Technical Reviews
Restoration ecology and conservation biology in agriculture (PART II)
[Note: Part 1, focusing on general theory, appeared in Sustainable Agriculture, Vol. 13, No. 3, Fall 2001.]
Farming Affects Birds
Fluetsch and Sparling (1994) conducted a replicated study of insecticide use, songbird diversity, and nesting success of Mourning dove (Zenaida macroura) and American robin (Turdus migratorius) in organic vs. conventional apple orchards of Pennsylvania. Their data reflect heavy use of organophosphorus insecticides and document insecticide deposition in bird nests. Further, daily survival rate of immature birds and bird diversity were significantly greater in organic orchards. Intermediate forms of orchard management that are not organic have also eliminated organophosphorus insecticides (Santer 1995), and might be expected to support bird life better than conventional orchard management.
In the Prairie Pothole region of southeastern North Dakota, Lokemoen and Beiser (1997) evaluated bird densities, bird nest densities, and daily survival rates on conventional, minimum-tillage, and organic farms (sunflower and wheat production; organic farms used yellow sweetclover as a green manure or fallow). Mean number of nesting species and mean nest densities were higher on minimum-tillage and organic than on conventional farms. Significantly greater densities of birds were observed on reduced-tillage than on conventional farms. Daily survival rate for shore birds was greater in minimum-tillage than in organic fields. No other differences among farm type were observed. For organic fields, there was a negative correlation between tillage treatments (numbers of diskings) and bird nest densities. In light of the low nesting success rate, the authors suggested use of late-maturing legumes to allow delayed plowing down of organic fallow fields until late June, which would in turn enable bird nesting. The authors also recommended further reduction of tillage.
In England and Wales, Chamberlain et al. (1999) compared replicated pairs of organic and nearby conventional farms. The same observer assessed each pair of farms, to avoid systematic bias. Farms were not matched by crop rotation, because rotation is an intrinsic difference between the organic and conventional approaches, as are pesticide and fertilizer regimes and management of non-crop areas (e.g., hedgerows). In the spring and summer (the breeding season), the number of territorially active birds were estimated. During fall and winter, numbers of birds in boundaries (edges) of fields were assessed. On-farm habitat was characterized, including area, crop types, and boundary conditions. The study included fall and winter data for 1992/93-1993/94 (two years) as well as breeding-season data for 1992-1994 (three years). There was no overall pattern in the Shannon-Weiner species diversity index. Pair densities for 17 species over three breeding seasons showed 43 of 51 differences in favor of the organic farms, and data from 1992 (15 of 17 comparisons) and 1994 (16 of 17) were significant individually, whereas data from 1993 (12 of 17) were not, by themselves, significant. During fall and winter, bird density was greater on organic than conventional in 56 of 64 cases. Nine of these individual cases showed statistical significance, and sign tests showed significantly greater overall densities for organic in three of the four years of sampling. Outside the breeding season, bird densities in fields were greater for organic in 50 of 68 cases, but only in the winter of 1992 was a difference detected for a given time period. The researchers infer that territorial exclusion may lead to smaller differences between organic and conventional during the breeding season, but that in fall and winter territorial behavior is relaxed for most species, so densities will better reflect resource availability.
Shutler et al. (2000) evaluated bird diversity and density over an approximately one-year period on Saskatchewan, Canada wild sites and on organic, reduced-tillage, and conventional fields. Geographic area was used as a blocking variable to provide local clusters of fields representing all four treatments. A fixed-radius, point-count method was used, and several ecological covariates were measured within the quartersections containing the fields where bird density was assessed. Values for these variables were based on whether wetlands or woodlands were located within a 200-meter radius of the center of each sampling area, and on the area of wetlands and woodlands and an estimate of habitat complexity within the quartersection containing each sampling site. Several species of birds showed significantly higher densities on organic farms or the adjoining wetlands. For example, black-billed magpie, American robin, Le Conte’s sparrow, and Vesper sparrow, all had significantly greater densities on organic than on one or more of the other three treatments. Relative abundance of birds on organic farms was significantly greater than on conventional or reduced tillage farms, and was not significantly less than for wild sites. Wetlands and their margins adjoining organic fields had significantly greater relative abundance of birds than did those adjoining reduced tillage or conventional fields.
