COS 293-5 - Bottom-up effects of nutrients on herbivory are negated by warming

Abstract: Climate warming has the potential to reshape ecological communities by altering species interactions. We evaluated this possibility by assessing effects of experimental warming and nutrient additions on consumption of algae by intertidal herbivores. We quantified herbivory in tide pools at Corona del Mar State Beach (CDM) in Southern California, USA, and Bodega Marine Reserve (BMR) in Northern California, locations characterized by differences in nutrient availability (concentrations 7-10 times higher at BMR) and temperature (max daily temperatures ~4 °C warmer at CDM). We predicted that nutrient additions would have stronger effects on herbivory at CDM than at BMR due to the potential for amelioration of nutrient limitation at CDM where ambient nutrient concentrations were low. We conducted a month-long factorial experiment manipulating herbivore abundances and nutrient availability at each location and found just the opposite. At BMR, herbivore exclusions enhanced algal abundances when nutrients were added, but there was no effect of herbivores in control pools. In contrast, herbivore exclusions had no effect at CDM, regardless of whether nutrients were added. These results were consistent with an effect of temperature – the other factor that differed substantially between the locations – and we tested this possibility by adding temperature as an additional factor in our experiment at BMR, raising average daily maximum temperatures by 1.4 °C. In contrast to un-warmed pools, where nutrient additions enhanced herbivory, nutrient additions did not affect herbivory in warmed tide pools at BMR. Effects of bottom-up processes on top-down processes are important aspects of ecological communities, and our results highlight the potential for warming to negate these effects.