Responses of stream macroinvertebrates and ecosystem function to conventional, integrated and organic farming.
Organic farming practices can benefit a wide range of terrestrial biota in comparison to conventional farming but we do not know whether this benefit extends to streams flowing through farmed landscapes. We measured the impacts of organic, integrated management and conventional farming on the taxonomic and trait composition of macroinvertebrate communities and on stream ecosystem functioning (algal accrual on tiles and leaf breakdown). Our study design included five replicates of each farming practice, arranged in blocks, in agricultural land dedicated to the farming of sheep and beef cattle on pasture in southern New Zealand. In each farm stream, we studied both upstream and downstream reaches within the farm's boundaries. The different farming practices were reflected in contrasting stream physicochemistry (total dissolved nitrogen, fine sediment on the bed and glyphosate concentrations in the bed sediment) and in differences in both the taxonomic organization and trait representation of stream invertebrate communities. Conventional farm streams showed the strongest negative responses, whilst the condition of organic and integrated farm streams was similar. Invertebrate trait measures proved as effective as taxonomic measures in their response to agricultural intensity, whereas ecosystem function measures were least sensitive. There were no overall physicochemical differences between upstream and downstream sites within the farms and few notable longitudinal patterns in ecological response variables. Synthesis and applications. Conventional farming had the strongest adverse consequences for stream condition in our study. In contrast, an integrated management system (aimed at reducing pesticide use, increasing beneficial pest predators and encouraging environmentally responsible soil, water and energy management) proved at least as effective as organic farming and, together with organic farming, can be considered better practice.