Climate Change Impacts on Interactions between Native and Exotic Marine Species

Purpose of the study: Species invasion is one of the main threats to marine environments and the relationship between individual stressors and species introductions is relatively well known. However, in nature, multiple stress factors play a role simultaneously. How multiple stressors related to climate change affect marine epibenthic communities and species invasion success as mediated by interspecific competition is still unclear. Procedure: To understand if multiple stressors due to climate change favor exotic species in terms of their growth rate and competitive ability, I set up PVC plastic plates in two biologically similar and physically different adjacent sites (Tomales Bay and Bodega Harbor) and observed naturally recruited epibenthic species for four months by taking and analyzing weekly photographs and monitoring environmental conditions. I also ran a laboratory experiment to determine the individual and combined effects of water temperature and the availability of phytoplankton (a primary food source for sessile marine epibenthic animals), measured as chlorophyll a (chi-a). Findings: In the field, the growth rate of exotic species in the absence of other species, was positively associated with increased water temperature, while that of native species were negatively correlated. Exotic species grew fastest under conditions of high water temperature and high chl-a. The apparent negative effects of water temperature on native species growth rates were reduced when chl-a was high. The results of the lab experiment were similar for temperature, but effects of chl-a were not clearly evident. However, growth rates were very low in the lab compared to the field and the two chl-a treatment levels may have limited growth overall. Sea water temperature and chl-a levels also appeared to influence the outcome of competition between native and exotic species. The exotic species Botryllus schlosseri and Watersipora subtorquata quickly outcompeted native species for space and became dominants in the community, under the warmer temperatures and higher chi-a levels observed in Tomales Bay compared to Bodega Harbor. Conclusions: Temperature and food availability have a synergistic effect on marine exotic species in terms of their growth rate and competitive ability. And by favoring exotic invasive species in marine ecosystems, climate change will likely increase coastal economic, ecologic and cultural losses.