Accurate predictions of coexistence in natural systems require the inclusion of facilitative interactions and environmental dependency

Abstract

1. Coexistence between plant species is well known to depend on the outcomes of species interactions within an environmental context. The incorporation of environmental variation into empirical studies of coexistence are rare, however, due to the complex experiments needed to do so and the lack of feasible modelling approaches for determining how environmental factors alter specific coexistence mechanisms.2. In this article, we present a simple modelling framework for assessing how variation in species interactions across environmental gradients impact on niche overlap and fitness differences, two core determinants of coexistence. We use a novel formulation of an annual plant population dynamics model that allows for competitive and facilitative species interactions and for variation in the strength and direction of these interactions across environmental gradients. Using this framework, we examine outcomes of plant-plant interactions between four commonly co-occurring annual plant species from Western Australian woodlands. We then determine how niche overlap and fitness differences between these species vary across three environmental gradients previously identified as important for structuring diversity patterns in this system: soil phosphorus, shade and water.3. We found facilitation to be a widespread phenomenon and that interactions between most species pairs shift between competitive and facilitative across multiple environmental gradients. Environmental conditions also altered the strength, direction and relative variation of both niche overlap and fitness differences in nonlinear and unpredictable ways. Synthesis.We provide a simple framework for incorporating environmental heterogeneity into explorations of coexistence mechanisms. Our findings highlight the importance of the environment in determining the outcome of species interactions and the potential for pairwise coexistence between species. The prevalence of facilitation in our system indicates a need to improve current theoretical frameworks of coexistence to include noncompetitive interactions and ways of translating these effects into explicit predictions of coexistence. Our study also suggests a need for further research into determining which factors result in 1840 | Journal of Ecology BIMLER Et aL.

Distinct responses of niche and fitness differences to water availability underlie variable coexistence outcomes in semi‐arid annual plant communities

“…This negative result implies that the population models we parameterized likely do not capture the key mechanisms of coexistence in this system. In all, our findings point to challenges in operationalizing current coexistence theory for forecasting the impact of global change on diversity dynamics, a finding consistent with other studies of this system (Bimler et al, ). However, our findings also highlight the importance of considering how species responses to water availability are mediated both directly and indirectly through impacts on vital rates and relative sensitivities to interactions.…”

“…Additionally, our population model does not include facilitative or higher order interactions, which may have buffered the strength of direct competitive effects on these focal species. The contributions of noncompetitive and nonadditive interactions to coexistence in natural communities warrant greater exploration (Bimler et al, ; Mayfield & Stouffer, ), and point to challenges in operationalizing current coexistence theory in systems where competitive impacts may not be strongly density‐mediated (Hart et al, ).…”

“…By contrast, other studies focus on individual species’ responses to climate change (e.g., Llorens & Penuelas, ; Morin, Roy, Sonié, & Chuine, ), and do not address how climate change might alter interaction‐driven niche differences. While some empirical studies have examined how interactions in plant communities may be altered by climatic factors (e.g., Dormann, Wal, & Woodin, ; Suttle, Thomsen, & Power, ), few have explicitly examined the implications for coexistence in naturally occurring communities (but see Bimler, Stouffer, Lai, & Mayfield, ; Matias, Godoy, Gómez‐Aparicio, Pérez‐Ramos, & Allan, ), which requires study of potential changes to both niche and fitness differences (Siepielski & McPeek, ).…”

“…However, recent empirical work has demonstrated that relationships between resource availability and coexistence outcomes may not be straightforward. For example, Bimler et al () and Hart, Freckleton, and Levine () suggest that current coexistence models may not be appropriate in systems where competition intensity and the density of competing individuals are not strongly correlated. Furthermore, Matias et al () found that among annual plants, experimental drought reduced fitness inequalities, but contrary to expectations, ultimately reduced coexistence relative to control conditions due to increased niche overlap.…”

Corrigendum

“…In the paper by Bimler, Stouffer, Lai, and Mayfield (), several inadvertent errors were made which the authors would like to correct. Please note that none of these changes impact on the conclusions made by the paper but are necessary to correct for accuracy.…”