Cracking the AP Biology Exam


Animal Behavior and Ecology


Population ecology is the study of how populations change. Whether these changes are long-term or short-term, predictable or unpredictable, we’re talking about the growth and distribution patterns of a population.

When studying a population, you need to examine four things: the size (the total number of individuals), the density (the number of individuals per area), the distribution patterns (how individuals in a population are spread out), and the age structure.

One way to understand the growth pattern and make predictions about the population growth of a country is by examining age structure histograms. For example, in underdeveloped countries, where the population increase is high, the base of the histogram is very wide compared to countries that show moderate growth.

Another way to study the changes in a population is by looking at survivorship curves. These curves are graphs of the numbers of individuals surviving to different ages, indicating the probability of any individual living to a given age.

For example, the graph below shows there is a high death rate among the young of oysters, but those that survive do well. On the other hand, there is a low death rate among the young of humans, but, after age 60, the death rate is high.

The growth of a population can be represented as the number of crude births minus the number of crude deaths divided by the size of the population:

r = (births – deaths)/N

(r is the reproductive rate, and N is the population size)

Each population has a carrying capacity—the maximum number of individuals of a species that a habitat can support. Most populations, however, don’t reach their carrying capacity because they’re exposed to limiting factors.

One important factor is population density. The factors that limit a population are either density-independent or density-dependent. Density-independent factors are factors that affect the population regardless of the density of the population. Some examples are severe storms and extreme climates. On the other hand, density-dependent factors are those with effects that depend on population density. Resource depletion, competition, and predation are all examples of density-dependent factors. In fact, these effects become even more intense as the population density increases.

Exponential Growth

The growth rates of populations also vary greatly. There are two types of growth: exponential growth and logistic growth. Exponential growth occurs when a population is in an ideal environment. Growth is unrestricted because there are lots of resources, space, and no disease or predation. Here’s an example of exponential growth. Notice that the curve arches sharply upward—the exponential increase.

Exponential growth occurs very quickly, resulting in a J-shaped curve. A good example of exponential growth is the initial growth of bacteria in a culture. There’s plenty of room and food, so they multiply rapidly—every 20 minutes.

However, as the bacterial population increases, the individual bacteria begin to compete with each other for resources. The population reaches its carrying capacity, and the curve levels off.

The population becomes restricted in size because of limited resources. This is referred to as logistic growth. Notice that the growth forms an S-shaped curve. These growth patterns are associated with two kinds of life-history strategies: r-selected species and k-selected species.

We’ve already mentioned that organisms that grow exponentially approach the carrying capacity. These organisms tend to thrive in areas that are barren or uninhabited. Once they colonize an area, they reproduce as quickly as possible. Why? They know they’ve got to multiply before competitors arrive on the scene! The best way to ensure their survival is to produce lots of offspring. These organisms are known as r-strategists. Typical examples are common weeds, dandelions, and bacteria.

At the other end of the spectrum are the k-strategists. These organisms are best suited for survival in stable environments. They tend to be large animals such as elephants, with long lifespans. Unlike r-strategists, they produce few offspring. Given their size, k-strategists usually don’t have to contend with competition from other organisms.


Communities of organisms don’t just spring up on their own; they develop gradually over time. Ecological succession refers to the predictable procession of plant communities over a relatively short period of time (decades or centuries).

Centuries may not seem like a short time to us, but if you consider the enormous stretches of time over which evolution occurs, hundreds of thousands or even millions of years, you’ll see that it is pretty short.

The process of ecological succession where no previous organisms have existed is called primary succession.

The Pioneers

How does a new habitat full of bare rocks eventually turn into a forest? The first stage of the job usually falls to a community of lichens. Lichens are hardy organisms. They can invade an area, land on bare rocks and erode the rock surface, and over time turn it into soil. Lichens are considered pioneer organisms.

Once lichens have made an area more habitable, they’ve set the stage for other organisms to settle in. Communities establish themselves in an orderly fashion. Lichens are replaced by mosses and ferns, which in turn are replaced by tough grasses, then low shrubs, then deciduous trees, and finally, the evergreen trees. Why are lichens replaced? Because they can’t compete with the new plants for sunlight and minerals.

The entire sequence is called a sere. The final community is called the climax community. The climax community is the most stable. In our example, the evergreens are part of the climax community.

Now what happens when a forest is devastated by fire? The same principles apply, but the events occur much more rapidly. The only exception is that the first invaders are usually not lichens but grasses, shrubs, saplings, and weeds. When a new community develops where another community has been destroyed or disrupted, this event is called secondary succession.