Environmental Engineering Reference
In-Depth Information
chances of mating. Lower population densities decrease the chance of competition and the spread of dis-
ease but may also make it more difficult to find a mate.
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Age structure:
Examines the number of organisms in each age range within a population. This distribu-
tion will affect whether a population grows, declines, or remains stable over time. If most individuals are
younger, the population will most likely increase in number. When a population is comprised mostly of
older individuals past reproductive age, the population will probably decline. Even age distribution re-
flects a stable population. The age structure of a population is shown in age-structure diagrams or age
pyramids.
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Sex ratio:
The number of males to females in a population. This can affect whether a population of or-
ganisms will increase or decrease over time because it affects the chances of mating.
Carrying Capacity
Given the right situation and enough resources, a population can grow. In
exponential growth,
a population
increases by a fixed percentage per unit of time. Exponential growth is represented on an exponential growth
curve (see the following figure). A population can grow exponentially when it's using an unused resource or
colonizing a new environment. Exponential growth cannot last indefinitely, though, because resources are fi-
nite. Humans are an example of a population that has experienced exponential growth. The question is, when
will our resources run out?
Because resource availability will eventually decrease, exponential growth will be halted when a population
reaches
carrying capacity (K).
This is the maximum number of organisms in a species that an environment
can support indefinitely. Carrying capacity is not a set number and can vary depending on the environment,
populations, and limiting factors.
A population reaches its carrying capacity based on
limiting factors,
which are the factors that control a popu-
lation's growth. Limiting factors can be the availability of food, shelter, water, mates, or anything else an or-
ganism depends upon for survival. A population can also be limited by disease, predators, natural disasters,
sunlight, moisture, temperature, or nutrients. And in an aquatic ecosystem, salinity, sunlight, pollutants, dis-
solved oxygen, or temperature can play a role in restricting population growth. All limiting factors acting on a
population together are called
environmental resistance.
Sometimes a species can alter its environment, thus
