Birds and butterflies respond to soil-induced habitat heterogeneity in experimental plantings of tallgrass prairie species managed as agroenergy crops in Iowa, USA

Abstract

The maintenance of habitat heterogeneity in agricultural landscapes has been promoted as a key strategy to conserve biodiversity. Animal response to grassland heterogeneity resulting from spatiotemporal variation in disturbance is well documented; however, the degree to which edaphic variation generates heterogeneity detectable by grassland wildlife has proven more difficult to study in natural settings. We conducted a field experiment to study how soils directly affect vegetation structure and composition and indirectly affect bird and butterfly assemblages using plantings of tallgrass prairie species managed as agroenergy crops in Iowa, USA. The experimental design included four vegetation treatments of varying species richness replicated on three soil types. Habitat characteristics varied widely among soils. Crops on sandy loam, the driest, most acidic soil with the lowest nutrient content, developed shorter, less dense vegetation with sparse litter accumulation and more bare ground compared to crops on loam and clay loam. Birds and butterflies responded similarly to soil-induced variation in habitat characteristics. Their abundance and species richness were similar on all soils, but their assemblage compositions varied among soils in certain vegetation treatments. In low-diversity grass crops, bird assemblages using sandy loam were dominated by species preferring open ground and sparse vegetation for foraging and nesting, whereas assemblages using loam and clay loam were dominated by birds preferring tall, dense vegetation with abundant litter. In high-diversity prairie crops, the species composition of forbs in bloom varied among soils and strongly influenced butterfly assemblages. Synthesis and applications. Prairie agroenergy crops established with identical management practices developed variable habitat characteristics due to natural edaphic variation, and this heterogeneity influenced the spatial distribution of bird and butterfly assemblages due to differential habitat use among species. This finding suggests that if unfertilized prairie crops were grown for agroenergy by land managers large-scale, soil-induced habitat heterogeneity would promote wildlife diversity within and among fields, further increasing the habitat value of these crops compared to the fertilized, annual monocultures that currently dominate the agricultural landscape. Our study also highlights the need for managers to consider soil properties when selecting sites to restore grassland habitat for species of conservation concern. Prairie agroenergy crops established with identical management practices developed variable habitat characteristics due to natural edaphic variation, and this heterogeneity influenced the spatial distribution of bird and butterfly assemblages due to differential habitat use among species. This finding suggests that if unfertilized prairie crops were grown for agroenergy by land managers large-scale, soil-induced habitat heterogeneity would promote wildlife diversity within and among fields, further increasing the habitat value of these crops compared to the fertilized, annual monocultures that currently dominate the agricultural landscape. Our study also highlights the need for managers to consider soil properties when selecting sites to restore grassland habitat for species of conservation concern.