Multi-scale factors affecting bird use of isolated remnant oak trees in agro-ecosystems
Introduction
Agricultural conversion of natural environments is a major factor in the current global decline of biodiversity (Tilman et al., 2001, Foley et al., 2005). Approximately half of the earth’s habitable land has been modified to some degree for agricultural purposes (Clay, 2004) and current trends indicate that the global agricultural footprint could increase a further 18% by 2050 (Tilman et al., 2001). This high degree of agricultural impact on global terrestrial ecosystems suggests that conservation of biodiversity can no longer be solely focused on protected areas (Fischer et al., 2006, Vandermeer and Perfecto, 2007). Moreover, in many highly modified landscapes, existing reserve networks may be insufficient for conservation of biodiversity (Brooks et al., 2004, Rodrigues et al., 2004). Consequently, an emerging research theme in conservation biology has been the assessment of structural elements thought to be important for conserving biodiversity in agricultural systems (Harvey et al., 2006, Sekercioglu et al., 2007, Haslem and Bennett, 2008).
In North America, agricultural conversion has particularly impacted oak savanna, one of the continent’s most imperiled ecosystems (Noss et al., 1995). In many agricultural areas, biological legacies (sensu Franklin et al., 2000) from historic landscapes exist as scattered large trees which have often been retained by landowners for cultural reasons (Harvey and Haber, 1999, Fischer and Bliss, 2008). Previous studies in tropical and Australian agricultural systems demonstrate that isolated remnant trees provide numerous ecological functions important to birds including landscape connectivity for woodland species (Fischer and Lindenmayer, 2002b, Robertson and Radford, 2009), foraging sites (Luck and Daily, 2003), and nesting sites (Manning et al., 2004). Manning et al. (2006) further suggest that isolated trees are keystone structures in human-modified landscapes because an individual tree’s ecological influence is disproportionate to its actual physical footprint. Within scattered tree landscapes critical management priorities are to determine: (i) an appropriate spatial pattern of trees that best maintains landscape-level biodiversity (Manning et al., 2006); (ii) the characteristics of individual trees that best provide wildlife habitat (Tews et al., 2004); (iii) the influence of the surrounding matrix on wildlife use of individual isolated trees (Ricketts, 2001, Kupfer et al., 2006).
Here, we investigated the potential role that isolated remnant oak trees play in conserving oak savanna-associated bird diversity in a North American agro-ecosystem. We compared bird use of isolated Oregon white oak (Quercus garryana) remnant trees in three different landscape contexts within the agricultural matrix of the Willamette Valley, Oregon, USA. We evaluated the relative importance of site-specific and landscape-level factors thought to influence bird use of individual remnant trees. We ranked the following four factors on how well each could explain bird use of individual remnant trees: (i) tree architecture, (ii) tree isolation, defined as the distance to the nearest tree or patch, (iii) tree cover in the surrounding landscape, and (iv) landscape context, defined as the dominant land use in the surrounding landscape. We investigated species-specific responses and 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) species richness of aerial- and ground-foraging birds, grouped collectively as species that do not typically forage on trees or within tree canopies.
Section snippets
Study area
We conducted our study in the southern half of the Willamette Valley (43o56′–44o54′W, 122o53′–123o22′N), which lies between the Cascade and Coast Ranges in western Oregon. The Willamette Valley (elevation 70–120 m) has a Mediterranean temperate climate characterized by long wet winters (mean annual precipitation = 110.9 cm) and short dry summers (OCS, 2006). Outside of urban development, predominant land uses in our study area are grass seed production and, to a lesser extent, livestock grazing.
Results
We recorded 47 species of birds using remnant trees from 528 detections (see Appendix). European Starling (n = 20 sites; see Appendix for scientific names) was most frequently encountered followed by American Robin (n = 18) and American Goldfinch (n = 17). Among oak savanna associates, American Goldfinch and Lazuli Bunting (n = 11) were most frequently observed. Bullock’s Oriole (n = 10) was the most frequently observed tree foraging species. The majority of species were detected at <10 sites. Of the 23
Discussion
The ability of agricultural fields to contribute to regional conservation is a key goal of many sustainable agriculture programs. We documented 47 bird species using remnant trees in agro-ecosystems, suggesting that these trees are serving some function in supporting species that would not persist in otherwise treeless agricultural landscapes. Importantly, 16 oak savanna-associated species used these trees, including species of regional conservation concern such as White-Breasted Nuthatch and
Acknowledgements
This project would have been impossible without the assistance of many private landowners and public agencies that facilitated access to their properties. We thank G. Fitzpatrick for initiating discussions on research of isolated oak trees and J. Miller and S. Zack for their review of field methods and earlier drafts of the manuscript. We also thank M. Betts, J. Hagar, D. Robinson, and two anonymous referees for further reviews of the manuscript. This research was supported in part by
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