Farmland Heterogeneity Benefits Birds in American Mid-west Watersheds

It is now well known that biodiversity in agricultural landscapes can be increased by converting production lands (i.e., farmland) into more natural habitat. However, it remains relatively unknown to what extent biodiversity can also be enhanced by changing the composition and configuration of farmland per se. We examined relationships between farmland structure and avian biodiversity in six watersheds representing a gradient in rowcrop intensity in the American Mid-west. We used canonical correspondence analysis (CCA) to model relationships at the site (0.8 ha), field (21 ha), section (165 ha) and watershed (4000-18,000 ha) extents to explore these questions: (1) At what spatial extent do birds respond most strongly to farmland structure? (2) Which farmland variables are most important? (3) Does avian biodiversity increase with increasing farmland heterogeneity? Most variation in bird assemblages was explained at the field versus the site or section extents. Of the top 20 field-extent variables in CCA, most important were: % woodland, an index of compositional heterogeneity (the modified Simpson's evenness index, MSIEI), % rowcrop, % core pasture (i.e., away from an edge), and edge density. Eighteen species were associated with woodland; their richness increased with woodland from 0% to 10%. Nine species were associated with rowcrop; their richness decreased two-fold with a change in rowcrop from 80% to 3%. Twelve species were associated with farmland heterogeneity; their richness increased four-fold with more field cover heterogeneity (MSIEI) and decreased three-fold with increasing mean field size from 1.2 to 7 ha. Field-extent alpha and beta species richness were significantly higher than expected in the null model in the two most heterogeneous watersheds and lower in the two most intensely farmed watersheds. Differences among watersheds were significant and evident in the CCA ordination plot. Our results show empirically that policies to influence farmland heterogeneity could provide a conservation benefit to avian biodiversity.