9-1 Population Dynamics and Carrying Capacity

What Are the Major Characteristics of a Population? Changing and Clumping

Population dynamics is a study of how populations change in size, density, and age distribution in response to changes in environmental conditions. Populations can also change in how they are distributed in their habitat. Three general patterns of population distribution or dispersion in a habitat are:

  • Clumping

  • Uniform dispersion

  • Random dispersion

The populations of most species live in clumps or groups. Examples are patches of vegetation, cottonwood trees clustered along streams, wolf packs, etc. There are 4 main reasons for this:

  1. The resources a species needs vary greatly in availability from place to place.

  2. Living in herds, flocks, and schools can provide better protection from predators.

  3. Living in packs gives some predator species such as wolves a better chance of getting a meal.

  4. Some animal species form temporary groups for mating and caring for their young.

What Factors Govern Changes in Population Size? Entrances and Exits on the Global Stage

Four variables govern changes in population size:

  1. Births

  2. Deaths

  3. Immigration

  4. Emigration

Where:

Population Change = (Births + Immigration) - (Deaths + Emigration)

A population’s age structure can have a strong effect on how rapidly its size increases or decreases. Age structures are usually described in terms of organisms that are not mature enough to reproduce (the prereproductive stage), those that are capable of reproduction (the reproductive stage), and those that are too old to reproduce (the postreproductive stage).

What Limits Population Growth? Resources and Competitors

Populations vary in their capacity for growth, also known as the biotic potential of a population. The intrinsic rate of increase (r) is the rate at which a population would grow if it had unlimited resources.

Individuals in populations with a high rate of growth typically reproduce early in life, have short generation times (the time between successive generations), can reproduce many times (have a long reproductive life), and have many offspring each time they reproduce.

Environmental resistance consists of all factors that act to limit the growth of a population. The size of the population of a particular species in a given place and time is determined by the interplay between its biotic potential and environmental resistance.

Together, biotic potential and environmental resistance determine the carrying capacity (K).

What Is the Difference between Exponential and Logistic Population Growth? J-Curves and S-Curves

A population with few if any resource limitations grows exponentially. In exponential growth, a population grows at a fixed rate such as 1% or 2%. It starts slowly and grows faster as the population increases because the base size of the population is growing. Plotting this yields a “J-curve”

Logistic growth involves rapid exponential population growth followed by a steady decrease in population growth with time until the population size levels off. This occurs as the population encounters environmental resistance and its rate of growth decreases as it approaches the carrying capacity of its environment. Plotting this yields an “S-curve”.

What Happens If the Population Size Exceeds the Carrying Capacity? Diebacks

The populations of some species do not make a smooth transition from exponential growth to logistic growth. Instead they use up their resource supplies and temporarily overshoot, or exceed, the carrying capacity of their environment.

In such cases the population suffers a dieback, or crash, unless the excess individuals can switch to new resources or move to an area with more resources.

How Does Population Density Affect Population Growth? Some Effects of Clumping

Density-independent population controls affect a population’s size regardless of its density. Such controls include floods, hurricanes, unseasonable weather, fire, habitat destruction (such as clearing a forest of its trees or filling in a wetland), pesticide spraying, and pollution.

Some factors that limit population growth have a greater effect as a population’s density increases. Examples of such density-dependent population controls include competition for resources, predation, parasitism, and infectious disease.

How Do Wildlife Managers Apply the Concepts of Growth Curves and Carrying Capacity?

Wildlife managers are called upon to use their knowledge of ecology, biology, and in particular population growth to achieve desired outcomes.

A wildlife manager might wish to increase carrying capacity by improving habitat or adding food. Another strategy would be to decrease environmental resistance—perhaps through predator control— although there is a risk that the population will overshoot its carrying capacity and crash to low numbers.

Yet another strategy would be to harvest about 50% of the animals, pushing population numbers about halfway down the curve where the biotic potential again favours rapid population growth

Since this approach runs the risk of depleting the population beyond its capacity to rebound quickly, many wildlife managers prefer to use optimum sustained yield. This typically involves harvesting 30% or fewer of the animals, and allowing predators to fulfill a role by providing some natural selection and culling unhealthy animals.

What Kinds of Population Change Curves Do We Find in Nature? Variety Is the Spice of Life

In nature we find four general types of population fluctuations:

  • Stable

  • Irruptive

  • Cyclic

  • Irregular

Some species, such as the raccoon and feral house mouse, normally have a fairly stable population. However, their population growth may occasionally explode, or irrupt, to a high peak and then crash to a more stable lower level.

The third type consists of cyclic fluctuations of population size over a regular time period.

Finally, some populations appear to have irregular behaviour in their changes in population size, with no recurring pattern

Do Predators Control Population Size? The Lynx–Hare Cycle

Some species that interact as predator and prey undergo cyclic changes in their numbers: sharp increases are followed by crashes. For decades, predation has been the explanation for the 10-year population cycles of the snowshoe hare and its predator, the Canadian lynx. According to this top-down control hypothesis, lynx preying on hares periodically reduce the hare population. The shortage of hares then reduces the lynx population, which allows the hare population to build up again.

Another (bottom-up) theory is that periodic crashes in the hare population are influenced by their food supply. Large numbers of hares can die following a period when they consume food plants faster than the plants can be replenished, especially during winter.

These theories are not mutually exclusive