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: This theory that the population dynamics of phy- toplankton species in monoculture can be used to make a priori predictions of the dynamics and outcome of competition for light is tested.
Abstract: According to recent competition theory, the population dynamics of phy- toplankton species in monoculture can be used to make a priori predictions of the dynamics and outcome of competition for light. The species with lowest ''critical light intensity'' should be the superior light competitor. To test this theory, we ran monoculture experiments and competition experiments with two green algae (Chlorella vulgaris and Scenedesmus protuberans) and two cyanobacteria (Aphanizomenon flos-aquae and a Microcystis strain) in light-limited continuous cultures. We used the monoculture experiments to estimate the critical light intensities of the species. Scenedesmus had by far the highest critical light intensity. The critical light intensities of Chlorella, Aphanizomenon,and Microcystis were rather similar. According to observation, Aphanizomenonhad a slightly lower critical light intensity than Chlorella and Microcystis. However, according to a model fit to the mono- culture experiments, Chlorella had a slightly lower critical light intensity than Microcystis, which in turn had a slightly lower critical light intensity than Aphanizomenon.These subtle differences between observed and fitted critical light intensities could be attributed to differences in the light absorption spectra of the species. The competition experiments were all consistent with the competitive ordering of the species according to the fitted critical light intensities: Chlorella displaced all three other species, Microcystis displaced both Aphanizomenon and Scenedesmus, and Aphanizomenon only displaced Scenedesmus. Not only the final outcomes, but also the time courses of competition predicted by the theory, were in excellent agreement with the experimental results for nearly all species combi- nations.
341 citations
"Niches versus neutrality: uncoverin..." refers background in this paper
...Indeed, they co-occur in space and time in similar hydrodynamic regimes and conditions (Huisman et al. 1999; Rodriguez et al. 2001), share common predators (Armstrong 2003) and have similar critical concentration thresholds for nutrients (Chisholm 1992)....
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TL;DR: An empirical model relates the magnitude of mesoscale vertical motion to the slope of the size–abundance spectrum of phytoplankton in a frontal ecosystem and indicates that the relative proportion of large cells increases with the magnitude of the upward velocity.
Abstract: Phytoplankton size structure is acknowledged as a fundamental property determining energy flow through ‘microbial’ or ‘herbivore’ pathways1. The balance between these two pathways determines the ability of the ecosystem to recycle carbon within the upper layer or to export it to the ocean interior1. Small cells are usually characteristic of oligotrophic, stratified ocean waters, in which regenerated ammonium is the only available form of inorganic nitrogen and recycling dominates. Large cells seem to characterize phytoplankton in which inputs of nitrate enter the euphotic layer and exported production is higher2,3,4. But the size structure of phytoplankton may depend more directly on hydrodynamical forces than on the source of available nitrogen5,6,7. Here we present an empirical model that relates the magnitude of mesoscale vertical motion to the slope of the size–abundance spectrum8,9,10 of phytoplankton in a frontal ecosystem. Our model indicates that the relative proportion of large cells increases with the magnitude of the upward velocity. This suggests that mesoscale vertical motion—a ubiquitous feature of eddies and unstable fronts—controls directly the size structure of phytoplankton in the ocean.
216 citations
"Niches versus neutrality: uncoverin..." refers background in this paper
...Indeed, they co-occur in space and time in similar hydrodynamic regimes and conditions (Huisman et al. 1999; Rodriguez et al. 2001), share common predators (Armstrong 2003) and have similar critical concentration thresholds for nutrients (Chisholm 1992)....
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TL;DR: It is shown that coral assemblages differ markedly from neutral-model predictions for patterns of community similarity and the relative abundance of species, which supports spatio-temporal environmental stochasticity as a major driver of diversity patterns on coral reefs.
Abstract: The global decline of coral reefs highlights the need to understand the mechanisms that regulate community structure and sustain biodiversity in these systems. The neutral theory, which assumes that individuals are demographically identical regardless of species, seeks to explain ubiquitous features of community structure and biodiversity patterns. Here we present a test of neutral-theory predictions with the use of an extensive species-level data set of Indo-Pacific coral communities. We show that coral assemblages differ markedly from neutral-model predictions for patterns of community similarity and the relative abundance of species. Within local communities, neutral models do not fit relative abundance distributions as well as the classical log-normal distribution. Relative abundances of species across local communities also differ markedly from neutral-theory predictions: coral communities exhibit community similarity values that are far more variable, and lower on average, than the neutral theory can produce. Empirical community similarities deviate from the neutral model in a direction opposite to that predicted in previous critiques of the neutral theory. Instead, our results support spatio-temporal environmental stochasticity as a major driver of diversity patterns on coral reefs.
