Single trees in agricultural systems give big boost to birds and bats

Lazuli bunting (Passerina amoena) frequently found in remnant oak trees in Oregon.Conservationists often place the greatest value on protecting large contiguous habitat areas. However, as two new studies in the journal Biological Conservation show, single, isolated trees on agricultural lands can give a big boost to wildlife species.

First, research by Joern Fischer and fellow scientists from Australia shows that individual trees can have a disproportionately large positive impact on birds and bats.

The researchers conducted their study on livestock grazing pastures in Eastern Australia and measured avian and bat species richness at specific areas with tree density ranging from 0 to 100 trees per site.

They found that as the number of trees at a site increased from 0 to 1, the number of bird species doubled from 5 to 10. From thereon, adding each additional tree made a much smaller contribution to species richness.

The findings were similar for bats - species richness went from 2 at treeless sites to 5 at sites with one or two trees before leveling off at 7 to 8 species at sites with 5 trees. Similarly they looked at bat activity and found that it increased ten-fold from 0 to 2 trees and then increased ten-fold again with the presence of two or three more trees before leveling off.

These findings provide support for the concept that individual remnant trees serve as keystone structures in which their ecological impact is disproportionately large relative to the space that they take up.

The findings also provide support for retaining scattered remnant trees on agricultural lands. The researchers concede that species richness alone is not a definitive measurement of habitat quality.

They note however that even if these areas do not serve as sources for population growth, "the presence of birds and bats throughout the landscape is likely to provide valuable ecosystem services, including pest control and seed dispersal.

Meanwhile a new study by Craig De Mars and fellow scientists provides additional support for the importance of retaining remnant trees on agricultural lands.

The scientists looked at bird species richness among isolated, individual oak trees in three landscape settings - cropland, pastures, and oak savannah reserves - in the Willamette Valley of Oregon. Oak savannah habitat in the region - and much of the U.S. - has been largely lost.

The researchers documented 47 species including 16 oak-savannah birds. They found little difference in species at remnant oak trees in any of the landscape settings.

Conversely they found that tree architecture - i.e. size and complexity - was an important explanatory factor in terms of species richness and composition. The authors write,

"Our results suggest that oak savanna restoration in agricultural systems does not necessarily need to be an all-or-nothing proposition. Large savanna-form oak trees scattered in agricultural fields have wildlife value, particularly for many oak-associated birds. Moreover, individual trees have a relatively small physical footprint thus allowing minimal impact on agricultural production and contributing to biological diversity at a small cost to production." 

--by Rob Goldstein

Fischer, J., Stott, J., & Law, B. (2010). The disproportionate value of scattered trees Biological Conservation DOI: 10.1016/j.biocon.2010.03.030