untitled

and replacement has been repeated many times in the Pliocene and Pleistocene and

probably earlier, whilst the tropics have been extant throughout.

3 Productivity. The warmer temperatures and higher rainfall of tropical regions

allows species to fit into narrow niches over small areas while still maintaining a large

enough population to avoid extinction. In contrast, low productivity at high latitudes

means that fewer species must maintain broad niches over wide areas to maintain

the same population size.

4 Predation. The higher biodiversity of the tropics includes a higher number of pred-

ators. These impose a top-down regulation on prey species (see Section 21.8.4), which

allows a greater number of prey species to coexist because they are not competing

with each other. This process is called predator-mediated coexistence.

5 Environmental stability. High latitudes fluctuate considerably between summer and

winter in environmental parameters such as temperature. Large populations are required

to withstand such fluctuations and this means few species can live there. Stable

environments in the tropics allow smaller populations to survive. These small

populations can have smaller niches and so more species can fit into the system.

6 Intermediate disturbance. This is a variation of (5). While agreeing that higher

latitudes experience frequent major disturbances, and so support fewer species, it

differs in considering that the tropics have some smaller disturbances (hence

intermediate between few and many) that allow both early succession and highly

competitive species to coexist (Connell 1978).

In summary, no single hypothesis explains all of the observed distributions. It is likely

that different hypotheses apply in different locations, and also that more than one

process occurs at one location.

The total diversity within a large area, a region, is called the gammadiversity. This

is determined by two components: (i) alphadiversity, which is the number of

species in a local area or habitat; and (ii) betadiversity, which is the reciprocal of

the mean number of habitats or localities occupied by a species. Thus:

gamma diversity =average diversity per habitat (alpha) ×1/mean number of

habitats occupied by a species (beta) ×total number of

habitats

378 Chapter 21

600

500

400

300

200

100

0

020406080

Mean latitude north

Number of bird species

Fig. 21.2The number
of terrestrial bird species
in North America
declines from the
tropics to the Arctic.
(Data from MacArthur
1972.)

21.12.2Local and
regional diversity