untitled

Green (1998) found complementarity in ducks along habitat and feeding beha-

vior axes. He found in dabbling and diving ducks in Turkey that pairs with similar

habitat had dissimilar feeding mechanisms.

We have considered only two dimensions of a niche so far, but clearly the niche

must include every aspect of the environment that would limit the distribution of

the species. We cannot draw all these dimensions on a graph but we could perhaps

imagine a sort of sphere or volume with many dimensions, which could theoretically

describe the complete niche. Hutchinson (1957) described this as the n-dimensional

hypervolume. This is the fundamental niche of the species and is defined by the set

of resources and environmental conditions that allow a single species to persist in a

particular region (Schoener 1989; Leibold 1995). This suggests that the niche is in

some way discrete. However, resource measures are usually continuous so the dis-

creteness does not come from these. Rather it comes from the constraints of the species

in terms of their morphology, physiology, and behavior – a species is more efficient

at using some combinations of a resource than other combinations, while other species

have different combinations where they are most efficient. These peaks of efficiency,

then, are the adaptive peaks exhibited by a species (Schluter 2000).

The fundamental niche is rarely if ever seen in nature because the presence of com-

peting species restricts a given species to a narrower range of conditions. This range

is the observed or realized nicheof the species in the community. It emphasizes that

interspecific competition excludes a species from certain areas of its fundamental niche.

In terms of the Lotka–Volterra diagrams (see Fig. 9.2) the weaker competitor has no

realized niche in Figs 9.2a and 9.2b, and for Fig. 9.2d parts of the fundamental niche

are not used.

The difference between the two types of niches can be seen in a study by Orians

and Willson (1964) of red-winged blackbirds (Agelaius phoeniceus) and yellow-

headed blackbirds (Xanthocephalus xanthocephalus). Both species make their nests among

reeds in freshwater marshes of North America, and, if alone, both will use the deep-

water parts of the marsh (there is greater protection from mammalian predators here).

However, when the two species occur together, the yellow-heads exclude the red-

winged blackbirds, which are then restricted to nesting in the shallow parts. Thus,

the fundamental niche for nesting red-winged blackbirds is the whole marsh, but the

COMPETITION AND FACILITATION BETWEEN SPECIES 145

1.0

0.8

0.6

0.4

0.2

0

1.0

0.8

0.6

0.4

0.2

0

0 0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0

Food overlap (aggregate %)

Winter 1984 Summer 1984

Habitat overlap

Food overlap (aggregate %)

(a) (b)

Habitat overlap

Fig. 9.9Resource
overlaps in seven
species of dabbling
ducks. Below the
broken line there is
complementarity in
overlaps. (a) In winter,
high habitat overlap
between pairs of species
tends to be associated
with low food overlap,
demonstrating
complementarity.
(b) In summer there is
simultaneous overlap
in both dimensions.
(After DuBowy 1988.)