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7.5 Carrying capacity and factors affecting population sizes within communities/ecosystems
Population growth and carrying capacity
In most ecosystems, the number of individuals of a species (its population) will fluctuate over time, but this is not a strict
rule. Sometimes, population sizes can remain fairly constant over a long period, and in other cases, a sudden event can
cause a dramatic increase or decrease in the number of individuals. The size of a population boils down to the difference
between the mortality rate (death rate) and reproduction rate (birth rate).
lag phase log phase stationary phase
During the lag phase there may only be a few individuals, still acclimatising to their habitat, and at this point the
reproduction rate (r) is low and the growth rate of the population is slow
During the log phase (exponential phase), resources are plentiful and conditions are good, and the reproduction rate
increases significantly, exceeding mortality rate, and as a result, population size increases rapidly during this phase
During the stationary phase the habitat itself cannot support a larger population size, which has levelled out to the
point where the habitat is said to have reached its carrying capacity (k), so the rates of reproduction and mortality are
equal, so the population size stays about the same, with very small fluctuations
When a species within a habitat reaches its carrying capacity, the habitat cannot support a larger population because of
any combination of limiting factors, such as: the availability of food/prey, suitable space, weather and abiotic factors,
predation, competition, disease and parasites, etc. The carrying capacity is the upper limit that these factors place upon
the population size.
k- and r-strategists
Species whose population size is determined by the carrying capacity are often called k-strategists. For these populations,
the limiting factors exert a more and more significant effect the closer the population approaches its carrying capacity,
causing the population size to gradually level out.
However, some species adopt a different type of population growth. In
these cases, the population size increases so quickly that it can exceed
the carrying capacity of the habitat before the limiting factors have time
to come into effect. Unfortunately for the species, once the carrying
capacity has been exceeded, this clearly means there will not be enough `boom'
resources to allow many members to reproduce (or even survive), and an
excessive build up of waste products may begin to poison them, entering
the species into the death phase. This is a boom and bust type of Time
population growth, and the species which adopt this are called r-strategists.
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For r-strategists the most important factor affecting growth rate within the population is the rate at which they can
reproduce (r). This is a characteristic attribute for species with short generation times, such as bacteria, and also of
pioneer species (see 7.3 Succession). Quick population growth means a pioneer r-strategist species can occupy a
disturbed habitat before a species of k-strategists, dispersing to other habitats once limiting factors start to take effect.
Predator and prey relationships
Predators hunt prey.…read more