UCLA

Ecological interactions are context dependent. The net effect of species interactions includes the multifaceted impacts of community composition and abiotic influences of the environment. I examined how both biotic and abiotic stressors, in the form of herbivore composition and salinity stress, respectively, elicited positive interactions between species. In a kelp forest I examined how the relationship between kelp and an encrusting bryozoan, Membranipora membranacea , shifted to an associational defense depending on herbivore guild composition. Using a combination of choice experiments and surveys of grazing damage, I demonstrated that the mesograzers Lacuna unifasciata, Perampithoe humeralis, and Idotea resecata almost entirely avoided (<1% of total diet) encrusted kelp. The large snail Norrisia norrisi also preferred clean kelp, but kelp crabs, Pugettia producta, targeted encrusted kelp. Field surveys in a mesograzer dominated canopy found 2.5 times more grazing damage on sparsely encrusted kelp than on heavily encrusted kelp and far more grazing in the upper canopy than on the blades from the vertical stipes below the canopy. These results indicate that, when mesograzers are dominant, Membranipora provides kelp associational resistance to grazing. Such protection may be more prominent in the upper canopy. Additionally, I examined the role of abiotic stress on positive interactions in a hypersaline salt marsh. Following disturbance that removes established vegetation, salt-tolerant species can ameliorate harsh soil salinities for less tolerant species and therefore promote secondary succession. However, when abiotic stress is extremely high amelioration may be inadequate to improve growth of associated neighbors. Using clipping manipulations of Batis maritima, an early successional halophyte, I tested whether B. maritima facilitates secondary succession in an excessively hypersaline salt marsh in southern California. Experimental plots with B. maritima present recovered faster (27% compared to 14% revegetation by matrix species) and had lower increases in soil salinity. Salicornia pacifica and Arthrocnemum subterminale were the dominant recovery species in both treatments and no differences were found in species richness, diversity, or evenness of recovery species between treatments.

Overall, my research indicates positive interactions play a prominent role in these coastal ecosystems though that role will depend on the specific nature of the community.