214 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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01 Jan 1989
TL;DR: In this paper, the impact of essentially physical variables on the population dynamics of individual species and it seeks to establish the particular properties of the organisms for which each selects is discussed, and the role of physical factors in regulating seasonal succession of phytoplankton generally.
Abstract: Just as the composition of phytoplankton assemblages depends upon the presence and relative abundances of populations of individual species, so temporal changes in their composition are brought about by differences in the relative rates of augmentation and attrition of each population. These rates respond to a complex of interactions among various physical, chemical, and biotic environmental factors, operating at a variety of intensities and frequencies. This chapter addresses the impact of essentially physical variables on the population dynamics of individual species and it seeks to establish the particular properties of the organisms for which each selects. Factual information relating the performances of algae to quantifiable aspects of the physical environment is drawn largely from observations made in controlled laboratory experiments. Realistic potential combinations of the relevant physical factors are suggested in order to simulate the likely responses of specific populations in natural waters. The outcomes of such simulations are then compared with the PEG-model of phytoplankton succession (see Section 1.2) propounded by Sommer et al. (1986), which was originally elaborated to explain the pattern of seasonal change in species dominance, as regularly observed in Lake Constance (the Bodensee). A concluding section assesses the role of physical factors in regulating seasonal succession of phytoplankton generally. At the end of the chapter, beginning on page 52, there are three appendices. The first one, Appendix 2.1, defines the units used in this chapter. The second, Appendix 2.2, identifies the symbols used, and Appendix 2.3 explains the abbreviations used for algal names.
208 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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...Phytoplankton communities are particularly well-suited to test for a combined influence of niche-based and neutral processes because they consist of a set of apparently similar species that respond in a predictable way to the environment (Reynolds 1989)....
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TL;DR: It is demonstrated that neither traditional low-dimensional tradeoffs nor neutrality can resolve the biodiversity paradox, in part by showing that they do not properly interpret stochasticity in statistical and in theoretical models.
Abstract: The paradox of biodiversity involves three elements, (i) mathematical models predict that species must differ in specific ways in order to coexist as stable ecological communities, (ii) such differences are difficult to identify, yet (iii) there is widespread evidence of stability in natural communities. Debate has centred on two views. The first explanation involves tradeoffs along a small number of axes, including ‘colonization-competition’, resource competition (light, water, nitrogen for plants, including the ‘successional niche’), and life history (e.g. high-light growth vs. low-light survival and few large vs. many small seeds). The second view is neutrality, which assumes that species differences do not contribute to dynamics. Clark et al. (2004) presented a third explanation, that coexistence is inherently high dimensional, but still depends on species differences. We demonstrate that neither traditional low-dimensional tradeoffs nor neutrality can resolve the biodiversity paradox, in part by showing that they do not properly interpret stochasticity in statistical and in theoretical models. Unless sample sizes are small, traditional data modelling assures that species will appear different in a few dimensions, but those differences will rarely predict coexistence when parameter estimates are plugged into theoretical models. Contrary to standard interpretations, neutral models do not imply functional equivalence, but rather subsume species differences in stochastic terms. New hierarchical modelling techniques for inference reveal high-dimensional differences among species that can be quantified with random individual and temporal effects (RITES), i.e. process-level variation that results from many causes. We show that this variation is large, and that it stands in for species differences along unobserved dimensions that do contribute to diversity. High dimensional coexistence contrasts with the classical notions of tradeoffs along a few axes, which are often not found in data, and with ‘neutral models’, which mask, rather than eliminate, tradeoffs in stochastic terms. This mechanism can explain coexistence of species that would not occur with simple, low-dimensional tradeoff scenarios.
197 citations
"Niches versus neutrality: uncoverin..." refers background in this paper
...Finally, the high dimensionality hypothesis (Clark et al. 2007) proposes that coexistence of apparently similar species occurs because of hidden additional niche dimensions along which differences exist....
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...…hypothesis was first proposed as a way to tackle explicitly the observed complexity of the natural world and an alternative to other, low dimensionality models that may be misleading where lack of evidence for differences in species niches does not equate actual similarity (Clark et al. 2007)....
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