Thirty-four percent of Earth’s arable land has been converted to agricultural uses, and increased agricultural intensification has been correlated with contemporary decreases in avian abundance and functional diversity. Farm-scale biodiversity enhancement features such as native woody plant hedgerows have been planted in crop margins with the expectations of attracting beneficial predators and pollinators, and of improved pest control and pollination services in adjacent crops. Despite ongoing investment and perceived benefits of these types of enhancements for biodiversity, few evaluations have tested their effectiveness at increasing avian diversity. A growing body of research has quantified crop pest reduction by birds with indirect benefits for yield, yet few studies have evaluated whether farm-scale conservation activities improve effect sizes of these services. Further, whether or not crop margin habitats confer fitness benefits necessary for avian population viability (i.e., survival and reproduction) remains almost entirely unstudied. On each of these fronts, landscape effects can counteract or interact synergistically with the effects of local activities, and assessment of farmland habitat enhancement must be done with explicit reference to landscape context. I conducted field research in a system of extant woody hedgerows and semi-natural riparian habitat patches among farmlands of the Sacramento Valley of California’s Central Valley, USA to investigate three questions. Does farm-scale biodiversity enhancement and retention of semi-natural landscape habitat 1) influence patterns in avian abundance and diversity, 2) increase the rate of pest reduction by birds in adjacent crops, and 3) provide quality habitat and confer fitness benefits for avian occupants? In Chapter 1, and with collaborators, I sampled birds and habitat characteristics in 111 crop margins and landscape buffers during two winter and breeding seasons. We found that margins with hedgerows, treelines, or remnant riparian habitat harbored 2–3 times as many bird species and 3–6 times greater abundance than bare or weedy margins. Margin habitat type interacted with distance from semi-natural woodlands; hedgerow or riparian margins further from woodlands harbored more bird species. In Chapter 2, I performed a sentinel prey exclosure experiment in walnuts to compare pest cocoon predation rates by birds in 10 orchards with and 10 orchards without woody vegetation patches in their margins, and I characterized semi-natural cover within landscape buffers. Avian predator richness and abundance was greater in habitat orchard margins than in bare margins, and birds were confirmed predators of 23±29% pest cocoons per orchard (range 0 – 80%). Pest predation rates did not increase with the presence of woody margin habitat. Instead, predation rates increased with the increasing size of orchard trees, avian predator abundance, and percentage of semi-natural cover in the landscape. In Chapter 3, I used a suite of environmental, body condition, and population measures in long-distance migratory Zonotrichia sparrows to quantify habitat quality at hedgerows and natural reserves along a gradient of connectivity and landscape habitat amount. Abundance and within-winter apparent survival was highest in connected hedgerows and natural reserves with the most woodland landscape cover. Isolated hedgerows were of poor quality for first year Gambel’s white-crowned sparrows (Z. leucophrys gambelii) and the differences in within-winter apparent survival between first year and adult birds decreased significantly with increasing woodland landscape cover. The combined results suggest that farm scale habitat enhancement can be beneficial for birds in terms of local abundance and diversity, and within-winter apparent survival if connected to and among a sufficient percentage of other similar habitats. Farm scale enhancements can also be beneficial to growers by increasing the number of avian predators of crop pests. Yet, growers appear to benefit most by having crops located in landscapes with greater percentages of semi-natural landscape cover, where avian predation rates of crop pests were highest.
|Commitee:||Lawler, Sharon P., Williams, Neal M.|
|School:||University of California, Davis|
|School Location:||United States -- California|
|Source:||DAI-B 80/03(E), Dissertation Abstracts International|
|Subjects:||Wildlife Conservation, Ecology, Conservation biology, Agriculture|
|Keywords:||Apparent survival, Birds, Hedgerows, Landscape ecology, Pest control|
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