Asymmetric sensitivities in environmental responses: Its role in creating temporal niche partitioning for species coexistence

Background/Question/Methods

It is undeniable that species live in an ever changing physical environment. The role of this changing environment on species coexistence is not well understood. Theory says that two paralleling mechanisms, the storage effect and relative nonlinearity, arise with recruitment variation in temporal varying environments. The storage effect is predicted to be the stronger mechanism in most situations, and formalizes the concept of temporal niche partitioning. A common opinion is that effective temporal partitioning requires species to have negatively correlated responses to environmental fluctuations. This casts doubt on the role of niche partitioning in system with high species diversity because at most a few species can then be strongly negatively correlated. However, even when species’ responses are highly positively correlated, species can have large sensitivity differences in their environmental responses: some species’ responses vary widely between years, while others are more consistent. Effective temporal niche partitioning is possible with sensitivity differences between species, rather than negative  correlations in response to the environment. We studied how sensitivity differences affect species coexistence in lottery models, which is a general models for perennial plant communities. The storage effect and relative nonlinearity are quantified under different combination of species’ environmental responses and demographic traits.

Results/Conclusions

When species are equally sensitive to environmental variation, the storage effect decreases linearly with increasing between-species correlations in environmental responses, and becomes zero when species are fully synchronized. However, when species differ in sensitivity to environmental variation, the storage effect is not zero even with full correlation. When species have similar longevities, the storage effect impacts species with different sensitivities in contrasting ways. Relatively insensitive species are strongly advantaged by the storage effect even when species are fully correlated. The more sensitive species are disadvantaged by the storage effect. Nevertheless, the advantage of the insensitive species overrides the disadvantages of the sensitive species. As a result, the strength of coexistence from the storage effect increases with increasing species differences in sensitivity to the environment. This conclusion can be generalized to long-lived perennial plant communities where species differ in longevity. In these situations, relative nonlinearity also has the potential to promote coexistence when species have highly correlated environmental responses.