Understanding of colonization and breakdown of leaves by invertebrates in a tropical stream is enhanced by using biomass as well as count data

We hypothesized that (i) the importance of shredders for leaf breakdown is more evident in terms of their biomass than their abundance, due to the large bodies and high-feeding efficiencies of some typical shredders; (ii) non-shredder invertebrates select more refractory leaves because these are a more stable substrate for colonization or to obtain other forms of food. To test these hypotheses, we performed a decomposition experiment with leaves of contrasting chemical composition in a tropical stream, and determined the changes in the ash-free dry mass (AFDM) of the litter, and the invertebrate abundance and biomass during a 44-day period. The biomass of shredders showed a positive relationship with AFDM remaining, whereas their abundance was unrelated to AFDM. While shredder abundance represented only 4-12% of total invertebrate abundance, shredder biomass constituted 19-36% of total invertebrate biomass. We conclude that (i) shredder biomass expresses better than abundance the role of this guild in the decomposition of leaf detritus, demonstrating that they are important for the functioning of tropical streams; (ii) incubation time rather than stability of leaf litter as a substrate influences colonization by non-shredder invertebrates.