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Competition can result in evolutionary changes to coexistence between competitors, yet there are no theoretical models that predict how the components of coexistence change during this eco-evolutionary process. We study the evolution of the coexistence components, niche overlap and competitive differences, in a two-species eco-evolutionary model based on consumer-resource interactions and quantitative genetic inheritance. Species evolve along a one-dimensional trait axis that allows for changes in both niche position and species intrinsic growth rates. There are three main results. First, the breadth of the environment has a strong effect on the dynamics, with broader environments leading to reduced niche overlap and enhanced coexistence. Second, coexistence often involves either a reduction in niche overlap while competitive differences stay relatively constant, or vice versa: a change in competitive differences while niche overlap does not change much. Large simultaneous changes in niche overlap and competitive difference often result in one of the species being excluded. Third, provided that the species evolve to a state where they coexist, the final niche overlap and competitive difference values are independent of the system's initial state, though they do depend on the model's parameters. The model suggests that evolution is often a destructive force for coexistence due to evolutionary changes in competitive differences, a finding that expands the paradox of diversity maintenance.

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The importance of predators in influencing community structure is a well-studied area of ecology. However, few studies test ecological hypotheses of predation in multi-predator microbial communities. The phytotelmic community found within the water-filled leaves of the pitcher plant, Sarracenia purpurea, exhibits a simple trophic structure that includes multiple protozoan predators and microbial prey. Using this system, we sought to determine whether different predators target distinct microorganisms, how interactions among protozoans affect resource (microorganism) use, and how predator diversity affects prey community diversity. In particular, we endeavored to determine if protozoa followed known ecological patterns such as keystone predation or generalist predation. For these experiments, replicate inquiline microbial communities were maintained for seven days with five protozoan species. Microbial community structure was determined by 16S rRNA gene amplicon sequencing (iTag) and analysis. Compared to the control (no protozoa), two ciliates followed patterns of keystone predation by increasing microbial evenness. In pairwise competition treatments with a generalist flagellate, prey communities resembled the microbial communities of the respective keystone predator in monoculture. The relative abundance of the most common bacterial Operational Taxonomic Unit (OTU) in our system decreased compared to the control in the presence of these ciliates. This OTU was 98% similar to a known chitin degrader and nitrate reducer, important functions for the microbial community and the plant host. Collectively, the data demonstrated that predator identity had a greater effect on prey diversity and composition than overall predator diversity.

Competition-colonization trade-offs are theorized to be a mechanism of coexistence in communities structured by environmental fluctuations. But many studies that have tested for the trade-off have failed to detect it, likely because a spatiotemporally structured environment and many species assemblages are needed to adequately test for a competition-colonization trade-off. Here, we present a unique 32-year study of rock-dwelling lichens in New Mexico, USA, in which photographs were used to quantify lichen life history traits and interactions through time. These data allowed us to determine whether there were any trade-offs between traits associated with colonization and competition, as well as the relationship between diversity and disturbance in the community. We did not find evidence for a trade-off between competitive ability and colonization rate or any related life history traits. Interestingly, we did find a peak in all measures of species diversity at intermediate levels of disturbance, consistent with the intermediate disturbance hypothesis pattern. We suggest that the coexistence of the dominant species in this system is regulated by differences in persistence and growth rate mediating overgrowth competition rather than a competition-colonization trade-off.

Citation: Pastore, A. I., and B. P. Scherer. 2016. Changes in community phylogenetic structure in a North American forest chronosequence. Ecosphere 7(12):e01592. 10.1002/ecs2.1592Abstract. Several studies of succession in tropical and subtropical climates include phylogenetic analyses of the plant communities; the majority of these studies find a shift from more closely related to less closely related assemblages over succession. It has been suggested that this pattern indicates a shift from abiotic to biotic filters structuring communities over time, but there is considerable debate surrounding this interpretation. Conducting analyses for multiple components of plant assemblages can provide insight into the processes structuring communities. Here, we present community phylogenetic analyses of a deciduous forest chronosequence for three community components: standing vegetation, seed bank, and vegetation regenerated after small-scale disturbance. We constructed a phylogeny from 228 taxa present in the community data of a chronosequence obtained from previously published research. In the standing vegetation, we found a shift from more closely related to less closely related vegetation over the chronosequence. These results are consistent with other studies of chronosequences in tropical forests, lending support to the ubiquity of such shifts in relatedness over succession under different climatic conditions. However, the seed bank and vegetation regenerated after small-scale disturbance showed no consistent pattern with stand age, suggesting recruits are experiencing different forces than surrounding vegetation. These phylogenetic analyses of seed banks and vegetation regenerated after small-scale disturbance over a chronosequence provide additional evidence into the mechanisms driving forest succession.

Predicting the outcome of interactions between species is central to our current understanding of diversity maintenance. However, we have limited information about the robustness of many model-based predictions of species coexistence. This limitation is partly because several sources of uncertainty are often ignored when making predictions. Here, we introduce a framework to simultaneously explore how different mathematical models, different environmental contexts, and parameter uncertainty impact the probability of predicting species coexistence. Using a set of pairwise competition experiments on annual plants, we provide direct evidence that subtle differences between models lead to contrasting predictions of both coexistence and competitive exclusion. We also show that the effects of environmental context-dependency and parameter uncertainty on predictions of species coexistence are not independent of the model used to describe population dynamics. Our work suggests that predictions of species coexistence and extrapolations thereof may be particularly vulnerable to these underappreciated founts of uncertainty.

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