Clusters of ecologically similar species emerge along a water availability gradient in a semiarid grassland as a result of species interactions

Alejandra Martínez-Blancas, Facultad de Ciencias, Universidad Nacional Autónoma de México, Mexico City, DF, Mexico and Carlos Martorell, Departamento de Ecología y Recursos Naturales, Facultad de Ciencias, Universidad Nacional Autónoma de México, Mexico City, DF, Mexico

Background/Question/Methods
Theoretical studies of competition have shown that clusters of ecologically similar species emerge as a result of competition. This pattern, termed emergent neutrality, may seem counterintuitive because there will be strong competition between species within the same cluster. However, clumps of similar species may emerge to avoid competition because species within clusters may facilitate each other by suppressing common competitors. Additionally, species within clusters may be able to coexist through similarities in their competitive abilities that prevent one species from excluding others. Coexistence between species of different clusters may be possible through niche differentiation. We hypothesized that species in a grassland community assemble into clusters along an important niche axis, a soil depth gradient related to water availability, as a result of species interactions. To determine if clusters of similar species emerge, we calculated population growth rates and interaction coefficients of 36 herbs along the gradient using a population growth model parameterized with field data. These parameters were then used to simulate changes in species abundances along the gradient with and without species interactions. Resulting abundances were used to assess if species aggregated into clusters along the gradient. Population growth rates and interaction coefficients were used to calculate coexistence mechanisms.
Results/Conclusions
After simulating our community, 27 species persisted and 10 went extinct. In the absence of species interactions no species clusters emerge. However, when interactions were taken into account, 4 species clusters emerge along the soil depth gradient. Niche differences were smaller between species that occupy portions of the gradient where these clusters occur. This suggests that species within clusters coexist because of their similarities in competitive abilities and low niche differentiation. There were also large differences in competitive abilities at portions of the gradient in between where clusters occur. This suggests that gaps between clusters may be the worst place to establish because of strong differences in competitive abilities occurring there, causing the exclusion of species with lower competitive abilities. Our study is the first to find emergent neutrality as a result of competition in plant communities. It also highlights the importance of coexistence mechanisms in structuring communities into clusters of ecologically similar species, and confirms the importance of hydrological niches in structuring plant communities.