Potential consequences of grassland bioenergy crop production on bird communities in Wisconsin
Agroenergy, bioenergy crops, biomass, grassland birds, landscape scenarios, land-use change, species of greatest conservation need, Switchgrass, warm-season grasses
Increased demand and government mandates for bioenergy crops in the United States could require a large allocation of agricultural land to bioenergy feedstock production and substantially alter current landscape patterns. Corn is currently the dominant bioenergy crop, but perennial grasslands could produce renewable bioenergy resources and enhance biodiversity. The ecological consequences of increased grassland bioenergy crop production on working lands remain unclear. Grassland bird populations, which have substantially declined in recent decades, may particularly benefit from perennial grasslands grown for bioenergy. We conducted a study to assess how breeding grassland birds could be affected by an increase in perennial grassland bioenergy production in southern Wisconsin.
The study included two phases. In the first phase, we conducted a field study of bird communities, vegetation characteristics, and biomass yields in potential warm-season grassland bioenergy crop fields. In the second phase, we constructed spatially explicit alternative landscape scenarios of bioenergy crop production in southern Wisconsin and assessed implications for grassland bird communities. Among the questions of interest were: How do bird communities differ among cornfields and potential bioenergy grasslands? How do vegetation and landscape context influence bird communities in potential bioenergy grasslands? What are the tradeoffs between producing biomass and providing bird habitat? How do bioenergy crop type, percent conversion to bioenergy crops, and the spatial arrangement of bioenergy crops influence bird communities?
We focused on three potential grassland bioenergy field types along a gradient of plant diversity: grass monocultures, grass-dominated fields, and forb-dominated fields (Fig. 1). We also compared the bird communities in perennial grassland fields to nearby cornfields.
Cornfields had few birds compared to perennial grassland fields. Ten bird Species of Greatest Conservation Need (SGCN) were observed in grassland fields (Fig. 2). Bird species richness and total bird density increased with forb cover and were greater in forb-dominated fields than grass monocultures (Fig. 3). SGCN density declined with increasing vertical vegetation density, indicating that tall, dense grassland fields managed for maximum biomass yield would be of lesser value to imperiled grassland bird species (Fig. 4). The proportion of grassland habitat within 1 km of study sites was positively associated with bird species richness and the density of total birds and SGCNs, suggesting that grassland bioenergy fields may be more beneficial for grassland birds if they are established near other grassland parcels.
We developed 36 spatially explicit landscape scenarios of increased bioenergy crop production in an 80-km radius landscape centered on a potential biomass-processing energy facility in Madison, WI. Our scenarios included conversion of existing annual row crops to perennial bioenergy grasslands (Fig. 5) and conversion of existing grasslands to annual bioenergy row crops, and explored combinations of bioenergy crop types (the three potential grassland crops and one annual row crop [corn]), three land conversion percentages to bioenergy crops (10%, 20%, or 30% of row crops or grasslands), and three spatial configurations of biomass crop fields (random, clustered near similar field types, or closest to the processing plant). For each scenario we predicted the impact on three bird community metrics: bird species richness, total bird density, and the density of Species of Greatest Conservation Need (SGCN).
Bird community metrics in the study landscape consistently increased with conversion of row crops to bioenergy grasslands (Fig. 6) and consistently decreased with conversion of grasslands to bioenergy row crops. Spatial arrangement of bioenergy fields had strong effects on the bird community. Clustering bioenergy crops closer to similar field types had the most positive effects on bird community metrics.
Bioenergy grasslands promote agricultural multifunctionality and conservation of biodiversity in working landscapes. Perennial grassland fields could produce bioenergy feedstocks while providing bird habitat. Expansion of bioenergy grasslands onto marginal agricultural lands will likely benefit grassland bird populations, and bioenergy landscapes could be designed to increase biodiversity while meeting targets for biomass production. Incorporating ecological concerns into bioenergy policy will be an important step towards ensuring the sustainability of bioenergy development in agricultural landscapes.
Funding was provided from the Wisconsin Department of Natural Resources Federal Wildlife Restoration Grant W-160-P.
Blank, P.J., D.W. Sample, C.L. Williams, and M.G. Tuner. 2014. Bird communities and biomass yields in potential bioenergy grasslands. PLoS ONE 9(10): e109989. doi:10.1371/journal.pone.0109989
Blank, P. J., C. L. Williams, D. W. Sample, T. D. Meehan, and M. G. Turner. In press. Alternative scenarios of bioenergy crop production in an agricultural landscape and implications for bird communities. Ecological Applications.