Processing of transgenic crop residues in stream ecosystems.
Research in agricultural ecosystems is uncovering how the management of crop fields leads to the delivery of transgenic crop residues to adjacent waterways. Aquatic consumers encountering this material may be reduced in abundance and/or limit their feeding activity, subsequently altering organic matter breakdown rate, which is a key ecosystem process in streams. We investigated the effect of the transgenic nature of senesced corn (Zea mays L.), tissue on breakdown rates, invertebrate abundance and invertebrate community composition in nine streams draining agricultural fields over 2 years (2004-2006). We studied corn tissue modified to express protein toxins from the bacterium Bacillus thuringiensis (Bt) from four hybrid families, each with its single, stacked and non-Bt near-isoline. In 2004, we identified two instances whereby Bt leaf litter degraded slower (67-68%) than corresponding near isolines. At one site this was associated with significantly fewer individuals of Pycnopsyche sp., a leaf-chewing caddisfly. In 2005-2006, no differences in breakdown were found between Bt and non-Bt near isolines. Multivariate analysis of invertebrate communities found no difference associated with Bt treatment. Principle components analysis identified important abiotic factors as explanatory variables influencing breakdown, but no interaction was found between these and Bt treatment. Breakdown was strongly related to total invertebrate abundance occurring on experimental litter bags, but this did not interact with Bt treatment across all hybrid × isoline × site combinations. Synthesis and applications. Ecological interactions facilitate breakdown of allochthonous detritus, and understanding the potential disruption of these interactions is important to the management of ecosystem processes. The results from our study suggests that corn tissue breakdown is unlikely to be altered by Bt, but more so by hybrid- and site-specific factors such as nutrients. Management of agricultural streams will need to consider multiple sources of stress at larger scales, such as nutrient loading and temperature, which probably overwhelm the potential for consumer mediation of ecosystem processes in these ecosystems.