The mathematical models that we looked at in the previous section are highly simplistic. Even the logistic model, which does limit population growth, doesn’t attempt to account for the different environmental factors that may affect, or regulate, the population size. Density-dependent factors intensify as the population gets larger. Density-independent factors are not related to population size.

Here are six factors that may affect the size of populations of organisms.

Click the check box next to the factors that you think are density-dependent. Then click Submit.

Correct: That’s right!

Incorrect: That’s not quite right

All: Scarce food supplies, overcrowding, and the availability of nesting sites are all density-dependent factors. If there is little food, the growth of the population may be limited by malnutrition or starvation. Gardeners know that overcrowding produces smaller plants and poorer yields of produce. Sea birds that nest on rocky ledges won’t be able to nest if all the ledges are already occupied.

On the other hand, the effects of weather, pests, and hurricanes are all largely independent of population density. Whether a grower has 20 or 5000 fig trees in an acre, the entire crop will be lost with a hard winter freeze. Similarly, a hurricane can destroy almost any population in its path, from growing crops to humans.

Two other factors that can affect population size are disease and predation. These factors may be influenced by population density, but sometimes not in the way you might expect.

A herd of wildebeest on the African savanna will group together to protect the young animals in the center of the group. A lion might not attack the herd for fear of being injured by a number of the wildebeest. However, a single young wildebeest on its own would be no match for a hungry lion.

In this example, a large population would be less likely to suffer losses through predation than a small group.

Disease is more often associated with highly dense populations. Any contagious disease will be spread more quickly if the individuals come into close contact. Diseases that are spread by a carrier, such as the rat- and fleaborne plagues of the Middle Ages, are also more likely to cause serious outbreaks in crowded cities.

The population of the snowshoe hare of Canada and Alaska has been measured since the 1850s, because the hares used to be prized for their pelts. The snowshoe hare, like a number of other animal species, goes through periodic cycles of population changes. The population steadily grows and then suddenly drops to very low numbers. There are many different reasons for population changes of this kind. Let’s look at several of them.

Look at the graph showing the numbers of snowshoe hares over time. Use the graph to estimate the length of the hare’s population cycle. Is it 5 years, 10 years, or 20 years?

Correct: That’s right!

Not correct: That’s not quite right!

All: The periodic cycle is about 10 years in length.

Lynx are members of the cat family that eat snowshoe hares. Do you think the lynx population cycle will:

A. Follow the same highs and lows as the hare population cycle?

B. Follow the same highs and lows as the hare population cycle, but be offset by a year or two?

Or

C. Remain steady throughout the whole period shown in the graph?

Click on your answer, then click Submit.

Correct: That’s right!

Incorrect: That’s not quite right!

All: The correct answer is B. The lynx population will rise as the available food supply—the hares—rises, but there will be a lag of a year or two until the well-fed lynx produce more offspring. When the hare population decreases again, food becomes scarce, and fewer lynx are born.

Scientists believe that the snowshoe hare cycle is due to the hares exhausting their winter food supply of willow and birch plants. This isn’t surprising, because the number of hares in a peak year has been as high as 6500 animals per square mile! However, the cycle in the lynx population is probably due to the fluctuating numbers of snowshoe hares.

There always seems to be a factor that prevents any organism from growing exponentially for a long period. Even bacteria, which have life cycles measured in minutes, can only divide until their growth medium is exhausted.

The exception to this rule is our own species. We have developed ways to increase food production and to use the raw materials of our planet to construct everything from housing to shelter to airplanes so we can travel vast distances in a short time.

The one problem we haven’t solved is how to slow down the exponential growth of the human population, and the phenomenal use of the Earth’s resources needed to fuel this growth.

It's up to us—as individuals and as citizens— to decide how to slow down the growth rate. There is certainly a carrying capacity for our population on the planet Earth, but whether we will ever reach this capacity may be in doubt. Unless we take steps to control our industrial growth, we’ll probably run out of resources well before we run out of space!

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education