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  1. Abstract

    The temporal storage effect—that species coexist by partitioning abiotic niches that vary in time—is thought to be an important explanation for how species coexist. However, empirical studies that measure multiple mechanisms often find the storage effect is weak. We believe this mismatch is because of a shortcoming of theoretical models used to study the storage effect: that while the storage effect is described as having just three requirements (partitioning of temporal variation, buffered population growth, and a covariance between environment and density‐dependence), models used to study the storage effect make four assumptions, which are mathematically subtle but biologically important. In this paper, we examine those assumptions. First, models assume that environmental variation leads to a rapid impact on density‐dependence. We find that delays in density‐dependence (including delays caused by competition between cohorts) weaken the storage effect. Second, models assume that intraspecific competition is almost identical to interspecific competition. We find that unless resource or predator partitioning are virtually absent, then variation‐independent mechanisms will overshadow the benefits of the storage effect. Third, models assume even though there is vast variation in the environment, species are equally adapted on average (i.e., zero fitness‐differences). We show that fitness differences are particularly problematic in the storage effect because specializing on temporally rare niches is far less effective than specializing on other types of rare niches. Finally, models assume that stochastic extinctions can be ignored, and invader growth can determine coexistence. We show that storage effects tend to reduce mean persistence times, even if invader growth rates are positive. These results suggest that the assumptions needed for the storage effect are strict: if the first or second assumption is relaxed, it will greatly weaken the stabilizing mechanism; if the third or fourth assumption is relaxed, it will create a diversity‐destroying effect that may undermine coexistence. We examine three real‐world communities—annual plants, tropical forests, and iguanid lizards—and find that empirical studies suggest that all three communities violate multiple assumptions. This suggests that the temporal storage effect is probably not an important explanation for species diversity in most systems.

     
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  2. Abstract

    Intraspecific trait variation (ITV) is a widespread feature of life, but it is an open question how ITV affects between‐species coexistence. Recent theoretical studies have produced contradictory results, with ITV promoting coexistence in some models and undermining coexistence in others. Here we review recent work and propose a new conceptual framework to explain how ITV affects coexistence between two species. We propose that all traits belong to one of two categories: niche traits and hierarchical traits. Niche traits determine an individual's location on a niche axis or trade‐off axis, such that changing an individual's trait makes it perform better in some circumstances and worse in others. Hierarchical traits represent cases where conspecifics with different traits have the same niche, but one performs better under all circumstances, such that there are winners and losers. Our framework makes predictions for how intraspecific variation in each type of trait affects coexistence by altering stabilizing mechanisms and fitness differences. For example, ITV in niche traits generally weakens the stabilizing mechanism, except when it generates a generalist–specialist trade‐off. On the other hand, hierarchical traits tend to impact competitors differently, such that ITV in one species will strengthen the stabilizing mechanism while ITV in the other species will weaken the mechanism. We re‐examine 10 studies on ITV and coexistence, along with four novel models, and show that our framework can explain why ITV promotes coexistence in some models and undermines coexistence in others. Overall, our framework reconciles what were previously considered to be contrasting results and provides both theoretical and empirical directions to study the effect of ITV on species coexistence.

     
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  3. Abstract

    Conspecific negative density dependence (CNDD) is thought to promote plant species diversity. Theoretical studies showing the importance ofCNDDoften assumed that all species are equally susceptible toCNDD; however, recent empirical studies have shown species can differ greatly in their susceptibility toCNDD. Using a theoretical model, we show that interspecific variation inCNDDcan dramatically alter its impact on diversity. First, if the most common species are the least regulated byCNDD, then the stabilising benefit ofCNDDis reduced. Second, when seed dispersal is limited, seedlings that are susceptible toCNDDare at a competitive disadvantage. When parameterised with estimates ofCNDDfrom a tropical tree community in Panama, our model suggests that the competitive inequalities caused by interspecific variation inCNDDmay undermine many species’ ability to persist. Thus, our model suggests that variableCNDDmay make communities less stable, rather than more stable.

     
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