In Ontario, Canada, Freemark and Kirk (2001) studied bird diversity on 10 pairs of organic and conventional farms. Mean bird diversity and species richness were significantly higher and eight species were more abundant on organic farms. Two species were significantly more abundant on conventional farms. Statistical trends (0.05<P<0.10) suggested four additional species were more abundant on organic and three on conventional farms. Further statistical analyses revealed the importance of non-crop habitats, permanent crop cover, and the negative effect of intensive, conventional management practices on bird species diversity.
In a review article, Vickery et al. (2001) highlighted probable reasons for decline of bird diversity and density on lowland neutral grasslands in England, apparently correlated with intensification of forage-production practices. The following points were emphasized:
- Transition from cattle to sheep has led to simplification of the sward floristic composition.
- Transition from grazing to silage production has led to monocultures of high-statured plants subject to frequent close mowing for harvest.
- Addition of fertilizer nitrogen leads to simplified stand composition, uniform, high stature, and reduced densities of several key arthropod prey of birds.
- Use of the anti-helminthic (de-worming) medicine Avermectin makes animal dung less hospitable to various arthropods that are a food base to birds.
The authors note that low-input grazing operations have long been recognized as a major reservoir for native biodiversity, but that most on-farm studies of bird ecology have focused on arable (tillage-based) agriculture rather than extensive grasslands. The authors advocate the conservation and restoration of extensive, low-input grazing operations, the protection of field edges from grazing, and the exploration of alternative anti-helminthic medicines to the Avermectin now used to treat stock animals.
References Cited
Chamberlain, D.E., J.D. Wilson, R.J. Fuller. 1999. A comparison of bird populations on organic and conventional farm systems in southern Britain. Biological Conservation 88:307-320.
Fluetsch, K.M. and D.W. Sparling. 1994. Avian nesting success and diversity in conventionally and organically managed apple orchards. Environmental Toxicology and Chemistry 13:1651-1659.
Freemark, K.E. and D.A. Kirk. 2001. Birds on organic and conventional farms in Ontario: partitioning effects of habitat and practices on species composition and abundance. Biological Conservation 101:337-350.
Lokemoen, J.T. and J.A. Beiser. 1997. Bird use and nesting in conventional, minimum-tillage, and organic cropland. Journal of Wildlife Management 61:644-655.
Santer, L. (Editor). 1995. BIOS for Almonds: A Practical Guide to Biologically Integrated Orchard Systems Management. Community Alliance with Family Farmers Foundation, Davis, California / Almond Board of California, Modesto, Calif.
Shutler, D., A. Mullie and R.G. Clark. 2000. Bird communities of prairie uplands and wetlands in relation to farming practices in Saskatchewan. Conservation Biology 14: 1441-1451.
Usher, M.B. and S.W.J. Keiller. 1998. The Macrolepidoptera of farm woodlands: determinants of diversity and community structure Biodiversity and Conservation 7(6): 725-748.
Vickery, J.A., J.R. Tallowin, R.E. Feber, E.J. Asteraki, P.W. Atkinson, R.J. Fuller and V.K. Brown. 2001. The management of lowland neutral grasslands in Britain: effects of agricultural practices on birds and their food resources. Journal of Applied Ecology 38:647-664.
Other suggested reading:
Elphick, C.S. 2000. Functional equivalency between rice fields and seminatural wetland habitats. Conservation Biology 14:181-191.
Reddersen, J. 1997. The arthropod fauna of organic versus conventional cereal fields in Denmark. Biological Agriculture and Horticulture 15: 61-71.
Rodenhouse, N.L., L.B. Best, R.J. O’Connor, and E.K. Bollinger. 1995. Effects of agricultural practices and farmland structures. Pp. 269-293 in: T.E. Martin and D. M. Finch (eds.). Ecology and management of neotropical birds. Oxford University Press, New York.
Sotherton, N.W. 1998. Land use changes and the decline of farmland wildlife: An appraisal of the set-aside approach. Biological Conservation 83:259-268.
Van Elsen, T. 2000. Species diversity as a task for organic agriculture in Europe. Agriculture Ecosystems and Environment. 77:101-109.
