Modern Coexistence Theory Research Articles

Linking modern coexistence theory and contemporary niche theory

AbstractModern coexistence theory and contemporary niche theory represent parallel frameworks for understanding the niche's role in species coexistence. Despite increasing prominence and shared goals, their compatibility and complementarity have received little attention. This paucity of overlap not only presents an obstacle to newcomers to the field, but it also precludes further conceptual advances at their interface. Here, we present a synthetic treatment of the two frameworks. We review their main concepts and explore their theoretical and empirical relationship, focusing on how the resource supply ratio, impact niche, and requirement niche of contemporary niche theory translate into the stabilizing and equalizing processes of modern coexistence theory. We show, for a general consumer–resource model, that varying resource supply ratios reflects an equalizing process; varying impact niche overlap reflects a stabilizing process; and varying requirement niche overlap may be both stabilizing and equalizing, but has no qualitative effect on coexistence. These generalizations provide mechanistic insight into modern coexistence theory, while also clarifying the role of contemporary niche theory's impacts and requirements in mediating coexistence. From an empirical perspective, we recommend a hierarchical approach, in which quantification of the strength of stabilizing mechanisms is used to guide more focused investigation into the underlying niche factors determining species coexistence. Future research that considers alternative assumptions, including different forms of species interaction, spatiotemporal heterogeneity, and priority effects, would facilitate a more complete synthesis of the two frameworks.

Grazing and temporal turnover in herbaceous communities in a Mediterranean landscape

AbstractQuestionWhat are the direct and indirect effects of cattle grazing on temporal turnover in Mediterranean herbaceous communities, and are there differences between species (taxonomic) and functional turnover rates? Are turnover rates related to changes in precipitation patterns?LocationThe long term ecological research (LTER) station, Ramat Hanadiv Nature Park, Israel.MethodsWe used data from a Mediterranean LTER site to analyse temporal turnover in herbaceous communities under cattle grazing and control conditions, while accounting for the potential effects of precipitation and woody cover. We analysed two types of temporal turnover: taxonomic turnover (changes in species composition) and turnover in functional traits (changes in the relative abundance of species with different functional traits).ResultsWe found that communities exhibited rapid taxonomic turnover between sampling years, which was mostly explained by changes in precipitation patterns. Furthermore, there was a strong and consistent directional trend in taxonomic turnover (21% per decade) throughout the study period, which was correlated with a long‐term trend of decreasing precipitation. This trend, however, was not significantly explained by grazing, woody cover change or their interaction. Grazing did have a significant effect on short‐term turnover rates, but its effects varied in both direction and magnitude among sites located in different habitats. Temporal turnover in functional traits was much slower than taxonomic turnover, and was also inconsistent among traits and sites. Grazing reduced the bi‐annual turnover rates in the composition of different life forms, and reduced the decadal turnover rate in the composition of different height groups in the community.ConclusionsThe results of this analysis suggest that the effect of cattle grazing on the temporal dynamics of herbaceous communities in Mediterranean ecosystems is habitat‐ and trait‐specific, and unrelated to changes in woody cover. In contrast, inter‐annual changes in precipitation patterns have a much stronger effect on turnover rates. The relationship between temporal species turnover and precipitation in these communities is therefore in line with modern co‐existence theory, by which fluctuating environmental conditions facilitate long‐term species co‐existence.

Potential mechanisms of coexistence in closely related forbs

The stable coexistence of very similar species has perplexed ecologists for decades and has been central to the development of coexistence theory. According to modern coexistence theory, species can coexist stably (i.e. persist indefinitely with no long‐term density trends) as long as species' niche differences exceed competitive ability differences, even if these differences are very small. Recent studies have directly quantified niche and competitive ability differences in experimental communities at small spatial scales, but provide limited information about stable coexistence across spatial scales in heterogeneous natural communities. In this study, we use experimental and observational approaches to explore evidence for niche and competitive ability differences between two closely related, ecologically similar and widely coexisting annual forbs: Trachymene cyanopetala and T. ornata. We experimentally tested for stabilizing niche differences and competitive ability differences between these species by manipulating species' frequencies, under both well‐watered and water‐stressed conditions. We considered these experimental results in light of extensive field observations to explore evidence of niche segregation at a range of spatial scales. We found little evidence of intra‐specific stabilization or competitive ability differences in laboratory experiments while observational studies suggested niche segregation across pollinator assemblages and small‐scale microclimate heterogeneity. Though we did not quantify long‐term stabilization of coexisting populations of these species, results are consistent with expectations for stable coexistence of similar species via a spatial storage effect allowing niche differences to overcome even small (to absent) competitive ability differences.

