Competing for resources
by Liv Romoren
What happens in nature when many individuals exploit the same limited
resources?
These resources can be things like food, feeding places, shelter and so
on.
Surely the decisions made by one competitor may be influenced by what
the others are doing and visa versa?
I think it is very interesting how animals exist together in different
environments, and how they change their behaviour according to the conditions
around them, so that they can make the best out of the situation they
are in.
For example, is it always a good idea to compete with others, or is it
better to keep a low profile instead of indulging a conflict?
I therefore decided to look at the different kinds of competition that
exists, and how different kinds of animals live and cope with them.
The simplest form of competition is exploitation, which means “ to use
up the resources “
A good example is to imagine two habitats, where habitat 1 is a rich one
containing a lot of resources and habitat 2 is a poor one containing few.
Each individual can choose to exploit the habitat in which it can achieve
the higher pay-off, measured as a rate of consumption of resource.
With no competitors it is clearly that an individual will go to the best
habitat. But because there is no territoriality or fighting, also later
arrivals will come here. However, the more competitors that occupy the
rich habitat, the more resources will be used up, and the habitat will
be less useful for further newcomers.
Eventually a point will be reached where the next arrivals will do better
by occupying the poorer quality habitat where, although the resource is
in shorter supply, there will be less competition.
The two habitats will therefore be filled so that the profitability for
an individual is the same in each one.
Competitors in this way adjust their distribution in relation to habitat
quality so that each individual enjoys the same rate of acquisition of
resources.
This pattern of distribution of competitors between resources has been
termed the “ideal free distribution” because it assumes that animals are
free to go where they will do best and that the animals are ideal in having
complete information about the availability of resources
Another form of competition is resource defence, in which animals keep
others away from resources by fighting or by aggressive displays.
One can consider the same situation as before, two habitats, one rich
and one poor.
But this time the first competitors to settle in the rich habitat defend
resources by establishing territories so that later arrivals are forced
to occupy the poor habitat even though they do less well there than the
individuals in the rich area.
When also the poor habitats are filled up with territory defending individuals,
the latest arrivals my end up being excluded from the resource altoghether.
An example of this is in Wytham Woods, near Oxford, England, where the
best breeding habitat for great tits is in oak woodland. This is quickly
occupied in the spring and becomes completely filled with territories.
This leads to excluding of some individuals from the oak wood to the hedgerows
nearby, where there is less food and as a consequence lower breeding success.
Experiments have shown that if great tits are removed from the best habitat,
birds will rapidly move in from the hedgerows to fill the vacancies. (Krebs
1971 )
Another experiment involves the red grouse, which are territorial birds defending the richest areas of the heather moors as feeding and breeding territories. Here excluded birds have to go about in flocks and exploit poor habitats where their chances of survival is low. But if a territory owner is removed, its place is quickly taken by a bird from the excluded flock, showing the same result as in the previous example ( Watson 1967 )
Resource defence is one kind of competition that comes under the general heading of interference, which occurs whenever competitors interact with one another in such a way as to reduce their efficiency at searching for or exploiting resources.
As we have seen, some compete for resources by exploitation, and others
by territoriality. But is there some way to find out when the latter form
for competition should be adopted instead of the former?
When does it pay to compete by means of territorial defence?
Defending a resource has different costs, like energy expenditure, risk
of injury and so on, in addition to the benefits of priority of access
to the resource.
So logically, territorial behaviour should be favoured by selection whenever
the benefits are greater than the costs.
To examine this, it has been useful to look at birds such as hummingbirds
and sunbirds, in which costs and benefits can be measured in calories
in the field.
An experiment by Frank Gill and Larry Wolf ( 1975 ) were done on golden-winged
sunbirds in East Africa.
They were able to measure the nectar content of the territories, in which
the sunbirds defend patches of Leonotis flowers.
From time budget studies and laboratory measurements of the energetic
costs of different activities like flight, sitting and fighting, they
also calculated how much a sunbird expends in a day.
