Measuring the wildlife benefits
of buffer strips

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If you travel much in rural Iowa, you've seen those long, green strips that separate crop fields from rivers and streams.

Aided by federal farm programs, Iowa leads the nation in planting these non-cropped strips-also known as riparian filter strips or buffer strips. Their primary goal is buffering our waterways from pollutants and sediment carried by field runoff. Filter strips also provide wildlife habitat-but how good is that habitat and how could it be better? New research by Iowa State University's John C. Henningsen (graduate student) and professor Louis B. Best explores those questions. The National Fish and Wildlife Foundation (NFWF) and the Leopold Center for Sustainable Agriculture funded their research, and NFWF funds were administered through the Iowa Natural Heritage Foundation.

Buffer strips protect water quality while offering some wildlife benefits.

Study Method and Results The researchers monitored more than 600 bird nests in 33 Iowa filter strips. They recorded the abundance of individual bird species and successful nests (nests where at least one offspring reached fledging stage). They evaluated three variables: planting mixture (cool-season vs. warm-season grasses), adjacent edge type (wooded vs. non-wooded) and strip width (25 to 120 feet). The overall results were discouraging: "Our nest success estimates were generally low (27% for all species combined), and nest success did not vary much within the range of variables we studied." Predators like raccoons, skunks, snakes and rodents caused 85% of the nest failures-a predation level often linked to narrow habitats. Management Implications The researchers conclude that, though filter strips offer only marginal habitat, their impact on wildlife is still significant in intensively farmed areas. And, though these narrow strips favor "generalists" (common species that adapt to many habitats), they can aid less common "specialists." The researchers recommend continued establishment of filter strips-along with management advice for improved wildlife benefits. Ecosystem matters Landowners seeking more ring-necked pheasants or a greater variety of birds should establish filter strips along wooded areas-but these birds will tend to be the more common generalists. Landowners who want to help less common grassland birds and songbirds should place filter strips along streams without wooded banks-and/or remove woody encroachments regularly. Diversity matters More diverse plantings yield more diverse bird communities-whether landowners choose cool-season grasses (like brome) or warm-season grasses (like switchgrass). Carefully managed mowing or burning may help maintain that diversity. Size matters Landowners seeking to lower nest predation caused by "edge effect" should develop wider natural areas. Unfortunately, even the widest strips under current regulations have edge effects. If habitat is a significant goal, government programs may need to increase strip width requirements. In the meantime, filter strips are helpful-but no substitute for larger, undisturbed habitat. The main goal of buffer strips is protecting our water quality, but-with better planning and management-they can help sustain our wildlife as well.

Released 10/24/03

For more information, e-mail Cathy Engstrom, Director of Communications, or call (515) 288-1846.