realized niche is the shallow-water reedbed. Coexistence occurs from the partition-
ing of the resource (nesting habitat), and the divergence of realized niches.In 1934 Gause stated that “as a result of competition two species hardly ever occupy
similar niches, but displace each other in such a manner that each takes possession
of certain kinds of food and modes of life in which it has an advantage over its com-
petitor” (Gause 1934). In short, two species cannot live in the same niche, and if
they try one will be excluded; second, coexisting species live in different niches. This
is known as the principle of competitive exclusion, or Gause’s principle (Hardin 1960),
and has become one of the fundamental tenets of ecology. It proposes that species
can coexist if adaptations arise to effectively partition resources. Examples of such
adaptations include the use of different microhabitats, different components of
prey, different ways of feeding, different life stages of the same prey, different time
periods in the same habitat, or taking advantage of disturbance, and interference com-
petition (Richards et al. 2000). Therefore, Gause’s principle has become the basis
for studies of resource partitioning and overlap as a way of measuring interspecific
competition.
There are, however, two serious problems with Gause’s statement. The first is that
it is a trivial truism, because we have already identified the two coexisting popula-
tions as being different by calling them different species, and, therefore, if we look
hard enough we are likely to find differences in their ecology as well. This is called
a tautology: having defined the species as being different, it should be no surprise to
find they are different.
The second problem is that the principle is untestable. It cannot be disproved because
either result (exclusion or coexistence) can be attributed to the principle. To
disprove the principle it is necessary to demonstrate that the niches of two species
are identical. Yet, as we can see from Fig. 9.8, what appears to be overlap, even
complete overlap, may not be so when an additional axis is taken into account. Since
we can never be sure that we have measured all relevant axes in describing the niches
of two species, we can never be sure that the two niches are the same. Hence, we
cannot disprove the principle.Despite these problems with the competitive exclusion principle, it underlies the numer-
ous studies of habitat partitioning amongst groups of coexisting species. Lamprey (1963)
described the partitioning of habitats by species of savanna antelopes in eastern Africa.
A similar study by Ferrar and Walker (1974) showed how various antelopes in
Zimbabwe used the three habitat types of grassland, savanna, and woodland
(Fig. 9.10). In both cases there was partitioning as well as overlap.
Similar studies by Wydeven and Dahlgren (1985) show partitioning of both habi-
tat and food in North American ungulates (Fig. 9.11). In Wind Cave National Park
elk and mule deer have similar winter habitat choices, as do pronghorn and bison,
but these pairs have very different diets. For example, the diet of bison contains 96%
grass as against 4% for pronghorn.
Interspecific overlap in the diet niches of two sibling bat species (Myotis myotis,
M.blythii) of Switzerland shows niche partitioning: M. myotisfeeds largely on
ground insects (carbid beetles) whereas M.blythiifeeds mostly on grass-dwelling insects
(bush crickets). This allows coexistence within the same habitats (Arlettaz et al. 1997).
MacArthur (1958), in a now classic paper, described the different feeding positions146 Chapter 9
9.6 Resource partitioning and habitat selection
9.6.1Habitat
partitioning