Wildlife Response to Different Kinds of Residential Development

Oaks ‘n Folks – Volume 14, Issue 1 – March, 1999

Residential development is expanding in California, resulting in a larger number of houses and roads in wooded areas. This increased demand for property in rural areas has resulted in land fragmentation and conversion of oak woodlands to housing, roads, and recreational development.

Sonoma County, where there are over 300,000 acres of hardwood forest, is one of the fastest growing counties in California with a population increase of 282% from 1960 to 1995. In addition, over 90% of Sonoma County is privately owned, making integration of wildlife conservation with private land management imperative. Continued subdivision or “parcelization” of large, private ranches to meet the housing needs of a growing population has led to increased habitat fragmentation and land modification.

To examine the effects of this type of habitat fragmentation on biodiversity, 12 sites of similar elevation, slope, and vegetation were identified using remote sensing and a geographic information system in Sonoma County’s oak woodlands. Four sites were selected in (1) relatively undisturbed hardwood rangeland in private parcels greater than 300 acres; (2) ranchettes on 10- to 40-acre lots; and (3) suburban areas with single-family homes on 0.5- to 2.5-acre lots. With the exception of one site located west of Hwy 101 and two sites in the Sonoma Mountains, all sites were located in the foothills of the Mayacmus Mountains and Mt. Hood Range east of highways 101 and 12. The predominant land uses in the surrounding areas were rural residential, vineyards, rangeland, and wildlands. Common tree species at these sites included coast live oak, black oak, blue oak, Oregon oak, madrone, and California bay. The shrub layer was dominated by poison oak and toyon. Exotic and native grasses and forbs characterized the understory.

This study was conducted to determine if (1) hardwood cover measured from thematic mapper (TM) satellite data; (2) tree density; (3) plant species richness and composition; (4) bird species richness, abundance, and composition; and (5) butterfly species richness and composition differed among different land-use types and lot sizes.

Percent hardwood cover detected by TM imagery varied among sites and land-use types; however, no statistical difference was detected among land-use types. This suggests that remotely sensed imagery should not be used as a surrogate measure of disturbances such as housing density, and cannot always provide an accurate estimate of fragmentation in oak woodlands. Tree density decreased with increasing housing density and suburban areas had a marked increase in exotic plants because of residential gardens.

The number of plant, bird, and butterfly species also was similar among land-use types. This illustrates the difficulties of using species richness to detect changes in ecosystem health or effects of disturbance. Often, species adapted to a higher level of disturbance replace species that require undisturbed habitat leading to a change in species composition without an overall change in richness. In addition, habitat that is more frequently disturbed or has a greater diversity of vegetation types within it often will support a larger number of species as a result of increased habitat diversity.

Species composition of bird communities differed among land-use types, illustrating one consequence of subdividing private land. For example, the percent of neotropical migrant birds, species that winter in Central and South America, was significantly higher at undeveloped sites than at ranchettes and small suburban lots. Declining neotropical bird populations have been a concern in North America for the past 15 years. Many explanations for these declines have been proposed, including habitat fragmentation in both breeding and non-breeding habitats. Our data suggest that smaller property sizes and associated disturbances (e.g., increased road density, impact of house cats, and human activity) likely reduce the diversity and abundance of rarer bird species, including some neotropical migrants. Detecting differences in butterfly community composition between our treatments was difficult. The low number of butterfly species detected severely limited our ability to analyze these data and no differences were detected.

The future of California’s oak woodlands depends on maintaining large, continuous parcels of privately owned land. Therefore, reducing habitat fragmentation and employing practices that maintain continuous habitat is important. Economic incentives and county planning initiatives that minimize property subdivision are two ways for Californians to maintain the ecological integrity of their privately owned oak woodland.

This research would have been impossible without the consent and support of landowners who graciously allowed us access onto their property. Their willingness to participate in this study has furthered our understanding of the complex nature of oak woodlands.

prepared and edited by Richard B. Standiford, Justin Vreeland, and Bill Tietje

Adina Merenlender, Natural Resource Specialist
Kerry Heise, Staff Research Associate, U.C. Hopland Research and Extension Center