Environmental Engineering Reference
In-Depth Information
increasing its carrying capacity and decreasing its environmental resistance. Humans are an example of a spe-
cies that has created ways of bettering survival through alteration of the environment and through invention.
Some limiting factors are related to the density of a population. These factors are considered
density-depend-
ent factors
and include disease, availability of mates, and predation. A population that is denser is at an in-
creased risk of predation or the transmission of disease, yet it has a higher probability of finding mates. On the
other hand,
density-independent factors
do not depend on the density of a population. For example, natural
disasters, extreme temperature fluctuations, or lack of sunlight can affect the numbers of a species, regardless
of whether the organisms are in a crowded population.
In order to represent a population that grows exponentially and then reaches its carrying capacity, a
logistic
growth curve
(see the following figure) is used. Initially, a population increases quickly; then it levels off due
to limiting factors. The logistic growth curve is generally a theoretical model because actual populations of or-
ganisms don't behave as the curve suggests. Different populations act in a variety of ways depending on the en-
vironment, the species, and limiting factors. More realistic logistic growth curves show populations that:
• Fluctuate above and below carrying capacity for an indefinite amount of time
• Rise quickly and then decrease abruptly
• Fluctuate for a period of time, and then start to experience less dramatic changes and stabilize to an extent
• Exhibits a pattern similar to the theoretical model
Reproductive Strategies
Limiting factors affect a population's growth and decline, along with an organism's
biotic potential,
the ability
of an organism to produce offspring. There are two types of reproductive strategies:
•
K-selected:
K-selected species are species that have relatively few offspring and devote a large amount
of time, energy, and resources toward nurturing and raising their young. These organisms usually are lar-
ger, have long gestation periods, and live longer. Because only a few offspring are produced, k-selected
species have low biotic potential. Overall, these populations also remain close to carrying capacity and
have a relatively constant population size. Examples include humans, elephants, horses, and cows.
