaches for inferring the effects of environmental filtering and competitive exclusion. Here, we relate modern coexistence theory to phylogenetic community assembly approaches to refine expectations for how species relatedness influences the outcome of competition. We argue that two types of species differences determine competitive exclusion with opposing effects on relatedness patterns. Importantly, this means that competition can sometimes eliminate more different and less related taxa, even when the traits underlying the relevant species differences are phylogenetically conserved. Our argument leads to a reinterpretation of the assembly processes inferred from community phylogenetic structure.
1,321 citations
Cites background from "A niche for neutrality."
...A more technical and extensive explanation of this relationship can be found in Adler et al. (2007) and Chesson (2000)....
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...2; Hubbell 2001; Adler et al. 2007)....
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...Still, the more general question of how species differences influence the outcome of competition, independent of phylogeny, remains a fundamental empirical problem, central to debate between niche and neutral theory (Adler et al. 2007)....
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...Based on the arguments presented here, answering this question will require separating the influence of niche and competitive ability differences, the details of which are explained in several recent papers (Chesson 2000; Leibold & McPeek 2006; Adler et al. 2007; Levine & HilleRisLambers 2009)....
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...All these niche differences facilitate coexistence by favouring species when they drop to low density and are threatened with extinction (frequency-dependent regulation; Adler et al. 2007)....
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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: An analytical framework is developed for interrogation of subsurface microbial communities distributed across two geologically distinct formations of the unconfined aquifer underlying the Hanford Site in southeastern Washington State that quantitatively estimate influences of Drift, Selection and Dispersal.
Abstract: Spatial turnover in the composition of biological communities is governed by (ecological) Drift, Selection and Dispersal. Commonly applied statistical tools cannot quantitatively estimate these processes, nor identify abiotic features that impose these processes. For interrogation of subsurface microbial communities distributed across two geologically distinct formations of the unconfined aquifer underlying the Hanford Site in southeastern Washington State, we developed an analytical framework that advances ecological understanding in two primary ways. First, we quantitatively estimate influences of Drift, Selection and Dispersal. Second, ecological patterns are used to characterize measured and unmeasured abiotic variables that impose Selection or that result in low levels of Dispersal. We find that (i) Drift alone consistently governs ∼25% of spatial turnover in community composition; (ii) in deeper, finer-grained sediments, Selection is strong (governing ∼60% of turnover), being imposed by an unmeasured but spatially structured environmental variable; (iii) in shallower, coarser-grained sediments, Selection is weaker (governing ∼30% of turnover), being imposed by vertically and horizontally structured hydrological factors;(iv) low levels of Dispersal can govern nearly 30% of turnover and be caused primarily by spatial isolation resulting from limited exchange between finer and coarser-grain sediments; and (v) highly permeable sediments are associated with high levels of Dispersal that homogenize community composition and govern over 20% of turnover. We further show that our framework provides inferences that cannot be achieved using preexisting approaches, and suggest that their broad application will facilitate a unified understanding of microbial communities.
1,110 citations
Cites background from "A niche for neutrality."
...Although often rebuked and rejected (for example, Ricklefs and Renner, 2012), Hubbell’s (2001) neutral theory encouraged broader recognition of Drift and Dispersal....
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...These studies provide important insights, but continued progress requires that we characterize how multiple processes simultaneously govern ecological systems (Gravel et al., 2006; Adler et al., 2007; Vellend, 2010; Stegen and Hurlbert, 2011)....
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...As a consequence, it is now broadly recognized that Selection works alongside Drift and Dispersal (Cottenie, 2005; Gravel et al., 2006; Adler et al., 2007; Legendre et al., 2009; Dumbrell et al., 2010; Chase and Myers, 2011)....
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References
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Book•
01 Jan 1982
TL;DR: This book builds a mechanistic, resource-based explanation of the structure and functioning of ecological communities and explores such problems as the evolution of "super species," the differences between plant and animal community diversity patterns, and the cause of plant succession.
