Can organic and conventional agricultural systems affect wetland macroinvertebrate taxa in rice fields?

Abstract

The use of agrochemicals, water management and intense mechanization is compromising species diversity in rice fields. Organic agriculture has been proposed as a way to reduce the pressure on biodiversity in agricultural landscapes. Therefore, the following hypotheses were tested: (1) Taxonomic richness and abundance of macroinvertebrates are lower in rice fields than in natural ponds; (2) Conventional rice crops decrease taxonomic richness and abundance of macroinvertebrates compared with organic crops; (3) Taxonomic macroinvertebrate composition in organic rice crops is more similar to the composition in natural ponds than conventional fields; and (4) A reduction of taxonomic richness, abundance as well as a modified composition during soil preparation and the initial growth phases of the rice cultivating cycle were expected. To test these hypotheses, four natural ponds, four organic and four conventional rice fields were selected in southern Brazil. In each area, six collections were carried out over the rice cultivation cycle (August 2010 to August 2011). A total of 25,449 individuals from 73 different taxa were sampled. The richness and abundance were higher in ponds than in rice crops in the anterior and posterior off-season and in the soil preparation periods. However, organic and conventional rice fields showed similar taxonomic richness and abundance. Although the taxonomic composition differed only between rice crops and ponds, the presence of many taxa that only occur in organic crops and ponds indicates that organic management favors the establishment of some pond macroinvertebrate taxa in rice agroecosystems, mainly predators. Our results should be seen as an incentive for more sustainable production with less impact on the environment.