Are Tropical Plants Better Defended? Palatability and Defenses of Temperate vs. Tropical Seaweeds

Investigators working in both terrestrial and marine systems hypothesize that prey defenses are better developed in tropical than in temperate habitats because tropical consumers are more active and diverse. To investigate the hypothesis that chemical defenses are more potent in tropical than in temperate seaweeds, we conducted 38 feeding assays involving seven genera of algae. We offered tropical and temperate sea urchins the choice of a temperate alga from North Carolina or a closely related tropical alga from the Bahamas. Plants were freeze-dried, ground to a fine powder, embedded in agar, and presented in congeneric pairs (North Carolina species vs. Bahamian species), or same-species pairs, to the temperate urchin Arbacia punctulata and the tropical urchin Lytechinus variegatus. For the genera we tested (the red alga Amphiroa, the green alga Udotea, and the brown algae Dictyopteris, Dictyota, Lobophora, Padina, and Sargassum), we documented a general pattern in which the mean amount of North Carolina seaweeds eaten was approximately twice that of their Bahamian congeners. Dictyota was an exception to the overall pattern, in that mean consumption of temperate and tropical species was equivalent. In addition to showing a latitudinal difference in seaweed palatability, we found that some species exhibited significant, between-population variation in herbivore resistance on a local scale. The most notable example was Dictyota menstrualis from North Carolina. When collected from a shallow inshore site, this species was significantly more palatable than tropical species of Dictyota in four of six contrasts. When collected from a deeper offshore site, it was significantly less palatable than the tropical species in four of six contrasts. Thus, for this species, the between-population variance in palatability within a geographic region (North Carolina) bracketed the variance in palatability that we found among species and between regions. Feeding assays with both temperate and tropical urchins produced similar results, indicating that choices were made based on fundamental characteristics of the algae rather than on greater familiarity with local prey species. To see if chemical defenses could explain the differences in herbivore resistance that we noted using freeze-dried algae, we tested the effects of lipid-soluble and water-soluble extracts from each alga on urchin feeding by incorporating extracts into a standard palatable food. We also measured protein content and percent ash-free dry mass of the seaweeds in order to correlate urchin feeding preferences with these measures of food quality. Lipophilic chemical extracts explained urchin preferences in 9 of 15 (60%) North Carolina-Bahamas contrasts. Water-soluble extracts, protein content, and percent ash-free dry mass generally did not account for feeding patterns seen in the original assays. Our results support the general hypothesis that tropical seaweeds have stronger chemical defenses than temperate seaweeds. For some genera, however, smaller-scale local variability in herbivore resistance within a species can be as striking as overall latitudinal differences.