A role for biotic filtering in driving phylogenetic clustering in soil bacterial communities

AbstractAimPhylogenetic clustering, the coexistence of evolutionarily related organisms, appears to be common in soil bacteria. This pattern has traditionally been attributed to the habitat‐filtering of bacteria that are able to survive under particular abiotic settings. According to the modern coexistence theory, however, phylogenetic clustering can also arise from biotic interactions such as the competitive exclusion of large clades with low competitive abilities. Here, we used phylogeny‐based methods to discern whether the coexistence of evolutionarily related soil bacteria results from abiotic and/or biotic filtering.LocationWorldwide.MethodsWe performed a Bayesian meta‐analysis based on a literature review (n = 231) to assess whether the net relatedness index (NRI) or the nearest taxon index (NTI), two measures of the phylogenetic relatedness of taxa in local assemblages, deviate from those in randomly configured communities. We then sought the best abiotic (pH, total organic carbon and total nitrogen) and biotic predictors (relative abundance of Proteobacteria, Actinobacteria and Acidobacteria) of NRI and NTI.ResultsPhylogenetic clustering is pervasive in soil bacterial communities regardless of the spatial and taxonomic scales (NRI = 2.29; 95% CI [1.43, 3.29]; P < 0.001). Clustering is accentuated by productivity; that is, more fertile soils hold communities with more closely related bacteria (estimate = 1.05 [0.03, 2.15]; P < 0.05). Proteobacterial abundance, which increases with organic carbon enrichment, leads to higher relatedness among coexisting bacteria (estimate = 0.1 [0.02, 0.17]; P < 0.01) through the competitive exclusion of distantly related deep‐branching clades.Main conclusionsOur results, together with the dominance of proteobacterial lineages in soils worldwide, suggest that the overrepresentation of this clade underlies the widespread coexistence of phylogenetically related bacteria. These results are consistent with phylogenetic clustering arising via differences in competitive ability as predicted by the coexistence theory. This supports the idea that biotic filtering might have a role in driving the phylogenetic community assembly of soil prokaryotes.

Environmental stress, facilitation, competition, and coexistence

The major theories regarding the combined influence of the environment and species interactions on population and community dynamics appear to conflict. Stress/ disturbance gradient models of community organization, such as the stress gradient hypothesis, emphasize a diminished role for competition in harsh environments whereas modern coexistence theory does not. Confusion about the role of species interactions in harsh environments is perpetuated by a disconnect between population dynamics theory and data. We linked theory and data using response surface experiments done in the field to parameterize mathematical, population-dynamic competition models. We replicated our experiment across two environments that spanned a common and important environmental stress gradient for determining community structure in benthic marine systems. We generated quantitative estimates of the effects of environmental stress on population growth rates and the direction and strength of intra- and interspecific interactions within each environment. Our approach directly addressed a perpetual blind spot in this field by showing how the effects of competition can be intensified in stressful environments even though the apparent strength of competition remains unchanged. Furthermore, we showed how simultaneous, reciprocal competitive and facilitative effects can stabilize population dynamics in multispecies communities in stressful environments.

Opposing effects of competitive exclusion on the phylogenetic structure of communities

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.

A novel theory to explain species diversity in landscapes: positive frequency dependence and habitat suitability.

Theories to explain the diversity of species have required that individual species occupy unique niches and/or vary in their response to environmental factors. Positive interactions within a species, although common in communities, have not been thought to maintain species diversity because in non-spatial models the more abundant species always outcompetes the rarer species. Here, we show, using a stochastic spatial model, that positive intraspecific interactions such as those caused by positive frequency dependence and/or priority effects, can maintain species diversity if interactions between individuals are primarily local and the habitat contains areas that cannot be colonized by any species, such as boulders or other physical obstructions. When intraspecific interactions are primarily neutral, species diversity will eventually erode to a single species. When the landscape is homogeneous (i.e. does not contain areas that cannot be colonized by any species), the presence of strong intraspecific interactions will not maintain diversity.