the middle of the wet season 9 months after the previous rains; but both occur when
the swamp grasslands are most available.So far we have discussed range limitation by abiotic environmental factors. However,
these abiotic factors can interact with biotic processes such as predation and com-
petition to further limit a species’ range. For example, the geographic distribution of
arctic fox (Alopex lagopus) is largely in the tundra of the Holarctic and is separate
from that of the more southerly red fox (Vulpes vulpes). However, their ranges over-
lap in some areas of North America and Eurasia. The northern limit of the red fox’s
range is determined directly by resource availability, which is determined by climate.
The southern limit of the arctic fox’s range is determined by interspecific competi-
tion with the more dominant red fox (Hersteinsson and Macdonald 1992).A major pattern in ecology is the positive relationship between the range of a species
and its abundance. In general, locally abundant species have wide ranges whereas rare
species have narrow ranges (Brown 1995; Gaston and Blackburn 2000). This observa-
tion has led to Rapoport’s rule, namely that the latitudinal extent of a species’ range
increases towards the poles (Rapoport 1982). This general pattern is modified by species
richness, rainfall, vegetation, and land surface as seen in studies of birds (Gentilli
1992) and mammals (Pagel et al. 1991; Letcher and Harvey 1994; Smith et al. 1994).
Of great importance in conservation management is what happens to a species’ range
when the population declines. One expects that population densities tend to be higher
at the center of a population’s range than at the periphery. Geographic ranges should
collapse from the outside, with the center being the last to go (Brown 1995).
Analyses of range contractions in a wide variety of animals and plants suggest that
populations often collapse first in the center, leaving isolated fragments on the per-
iphery (Lomolino and Channell 1995; Channell and Lomolino 2000). These collapses
were due to the variety of causes outlined by Caughley (1994). Thus, peripheral
populations not only provide a refuge for endangered species but also represent genetic98 Chapter 7
120010008006004002000100806040200Births FloodLechwe
CalvesJ J ASOND J FMAMJ J ASOND J
1965 1966Number of lechweCalves / 100 femalesFig. 7.3The numbers
of lechwe, a flood plains
antelope of southern
Africa (, left axis),
increase on the flood
plains as water recedes
in the Chobe River
exposing the greatest
area of high-quality
food. The recruitment
of newborn per 100
females (, right axis)
shows that births occur
at this time. (After
Sinclair 1983, which is
after Child and von
Richter 1969.)
7.4.4Range limited
by biotic factors