Birds on organic and conventional farms in Ontario: partitioning effects of habitat and practices on species composition and abundance

Abstract

Population declines of farmland birds over recent decades in Europe, Canada and the USA have been attributed to more intensive agricultural management. We counted birds during the 1990 breeding season on 72 field sites in southern Ontario, Canada, paired between 10 organic and 10 conventional farms for local habitat to enhance our ability to detect effects of agricultural practices. Of 68 species recorded, 58 were on organic sites, 59 on conventional. Species richness and total abundance were significantly greater on organic than conventional sites based on log-linear regression. Of 43 species analyzed with log-linear regression, eight (18.6%) were significantly ( P<0.05) more abundant on organic than conventional sites and four (9.3%) approached significance (0.05< P<0.10). Eight of these 12 species had negative population trends for 1967–1998 Breeding Bird Surveys (BBS) in this region. Two of the 43 species analyzed (4.7%) were significantly more abundant on conventional than organic sites and three (7.0%) approached significance. Two of these five species had negative BBS population trends. A canonical correspondence analysis (CCA) of 13 practices and 13 habitat variables explained 44% of total variation (TV) accounted for in a detrended correspondence analysis of bird species composition and abundance. Practices contributed 23.7% of TV, habitat 26%; habitat and practices shared 5.7% with each other and 12% with farm ownership (i.e. clustering of field sites within farms). CCA ordinations indicated considerable mixing of organic and conventional sites across a gradient from sites with many birds species associated with greater habitat heterogeneity and more pasture, winter grain, farmstead and other non-crop habitats (hedgerow, woodland) to sites with few bird species associated with larger fields, more rowcrop and spring grain, more passes and tilling, and use of herbicides and chemical fertilizers. Our results re-emphasize the importance of non-crop habitats, more permanent crop cover, and less intensive management practices to the conservation of avian biodiversity on farmland.