Patterns in leaf herbivory on seagrasses

Abstract

We assess leaf herbivory on several populations of four temperate and five tropical seagrass species and examine, along with a compilation of published reports, the extent of herbivory variability and how it affects its general magnitude to help solve the apparent controversy about the importance of herbivory on seagrasses brought out by recent reports. We further test whether herbivory variability is related to differences in leaf specific growth rate (SGR) as a descriptor of leaf nutritional quality for herbivores. The extent of herbivory varied broadly, both within and among species, ranging from negligible values up to 50% of leaf production removed within some species. This variability, along with that shown by other published reports, claim that the classical statement that herbivory represents minor losses of seagrass production, being considered negligible in most cases, can be misleading and lead to the neglect of important seagrass–herbivore interactions. Differences among species in the percentage of leaf production removed were associated with differences in SGR, pointing to herbivore selective feeding upon faster-growing species resulting from their higher nutritional quality. This selection seems to be independent of leaf nutrient concentrations suggesting that, in agreement with past reports, nutrient levels are a poor descriptor of seagrass nutritional quality as most nutrients can be bound to indigestible fibre. No relationship between herbivory intensity and SGR was found among populations of a single species. On the other hand differences among species in the areal flux of production transferred to herbivores seemed related to differences in the level of production attained. These results point out that whereas SGR should be a descriptor of the variability among species in the extent of herbivore pressure (i.e. percentage of production removed), the level of production should be indicative of differences among species in their capacity to support herbivore production (i.e. flux of production channelled).