(3) Bye bye biogeography

2000 Landsat image of part of west Gippsland, Victoria, Australia. Almost the whole of the area shown was covered in native forest 150 years ago. Now it’s farms (light green and brown), a coal mine (gray on right), and forestry plantations (most of the dark green) established on former pasture. “p” marks the patch with the overlap zone of two native millipede species found only in native vegetation.

---

Species disappear one locality at a time, erasing piecemeal what used to be the geographic range of the species.

The range is the fundamental unit of species-level biogeography. Knowing where it is can tell you a lot about the habitat preferences of the species, about how the species interacts with other species, and often something about the species’ history as well.

As ranges are erased, so is biogeography. Forty years ago the German zoogeographer Gustaf de Lattin (1913-1968) called for an international effort to document ranges before they vanished:

Nature conservation measures, as useful and as necessary as they are for other reasons, do not help here. They can only preserve the threatened habitats and their species point-fashion in small reserves. The original form of the basic element of zoogeography, the natural range, would nevertheless be made unrecognisable... (p. 447; my translation)

de Lattin, G. 1967. Grundriss der Zoogeographie. Stuttgart: Gustav Fischer.

Range-erasing has often frustrated my own, specialist work on millipedes. A group of closely related millipede species is typically distributed mosaic-fashion on the map, with each species occupying its own mosaic “tile”. In relatively undisturbed landscapes, mosaics are complete and can be mapped on a fine scale. The “tile” boundaries, where two species ranges overlap, can be as narrow as 50 m, but are more usually wider, up to perhaps 5 km. (See my Tasmanian Multipedes website for examples of mosaics in the millipede genera Atrophotergum, Dasystigma and Gasterogramma; start at the Checklist page.)

With one millipede genus in the Australian state of Victoria, I struck it lucky. On a farm I sampled a small habitat remnant which preserved the overlap zone between two species otherwise found only in a few other scattered, nearby remnants (see image above; paper in press). There used to be a mosaic, but it’s been almost entirely erased, and one of the two species is now on the verge of extinction.