Weathering it together: Experimental test of the effect of climate variability on the neutral coexistence of barnacle competitors on Chilean rocky shores
Evidence continues to amass suggesting that the effect of climate variability on species interactions may have important or surprising consequences for natural systems. Recent work posits that interactions within non-trophically regulated communities may be differently affected by climate variability depending on the strength of niche differences among competitors. Specifically, the more functionally similar, or “neutral” competitors are, the more sensitive they should be to the indirect effects of climate variability on their interactions. We empirically tested this theory over a three year period at several rocky shore sites (separated by 10s-100s of km) in central Chile by manipulating local climate conditions at two shore elevations where two common barnacles, Jehlius cirratus and Notochthamalus scabrosus, coexist by primarily neutral means. Within experimental plots, we compared total barnacle recruitment rates, species-specific per capita growth rates, and competition coefficients in replicated shaded and unshaded areas of rocky intertidal platforms. We replicated our design at sites characterized by distinctive aerial and sea surface temperatures (SST) to see if climate variability indeed has a strong effect on the interaction between these two species and whether or not it can produce a shift in relative species dominance.
No latitudinal trend was found in barnacle response to manipulation of local climatic conditions. At sites characterized by cooler SST, recruitment of barnacles into experimental plots was low and similar between shaded and unshaded areas. At these sites we found no differences in species abundances among different local climate conditions or evidence of shifting species dominance. At sites characterized by warmer SST, recruitment rates were higher, especially under shaded areas. Adult abundance varied by site and between shaded and unshaded areas, yet we observed a shift in dominance toward a greater relative abundance of Notochthamalus in cooler areas. Evidence from thermal settlement collectors suggest that substrate surface temperature does not strongly influence settlement selection or early postsettlement survivorship of either barnacle, but cooler temperatures favor survival of juvenile Notochthamalus when recruitment is high. We conclude that climate variability has important indirect effects on Jehlius and Notochthamalus, but only when and where competition for space is most intense after an influx of new recruits. We also discuss the relative contribution of changes in per capita growth rates versus changes in strength or direction of species interactions in mediating the effects of climate variability on competitive interactions.