Rodent-Prey Content in Long-Term Samples of Barn Owl (Tyto Alba) Pellets from the Northwestern United States Reflects Local Agricultural Change
Lyman, R. Lee, The American Midland Naturalist
Rodent prey contained in two temporally distinct collections of Barn Owl (Tyto alba) pellets from the same roost in southeastern Washington state (USA) differ in terms of taxonomic abundances. Deer mice (Peromyscus maniculatus) dominate the fauna in the pellet sample deposited while much of the landscape was productive wheat field, and voles (Microtus spp.) are a distant second. The fauna in the pellet sample deposited after 20% of the surrounding landscape was placed in soil bank and converted to a grass non-producing field is dominated by voles with deer mice a close second. The coincident changes in local vegetation and in the rodent fauna are causally as well as temporally interrelated. Previous local studies have focused on the agricultural economics of coincident shifts in agricultural practices and rodent faunas. Results presented here indicate potential benefits to owl faunas of changes in agricultural practices and suggest that study of curated owl pellet faunas collected decades ago may reveal much about the long-term history of anthropogenic influences on rodent faunas.
Agricultural practices in the United States have evolved remarkably over the past century. Productivity per hectare has, for example, increased tremendously as a result of artificial fertilization and genetic engineering of plant crops. As productivity has increased food surpluses have been generated; simultaneously, soil conservation has become a concern. In the northwestern United States an initial step toward soil conservation was to adopt "no-till" cultivation in the late 1970s where the seed is planted directly through the residue remaining from the previous crop. Alternatively, the residue is burned prior to seeding (Papendick and Miller, 1977; Phillips et al, 1980). Burning was eventually abandoned because of air pollution. In the late 1990s, increasing amounts of previously productive farmland were taken out of production and seeded to grass for a decade or more under contract with the federal government. Such practice eliminates tillage and attendant loss of topsoil from the erosive forces of wind and water, in effect banking top soil for future use, hence the term "soil bank."
Although no-till practices have beneficial effects with respect to soil conservation, reducing surpluses, and raising crop prices, the effects of shifts in land use practices on mammal communities are only now becoming understood. Early studies (e.g., Johnson, 1987) suggested no-till practices had minimal impact on the density of voles, but later studies (e.g., Witmer et al, 2007) found that crop residue remaining in producing fields subject to no-till cultivation provided cover and insulation for many rodent taxa. Increased cover plus the lack of disruption of burrow systems by cultivation resulted in higher population densities of rodent pests and greater loss of new crops to these pests (Witmer et al, 2007). Fields in soil bank and with non-agricultural permanent grass cover may have the highest densities of rodents (Capelli, 2005).
Most previous studies have focused on the influence of tillage practices on agricultural economics and on shifts in the amount of crop destruction by rodent pests (Capelli, 2005; Witmer et al., 2007). Given ever-growing concerns over increased rates of biodiversity loss in particular and conservation biology issues in general, the influence of shifts in tillage practices over time on the taxonomic composition and abundances of rodent taxa has received surprisingly litde study. When it has received attention, it is largely from biologists with access to natural history collections with detailed collection-history data (e.g., Rowe, 2007; Moritz et al, 2008; Parra and Monahan, 2008; Tingley and Beissinger, 2009; Rowe et al., 2010). There is another source of long-term faunal data diat has, as yet, seen litde study with respect to the influence of agricultural land use practices on local faunas: rodent prey faunas in collections of owl pellets that span episodes of change in agricultural practices (Love et al, 2000). …