Australasian Science Magazine

By Jessica Sushinksy

How should we grow Australia’s cities to minimise their biodiversity impacts?

Should we build our cities up or out? That’s the question we asked when we considered the challenge of how a growing city can retain its wildlife. While it’s unrealistic to believe city living can co-exist with a full complement of bio­diversity, we wondered if there were differences in the impact of different growth strategies.

With half of the global population already living in cities, and predictions of increasing urban population in the future, this is far from an academic question. While the urbanisation that accompanies the movement of people into cities is one of most ecologically damaging and fastest-growing of any land use, little is known about how we should design the growth of our cities to minimise their ecological impact.

Australian cities are among the lowest density in the world. While this makes them liveable, being characterised by large backyards and leafy streets, it means that our cities take up a very large area for a given population size. Using the city of Brisbane as a case study, we asked whether a more compact type of urban growth, where new housing is concentrated within existing city borders, would be better for conservation.

Urban growth in Brisbane has historically been of a sprawling character with a very low density of houses. However, in the face of recent rapid population growth, the state government has adopted a strategy of more compact urban growth in an effort to reduce land conversion. To test what the impact of the two strategies might be on biodiversity, we modelled the effects of compact and sprawling urban growth on distributions of bird species in the city.

We made models of the current distributions of birds based on 636 bird surveys across the city. Then, using relationships between bird occurrence and key features of the urban environment (including housing density, extent of green space coverage, and vegetative cover), we projected those models forward under two alternative urban development scenarios.

Both scenarios considered what would happen if 84,642 dwellings were added to the city. In the compact scenario these dwellings were added through infill and subdivision of existing residential areas. The sprawling scenario directs these extra dwellings into areas of open space outside already developed areas as low density development. This sprawling scenario reflects a continuation of the pattern of urban growth that has previously occurred in Brisbane.

To measure changes in bird distributions under each urban growth scenario we calculated the change in the area of occupancy for each bird species between the current model and the two growth scenarios. We also measured how our experiences of birds would change. How many local extinctions would occur around where we live, and how would the two urban growth strategies change people’s access to green spaces and backyards?

Our analysis revealed that both scenarios of urban growth reduce overall bird distributions. However, compact development substantially slows these reductions at the city scale.

Urban-sensitive species in particular benefit from compact development because large green spaces were left intact. Under the sprawling development scenario, urban-sensitive species declined sharply while non-native species expanded. Overall, sprawling development resulted in significant changes in species’ distributions and major shifts in assemblage structure, while the impacts of compact development were much less pronounced.

Urban sprawl, therefore, results in the disappearance of many urban-sensitive birds – birds that only live in areas where there is native vegetation, such as parklands and woodlands. And it leads to an increase in feral birds, such as the common myna, as they tend to thrive in low density suburbs.

On the other hand, compact development retains more birds, including species such as Lewin’s honeyeater, grey shrike-thrush, red-backed fairy-wren and striated pardalote. This is because it keeps more of its parks and green areas.

Our results suggest that cities built to minimise per capita ecological impact are characterised by high residential density, with large interstitial green spaces and small backyards. However, there are important trade-offs between maintaining city-wide species diversity and people’s access to biodiversity in their own backyard.

If well planned, compact urban growth can preserve large intact green spaces and maintain an ecologically heterogeneous city that supports a large variety of bird species. However, without careful planning, increased residential density may just as easily result in the loss of these large green intact spaces and a loss of biodiversity. The choice is ours.