CONCEPTS IN BIOLOGY

PART IV. EVOLUTION AND ECOLOGY

17. Population Ecology

White-tailed Deer Are Becoming Urban Pests

Residents look for solutions.

Many urban areas in eastern North America have a white-tailed deer population problem. Suburban areas with parks, nature preserves, and large lots are particularly hard-hit. The deer eat shrubs and other plantings in parks and in the yards of homes. Car-deer collisions result in millions of dollars of damage yearly and many human and deer deaths. In some areas there is concern about the role deer play in the spread of lyme disease.

Biologists point out that large urban deer populations are a case of “nature out of balance.” There are few predators of deer in these areas and hunting is not allowed. So, with low mortality, populations have increased greatly. In more rural areas, there is some natural predation and hunters also play the role of predators.

There are few options to alleviate the problem. Currently, the administration of birth control measures to female deer is too expensive and difficult to achieve. Removal by trapping is expensive and has the additional problem of finding a suitable place to release the deer. Some cities have trapped deer and slaughtered them. More and more metropolitan areas have come to the conclusion that the most cost-effective method of population control is to allow controlled hunting or to hire specially trained sharpshooters to harvest deer. This is particularly effective in lowering populations if the females and young are harvested. In addition, the meat from the deer harvest is often donated to organizations that provide emergency food aid to local residents.

• What conditions allow these populations to become so large that they become a public nuisance and cause serious economic damage?

• If increased mortality is not allowed, what methods could be used to reduce the number of births?

• Should cost be an issue in choosing the method used to control these populations?

ü  Background Check

Concepts you should already know to get the most out of this chapter:

• The difference between sexual and asexual reproduction (chapter 9)

• Organisms acquire matter and energy from their surroundings (chapter 16)

• Organisms change their surroundings (chapter 16)

17.1. Population Characteristics

A population is a group of organisms of the same species located in the same place at the same time. Examples are the dandelions in a yard, the rat population in your city sewer, and the number of students in a biology class. On a larger scale, all the people of the world constitute the world human population.

The terms species and population are interrelated, because a species is a population—the largest possible population of a particular kind of organism. The term population, however, is often used to refer to portions of a species by specifying a space and time. For example, the size of the human population in a city changes from hour to hour during the day and varies according to the city’s boundaries. Because each local population is a small portion of its species, and each population is adapted to its local conditions, it is common that local populations differ from one another in the characteristics they display.

Gene Flow and Gene Frequency

Recall from chapter 12 that a species is a group of organisms capable of interbreeding and producing fertile offspring. Thus, within a species, genes flow from one generation to the next through reproduction. In addition to gene flow from one generation to the next, genes also can flow from one place to another as organisms migrate or are carried from one locality to another. Typically, both kinds of gene flow happen together as individuals migrate to new regions and reproduce, passing on their genes to the next generation in the new area (figure 17.1).

FIGURE 17.1. Gene Flow

Gene flow within a species occurs in two ways. (a) Genes flow from place to place when organisms migrate. (b) Genes flow from generation to generation as a result of reproduction.

As genes flow from generation to generation by reproduction or from one location to another by migration, it is possible that separate populations may develop differences in the frequency of specific genes. For example, many populations of bacteria have high frequencies of antibiotic-resistance genes whereas others do not. Methicillin-resistant Staphylococcus aureus (MRSA) has become a serious health problem, since this strain (population) of bacterium is resistant to methicillin and many other similar antibiotics. Populations of this strain of S. aureus are most common in hospitals and other healthcare facilities. The frequency of the genes for tallness in humans is greater in certain African tribes than in any other human population. The frequency of the allele for type B blood differs significantly from one human population to another (figure 17.2).

FIGURE 17.2. Distribution of the Allele for Type B Blood

The allele for type B blood is not evenly distributed in the world. This map shows that the type B allele is most common in parts of Asia and East and parts of Europe and Africa. There has been very little flow of the allele to the Americas.

Age Distribution

Age distribution is the number of organisms of each age in a population (figure 17.3). Often, organisms are grouped into three general categories based on their reproductive status:

1. Prereproductive juveniles (e.g. insect larvae, plant seedlings, and babies)

2. Reproductive adults (e.g. mature insects, plants producing seeds, and humans in early adulthood)

3. Postreproductive adults no longer capable of reproduction (e.g. annual plants that have shed their seeds, salmon that have spawned, and many elderly humans)

FIGURE 17.3. Age Distribution in Human Populations

The relative numbers of individuals in each of the three categories (prereproductive, reproductive, and postreproductive) are good clues to the future growth of a population. Kenya has a large number of young individuals who will become reproducing adults. Therefore, this population is likely to grow rapidly. The United States has a large proportion of reproductive individuals and a moderate number of prereproductive individuals. Therefore, this population is likely to grow slowly. Germany has a declining number of reproductive individuals and a very small number of prereproductive individuals. Therefore, its population has begun to decline.

A population does not necessarily have an age distribution that is divided into equal thirds. Some populations are made up of a majority of one age group (figure 17.4).

