Global change, response variability, and their implications for coastal ecosystem resilience and functioning
Keywords: Biology.

Global human stressors are forecast to negatively impact biodiversity and drive massive ecosystem changes on land and sea. Predictions of their long-term impacts require an understanding of how the responses of multiple species combine to modify ecosystem structure and functioning. However, this "scaling-up" from species to ecosystems would be unrealistic without considering within-community variations in species' function and sensitivity to human stressors. Depending on how species playing strong and weak functional roles in given communities are affected, stressor-driven species loss or changes in species performances could trigger variable ecosystem changes. In my dissertation, I used field and laboratory experiments and meta-analysis to examine how variable species response within consumer assemblages (e.g., herbivores, predators, and detritivores), or "consumer response variability" modulates the resilience and functioning of coastal marine ecosystems under two global stressors (i.e., ocean acidification and habitat alteration). This research highlights the broad importance of consumer response variability in reorganizing community dynamics and ecosystem properties. First, I show that consumer response variability drives coastal ecosystem resilience under ocean acidification (OA). Using an experimental marine assemblage comprising a habitat-forming seagrass, epiphytic algae, and algae grazers, I demonstrate that a) ocean acidification elicits variable changes in the performance of different grazer species (e.g., feeding and growth), and b) a resilient, functionally important species maintains the consumer community function (i.e., top-down control of algae) and seagrass resilience under the stressor. Second, I utilize naturally acidified seagrass ecosystems as a proxy for future OA conditions and experimentally show that consumer response variability modulates important ecosystem functions pertaining to coastal nutrient dynamics (i.e., detritivore consumption and decomposition of seagrass detritus). Contrary to theoretical predictions, I find that a) ocean acidification can boost the decomposition function despite causing significant losses in consumer richness and diversity, and that b) this change is related to the increased abundance of dominant and functionally important detritivore species. Third, I employ a quantitative meta-analysis and show that consumer response variability underlies the global patterns of changes in vital food-web processes (i.e., predation and herbivory) following habitat alterations. The findings suggest that a) fragmentation, structural degradation, and loss of coastal biogenic habitats generally intensify consumer pressure on marine prey, and b) these patterns are driven by resilient, generalist herbivores and predators that are abundant in marine environments. Taken together, my research suggests that global stressors have a large potential to reorganize consumer-resource dynamics and modulate the resilience and functioning of coastal marine ecosystems. However, the direction and magnitude of these changes would depend on complex and variable species responses within ecological assemblages and functional groups. Researchers should thus explicitly consider variable relationships between species trait/function and stressor tolerance in predicting future ecosystem changes