© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1434-2944/10/102-0027 Internat. Rev. Hydrobiol. 95 2010 1 27–41 DOI: 10.1002/iroh.200911173 MANUEL A. S. GRAÇA* , 1 and CLAUDIA CRESSA 2 1 IMAR and Department of Life Sciences, University of Coimbra, Portugal; e-mail: mgraca@ci.uc.pt 2 Instituto de Biología Experimental, Facultad de Ciencias, Universidad Central de Venezuela, Caracas, Venezuela; e-mail: claudia.cressa@ciens.ucv.ve Research Paper Leaf Quality of Some Tropical and Temperate Tree Species as Food Resource for Stream Shredders key words: tropical streams, litter decomposition, invertebrates; microbial conditioning Abstract We tested the hypotheses that (1) plant defenses against consumers increase in the tropics, and that these differences in quality are perceived by detritivores; and (2) microbial conditioning of leaf litter is important for the feeding ecology of shredders from both geographical regions. We compared quality parameters of 8 tree species from Portugal and 8 from Venezuela. The tropical leaves were tougher, but did not differ from temperate leaves in terms of N, C : N, and polyphenols. In multiple-choice experiments, shredders from Portugal (Sericostoma vittatum and Chaetopteryx lusitanica) and from Venezuela (Nectopsyche argentata and Phylloicus priapulus) discriminated among conditioned leaves, preferentially consuming softer leaves. In another set of experiments, all shredders preferentially fed on conditioned rather than unconditioned leaves, grew faster when fed conditioned than unconditioned leaves and fed more on temperate than tropical leaves. We conclude that leaf litter from the tropics is a low-quality resource compared to leaves in temperate systems, because of differences in toughness, and that tropical shredders benefit from microbial colonization, as previously demonstrated for temperate systems. We suggest that leaf toughness could be one explanation for the reported paucity of shredders in some tropical streams. 1. Introduction The species richness of birds, mammals, insects, plants, and other taxonomic groups tends to increase towards the tropics (BEGON et al., 2006). The higher taxonomic richness and greater biomass in the tropics may be the result of strategies allocating more resources to competition and defense against consumers and parasites. Therefore, it is plausible that biological interactions also change across latitudes. For instance, herbivory in terrestrial systems seems to be more intense in the tropics (COLEY and BARONE, 1996), and it has been postulated that many tropical plants are better protected against consumers than are their temperate counterparts, through a combination of chemical defenses, greater toughness, and lower nutrient content (COLEY and BARONE, 1996; WANTZEN et al., 2002; ARDÓN et al., 2006; WANTZEN et al., 2008). Tannins are among the most common plant-defense com- pounds (LAVELLE et al., 1993; COLEY and AIDE, 1996; AERTS, 1997), and proanthocyanidins * Corresponding author