Biodiversity and development are often seen at mutually exclusive terms, with one prevailing at the expense of the other. Can biodiversity be maintained within the bounds of urban development by prioritising conservation of important mature trees? How do we know if a tree is mature and important? Well, we watch them of course.
The abundance and frequency of bird visits to certain trees is an indicator of ecological importance. Mature trees with hollows provide habitats for various bird species that cannot be supplied by young trees , hence older, more mature trees are believed to be more ecologically important that younger trees. We set out to further this quest for knowledge.
My work experience was punctuated by relaxing moments of reclining in the sun, panicked scribbling of bird related information and collective swearing when birds skipped our survey trees (quickly dubbed a ‘fly by’). Armed with a clipboard, limited knowledge on common bird species and a complicated record keeping system we began surveying the birds (tree watching).
In a project run by Phil Gibbons, multiple (primarily Eucaluptus ) trees were selected at random to be monitored for their abundance of bird species, with the aim of determining the features of trees that birds prefer, with the intention of advising developers on the value of preserving mature trees. We were entrusted with the job of recording bird species, direction of flight and activity whilst on the target tree. These target trees ranged from paddock trees in farmland, trees fringing urban development to those on reserves.
Mature trees with large, overhanging dead branches are dangerous to developers; branches fall on people, they’re not aesthetically pleasing and they get in the way of good old-fashioned urban sprawl. Never fear, biodiversity offsets are a foolproof mechanism to account for the loss of biodiversity (sarcasm).
Mature trees often do not develop large hollows before 220 years , meaning the effectiveness of offsets is restricted to future benefits, not accounting for biodiversity losses that impact native species now. This also means that trees are being removed faster than they can be regenerated. Offset proposals often don’t account for this time differential.
In one mature hollow bearing tree, we saw Cockatoos nesting in the hollow, Eastern Rosellas and Magpies over a 20 minute period. In addition to habitat provisioning, mature trees provide steppingstones for migration across a landscape. We experienced this watching Cockatoos fly across grassland in Goorooyarroo nature reserve, stopping at large Eucalupts before continuing their journey.
Never again will I look at mature trees the same way.
Fischer, J. and Lindenmayer, D. B. (2002) ‘The conservation value of paddock trees for birds in a variegated landscape in southern New South Wales. 2. Paddock trees as stepping stones’, Biodiversity & Conservation, 11(5), pp. 833-849.
Gibbons, P., Macintosh, A., Constable, A. and Hayashi, K. (2017) ‘Outcomes from 10 years of biodiversity offsetting’, Global Change Biology, 24.
Hannan, L., Roux, D., Milner, R. and Gibbons, P. (2019) ‘Erecting dead trees and utility poles to offset the loss of mature trees’, Biological Conservation, 236.
Le Roux, D., Ikin, K., Lindenmayer, D., Manning, A. and Gibbons, P. (2015) ‘Single large or several small? Applying biogeographic principles to tree-level conservation and biodiversity offsets’, Biological Conservation, 191, pp. 558–566.
Office of Environment and Heritage (2017) Loss of Hollow Bearing Trees. Threatened Species Office of Environment and Heritage Available at: https://www.environment.nsw.gov.au/threatenedspeciesapp/profile.aspx?id=20079 2019).