Climate change and context-dependent competitive outcomes drive alterations in community composition

Background/Question/Methods

Climate change has already led to widespread alterations of biological systems; thus, it is crucial to refine methods for predicting future changes, especially given that even higher rates of global warming are predicted in the next century. Recent, single-species studies have highlighted the idiosyncrasy and context-dependency of species’ responses while continuing to beg the question of how community dynamics will be affected via warming impacts on species interactions. We examined the impacts of ocean warming in a space-limited marine epibenthic community. Using laboratory mesocosms and field observations, we empirically estimated rates of recruitment, survival, growth into unoccupied space, and overgrowth competition (growth into occupied space). These values were used to parameterize spatially implicit competition models for 5 common species at a current, ambient temperature and two future, increased temperatures predicted by IPCC warming scenarios (+3°C and +4.5°C).

Results/Conclusions

There were significant effects of ocean warming, particularly the higher increase of +4.5°C, on all four processes responsible for the acquisition and maintenance of space, the limiting resource in this community. Of these 5 species, the tunicate Distaplia occidentalis was the only species for which survival decreased and recruitment did not increase (but, rather, tended to decrease) at increased temperature. Growth (into unoccupied space) increased for 4 of these species by 50 to 140%. In contrast, growth into occupied space (overgrowth competition) was negative for two species, the tunicates Distaplia and Diplosoma listerianum, indicating that they were the competitive subordinates at all three temperatures. Interestingly, the predicted increase in water temperature led to a change in the dominant competitor, from Didemnum vexillum at the current, ambient temperature to the bryozoan Watersipora subtorquata at the future, increased temperature. The combination of both greater recruitment and greater rates of overgrowth at the higher temperature led to an increase in community dominance of Watersipora under simulated warming conditions. These results indicate the potential for climate change to cause marked shifts in community composition due to emergent community-level processes such as reversals in competitive outcomes. In addition, changes in the tempo of competitive exclusion may have consequences for coexistence at the metacommunity scale.