Altering species interactions outweighs the effects of experimental warming in structuring a rocky shore community

- A field experiment was conducted to test the effects of temperature and species' interactions on rocky intertidal communities.
- Substratum temperatures were manipulated using black and white plates, whilst anti-fouling paint was used to exclude key grazer.
- Exclusions resulted in a significant increase in macroalgal biomass irrespective of thermal treatment, whilst barnacles were not affected by thermal stress (although barnacle densities were reduced by grazing on natural rock substrata).
- These results emphasise the importance of top-down control relative to warming in structuring communities within this system.

The strength and direction of interspecific interactions governing communities is expected to change with increasing global temperatures. However, in many ecosystems it remains unclear which species interactions are most likely to be altered under increasing thermal stress and what impacts, if any, such altered interactions will have on community structure. We investigated the interactive effects of increased temperature and altered species interactions on the structure of a rocky intertidal community by deploying black and white plates in areas with or without the dominant grazer, the limpet Cellana tramoserica. Plates effectively manipulated substratum temperature, whilst anti-fouling paint effectively manipulated the strength of algal-herbivore interactions. Plates without C. tramoserica quickly developed and maintained abundant macroalgal assemblages irrespective of thermal stress. By contrast, the abundance of barnacles remained relatively low and did not differ across thermal treatments, although they were slightly more abundant in plots where C. tramoserica had been excluded. These results support recent findings that altered species interactions are anticipated to have greater effects on community structure than the direct impacts of warming, challenging the view community structure under warming climatic conditions can be predicted from individual responses of populations to increasing thermal stress. Based on these and other recent findings we therefore advocate for greater inclusion of biotic interactions into climate change models that aim to predict the state of future ecological communities.