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Population Ecology: Chapter 53
A population is defined as a group of individuals of the same species, living in a general area.
These populations are characterized by the fact that they are likely to interact and breed.
Populations are often created by a physical barrier, such as a river or canyon.
The Population Density is the number of individuals per area or volume. This density is tracked
over the entire area the population inhabits. The pattern of spacing of the individuals within the
population's boundaries is known as dispersion.
In order to record the size of populations, ecologists use the Mark-Recapture Method. This
Tagged Turkey Vulture
method involves tagging a certain number of individuals of a population, waiting for the tagged
individuals to encorperate back into the population, then taking another group of individuals.
The ecologists can then take the percent of individuals that were recaptured to the percent of
those that were new individuals. Using these figures, they can extrapolate an estimate number
of individuals in a population.
These figures must be considered estimates, since there are to many variables to take a precise
measurement. These variables are birth, death, immigration, and emigration. Immigration is the
act of individuals from one population moving to another. Emigration is when a population losses
individuals due to immigration.
How organisms organize themselves in an area is known as Patterns of Dispersion. The three
patterns are clumped, uniform, and random. Clumping generally is due to food being plentiful in
certain areas. Uniform is usually caused by certain space being needed to prevent aggressive
Patterns of Dispersion
interactions between neighbors. Random is generally practiced by plants and other organisms
that do not decide where they inhabit, so there is little rhyme or reason to their layout.
Demography is the study of how the vital statistics of populations change over time, this is
based around birth and death rates mostly. To chart these, demographers use Life Tables,
which are age specific summaries. These tables contain information such as death rate, life
expectancy, and average deaths per year. The graphic method of expressing data found on
life tables, is widely known as a survivorship curve. A survivorship curve shows the proportion
of individual alive at each age. Demographers that focus primarily on sexually reproducing
species will often create reproductive tables, or a fertility schedule. These tables are age
specific like Life Tables, except reproductive tables chart the reproductive output of an average
Life table, Survivorship Curve
individual until its death.
When population growth is tracked, it is separated into two catagories. Times of zero population
and times of exponential population growth. Zero population growth is characterized by population
decreasing or plateauing. Exponential population growth is characterized by population growing.
Populations will stop growing at some point regardless of how good the conditions are, due to
restrictions placed on the population by the environment it is in. Ecologists call this a carrying
capacity. The slight changes in the environment can change the carrying capacity of that
environment, this triggers logistical population growth.
Each organism is effected by natural selection. The set of traits each organism has effects its
ability to survive and reproduce. This set of traits is known as a life history.
Reproduction is different between organisms. Some organisms that rely on semelparity, such as
salmon and many insects,reproduce once on a massive scale, then die. Where other organisms
rely on iteroparity, which is repeated reproduction. When birth rate is effected by population density,
it is known as density dependent, where if it is not, it is known as density independent.
Long term studies reflect that populations fluctuate in size naturally. This fluctuation is known as
population dynamics. Population dynamics is effected primarily by the ebb and flow of predators.
For example, as the population of moose decreases due to predation, so does the population of
wolves, which then allows the moose population to increase. Another contributor to the fluctuation
of population dynamics is interaction with the metapopulation. When many local populations are
within reach of eachother, individuals will immigrate and emigrate between populations. This
interaction forms a metapopulation.
Age Structure is another important demographic variable. Age Structure is most commonly
illustrated as a pyramid displaying percent of population to age.
As an ecosystem, the earth has a carrying capacity. Out of all the species on earth, humans have
caused the most stress on the ecosystem. Our need for food, water, shelter, fuel, and building
materials increases our ecological footprint drastically.
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