Biofuel Crops and Grassland Birds: Can They Coexist?
By Tina Gheen
With the demand for petroleum based products, such as plastics on the rise, more and more grasslands in the US are becoming managed agricultural lands for bioenergy crops. Since biodiversity loss has already been linked to agricultural expansion, this shift has caused great concern. But crop selection can make a difference. As the trend continues, scientists have identified a need to examine the affect of the choice of biofuel crop upon biodiversity conservation.
Corn is the most important biofuel crop in North America. But corn fields typically do not support a very high diversity of animal or insect species. The US Department of Energy has identified switchgrass (Panicum virgatum) as a model energy crop. This native grass of the North American tallgrass prairie supports a variety of arthropods, including insects and spiders, as well as several species of grassland birds. Since diversity in plants promotes diversity among in animals, it also follows that a mixed-grass prairie used for biofuel is also a viable option.
Scientists at the Smithsonian Migratory Bird Center, Michigan State University, and the Nature Conservancy embarked on a study to discover how well these three types of habitats (corn, switchgrass, and mixed-grass prairie) support grassland birds. Over the course of two years, the team studied characteristics such as the abundance, density, and species of birds in each habitat. They also looked at the characteristics and composition of the landscape and studied the diversity and quantity of arthropods in each field.
The team identified 52 species of birds across the three types of fields. The greatest diversity was in the mixed prairie, where the team detected 45 species. The swtichgrass fields fared better than the cornfields, with 35 species detected in switchgrass versus 29 in corn. However, several species on conservation lists such as the grasshopper sparrow, Henslow's sparrow, and northern harrier were found in both the prairie and switchgrass fields. Arthropod counts followed the same pattern with the highest numbers and diversity found in the mixed prairie habitat, then switchgrass, and finally, corn.
Henslow's Sparrow
Bobolink
Northern Harrier
Grasshopper Sparrow
Structurally, the mixed prairie provided more variety among plant height and density than switchgrass or corn, which is important to many birds. Since fields grown for biomass and biofuel production, like corn and switchgrass, are typically of uniform height and density, this poses a problem for grassland birds.
Overall, the scientists found the mixed prairie habitat was better able to support grassland birds. Since diversity in plants enables diversity in animals, this is not unexpected. But the study also showed that while switchgrass fields were not as beneficial as the mixed prairie, they do provide for greater abundance and biodiversity than the typical corn-ethanol field.
As agricultural boundaries continue to expand and open grasslands shrink, crop choice is becoming a critical consideration for farmers, land management specialists, and ecologists. Smart, carefully-weighed decisions about the types of plants we grow for biofuel will greatly impact the future of grassland birds in our environment.
Dr. Bruce Robertson's exclosure experiment in the prairie. The idea is to see whether and to what degree birds control insects in these habitats, and act as potential biocontrol agents.
This article summarizes the information in this publication:
Robertson, B.A., Doran, P.J., Robertson, J.R., Loomis, E.R. & Schemske, D.W. 2011. Perennial biomass feedstocks enhance avian diversity. GCB Bioenergy, 3: 235–246.
Federal mandates to increase biofuel production in North America will require large new tracts of land with potential to negatively impact biodiversity, yet empirical information to guide implementation is limited. Because the temperate grassland biome will be a production hotspot for many candidate feedstocks, production is likely to impact grassland birds, a group of major conservation concern. We employed a multiscaled approach to investigate the relative importance of arthropod food availability, microhabitat structure, patch size and landscape-scale habitat structure and composition as factors shaping avian richness and abundance in fields of one contemporary (corn) and two candidate cellulosic biomass feedstocks (switchgrass and mixed-grass prairie) not currently managed as crops. Bird species richness and species density increased with patch size in prairie and switchgrass, but not in corn, and was lower in landscapes with higher forest cover. Perennial plantings supported greater diversity and biomass of arthropods, an important food for land birds, but neither metric was important in explaining variation in the avian community. Avian richness was higher in perennial plantings with greater forb content and a more diverse vegetation structure. Maximum bird species richness was commonly found in fields of intermediate vegetation density and grassland specialists were more likely to occur in prairies. Our results suggest that, in contrast to corn, perennial biomass feedstocks have potential to provide benefits to grassland bird populations if they are cultivated in large patches within relatively unforested landscapes. Ultimately, genetic improvement of feedstock genets and crop management techniques that attempt to maximize biomass production and simplify crop vegetation structure will be likely to reduce the value of perennial biomass plantings to grassland bird populations.
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