Elsevier

Biological Conservation

Volume 143, Issue 6, June 2010, Pages 1485-1492
Biological Conservation

Multi-scale factors affecting bird use of isolated remnant oak trees in agro-ecosystems

https://doi.org/10.1016/j.biocon.2010.03.029Get rights and content

Abstract

With recent emphasis on sustainable agriculture, conservation of native biota within agricultural systems has become a priority. Remnant trees have been hypothesized to increase biological diversity in agro-ecosystems. We investigated how remnant Oregon white oak (Quercus garryana) trees contribute to conserving bird diversity in the agro-ecosystem of the Willamette Valley, Oregon, USA. We compared bird use of isolated oak trees in three landscape contexts – croplands, pastures, and oak savanna reserves – and ranked the relative importance of four factors thought to influence bird use of individual trees: (i) tree architecture; (ii) tree isolation; (iii) tree cover in the surrounding landscape; and (iv) landscape context, defined as the surrounding land use. We evaluated species-specific responses and four community-level responses: (i) total species richness; (ii) richness of oak savanna-associates; (iii) tree forager richness; and (iv) aerial and ground forager richness. We documented 47 species using remnant oaks, including 16 species typically occurring in oak savanna. Surprisingly, landscape context was unimportant in predicting frequency of use of individual trees. Tree architecture, in particular tree size, and tree cover in the surrounding landscape were the best predictors of bird use of remnant trees. Our findings demonstrate that individual remnant trees contribute to landscape-level conservation of bird diversity, acting as keystone habitat structures by providing critical resources for species that could not persist in otherwise treeless agricultural fields. Because remnant trees are rarely retained in contemporary agricultural landscapes in the United States, retention of existing trees and recruitment of replacement trees will contribute to regional conservation goals.

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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