“Priority effects and non-hierarchical competition shape species composition in a complex grassland community”

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Lawrence H. Uricchio, S. Caroline Daws, Erin R. Spear, and Erin A. Mordecai (Feb 2019)

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Complex outcomes of competition in multispecies communities: priority effects, competitive exclusion, and coexistence

The bunchgrass Stipa pulchra, the state grass of California, is found in patches in Jasper Ridge Biological Preserve. The authors studied the ecological processes that determine the success or failure of native grasses, and found that the order of arrival of invasive European grass species has a major impact on plant community composition.
(Credit: Reuben Brandt)

The biological world is surprisingly diverse: even in our own backyards, we often see multiple species coexisting despite intense competition for just a few limiting resources like light, water, and nitrogen. Despite a long history of conceptual models to explain how ecological processes like competition or predation determine which species can coexist, it’s still not well understood how these kinds of interactions between species scale up to affect the distribution of species in space and time. It is critical to understand these complex multi-species interactions in a time when human-mediated changes to habitat, climate, and global species distributions are driving rapid environmental changes for many species.

Uricchio et al. studied the controls over plant species diversity in California grasslands, an historically widespread and diverse ecosystem. While California grasslands are a reservoir of biodiversity and endemic species, human-caused disturbances, such as grazing and invasions by introduced grasses, have fundamentally altered their composition; in fact, the golden hills of California are gold in large part due to dominance of annual grasses native to Europe. And yet our diverse native grasses persist and coexist with invasive grasses in some regions. What determines the winners and losers in highly diverse ecosystems, like these Mediterranean grasslands? The authors find that the order of arrival of invading species had important consequences for the persistence of native species, since each native grass was more likely to coexist with some exotic species than with others. Their findings also show that the expected outcome of competition between two species can be misleading in complex multi-species communities where indirect competitive interactions are pervasive. California grasslands are just one of many important ecosystems globally that are comprised of highly diverse and complex groups of species. These results suggest that understanding pairwise competitive hierarchies will not be sufficient for predicting long-term ecosystem dynamics. Rather, understanding the order in which species arrive in a given landscape, and their indirect competitive interactions with the species already present there, will ultimately govern which species persist and which fade out due to competition.


Abstract

Niche and fitness differences control the outcome of competition, but determining their relative importance in invaded communities – which may be far from equilibrium – remains a pressing concern. Moreover, it is unclear whether classic approaches for studying competition, which were developed predominantly for pairs of interacting species, will fully capture dynamics in complex species assemblages. We parameterized a population dynamic model using competition experiments of two native and three exotic species from a grassland community. We found evidence for minimal fitness differences or niche differences between the native species, leading to slow replacement dynamics and priority effects, but large fitness advantages allowed exotics to unconditionally invade natives. Priority effects driven by strong interspecific competition between exotic species drove single-species dominance by one of two exotic species in 80% of model outcomes, while a complex mixture of non-hierarchical competition and coexistence between native and exotic species occurred in the remaining 20%. Fungal infection, a commonly hypothesized coexistence mechanism, had weak fitness effects, and is unlikely to substantially affect coexistence. In contrast to previous work on pairwise outcomes in largely native-dominated communities, our work supports a role for nearly-neutral dynamics and priority effects as drivers of species composition in invaded communities.