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
References
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TL;DR: The proposed removal of C. fornicata might be economically desirable from a fisheries economic viewpoint, but it clearly would be associated, in the present context of excessive N inputs, to a potential risk of harmful algal blooms during summer.
Abstract: Ecological shifts from diatoms to other phytoplankton species have been related to decreasing Si:N and Si:P nutrient ratios. The Bay of Brest is such a perturbated ecosystem where Si has become limiting but where diatoms continue to dominate the phytoplankton throughout the productive period. Several hypothesis have been invoked to account for this dilemma. The most recent combines suspension feeder activity and Si recycling. Suspension feeder activity, stimulated by the proliferation of the invasive species Crepidula fornicata, would be the driving force of a biologically active silicate pump which would (1) retain Si within the Bay during spring and (2) provide the Si necessary for diatoms during summer. During the year 2000, this hypothesis was successfully tested. Direct evidence of silicic acid limitation has been provided, and during summer, benthic fluxes measured at a site with a high density of C. fornicata are one order of magnitude higher than those measured at the site with no C. fornicata. Seasonal budgets of Si inputs and diatom demand demonstrated that diatom production during summer depends strongly on Si recycling at the sediment-water interface. Thus, if C. fornicata decreases benthic biodiversity and perturbates the development of the native Great Scallop (Pecten maximus), it also helps the Bay cope with elevated N inputs. The proposed removal of C. fornicata might be economically desirable from a fisheries economic viewpoint, but it clearly would be associated, in the present context of excessive N inputs, to a potential risk of harmful algal blooms during summer.
97 citations
"Niches versus neutrality: uncoverin..." refers background in this paper
...For example, at similar size dinoflagellates are more mobile than diatoms and differ in their nutrient requirements (Broekhuizen 1999; Ragueneau et al. 2002)....
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TL;DR: Niche complementarity may have acted to enhance ecosystem function and that it is important for species coexistence in these fish communities, and the method used may be easily applied to any sort of biological community and thus may have considerable potential for determining the generality of niche complementarity effects on community structure.
Abstract: 1. The mechanisms that structure biological communities hold the key to understanding ecosystem functioning and the maintenance of biodiversity. Patterns of species abundances have been proposed as a means of differentiation between niche-based and neutral processes, but abundance information alone cannot provide unequivocal discrimination. 2. We combined species niche information and species' relative abundances to test the effects of two opposing structuring mechanisms (environmental filtering and niche complementarity) on species' relative abundances in French lacustrine fish communities. The test involved a novel method comparing the abundance-weighted niche overlap within communities against that expected when relative abundances were randomized among species within the community. 3. Observed overlap was consistently significantly lower than expected at random for two (swimming ability and trophic status) of four primary niche axes across lakes of differing physical environments. Thus, for these niche axes, pairs of abundant species tended to have relatively low niche overlap, while rare species tended to have relatively high niche overlap with abundant species. 4. This suggests that niche complementarity may have acted to enhance ecosystem function and that it is important for species coexistence in these fish communities. The method used may be easily applied to any sort of biological community and thus may have considerable potential for determining the generality of niche complementarity effects on community structure.
96 citations
"Niches versus neutrality: uncoverin..." refers background or methods in this paper
...Iexp and rexp are obtained by attributing at random (following a uniform distribution) observed biomasses over the set of observed body masses for 10 000 simulations and taking respectively the average biomassweighted distance and its standard deviation across all randomizations (Mason et al. 2008)....
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...Prediction 1 The influence of niche overlap on species biomass was estimated using the standardized effect size methodology (Mason et al. 2008)....
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TL;DR: A novel way of representing community structure is used to show that abundance within closely related pairs of co-occurring tree species in a highly diverse Mexican forest is more equitable than is abundance within more distantly related pairs.
Abstract: Community structure refers to the number of species in a community and the pattern of distribution of individuals among those species. We use a novel way of representing community structure to show that abundance within closely related pairs of co-occurring tree species in a highly diverse Mexican forest is more equitable than is abundance within more distantly related pairs. This observation is at odds with the fundamental assumption of neutral models of community structure, i.e., that species are interchangeable. The observed patterns suggest niche apportionment, in which interaction is focused pairwise between congeners but falls away from the phylogenetic structure above the genus level. Thus niche processes may significantly affect community structure through regulating relative abundance in a substantial proportion of species, which in turn potentially enhances community stability. One such mechanism of stable coexistence has already been shown to be active in this forest.
85 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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TL;DR: Perturbations significantly affected the densities and relative abundance of the main invertebrate taxa and these effects were consistent with the known effects of enrichment and predation, but there was little evidence of significant treatment effects on the overall benthic biomass or abundance size spectrum, supporting the contention that the spectrum is conservative and is probably constrained by habitat architecture.
Abstract: Holling's (1992) proposition that discontinuities in biotic and abiotic processes generate structure in ecological systems is examined experimentally by imposing size-specific perturbations on marine sediment assemblages. Two kinds of perturbations were applied: organic enrichment and predation, each at two levels. Perturbations significantly affected the densities and relative abundance of the main invertebrate taxa and these effects were consistent with the known effects of enrichment and predation. However, there was little evidence of significant treatment effects on the overall benthic biomass or abundance size spectrum, supporting the contention that the spectrum is conservative and is probably constrained by habitat architecture.
66 citations
"Niches versus neutrality: uncoverin..." refers background in this paper
...The relationship between ecological processes and processes occurring at coarser temporal and spatial scales is rarely addressed in classic niche-based species diversity models (Tokeshi & Schmid 2002), yet they have a major influence on local distributions (Raffaelli et al. 2000; Warwick 2007)....
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61 citations