When the daily costs were compared with the extra nectar gained by defending
a territory and excluding competitors, it turned out that the territorial
birds were making a slight net energetic profit, which means the resource
was economically defendable.
This idea of economic defendability has also been used to predict the
level of resource availability which could lead to territorial defence.
If resources are very scarce, the gains from excluding others may not
be enough to pay for the cost of territorial defence
An alternative is for the animal to abandon its territory and move elsewhere.
An upper threshold of resource availability beyond which defence is not
economical may also exist, and this boundary can arise for several reasons:
If there are many intruders trying to invade rich areas, the defence costs
will be prohibitively high
If the owner cannot make use of the additional resources made available
by defence, there may be no advantage of territoriality at high resource
levels
Territory defence may have associated risks such as increased conspicuousness
to predators, so that when resource levels are high enough to allow an
animal to satisfy its need without excluding others, territories should
be abandoned
Territories are not always defended just by single individuals, very
often two or more competitors share the same territory.
Such individuals are often mated pairs, as in most territorial birds,
so here the reasons for shared defence may be unrelated to economic considerations
of costs and benefits of resource defence.
But, sometimes shared defence may occur because of economic considerations,
and one such case which has been studied is Nick Davis and Alasdair Houston`s
study of the pied wagtail.
These birds defend territories along stretches of the river Thames in
the winter, where they feed on insects washed up by the river onto the
bank.
After the bird has foraged in one spot and eaten the insects, the numbers
gradually build up again as new insects are washed ashore.
To exploit this kind of renewing resource efficiently the wagtail visit
each place only after food has had a chance to renew itself. The territory
owner therefore works systematically around its territory and revisits
each spot on average about every 40 min after the insect supply has been
replenished.
Territory defence obviously pays, because without exclusive use the wagtail
could return after about 40 min to an old spot only to find that another
bird has just been there.
But even so, the wagtail does not always defend exclusive territories,
but sometimes instead tolerates a second bird there, a so called “satellite”.
The two birds move around the territory out of phase with one another,
so the average return time to a site is halved to 20 min, resulting in
a lower feeding rate for the owner.
So to counterbalance this cost there is a benefit of sharing, because
the satellite chases away intruders, and the owner therefore saves defence
time leaving more time to feed.
Of course the net effect of these costs and benefits maximizing feeding
rate depends on the food supply. On days when the rate of renewal of food
is high, the cost of sharing is relatively low, and on days when intrusion
rate is high, the benefit of sharing is relatively large.
Intrusion rate is closely related to asymptotic food levels, so sharing
will tend to pay on days with a high rate of food renewal and a high asymptotic
level.
After reading about the different experiments I have to conclude that
animals definitely behave according to the situation they are in, and
that the same animal may behave differently in the same environment, because
of other individuals present there.
For example, an animal may have used the same foraging strategy for a
long time, but if conditions change, it may switch and find a new and
better strategy to suit the new situation.
It is also very interesting that when humans have done calculations on
what the maximal profit an animal can get out of a given resource, taking
into account the different costs and benefits, this is exactly what the
animal is doing, or at least trying to achieve as good as it can, for
example by choosing to defend a resource or abandon it ( and this being
without calculations and formulas ! )
This can again raise new questions, on how they can “know” this, if it
is instinct, experiments, experience, or maybe another factor?
But I guess that would be enough material for a new essay.
Regarding this essay, I feel I found answers to the questions I initially
asked, and in addition ended up with a bunch of new ones!
References
Essentials of ecology; John L. Harper and Michael Bergen
Ecology; Charles J. Krebs
Animal Behaviour; Drickhammer, Vessay, Meikle
Owners and satellites: the economics of territory defence in the pied
wagtail; Davies, N.B. and Houston
Economics of feeding territoriality in the golden-winged sunbird; Gill,
F.B and Wolf
Territory and population regulation in the red grouse; A. Watsen
Territorial defence in the great tit; J.R. Krebs