The role of asymmetric sensitivities in environmental responses on promoting species coexistence in variable environment

Background/Question/Methods

Organisms live in a changing physical environment. One fundamental question is how environmental variation affects species coexistence. Organisms respond to a variable environment in both direct and indirect ways. As the result, recruitment to key stages in the life cycle can fluctuate greatly over time. However, theoretical studies show that environmental variation could not promote coexistence unless species have temporal (or spatial) niche partitioning in their response to these variations. Correlations in environmental responses are often taken as the only measure for the strength of this temporal/spatial niche partitioning when studies make conclusions on the importance of environmental variation. In systems with high species diversity such as tropical forests, there is a limit in the degree that species can decouple in their correlations to environmental responses. As a result, doubt arises about the role of variable environments in those systems. It’s universal for some species to be more sensitive to environmental fluctuations than others, but most theoretical and empirical work has been ignoring these differences. To study whether sensitivity differences between species to the environmental variation are able to create effective temporal niche partitioning, we simulate lottery model with species fully correlated in environmental responses.

Results/Conclusions

Without sensitivity differences to environmental variation between species, there is no temporal niche partitioning when species are correlated. With sensitivity differences, we can potentially get a substantial magnitude of coexistence mechanisms. In the two species cases, coexistence regions are quite large and their sizes increase quadratically with sensitivity differences. Starting from a community with two coexisting species, we run simulation to search for another species that will allow all three species coexist under full correlation. There are unlimited numbers of species that satisfy the coexistence criteria. A trade-off exists between the mean and sensitivity in environmental responses for species in the candidate pool: we can always find a species with higher sensitive but lower mean environmental response to coexists with the other two species. We repeat this process after increasing the species number in the community by adding one species from the candidate pool each time, and keep searching for the candidate pool for the new species that allow all species coexists. Though coexistence tends to be marginal with extremely high number of species, there is no absolute limit on the number of species that can coexist under full correlation.