Australasian Science: Australia's authority on science since 1938

Looking after Our Nomadic Species

By Claire Runge

The range of many Australian nomadic birds can contract to a very small area, making them much more vulnerable to extinction.

Geographic range is often treated as a fixed attribute of a species when calculating extinction risk, with species occupying smaller geographic ranges assumed to have a higher risk of extinction.

However, many species move around the landscape. Migratory species move in relatively predictable to-and-fro migrations, while nomadic species move in complex and irregular movements.

We recently modelled the distributions of many Australian nomadic bird species, and found that at certain times their range contracts to a very small area, making them much more vulnerable than had been previously realised. This has important implications for how we calculate their risk of extinction.

Nomads move in complex patterns that are often associated with highly fluctuating resources, such as seasonal fruiting or irregular desert rainfall. Their movement strategies can be adjusted dynamically according to the prevailing conditions at each time and place. Australian bird nomads include the enigmatic and threatened grey falcon (Falco hypoleucos); the flock bronzewing (Phaps histrionica), which irrupts in incredible flocks of tens of thousands of birds in the outback; and the princess parrot (Polytelis alexandrae), a rarely seen but spectacular inland bird.

For many of these species we have only a rudimentary understanding of where they spend their time, when and why those places are important, and what drives them to move around the landscape. These movements can lead to substantial temporary expansions and contractions of their geographic range. Sometimes the size of the contraction can potentially pose an extinction risk. This is of particular concern here in Australia, where almost half of our bird species are migratory or nomadic.

Nomadic movements limit our ability to determine whether the population is increasing, decreasing or remaining constant, and consequently our ability to estimate risk on that basis. Many migratory species can be surveyed annually because of predictable movements to and from breeding grounds, which allows reasonably accurate measurements of population change and extinction risk. However, when and where we monitor nomadic species may dramatically influence our estimates of both their population abundance and trend.

In our investigation we used a species distribution modelling approach to predict the distribution of 43 Australian nomadic bird species. By combining existing data from citizen-science projects with remotely sensed data from the time of each species record, we were able to map monthly distributions for these nomads over an 11-year period, even in species with only a few sightings.

We found that the distributions of many species expand, contract and shift around the landscape throughout time, sometimes by one or two orders of magnitude. While many of these species have large ranges overall, at certain points in time (usually during periods of poor environmental conditions) they can be present in only small areas, making them vulnerable to threats in those places, whether through changing grazing regimes, loss of habitat to vegetation clearing or increases in feral predators. Indeed, all the species we examined exhibited significant bottlenecks at certain times.

These findings have implications for how we prioritise and conserve nomadic species. For example, we discovered that the scarlet-chested parrot (Neophema splendida) and the near-threatened chestnut-breasted whiteface (Aphelocephala pectoralis) contract to very small areas, leaving them with a much greater risk of extinction than suggested by their current IUCN status.

We believe that our approach to determining geographic range size is more appropriate for the assessment of extinction risk in nomadic species. This is even more important with climate change predicted to affect the pattern of resource fluctuations across much of the Southern Hemisphere, where nomadism is the dominant form of animal movement.

Our approach provides a tool for discovering spatial dynamics in highly mobile species, and can be used to unlock valuable information for improved extinction risk assessment and conservation planning. One of our recommendations is that extinction risk frameworks should assess nomadic species on the basis of their minimum range size.

Claire Runge is a member of the ARC Centre of Excellence for Environmental Decisions, and is based at The University of Queensland.