Salinity controls rocky intertidal community structure via suppression of herbivory

Abstract

Climate change impacts ecosystems directly through differences in species specific responses as well as indirectly through changes to the strength of species interactions. To predict how species will be impacted by ongoing environmental change, we need to better understand the relative roles of these direct and indirect effects. Salinity is a strong driver of ecological patterns and processes, and salinity regimes in coastal regions are expected to be altered by climate change through intensification of the hydrological cycle and via climate-driven shifts in the timing and strength of the spring freshet. We hypothesized that hyposalinity can indirectly affect the intertidal community by excluding a dominant herbivore. To test this hypothesis, we 1) conducted intertidal diversity surveys in regions of high vs. seasonally low salinity in the Strait of Georgia, British Columbia, 2) conducted laboratory salinity tolerance trials for two important grazers (Lottia pelta and Lottia digitalis) and one primary producer (Ulva sp.), and 3) experimentally manipulated the abundance of grazers in these two regions. We show that rocky intertidal shores from two regions of disparate salinity regimes are distinct in their intertidal communities: low salinity sites were composed primarily of Mytilus trossulus, Fucus distichus and Ulva sp., whereas high salinity sites were dominated by Chthamalus dalli, Lottia spp., and Mastocarpus sp. Our laboratory trials confirmed that freshwater inputs experienced in the low salinity region resulted in hyposaline levels which exceeded the tolerance of Lottia spp., but not that of Ulva sp. Further, we show that by excluding grazers in high salinity sites, these communities more closely resemble that of the low salinity sites than they do of other high salinity sites with grazers present. Together, these results demonstrate that the pattern of distinct estuarine intertidal communities in low vs. high salinity regions in the Strait of Georgia may be largely driven by the indirect effects of freshwater inputs, mediated by salinity-driven differences in herbivore population size and thus grazing pressure.