Aquatic litter diversity: Directional differences in diversity effects

How variation in leaf quality, in the form of leaf diversity, influences litter decay in stream ecosystems is not well known. We present results from field studies that show leaf diversity results in non-additive litter decay, but with directional differences due to abiotic and biotic differences among sites. Whether diversity resulted in accelerated or decelerated decomposition depended on site properties, litter quality and shredder feeding. Decay of speciose litter was estimated using mesh bags in 3 Arizona and 1 Maryland stream (USA). Replicate bags from each stream were collected over 80 d. In Arizona, 5 single-species, 3 three-species, and a single 5-species treatment were used. In Maryland, 6 single-species treatments were used, and 4 mixtures of 2-5-species combinations were employed. Mass of litter remaining was regressed on time to estimate treatment decay. Speciose litter decay either did not change or increased in the Arizona streams relative to estimates of single-species breakdown, while mixtures in the Maryland stream decayed slower than predicted. Diverse leaf litter decays differently when compared with expected values estimated from individual species, but the direction of this difference varies. This is potentially explained by site effects, species composition, and shredder feeding. Multiple further hypotheses are being explored, in addition to work in other forest types and with various shredder taxa.