Geographic range size and extinction risk
Geographic range size (the size of a species’ distribution) is often treated as a fixed attribute of a species for the purposes of calculating extinction risk (see the segment below, on EOO & AOO). All else being equal, species occupying smaller geographic ranges are assumed to have a higher risk of extinction. However many species move around the landscape. Sometimes their movements involve relatively predictable to-and-fro migrations (migratory species). But sometimes they involve complex irregular movements, with these species often being referred to as nomads (nomadic species). 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 realized (Runge et al, 2015). This has important implications for how we calculate their risk of extinction.
Nomads move in complex patterns, often associated with highly fluctuating resources such as seasonal fruiting or irregular desert rainfall. Movement strategies may be adjusted dynamically according to the prevailing conditions at each time and place.
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 expansion and contraction of geographic ranges. 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.
Australian bird nomads include the enigmatic and threatened grey falcon (Falco hypoleucos), the flock bronzewing (Phaps histrionica) – an outback pigeon that irrupts in incredible flocks of tens of thousands of birds and the princess parrot (Polytelis alexandrae), a rarely seen but spectacular inland bird.
“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 ”
Nomadic movements limit our ability to determine population dynamics 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 measurement of population change and extinction risk. However, for nomadic species when and where we monitor may dramatically influence our estimates of both 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 (see the segment below, on citizen science) 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 and 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 during some period in terms of low areas of occupation.
Implications for nomads
These findings have implications for the way we prioritise and conserve nomadic species. For example, we discovered that two species (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 is suggested by their current IUCN status.
Figure 1 shows the distribution of the scarlet-chested parrot. If its distribution is based on all sightings (pooled over time) then it geographical range (extent of occurrence) would be considered quite large. However, when its potential distribution is modelled at different times of the year we found that in spring of 2010 its potential distribution is large but in autumn 2010 its distribution is only a tiny fragment of that range, and its risk is significantly higher.
We believe 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. And we recommend that extinction risk frameworks for nomadic species be assessed on the basis of their minimum range size.
EOO & AOO
Several different measures of geographic range size exist. Estimations of extinction risk are typically calculated using static metrics such as extent of occurrence (EOO) or area of occupancy (AOO), which are based on a conceptualization of geographic range size as a fixed attribute of a species. EOO is a measure of the degree to which a species’ distribution, and hence its vulnerability to threats, is spread across geographic space. AOO is a measure of the area actually occupied by the species. With these metrics, species with smaller extents or areas are assumed to be more threatened. However, when a species is nomadic within its overall distribution, estimates of EOO or AOO based on pooling observations across time will often be larger than the geographic range size at any one point in time. This could lead to an erroneous conclusion that a nomadic species is safe from extinction when it is not. We examined the temporal variability in the AOO of nomadic species and explored the consequences of such dynamism for extinction risk assessments (Runge et al, 2015).
Citizen science to the rescue
Monitoring nomadic species is a major challenge all around the world but it’s an even bigger challenge in a large country with a small population like Australia. The sightings data that formed one crucial element of our analysis were largely derived from citizen-science projects like Eremaea eBird (http://ebird.org/content/australia/) and the BirdLife Atlas (http://birdlife.org.au/projects/atlas-and-birdata). Thanks to citizen-science initiatives such as these, for most Australian birds we now have the information we need to plan conservation actions imperative for the survival of nomads. By combining the desires of volunteers to travel and explore remote regions with analytical tools such as species distribution modelling, we can now predict and map the key sites for conservation action to ensure species’ ongoing persistence. [See our stories on citizen science and eBird for more details on these exciting developments.]
More info: Claire Runge firstname.lastname@example.org
Reference: Runge, CA, A Tulloch, E Hammill, HP Possingham & RA Fuller (2015). Geographic range size and extinction risk assessment in nomadic species. Conservation Biology http://onlinelibrary.wiley.com/doi/10.1111/cobi.12440/abstract