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Summary ZOL3702 learning unit 5
- Course
- ZOL3702 - Ecology (ZOL3702)
- Institution
- University Of South Africa (Unisa)
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ZOL 3702 LEARNING UNIT 5INTRASPECIFIC COMPETITION
Textbook reference: pg 132-162
Learning outcomes:
Explain the nature and characteristics of intraspecific competition.
Describe influence of intraspecific competition on density-dependent mortality, fecundity, density effect.
Describe and quantify intraspecific competition and the regulation of population growth.
Explain density dependent growth.
Give an overview of the use of mathematical models in population dynamics.
Describe and compare models for species with discrete and continuous reproductive seasons.
Intraspecific competition arises between organisms of the same species.
Competition is interaction among individuals because they have the same resource requirements.
THE NATURE AND CHARACTERISTICS OF INTRASPECIFIC COMPETITION pg 132-134
Individuals from the same species have the same needs for survival, growth, and reproduction. The
collective demand for resources may exceed the supply, and this leads to competition.
The ultimate effect of competition is a reduction in survivorship and fecundity because of the smaller
contribution to the next generation.
EXPLOITATION COMPETITION
Interaction is often indirect in sense that resources are reduced by individuals – exploitation competition.
Instead, individuals respond to levels of a resource, which has been depressed by presence and activity of
other individuals.
Where each individual is affected by the amount of resource that remains after that resource has been
exploited by others, we call it exploitation.
Exploitation can only occur if the resource in question is in limited supply.
The extent of the exploitation competition depends on the quantity of resources available.
INTERFERENCE COMPETITION
Individuals interact directly with each other, and one individual will prevent another from exploiting the
resources within a portion of the habitat.
Territorial behavior is involved.
In this case, space can be seen as a resource in limited supply.
Interference competition may occur for resource of value, where interference is accompanied by
exploitation, or for surrogate resource which is only valuable because of access it provides to real
resource.
With exploitation, the intensity of competition is closely linked with the level of resource present and level
required, but with interference, intensity may be high even when level of the real resource is not limiting.
ONE-SIDED COMPETITION
Whether they compete through exploitation or interference, individuals within a species may have
features in common , using similar resources and reacting in the same way to conditions.
However, intraspecific competition may be 1 sided. Eg. a strong early seedling welched a stunted late one.
The effect of competition is different for each individual.
Weak competitors may make only a small contribution to the next generation, or no contribution at all.
Strong competitors may have their contribution only negligibly affected.
The likely effect of intraspecific competition on any individual is greater the more competitors there are.
The effects of intraspecific competition are said to be density dependent.
This is called proportional contribution.
DENSITY DEPENDENT MORTALITY AND FECUNDITY pg 134-135
, Fig 5.3 shows pattern of mortality in flower beetle Tribolium confusum where cohorts were reared at a
range of densities.
Known number of eggs were placed in glass tubes with 0.5 g are they flour yeast mixture , and the number
of individuals that survived to become adults in each tube was noted.
Throughout region 1 (low density) the mortality rate remained constant as density increased.
The number's dying and numbers surviving both rose, but the proportion dying remained the same, which
accounts for the straight lines in region one of these figures.
Mortality in this region is said to be density independent.
Individuals died, but chance of individual surviving to become an adult was not changed by initial density.
Judged by this, there was no intraspecific competition between the beetles at these densities.
Such density independent deaths affect the population at all densities. They represent a baseline which
any density dependent mortality will exceed.
In region 2, the mortality rate increased with density - there was density dependent mortality.
The no. dying continued to rise with density, but unlike region 1 they did so more than proportionately.
The numbers surviving also continued to rise, but this time less than proportionally.
Over this range increases in egg density continued to lead to increases in total no. of surviving adults.
The mortality rate had increased, but it undercompensated for increases in density.
In region 3 intraspecific competition was even more intense.
The increasing mortality rate overcompensated for any increase in
density.
Over this range, the more eggs there were present, the fewer adults
survived - an increase in the initial number of eggs led to an even
greater proportional increase in the mortality rate.
If the range of densities had been extended, there would have been
tubes with no survivors - but developing beetles would have eaten all
the available food before any of them reached the adult stage.
Fig 5.4 shows relationship between density and mortality in trout.
At lower densities there was under compensating density
dependence, but at higher densities mortality never overcompensated.
Rather it compensated exactly for any increase in density - any rise in the number of fry was matched by
an exactly equivalent rise in the mortality rate.
The no. of survivors therefore approached and maintained a constant level, irrespective of initial density.
Region 1 (density independent) in low-density area the mortality rate remains the same, even if there is a
linear increase in no. of organisms that survive and die. Therefore, there is no density dependent or
intraspecific competition.
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