A non-native macroalga is less attractive for herbivores but more susceptible to light limitation and grazing stress than a comparable native species

Abstract

It has been suggested that non-native species are more tolerant towards abiotic stress than ecologically comparable native species. Furthermore, non-native marine macroalgae should be under lower grazing pressure than native seaweeds, because they left their co-evolved enemies behind. As a consequence, they generally need to allocate less energy to defences and can invest more into compensating the negative effects of abiotic stress or, assuming that grazing pressure is low but not zero, to defensive reactions following grazer attack. This, in turn, should make them more stress tolerant and less susceptible to herbivory. However, empirical evidence for both concepts is still scarce and very little is known about whether enemy release is commonly associated with an enhanced tolerance towards abiotic or biotic stress. We therefore ran an experimental study that (a) assessed attractiveness for grazers, (b) verified whether short-term low-light stress impairs growth and (c) investigated whether light limitation and previous grazing interactively affect the consumption of two macroalgae from Madeira Island, the native brown alga Stypopodium zonale and the non-native red alga Grateloupia imbricata by the sea urchin Paracentrotus lividus. To come to ecologically meaningful low-light stress levels, pilot studies were performed in order to determine the light compensation point of photosynthesis for each algal species and then we established six light regimes around this point by reducing the amount of incoming light. Simultaneously, we let one sea urchin graze on each algal individual to stimulate a chemical defence in the seaweeds if present. In parallel to this, we kept the same number of algal replicates in the absence of sea urchins. After 21 days, we compared algal growth in the absence of grazers as well as the attractiveness of previously grazed and non-grazed algal material for P. lividus across all light regimes. Algal attractiveness was assessed in no-choice feeding assays. The observation that the non-native alga was less consumed by the grazer than the native species generally confirms the concept of enemy release. However, light limitation reduced growth in the non-native but not in the native seaweed, while previous grazing reduced consumption of the native but enhanced it in case of the non-native alga. These findings do not corroborate the assumption that enemy release can, through the re-allocation of energy, enhance tolerance to abiotic (light limitation) or biotic (grazing) stressors in non-native marine macroalgae.