(i) Preamble.

We need to understand some basics concerning species’ distributions – the most basic of zoogeographical observations. These form the basis for much of the hypotheses and inferences concerning processes in animal distributions.

The geographic range: the basic observational unit of zoogeography. It encompasses the geographic extent of occurrences of a taxon (lineage within species, species, genus, etc) during part or all of its life cycle.

Generally, the range is limited to a specific set of environmental conditions which can result in extremely localized distributions (e.g. African Great Lakes cichlid (Pisces: Cichlidae) that are endemic to a single lake, Olympic mudminnow (Novumbra hubbsi) found only on the Olympic Peninsula in Western Washington, or the Devil's Hole pupfish (Cyprinodon diabolis) found in a single pool of less than 100 sq m in southwestern Nevada!) or, more rarely, may be "cosmopolitan" and found throughout extremely wide geographic areas (e.g. the peregrine falcon is found on all continents except Antarctica, the blue whale's range is estimated at 300,000,000 sq. km!).

Distributions are usually summarized in outline maps that show an irregular (usually) outline drawing to define the limits of the known (or inferred) distribution, dot maps that show more precise locations of known distributions. Sometimes abundance data can be combined with occurrence data to produce contour maps that give an indication of spatial variation in density (i.e. they can identify species "hotspots"). These maps can also apply to the geographic range of specific attributes of a taxon.

It is, of course, important to remember that range maps are pretty static and cannot convey the temporal changes in species distributions that may occur owing to environmental change even from season to season, much less on longer (evolutionary) time scales.

Geographic ranges are much more than simply static descriptions of the known or suspected distrubution of a taxon. Some comparative analyses of ranges have been conducted and general principles to explain the emergent patterns have been discussed (see Brown et al. 1996 for a good review). Here are some examples:

Most ranges are small to medium sized:

Here is a frequency histogram of range sizes in 90 or so species of pine tree (the same pattern holds for a number of brids and mammal species). The question is, why don't more species have larger ranges?

Also, the size of ranges show some associations with body size and abundance. The figures below show (marginal) positive associations of average range size with increasing body size and increasing abundance (average population density).

Finally, the shape of ranges can vary. Shown below are plots of maximum N-S range size versus maximum E-W range size for various birds and mammals in North America and Europe. The straight line represents a 1:1 relationships (as might be seen in a range in the shape of a perfect circle or square).

Points above the lines have exaggerated ranges in the N-S direction, those below the lines have exaggerated E-W sizes (particularly at larger range sizes). These different "shapes" may stem from the different shapes of the continents. Note the ranges for land birds in Europe tend to be exaggerated in an E-W direction relative to North American birds. This might relate to the fact that major dispersal barriers (mountains, rivers) tend to be oriented in a N-S direction in North America, but in an E-W direction in Europe.

(ii) What determines the geographic range of a taxon?

 (1) The "fundamental niche"

In 1957, G.E. Hutchinson, an aquatic ecologist, conceived of the idea of a fundamental niche, i.e. an n-dimensional hypervolume that defines that describes the set of conditions which limit the distribution of a species. This hypervolume is a function of the interaction among various abiotic and/or biotic environmental factors that influence the persistence of a particular species.

In it’s simplest form the response of a species (i.e. whether it shows population maintenance, growth, or decline) along an environmental gradient (e.g. temperature, salinity, predator abundance, etc) is usually described as roughly normally-distributed with some "optimum" value for survival and growth of individuals for the environmental factor in question.

A good example of possible range limitation by a single environmental factor is shown in the figure below. It depicts the close association between the northern limit of the eastern phoebe and the -4 C January isotherm.