Biology Reference
In-Depth Information
depending on their direct assessment of female arrival rate, or
indirect assessment based on pat age and competitor numbers.
Competition by resource
defence: the despotic
distribution
Rich habitat
Consider the same situation as before: two habitats, one rich
and one poor. This time, though, the first competitors to settle
in the rich habitat defend resources by establishing territories
(pieces of ground containing the resource), so later arrivals
are forced to occupy the poor habitat even though they do less
well there than the individuals in the rich area. When the poor
habitat fills up with territory-defending individuals the latest
arrivals of all may end up being excluded from the resource
altogether (Fig. 5.4). This kind of situation is very common in
nature. In Wytham Woods, near Oxford, UK, the best breeding
habitat for great tits is in oak woodland. This is quickly
occupied in the spring and becomes completely filled with
territories. Some individuals are excluded from the oak wood
and have to occupy the hedgerows nearby where there is less
food and, consequently, lower breeding success. If great tits are
removed from the best habitat then birds rapidly move in from
the hedgerows to fill the vacancies (Krebs, 1971). Similarly, in
red grouse (
Lagopus lagopus scoticus
) territorial birds defend the richest areas of the heather
moors as breeding and feeding territories. Excluded birds have to go about in flocks and
exploit poor habitats where their chances of survival are low. Once again, if a territory
owner is removed its place is quickly taken by a bird from the flock (Watson, 1967).
In these examples the strongest individuals are despots, grabbing the best quality
resources and forcing others into low quality areas or excluding them from the resource
altogether.
Poor habitat
a
b
Number of competitors
Fig. 5.4
Resource defence. Competitors
occupy the rich habitat first of all. At point
a this becomes full and newcomers are now
forced to occupy the poor habitat. When
this is also full (point b), further competitors
are excluded from the resource altogether
and become 'floaters'. After Brown (1969).
Removal
experiments show
that territorial
behaviour may
exclude some
competitors from
good habitats
The ideal free distribution with
unequal competitors
Most examples in nature will have features of both the simple models we have discussed
above. Perhaps the commonest situation will be where the best place to search depends
on where all the other competitors are, but within a habitat some individuals get more
of the resource than others. In the duck experiment, for example, population counts
showed a stable distribution of individuals but some ducks were better competitors than
others and grabbed most of the food (Harper, 1982). The stable distribution could come
about because of the way the subordinates distribute themselves in relation to the
despots. In effect, the despots are part of the habitat to which the subordinates respond
when deciding where to search.
