ECOLOGY: Musings on the home ranges of species

Saw an interesting article in PLOS Biology (DOI:10.1371/journal.pbio.0040208). It deals with with the range over which critters of a given species can be found, specifically birds. You'd expect a greater home range in areas with low productivity and more variable climate which enforces migration, which translates to closer-to-the-poles, both southern and northern (this commonsensical hypothesis is called Rapoport's Rule).

Not so! This pattern holds broadly true for some parts of the northern hemisphere, not coincidentally where most biologists are located. It falls apart with dramatic thoroughness in the southern hemisphere. Look at the article's Figure 2 and you'll quickly see why; the huge land masses of the upper northern hemisphere feature the largest home ranges, while the more restricted landmasses of the lower southern hemisphere show home ranges that wouldn't be out of place in the tropics. The reason appears to me to be obvious: The largest home ranges occur across the largest land masses, both in Eurasia and north America. Climatic variability is huge because the moderating influence of nearby oceans is small, seasonal food availability varies dramatically, ergo - larger home ranges, out of necessity. As Rapoport observed. But in the southern hemisphere, this doesn't hold because the land masses are far smaller.

The authors say that "spatial patterns in geographic range size may be due to geographical variation in the processes of speciation and extinction that ultimately generate biodiversity", but frankly, the simple fact of geophysical/climatic variability seems the parsimonious explanation for the observed home range effect. It also underlies the dynamics of speciation and extinction - and perhaps even the fundamental characteristics of metabolic rate, as my friend (and Sable Systems customer) Barry Lovegrove has been pointing out for years in a different context.