Quantitative assessment of animal-induced leaf damage: a test with three brackish crustaceans feeding on leaf detritus

A computerized image analysis technique was implemented to quantify small-scale leaf damage due to different feeding strategies of macrobenthic invertebrates. Additionally, a foraging strategy index (FSI) was developed to provide a quantitative estimation of inter-specific changes in feeding strategies. Laboratory feeding trials were run with the three brackish crustaceans Idotea baltica and Lekanesphaera hookeri (Isopoda) and Gammarus aequicauda (Amphipoda); the effect of their feeding activities were tested on decaying leaves of Cymodocea nodosa and Phragmites australis. Opposite foraging strategies characterised the isopods: Idotea baltica shredded the leaf blade heavily, whereas Lekanesphaera hookeri scraped the leaf surface removing the waxy cuticle and associated heterotrophic microflora. Qualitative observations corresponded to consistent variations in FSI values, with I. Baltica showing the lowest and S. hookeri the highest FSI values, respectively. Leaves consumed by Gammarus aequicauda, on the other hand, showed both shredding and scraping damages, with FSI values intermediate between those observed for I. Baltica and L. hookeri, respectively. The aforementioned differences in trophic strategy were independent from the trophic resource exploited inter-specific. The methodology and the index herein presented were effective in providing a quantitative assessment of species-specific differences in the feeding strategy of three ubiquitous components of Mediterranean macrobenthic communities, for which only qualitative, phenomenological descriptions of feeding effects on trophic resources have been to date provided