COS 7-8
Compensatory dynamics in nature: Do grass and forb years stabilize productivity across California rangelands?
Understanding factors that control community stability is a central goal in ecology. A key insight that has emerged is that stable communities can be comprised of highly unstable species populations. If populations fluctuate asynchronously, they can generate “compensatory dynamics” where a decrease in one species is compensated for by the rise of another. However, species asynchrony can arise through multiple mechanisms, only some of which are stabilizing. Asynchrony arising from differential species responses to environmental variability should stabilize community properties, whereas aynchrony driven by competition may have little compensatory effect. To date few studies have tested both the pattern and mechanism of species asynchrony, limiting our ability to understand the importance of compensatory dynamics in natural systems. Here we capitalized on a well-documented pattern of species asynchrony – “grass years” and “forb years” in California rangelands – to experimentally test whether this pattern generates compensatory dynamics. We used shelters to create dry conditions (associated with forbs) and irrigation to create wet conditions (associated with grasses), within which we manipulated functional group interactions (grass, forb, mixed, no-neighbor, control). This allowed us to isolate the environmental and competitive responses of each group and assess the implications for community stability.
Results/Conclusions
Our treatments successfully mimicked “grass years” and “forb years”, with high relative grass cover in the wet-treatment control plots and high relative forb cover in the dry. However, both the grass and forb monocultures exhibited similar positive responses to the wet treatment; competition with grasses drove a reduction in forb production in the mixed plots under wet conditions. As a result, grass-forb asynchrony did not stabilize community biomass across precipitation conditions. This effect was compounded by the fact that the potential biomass production of grasses versus forbs was highly unequal. However, although both functional groups shared similar intrinsic responses to precipitation, they responded at different times in the growing season (forb production peaked earlier the season). Consequently, although species asynchrony did little to stabilize biomass across precipitation conditions, community biomass was stabilized across the growing season. Our results indicate that competition is likely to be a primary factor shaping environment-species interactions in relation to limiting resources. Consequently, species asynchrony in similar systems is unlikely to generate strong compensatory dynamics.