untitled

by an upper and lower limiting temperature but it is demarcated within those

bounds by rainfall whose effect varies with temperature. High rainfall is tolerated only

in hot areas and low rainfall only in colder areas where evaporation is reduced.

The distribution of species C is controlled by a symmetric interaction of rainfall

and temperature. The species’ tolerance of high temperatures increases with increas-

ing annual rainfall, and the tolerance of the species to rainfall increases with tem-

perature. This is a two-way interaction.

A known range of tolerance to one or more factors like temperature and rain-

fall does not translate directly into a map of distribution because the factors may

interact as in example B and C of Fig. 7.2, the level of one factor determining the

effect of another. Whether distribution is determined by one or several factors

depends critically on the geographic dispersion of the levels of each factor.

Temperature can limit the distribution of animals through direct effects on their physi-

ology and indirectly by affecting resources. Some distributions can be described empir-

ically by temperature contours (isotherms). Thus, the southern limit to northern

hemisphere seals is set by sea surface temperatures never exceeding 20°C (Lavigne

et al. 1989). The reason is unclear, but most seals breed in regions of high marine

productivity and these are largely restricted to high latitudes. Similarly, the penguins

of the southern hemisphere inhabit seas with temperatures lower than 23°C. Most

penguin species inhabit latitudes between 45°S and 58°S where marine productivity

is high (Stonehouse 1967). They reach the equator at the Galapagos Islands off the

Pacific coast of South America, but only because those shores are bathed by the cold

Humboldt current.

The northern limit for rabbits (Oryctolagus cuniculus) in Australia is marked by

the 27°C isotherm. These temperatures coincide with high humidity, and the com-

bination of the two causes resorption of embryos so that the animals cannot breed.

Cold is clearly an important factor limiting species in the Arctic and subarctic.

Although the Arctic is an important breeding ground for birds, most leave during

94 Chapter 7

Hot

Cold

Dry Wet

A

B

C

Fig. 7.2Three
hypothetical envelopes
of adaptability of a
species to temperature
and moisture: A,
the two factors act
independently; B, the
level of one factor
influences the effect of
the other; C, the effect
of each factor varies
according to the level
of the other. (After
Caughley et al. 1988.)

7.4.1Range limited
by temperature