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