Effects of farmland heterogeneity on biodiversity are similar to—or even larger than—the effects of farming practices

Pressure to increase food production to meet the demands of a growing human population can make conservation-motivated recommendations to limit agricultural expansion impractical. Therefore, we need to identify conservation actions that can support biodiversity without taking land out of production. Previous studies suggest this can be accomplished by increasing “farmland heterogeneity”—i.e. heterogeneity of the cropped portions of agricultural landscapes—by, for example, decreasing field sizes. However, it is not yet clear whether policies/guidelines that promote farmland heterogeneity will be as effective as those targeting farming practices. Here, we estimated the relative effects of six practices—annual/perennial crop, fertilizer use, herbicide use, insecticide use, tile drainage, and tillage—versus two aspects of farmland heterogeneity—field size and crop diversity—on the diversity of herbaceous plants, woody plants, butterflies, syrphid flies, bees, carabid beetles, spiders, and birds in rural eastern Ontario, Canada. The strength of effect of farming practices and farmland heterogeneity varied among taxonomic groups. Nevertheless, we found important effects of both farming practices and farmland heterogeneity on the combined (multi) diversity across these groups. In particular, we found greater multidiversity in untilled, perennial crop fields than tilled, annual crop fields, and greater multidiversity in agricultural landscapes with smaller crop fields and less diverse crops. The directions of effect of these variables were generally consistent across individual taxonomic groups. For example, richness was lower in landscapes with larger fields and more diverse crops than in landscapes with smaller fields and less diverse crops for all taxa except spiders. The negative effect of crop diversity on multidiversity and the richness of most of the studied taxa indicates that this aspect of farmland heterogeneity does not necessarily benefit wildlife species. Nevertheless, a compelling implication of this study is that it suggests that policies/guidelines aimed at reducing crop field sizes would be at least as effective for conservation of biodiversity within working agricultural landscapes as those designed to promote a wildlife-friendly farming practice.