Landscape modification is a major global threat to terrestrial biodiversity. Managing human-modified landscapes in ecologically sustainable ways is crucial to avoid and mitigate biodiversity loss. However, practitioners (e.g. policymakers and developers) still urgently require research to inform targeted habitat protection policies, on-the-ground land management practices, and biodiversity offset strategies.
My research focused on identifying ways to strategically maintain and perpetuate habitat structures for wildlife in modified landscapes. I had three objectives: (1) measure and compare the current and future availability of habitat structures; (2) quantify the biodiversity value of scattered trees; and (3) test the effectiveness of artificial nest boxes as a biodiversity offset tool.
First, I conducted vegetation surveys at 300 plots in three dominant landscape contexts (reserves, pasture, urban greenspace). I found that in urban greenspace, the availability of multiple habitat structures (e.g. trees, logs, shrubs) depended upon by biota were significantly reduced compared with reserves, but comparable with agricultural land. Using a simulation model for tree populations, I also found that hollow-bearing trees were predicted to decline by an average of 87% in urban greenspace over the next 300 years under existing tree management policies. I identified that only a combination of tree management approaches can arrest this decline.
Second, I completed wildlife surveys at 72 individual trees of three sizes (small, medium, large) located in four landscape contexts (reserves, pasture, urban parklands, urban built-up areas). I recorded high invertebrate, bat and bird abundance and richness at scattered trees, representing a diversity of functional guilds. Furthermore, the biodiversity value of scattered trees in modified landscapes, including even small trees, was comparable or greater than that of trees located in reserves. I also found that several smaller trees could provide habitat compensation equivalent to that of a single large tree for some bird species and in certain landscape contexts (reserves and urban built-up areas). However, this was not a suitable offset strategy for a quarter of bird species and in other landscape contexts (pasture and urban parklands).
Finally, I conducted an experiment using 144 nest boxes with different entrance sizes (20, 35, 55, 75, 95 and 115 mm), secured to trees of three sizes (small, medium, large) located in four landscape contexts (reserves, pasture, urban parklands, urban built-up areas). I found that adding nest boxes to large trees resulted in an increase in tree visitation by hollow-nesting birds. However, the same response was not observed at small, medium or control trees. Nest boxes were also only occupied by common native and exotic species and are thus unlikely to be effective at ameliorating the residual impacts of hollow-bearing tree removal, especially for threatened taxa.
Based on my collective findings, I recommend: (1) adopting spatial zoning tactics that aim to resolve human-habitat conflicts and retain multiple habitat structures; (2) prioritising the conservation of scattered trees over the long-term by balancing both re-vegetation and mature tree preservation strategies; and (3) exercising caution in the wide-scale application of nest box offsets. These recommendations could assist practitioners in establishing more biodiversity-sensitive modified landscapes.