Click4Biology

5.3.1 Factors affecting population size

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Factors that increase population size:

Natality is recruitment to a population through reproduction
Immigration from external populations e.g. Bird migration

Factor reducing population size:

Mortality which is the death rate from any source e.g. predation
Emigration, where individuals leave the population for another habitat

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5.3.2 Population growth curve.

Model of population growth:

This models shows the changes in the population size against time. In this model the population is expanding into a habitat offering initial low resistance or low limiting factors

a) Exponential Phase of rapid population growth rate

b) A slowing yet still high growth rate against time or generations

c) Plateau stage in which the population remains stable with time. There maybe slight oscillations in the numbers

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5.3.3 Phases of the population growth curve.

a) Exponential Phase:

With low or reduced limiting factors the population expands exponentially into the habitat. The population may be increasing at 2ⁿwhere n= number of generations. The rate of natality + immigration is greater than Mortality + emigration. Note that it does not mean that death/ emigration= 0

This would be typical of a population of germinating annual plants in a new season.
Another example would be a a bacterial population during the initial phases of an infection.
The population of any species occupying a previously unoccupied habitat (succession)

b) Transition Stage: Resources are reduced and become limiting in the growth of the population:

As a population grows then there will be increased competition between the individuals of that population for the same resources.
This competition means that some individuals will obtain resources and survive and others will not.
Overall the effect is to produce a lower rate of population increase that observed during the exponential phase.
Notice the link here between ecology and evolutionary theory. There will not just be a reduced rate of population growth but also a selection (survival and reporduction) of these individuals within the population best suited to using, accessing the resources.

''........can we doubt (remembering that many more individuals are born than can possible survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and procreating their kind? On the other hand, we may feel sure that any variation in the least injurious would be rigidly destroyed. This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection' Darwin C. (1859) The Origin of Species

c) The Population plateau is where the population remains constant over time/ generations

A constant population means that Natality + Immigration = Mortality + Emigration
The population size is determined by the carrying capacity of the habitat at that point in time

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