Jared Walker Smith - 2005 Environmental Studies Senior Seminar Research Project
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One of the biggest threats to global biodiversity is the intensification and expansion of modern agriculture to provide for the needs of an ever growing and shifting human population (Hole et al., 2005, Green et al., 2005). Links have already been made between an increase in farming and the extinction of bird species, as well as the decrease in diversity of other vertebrate groups (Green et al., 2005). Generally, very few small mammals are adapted to the disturbance regimes of agriculture, keeping direct habitation of farm fields low, and increasing the edge effect on the surrounding natural habitat (Getz and Brighty, 1986; Stallman and Best, 1996). The type of agriculture, however, greatly influences the species composition found within and around the farm fields. The goal in this section is to discuss two general categories of agriculture, conventional and sustainable, and two specific agricultural methods, till and low- or no-till, and provide information regarding the respectful impacts on small mammal diversity and health.
It is hard to put a single definition to conventional farming, as the term is used to describe a wide range of agricultural practices. In general it is assumed to be any type of agriculture that requires high external energy inputs to achieve high yields, and generally relies upon technological innovations, uniform high-yield crops, and high labor efficiencies (Gold, 1999). Many view conventional agriculture less as a defined practice and more as a philosophical idea based on industrial agriculture (Stauber in Bird et al., 1995). The assumptions behind this philosophy include that "a) nature is a competitor to be overcome; b) progress requires unending evolution of larger farms and depopulation of farm communities; c) progress is measured primarily by increased material consumption; d) efficiency is measured by looking at the bottom line; and e) science is an unbiased enterprise driven by natural forces to produce social good" (Stauber in Bird et al., 1995).
How does conventional, high-intensity agriculture affect small mammal communities? While the definition of conventional agriculture is unclear, the answer to this question is not. Conventional agriculture typically has strong impacts on the diversity, structure, and roles of small mammal populations. Overall, conventionally managed agricultural fields have lower a lower biodiversity of small mammals than sustainably managed fields (Stallman and Best, 1996). Also, a lower abundance of small mammals is often prevalent; a study of bats showed that abundance decreased with the increase in agricultural intensity (Wickramasinghe, 2003). This is often due to the high use of pesticides and herbicides, which can eliminate essential habitat (ground cover) and food sources (green vegetation or insects). Dietary shifts to lower quality food, dispersal to new areas, and lowered reproductive success have also been noted in certain small mammals (Warner et al., 1989; Freemark and Boutin, 1995).
Conventional agriculture can also aid more disturbance-adapted species, such as deer mice and house mice. These species can often be more beneficial as insect pest and weed seed predators than detrimental as crop pests (Getz and Brighty, 1986). Furthermore, known agricultural pest species such as meadow voles and prairie voles are not well adapted to living in conventional fields. Their presence, if any, will be at the uncultivated edges of the fields, leading to virtually no detrimental impact on agricultural yields (Getz and Brighty, 1986).
While the tillage system is not necessarily exclusive to this type of agriculture, tilled fields are more often found in conventional agriculture than sustainable agriculture, and impose many of the same effects on small mammal populations as conventional agriculture does. Tillage is when the soil in a field is ploughed in order to reduce weed species and aid in planting. Tilled fields typically have lower species diversity, but with higher abundances of disturbance adapted species, such as the deer mouse, than do no-till fields. Similar to conventional agriculture, these species can often be more beneficial as pest and weed species controllers than detrimental to crops. Tilling also helps keep down pest species, such as prairie voles and meadow voles, which rely upon thick ground cover. These two species, if in large enough numbers, can cause serious economic problems (Getz and Brighty, 1986).
In general sustainable agriculture is diversified, ecologically sound, economically viable, socially just, culturally appropriate, and relies on the use of local renewable resources while minimizing external inputs (Gold, 1999). It "preserves biodiversity, maintains soil fertility and water purity, conserves and improves the chemical, physical and biological qualities of the soil, recycles natural resources and conserves energy" (NGO Sustainable Agriculture Treaty). Further definitions are available here. Similar to conventional agriculture, some feel there is a deeper philosophy behind sustainable agriculture, with the underpinnings including "a) integrating agriculture and nature b) more localized and regionalized food systems to provide greater community autonomy and accommodate environmental constraints c) less centralized control of agriculture and farm resources d) greater self-sufficiency of farm operators e) independence of farm operators from the nonfarm sectors (chemical input and marketing) f) greater cooperation among neighbors, and thus stronger farm communities" (Stauber in Bird et al., 1995).
Sustainably managed fields tend to promote a greater diversity and abundance of small mammals (Maisonneuve and Rioux, 2001; Wickramasinghe, 2003; Hole et al., 2005). This is often due to a decreased use of herbicides and pesticides, providing an increase in undergrowth and cover, as well as prey species, and a minimization of soil disturbances. Such practices such as strip cropping have been shown to be important in maintaining small mammal populations (Peles et al., 1997). While sustainable agriculture can provide habitat for more species, usually the deer mouse is still the most prevalent species in the field (Sterner et al., 2003). This presence of deer mice is not detrimental, as they are more likely to feed on pest insects and weed species than crop seed or vegetation. Still, sustainable practices may also help increase vole populations that are known to be agricultural pests (Getz and Brighty, 1986; Johnson, 1987; Jacob, 2003).
Low- or no-till agriculture is typical of sustainable agriculture, but can also be incorporated into conventional agricultural practices. Reduced till leaves ground cover and harvested waste on the fields, helping to reduce soil erosion and increase ground cover. Due to the reduced disturbance and increased cover, reduced tillage systems have been shown to increase the biodiversity and abundance of small mammal species in agricultural fields (Warburton and Klimstra, 1984; Sterner et al., 2003). Also, reduced tillage tends to leave waste behind, such as waste corn, which helps sustain active small mammal populations throughout the winter (Warner et al., 1989). Since waste corn left on fields is not desirable come planting time, the over-wintering animals become beneficial to agriculture.
While reduced till systems have the potential to increase the abundance of pest species such as voles and western harvest mice, a study in southwestern Iowa found that rodent damage to corn seedlings was less frequent than insect and weather damage in no-till agriculture (Clark and Young, 1986). The beneficial services the rodents provided in no-till agriculture were shown to outweigh the potential disadvantages of crop predation.