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PRINCIPLES OF POPULATION ECOLOGY

Because the human population is central to so many environmental problems, and their solutions, it is important that we understand how populations increase or decrease. The biological principles that affect the sizes of all animal and plant populations also apply to human populations.

The rate of change or growth rate (r), of a population is the birth rate (b) minus the death rate (d).

R= b-d

As an example, consider a hypothetical population of 10,000 in which there are 2,000 births per year (or 200 births per 1,000 people) and 1,000 deaths per year (or 100 deaths per 1,000 people).

R= 2000/10000  -  1000/10000

R= 0.2-0.1 = 0.1

A value of 0.1 for r means that the population has an annual percentage growth rate of 10 percent.

    Another way to express the growth rate of a population is to determine the doubling time—the amount of time it would take for the population to double in size, assuming that its rate of increase doesn't change. Doubling time (tj) is calculated as approximately 0.7 divided by the growth rate.

tj  = 0.7/r

In our example (r = 0.1), the doubling time would be 0.7/0.1 = 7 years.

In addition to the birth and death rates, migra­tion (movement from one region or country to an­other) must be considered when changes in popula­tions on a local scale are examined. There are two types of migration: immigration, by which individ­uals enter a population and thus increase the size of the population, and emigration, by which individu­als leave a population and thus decrease its size. The growth rate of a local population of organisms must take into account birth rate (b), death rate (d), immigration (i) and emigration (e}. The growth rate is equal to the value of birth rate minus death rate, plus the value of immigration minus emigration:

R = (b – d) + (i – e)

For example, the growth rate of a population of 10,000 that has 1,000 births, 500 deaths, 10 immi­grants, and 100 emigrants in a given year would be calculated as follows:

     r = (1,000/10,000 – 500/10,000) + (10/10,000 – 100/10,000)

     r = (0.l -0.05) + (0.001 -0.01)

     r = 0.05 - 0.009 = 0.041

Note that, although emigration exceeds immigra­tion in this example, the growth rate is still positive due to the very high birth rate. Can you calculate the doubling time for this population?

Maximum Population Growth

The maximum rate at which a population could increase under ideal conditions is known as its biotic potential. Different species have different biotic potentials. A particular species' biotic poten­tial is influenced by several factors, including the age at which reproduction begins, the percentage of the life span during which the organism is capable of reproducing, and the number of offspring pro­duced during each period of reproduction.

    Generally, larger organisms, such as blue whales and elephants, have lower biotic potentials, whereas microorganisms have the greatest biotic potentials. Under ideal conditions, certain bacteria can reproduce by splitting in half every 20 to 30 minutes. At this rate of growth, a single bacterium would increase to a population of more than 1,000,000 in just 10 hours (Figure 8-la), and the population from a single individual would exceed ! billion in 15 hours!

    If one were to plot this increase versus time, the graph would have the "J" shape that is characteris­tic of exponential growth, the constant reproduc­tive rate that occurs under optimal conditions. Regardless of the organism being considered, whenever its biotic potential is plotted versus time, the shape of the curve is the same. The only variable is time; that is, it may take longer for a sea lion population than for a bacterial popula­tion to reach a certain size, but its population will always increase exponentially under ideal condi­tions.

How Nature Limits Population Growth

Certain populations may exhibit exponential growth for a short period of time. However, organ­isms cannot reproduce indefinitely at their biotic potentials, because the environment sets limits, which are collectively called environmental resistance. Using the earlier example, bacteria would never be able to reproduce unchecked for an ex­tended period of time, because they would run nut of food and living space, and poisonous body wastes would accumulate in their vicinity. with crowding, they would also become more susceptible to parasites

and predators As their environment changed, their birth rare (b) would decline and their death rate (d) would increase due to shortages of food, increased predation, increased competition, and stress. The environmental conditions might worsen to a point where d would exceed b and the popula­tion would decrease. The number of organisms in a population, then, is controlled by the ability of the environment to support it.

    Over longer periods of time, the rate of population growth for most organisms decreases to around zero. This leveling out occurs at or near the limit of die environment's ability to support a population. The carrying capacity represents the highest popu­lation that can be maintained for an indefinite pe­riod of time by a particular environment.

    When population over a longer period of time is graphed, the curve has a character­istic "S" shape that shows the population's initial exponential increase (note the curve's "J" shape at the start), followed by a leveling out as the carrying capacity of the environment is approached. Al­though the S-curve is a simplification of actual population changes over time, it does appear to fit the population growth observed in many popula­tions that have been studied in the laboratory and a few that have been studied in nature. For example, G. F. Cause grew a single species, Paramecium caudatum, in a test tube. He supplied a constant but limited amount of food daily and re­plenished the media occasionally to eliminate the buildup of metabolic wastes. Under these condi­tions, the population of P. caudatum increased exponentially at first. The paramecia became so numerous that the water was cloudy with them. But then their rate of increase declined and their population leveled off.

Sometimes, when a population exceeds the car­rying capacity and environmental degradation re­sults, a population crash occurs. In 1910 a small herd of 26 reindeer was introduced on one of the Pribil of Islands of Alaska. The herd's population increased exponentially for about 25 years until there were approximately 2,000 reindeer, many more than the island could support. The reindeer overgrazed the vegetation until the plant life was almost wiped out. Then, in slightly over one dec­ade, as reindeer died from starvation, the number of reindeer plunged to eight, one-third the size of the original introduced population.