Conserving avian diversity in agricultural systems : the role of isolated Oregon\ud white oak legacy trees

Graduation date: 2009Over one third of the earth’s land surface has been modified to some extent for\ud agricultural purposes. The large global footprint of agriculture, combined with the\ud knowledge that existing reserve networks are likely insufficient for long-term\ud conservation of native biodiversity, has necessitated that agricultural systems\ud contribute to conservation of native biota. Current research paradigms have taken a\ud landscape-level view of conservation in agricultural systems, assessing the relative\ud contribution that various habitat elements make in conserving biodiversity in the\ud agricultural matrix. Within this context, I investigated the potential role that\ud individual Oregon white oak (Quercus garryana) trees play in conserving avian\ud diversity in the agricultural systems of the Willamette Valley, Oregon, U.S.A.\ud Retained by landowners primarily for cultural reasons, many of these trees pre-date\ud Euro-American settlement of the Willamette Valley and thus are biological legacies\ud from historic white oak habitats. I compared avian use of isolated white oak trees in\ud three different site contexts - croplands, pastures, and oak savanna reserves - and used\ud an information-theoretic model selection approach to determine the relative\ud importance of site-specific and landscape-level factors thought to influence avian use\ud of these individual trees. Specifically, I tested whether avian species presence on\ud Oregon white oak legacy trees could best be explained by: (i) tree architecture; (ii) the\ud distance of the tree to the nearest tree or patch; (iii) the density of trees in the\ud surrounding landscape; or (iv) the matrix in which the tree was embedded. I evaluated\ud species-specific responses as well as four community-level responses: (i) total bird\ud species richness; (ii) species richness of native birds associated with oak savanna; (iii)\ud species richness of tree foraging birds; and (iv) the combined species richness of\ud aerial- and ground-foraging birds. I sampled 35 individual white oak trees and\ud recorded 47 avian species using these individual trees, including a high number of oak\ud savanna-associated species such White-breasted Nuthatch (Sitta carolinensis) and\ud Chipping Sparrow (Spizella passerina). For the majority of these species, the\ud frequency of use of individual oak trees was similar among crop, pasture and reserve\ud sites. The most important factors for predicting avian use were tree size and tree\ud density in the surrounding landscape. In general, avian use increased with increasing\ud tree size and decreasing tree density. My findings suggest that individual white oak\ud legacy trees have the potential to positively contribute to landscape-level conservation\ud of a wide range of avian species within the Willamette Valley. Due to the declining\ud abundance of white oak legacy trees on the landscape, the conservation of existing\ud legacy trees and the recruitment of younger replacement trees should be a\ud management priority