Download Fluctuations in the size of a population are often difficult to measure

6. Stability in an ecosystem is a balance between competing effects. As a basis for understanding this
concept:
c. Students know how fluctuations in population size in an ecosystem are determined by the relative rates of
birth, immigration, emigration, and death.
Fluctuations in the size of a population are often difficult to measure directly but may be estimated by
measuring the relative rates of birth, death, immigration, and emigration in a population. The number of deaths
and emigrations over time will decrease a population’s size, and the number of births and immigrations over
time will increase it. Comparing rates for death and emigration with those for birth and immigration will
determine whether the population shows a net growth or a decline over time.
Notes:
What affects population growth?
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Fertility or birth rates
Death rates
Migration – immigration is individuals entering the population and emigration is individuals leaving the
population.
Population Growth (Change) = (births + immigration) - (deaths + emigration)
Population density is the number of organisms inhabiting a given area at a given time. As the population density
increases, density-dependent factors that limit population growth come into play. These factors include:
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Competition for resources and space.
Predators finding prey becoming easier. The density of the prey population determines the density of the
predator population.
Population growth is also affected by density.
Independent factors such as catastrophic events or events that cause the deprivation of a limiting resource
such as drought.
As resources become scarcer, competition for these resources becomes more intense. There are two different types
of competition which are:
1. Intraspecific competition - This is competition for available resources and space among members of the same
species. Although members of a species share similar characteristics, cacti individual posses its own characteristics
that distinguishes it from all the other members of the species. Some of these characteristics make some individuals
better able to survive than other members of their species. They are said to be better adapted to their environment.
Thus, if conditions become harsh, those members of the species who possess the most helpful adaptations will
survive. Those that are less adapted will not. This competition rarely ends in fatal injuries
2. Interspecific competition - This is competition between two different species. This occurs when the niches of
the two species overlap. The greater the overlap the more requirements they have in common. As resources being
competed for become scarcer, the competition becomes more intense. This may eventually result in the elimination
of one of the species from the ecosystem. The more successful or better adapted species occupies the niche.
Predation is another factor limiting population size. The predator’s population depends upon the size of its prey's
population. As the prey's reproductive cycle kicks in and begins to grow the population size of the predator will also
increase in tandem.