Abstract: One of the central questions of ecology is why there are so many different kinds of plants and animals Here David Tilman presents a theory of how organisms compete for resources and the way their competition promotes diversity Developing Hutchinson's suggestion that the main cause of diversity is the feeding relations of species, this book builds a mechanistic, resource-based explanation of the structure and functioning of ecological communities In a detailed analysis of the Park Grass Experiments at the Rothamsted Experimental Station in England, the author demonstrates that the dramatic results of these 120 years of experimentation are consistent with his theory, as are observations in many other natural communities The consumer-resource approach of this book is applicable to both animal and plant communities, but the majority of Professor Tilman's discussion concentrates on the structure of plant communities All theoretical arguments are developed graphically, and formal mathematics is kept to a minimum The final chapters of the book provide some testable speculations about resources and animal communities and explore such problems as the evolution of "super species," the differences between plant and animal community diversity patterns, and the cause of plant succession
5,795 citations
"A niche for neutrality." refers background in this paper
...One well-known example is a tradeoff in species ability to draw down two essential soil resources (Tilman 1982)....
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...Classic studies have shown that species may differ in their use of multiple-limiting resources (Tilman 1982; Grant 1986), their ability to colonize disturbed sites (Grubb 1977), and their response to temporal fluctuations in the environment (Caceres 1997)....
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TL;DR: Stabilizing mechanisms are essential for species coexistence and include traditional mechanisms such as resource partitioning and frequency-dependent predation, as well as mechanisms that depend on fluctuations in population densities and environmental factors in space and time.
Abstract: ▪ Abstract The focus of most ideas on diversity maintenance is species coexistence, which may be stable or unstable. Stable coexistence can be quantified by the long-term rates at which community members recover from low density. Quantification shows that coexistence mechanisms function in two major ways: They may be (a) equalizing because they tend to minimize average fitness differences between species, or (b) stabilizing because they tend to increase negative intraspecific interactions relative to negative interspecific interactions. Stabilizing mechanisms are essential for species coexistence and include traditional mechanisms such as resource partitioning and frequency-dependent predation, as well as mechanisms that depend on fluctuations in population densities and environmental factors in space and time. Equalizing mechanisms contribute to stable coexistence because they reduce large average fitness inequalities which might negate the effects of stabilizing mechanisms. Models of unstable coexitence...
5,240 citations
"A niche for neutrality." refers background or methods in this paper
...More generally, fitness inequality is the scaled difference in species per capita growth rates in the absence of stabilization; the scaling terms reflect differences in how species’ growth rates respond to shared limiting factors (Chesson & Huntly 1997; Chesson 2000; Snyder et al. 2005)....
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...For examples based on other models, and for the appropriate scaling of growth rates when species differ in their sensitivities to common limiting factors, see Chesson & Huntly (1997), Chesson (2000) and Snyder et al. (2005)....
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TL;DR: It is shown that when an individual dies, it may or may not be replaced by an individual of the same species, which is all‐important to the argument presented.
Abstract: SUMMARY
1According to ‘Gause's hypothesis’ a corollary of the process of evolution by natural selection is that in a community at equilibrium every species must occupy a different niche. Many botanists have found this idea improbable because they have ignored the processes of regeneration in plant communities.
2Most plant communities are longer-lived than their constituent individual plants. When an individual dies, it may or may not be replaced by an individual of the same species. It is this replacement stage which is all-important to the argument presented.
3Several mechanisms not involving regeneration also contribute to the maintenance of species-richness:
(a). differences in life-form coupled with the inability of larger plants to exhaust or cut off all resources, also the development of dependence-relationships,
(b) differences in phenology coupled with tolerance of suppression,
(c) fluctuations in the environment coupled with relatively small differences in competitive ability between many species,
(d) the ability of certain species-pairs to form stable mixtures because of a balance of intraspecific competition against interspecific competition,
(e) the production of substances more toxic to the producer-species than to the other species,
(f) differences in the primary limiting mineral nutrients or pore-sizes in the soil for neighbouring plants of different soecies, and
(g) differences in the competitive abilities of species dependent on their physiological age coupled with the uneven-age structure of many populations.
4The mechanisms listed above do not go far to explain the indefinite persistence in mixture of the many species in the most species-rich communities known.
5In contrast there seem to be almost limitless possibilities for differences between species in their requirements for regeneration, i.e. the replacement of the individual plants of one generation by those of the next. This idea is illustrated for tree species and it is emphasized that foresters were the first by a wide margin to appreciate its importance.
