Short communicationThe 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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