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Populations in the Ecosystem
Exponential Growth of Populations
A population's ability to grow depends partly on the rate at which its organisms can reproduce.
Bacteria are among the fastest-reproducing organisms. A single bacterium can reproduce every
20 minutes under laboratory conditions of unlimited food, space, and water. The bacteria would
be reproducing at it’s biotic potential. Biotic potential is the rate at which a population would
produce young if every new individual lived and reproduced at its maximum capacity. After just
36 hours, this rate of reproduction would result in enough bacteria to form a layer almost half a
meter deep covering the entire planet. In general, larger mammals reproduce at slower rates than
smaller mammals. For example, elephants produce fewer offspring than mice and have longer
time intervals between offspring. But even so, in theory, the descendants of a single pair of
mating elephants would number 19 million elephants within 750 years.
In these theoretical examples, the bacteria and elephant populations are undergoing exponential
growth, in which the population multiplies by a constant factor at constant time intervals.
Consider the bacteria example. The constant factor for this population is 2, because each parent
cell splits, forming two offspring cells. The constant time interval is 20 minutes. So every 20
minutes, the population is multiplied by 2. Graphing these data forms the J-shaped curve.
Therefore, bacteria are called a J-curve population.
Carrying Capacity
In nature, a population may start growing exponentially, but eventually one or more
environmental factors will limit its growth. The population then stops growing or may even begin
to decrease. For example, consider lily pads growing and spreading across the surface of a pond.
Once the pond is covered in lily pads, no more can grow. Space is one example of a limiting
factor, a condition that can restrict a population's growth. Other limiting factors include disease
and availability of food. When such environmental factors limit a population's growth rate, the
population is said to have reached its carrying capacity. The carrying capacity is the number of
organisms in a population that the environment can maintain, or carry, with no net increase or
decrease. As a growing population approaches carrying capacity, rapidly reproducing organisms
will overshoot or exceed the carrying capacity by a lot and the population will crash. Slower
reproducing species will not overshoot the carrying capacity as much and their population will
stabilize. Graphing the growth of a slower reproducing species forms an S-shaped curve and
organisms like these are called S-curve populations
Factors Affecting Population Growth are called Limiting Factors
In the laboratory, you can observe the effects of environmental factors such as food availability or
temperature on population growth. For example, if you put fruit flies in a container and add the
same amount of food each day, the population rapidly increases until the daily food supply cannot
support more flies. Food has become a limiting factor. There are two types of limiting factors.
Density-Dependent Factors Factors similar to those affecting laboratory fruit flies affect
natural populations. For example, the best nutrition for white-tailed deer is the new leaves and
buds of woody shrubs. When deer population density is low, this high-quality food is abundant,
and a large percentage of the females bear offspring. On the other hand, when the population
density increases, the nutritious food supply becomes scarce due to overgrazing, and many
females do not reproduce at all. The availability of high-quality food is one example of a densitydependent factor, a factor that limits a population more as population density increases. Another
example of a density-dependent factor is a disease that spreads more easily among organisms in a
dense population than in a less dense population.
Density-Independent Factors Factors that limit populations but are unrelated to population
density are called density-independent factors. Extreme weather events, such as hurricanes,
blizzards, ice storms, and droughts, are examples of density-independent factors. These
conditions have the same effect on a population regardless of its size
Population Growth Cycles
Some populations have "boom-and-bust" growth cycles: They increase rapidly for a period of
time (the "boom"), but then rapidly decline in numbers (the "bust"). Populations of various
rodents exhibit boom-and-bust cycles. A striking example is lemming populations, which can
cycle dramatically every three to five years. Some researchers hypothesize that natural change in
the lemmings food supply may be the underlying cause. Another hypothesis is that stress from
crowding during the "boom" may affect the lemmings' hormonal balance and reduce the number
of offspring produced, causing a "bust." Some populations' growth cycles appear to be influenced
by those of other populations in their environments. For example, in the forests of northern
Canada, both the lynx and the snowshoe hare follow boom-and-bust cycles. Hare populations
have a greater biotic potential than the lynx. As the hare population grows, more food is
available to the lynx. As the lynx population begins to grow, more hares are captured and their
population begins to decline. With less prey present, lynx begin to starve and their population is
lowered. When the numbers of predators are reduced, the hare population begins to increase
again and the process repeats itself.
Questions:
Answer the following questions on your own sheet of paper:
1. What is biotic potential? Under what conditions can a population reach its biotic
potential?
2. What is exponential growth?
3. What is a limiting factor?
4. What is an ecosystems carrying capacity?
5. How do S-curve species and J-curve species differ when they reach an ecosystem’s
carrying capacity?
6. What is overshoot?
7. What is a density-dependent limiting factor? List several density-dependent limiting
factors.
8. How does a density-independent limiting factor differ from a density-dependent limiting
factor? List several density-independent limiting factors.
9. Explain how the snowshoe hare and lynx populations affect each other.