Elsevier

Biological Conservation

Volume 143, Issue 6, June 2010, Pages 1564-1567
Biological Conservation

Short communication
The disproportionate value of scattered trees

https://doi.org/10.1016/j.biocon.2010.03.030Get rights and content

Abstract

Scattered trees are declining in agricultural landscapes worldwide. They are considered keystone structures because their effect on ecosystem functioning is believed to be disproportionate relative to the small area occupied by any individual tree. We empirically demonstrate the disproportionate value of scattered trees for birds and bats in an Australian livestock grazing landscape. We surveyed birds at 108 sites and bats at 63 sites. Sites spanned the full range of tree densities in the study area, from zero to over 100 trees per hectare. The marginal value of individual trees was highest when trees occurred at low densities. Compared to treeless sites, bird richness doubled with the presence of the first tree; bat richness tripled with the presence of 3–5 trees; and bat activity increased by a factor of 100 with the presence of 3–5 trees. Thereafter, the marginal effect of additional trees on birds and bats diminished rapidly. Although specialist species were restricted to large areas of dense tree cover, scattered trees effectively maintained moderate levels of bird and bat activity throughout largely cleared parts of the landscape. Future management activities should recognize the disproportionate value of scattered trees.

Introduction

Scattered trees represent the last remaining relicts of largely cleared woodlands and forests in agricultural landscapes around the world (Janzen, 1986, Harvey et al., 2006, Gibbons et al., 2008). They are considered keystone structures because their effect on ecosystem functioning is believed to be disproportionate relative to the small area occupied by any individual tree (Tews et al., 2004, Manning et al., 2006). However, unequivocal evidence of their disproportionately large effect is scarce. Because scattered trees are declining rapidly in agricultural landscapes around the world (Gibbons et al., 2008), explicit tests of their effects on species and ecosystem processes are urgently needed.

In this study, we examined the contribution of scattered trees to the species richness of birds and bats, as well as bat activity levels, in livestock grazing pastures in eastern Australia. Specifically, we investigated the change in bird and bat species richness, and bat activity, with an increasing density of trees. We calculated the mean marginal change in richness or activity per individual tree as the number of trees increased. If scattered trees do indeed have a disproportionate effect on birds and bats, the marginal contribution of trees should be highest where trees occur at low densities.

Section snippets

Study area

The study was conducted in the Upper Lachlan Catchment of New South Wales, Australia. Prior to European settlement, the region was dominated by relatively continuous woodland vegetation. Tree cover across the study area was approximately 15%, and approximately one third of remnant tree cover occurred as scattered trees (Fischer et al., 2010). We established 108 2-ha survey sites on a total of 33 farms. Sites were separated by at least several 100 m, and all were commercially grazed by livestock.

Results

Bird species richness increased with the number of trees in the site (Fig. 1a). Species composition also changed with an increase in tree density, with a decline in grassland specialists and an increase in species dependent on woody vegetation (Fig. S1). Notably, the mean species richness of sites with just a single isolated tree was approximately double the richness of a site with no tree (Fig. 1a). The marginal contribution of the first tree to species richness was disproportionately high,

Discussion

Our results confirm the disproportionate value of sparsely scattered trees in pastures for maintaining moderate levels of bat and bird activity throughout the landscape. Although specialist woodland species are restricted to extensive dense tree cover (Watson et al., 2001; Fig. S1), many species of birds and bats can use the resources associated with mature scattered trees (e.g. tree hollows) (Law et al., 2000, Fischer and Lindenmayer, 2002, Manning et al., 2004, Lumsden and Bennett, 2005). For

Conclusion

The potential keystone role of scattered trees warrants a balanced approach to conservation and restoration, in which scattered trees must be protected and perpetuated alongside the application of traditional conservation measures that focus on patches and corridors (Dorrough and Moxham, 2005, Gibbons et al., 2008, Fischer et al., 2009b). In the absence of a paradigm shift regarding the role of scattered trees, currently common keystone structures will be lost from agricultural landscapes

Acknowledgments

We are grateful for financial support from the Australian Research Council and the SeaWorld & Busch Gardens Conservation Fund; and for constructive comments by R. Kavanagh, A. Manning and K. Stagoll.

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