6The processes involved in the successful invasion of a gap by a given plant species and some characters of the gap that may be important are summarized in Table 2.
7The definition of a plant's niche requires recognition of four components:
(a) the habitat niche,
(b) the life-form niche,
(c) the phenological niche, and
(d) the regeneration niche.
8A brief account is given of the patterns of regeneration in different kinds of plant community to provide a background for studies of differentiation in the regeneration niche.
9All stages in the regeneration-cycle are potentially important and examples of differentiation between species are given for each of the following stages:
(a) Production of viable seed (including the sub-stages of flowering, pollination and seed-set),
(b) dispersal, in space and time,
(c) germination,
(d) establishment, and
(e) further development of the immature plant.
10In the concluding discussion emphasis is placed on the following themes:
(a) the kinds of work needed in future to prove or disprove that differentiation in the regeneration niche is the major explanation of the maintenance of species-richness in plant communities,
(b) the relation of the present thesis to published ideas on the origin of phenological spread,
(c) the relevance of the present thesis to the discussion on the presence of continua in vegetation,
(d) the co-incidence of the present thesis and the emerging ideas of evolutionists about differentiation of angiosperm taxa, and
(e) the importance of regeneration-studies for conservation.
4,057 citations
"A niche for neutrality." refers background in this paper
...Classic studies have shown that species may differ in their use of multiple-limiting resources (Tilman 1982; Grant 1986), their ability to colonize disturbed sites (Grubb 1977), and their response to temporal fluctuations in the environment (Caceres 1997)....
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TL;DR: In this paper, the authors used a well-replicated field experiment, in which species diversity was directly controlled, to show that ecosystem productivity in 147 grassland plots increased significantly with plant biodiversity.
Abstract: THE functioning and sustainability of ecosystems may depend on their biological diversity1–8. Elton's9 hypothesis that more diverse ecosystems are more stable has received much attention1,3,6,7,10–14, but Darwin's proposal6,15 that more diverse plant communities are more productive, and the related conjectures4,5,16,17 that they have lower nutrient losses and more sustainable soils, are less well studied4–6,8,17,18. Here we use a well-replicated field experiment, in which species diversity was directly controlled, to show that ecosystem productivity in 147 grassland plots increased significantly with plant biodiversity. Moreover, the main limiting nutrient, soil mineral nitrogen, was utilized more completely when there was a greater diversity of species, leading to lower leaching loss of nitrogen from these ecosystems. Similarly, in nearby native grassland, plant productivity and soil nitrogen utilization increased with increasing plant species richness. This supports the diversity–productivity and diversity–sustainability hypotheses. Our results demonstrate that the loss of species threatens ecosystem functioning and sustainability.
2,541 citations
"A niche for neutrality." refers methods in this paper
...Finally, the same manipulations used in biodiversity–ecosystem function experiments (e.g. Tilman et al. 1996) could quantify frequency-dependent growth, at least for communities of short-lived organisms....
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TL;DR: The Essay concludes that practitioners theorize, and theorists practice, use these intellectual tools differently because the goals and orientations of theorists and practitioners, and the constraints under which they act, differ.
Abstract: Much has been written about theory and practice in the law, and the tension between practitioners and theorists. Judges do not cite theoretical articles often; they rarely "apply" theories to particular cases. These arguments are not revisited. Instead the Essay explores the working and interaction of theory and practice, practitioners and theorists. The Essay starts with a story about solving a legal issue using our intellectual tools - theory, practice, and their progenies: experience and "gut." Next the Essay elaborates on the nature of theory, practice, experience and "gut." The third part of the Essay discusses theories that are helpful to practitioners and those that are less helpful. The Essay concludes that practitioners theorize, and theorists practice. They use these intellectual tools differently because the goals and orientations of theorists and practitioners, and the constraints under which they act, differ. Theory, practice, experience and "gut" help us think, remember, decide and create. They complement each other like the two sides of the same coin: distinct but inseparable.
2,077 citations
"A niche for neutrality." refers background in this paper
...This result can occur when dispersal limitation interacts with spatial environmental heterogeneity, demographic stochasticity, or strong, asymmetric competition (Hurtt & Pacala 1995; Bolker & Pacala 1999; Levine & Rees 2002; Snyder & Chesson 2003)....
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