FIGURE 17.4. Age Distribution in Selected Populations

Some populations are composed of many individuals of the same general age. The caterpillars are a population of prereproductives. The flock of sheep has a small number of reproductive adults and a large number of prereproductive juveniles. The population of yellow bladderpod plants is dominated by reproductive adults.

Many populations of organisms that live only a short time have high reproductive rates. Thus, they have population age distributions that change significantly in a matter of weeks or months. For example, many birds have a flurry of reproductive activity during the summer months. Therefore, samples of the population of a particular species of bird at different times during the summer would show widely different proportions of reproductive and prereproductive individuals. In early spring before they have started to nest, all the birds are reproductive adults. In late spring through midsummer, there is a large proportion of prereproductive juveniles.

Similarly, in the spring, annual plants germinate from seeds and begin to grow—all of the individuals are prereproductive juveniles. Later in the year, all the plants flower—they become reproducing adults. Finally, all the plants become postreproductive adults and die. But they have left behind seeds—prereproductive juveniles—which will produce the next generation.

Age distribution can have a major effect on how the population grows. If most of the population is prereproductive, a rapid increase in its size can be anticipated in the future as the prereproductive individuals reach sexual maturity. If most of the population is reproductive, the population should be growing rapidly. If most of the population is postreproductive, a population decline can be anticipated.

Sex Ratio

The sex ratio of a population is the number of males in a population compared with the number of females. In many kinds of animals, such as bird and mammal species in which strong pair-bonding occurs, the sex ratio may be nearly 1 to 1 (1:1). Among mammals and birds that do not have strong pairbonding, sex ratios may show a larger number of females than males. This is particularly true among game species in which more males than females are shot. This hunting practice leads to a higher proportion of surviving females. Because one male can fertilize several females, the population can remain large even though the females outnumber the males. In addition to these examples, many species of animals, such as bison, horses, elk, and sea lions, have mating systems in which one male maintains a harem of females. The sex ratio in these small groups is quite different from a 1:1 ratio (figure 17.5).

FIGURE 17.5. Sex Ratio

In some species of animals, males defend a harem of females; therefore, the sex ratio in these groups is several females per male. This male Steller sea lion is defending a harem of several females.

In many kinds of insect populations, such as bees, ants, wasps, and termites, there are many more females than males. Generally, in a colony of these organisms there is one or a few reproductive females, a large number of worker females, and very few males.

There are very few situations in which the number of males exceeds the number of females. In some human and other populations, there may be sex ratios in which the males dominate if female mortality is unusually high or if a special mechanism separates most of one sex from the other.

Many kinds of animals and plants are hermaphroditic, having both kinds of sex organs in the same body (e.g. earthworms and many flowering plants). Thus, the concept of sex ratio does not apply to them. Also, some species of animals— oysters, some fish, and others—change their sex at different times of their lives. They spend part of their lives as males and part as females.

Population Distribution

Population distribution is the way individuals within a population are arranged with respect to one another. There are basically three kinds of arrangements: even, random, and clumped. Even distributions occur under circumstances in which the organisms arrange themselves by very specific rules. In many birds that form dense breeding colonies, each nest is just out of reach of the neighbors. Random distributions are typical for many kinds of organisms that do not form social groups and have widely dispersed individuals. Many plants—particularly those that have seeds that are distributed by wind—have random distribution. Clumped distributions are typical for many kinds of plants and animals. In plants that have large seeds, the seeds are likely to fall near the parent plant and a clumped distribution results. In addition, plants that reproduce asexually produce local collections of organisms. Animals that form family groups, social groups, herds, or flocks typically show clumped distributions as do organisms that congregate near valuable resources, such as food or water (figure 17.6).

FIGURE 17.6. Population Distribution

The way organisms are distributed in their habitat varies. A few are evenly distributed. Some are randomly distributed. Many show some degree of clumped distribution.

Population Density

Population density is the number of organisms of a species per unit area. For example, the population density of dandelions in a park can be measured as the number of dandelions per square meter; the population density of white-tailed deer can be measured as the number of deer per square kilometer. Depending on the reproductive success of individuals in the population and the resources available, the density of populations can vary considerably. Some populations are extremely concentrated in a limited space; others are well dispersed. As reproduction occurs, the population density increases, which leads to increased competition for the necessities of life. Intense competition in dense populations is likely to lead to the death of some individuals and dispersal of individuals into less-populated areas.

Population pressure is the concept that increased intensity of competition, resulting from increased population size, causes changes in the environment and leads to the dispersal of individuals to new areas or the death of some individuals. Dispersal can relieve the pressure on the home area and lead to the establishment of new populations. Among animals, it is often the juveniles that participate in dispersal. For example, female bears generally mate every two years and abandon their nearly grown young the summer before the next set of cubs is to be born. The abandoned young bears tend to wander and disperse to new areas. Similarly, young turtles, snakes, rabbits, and many other common animals disperse during certain times of the year. That is one of the reasons so many animals are killed on the roads in the spring and fall.

If dispersal cannot relieve population pressure, there is usually an increase in the rate at which individuals die because of predation, parasitism, starvation, and accidents. For example, plants cannot relieve population pressure by dispersal. Instead, the death of weaker individuals usually results in reduced population density, which relieves population pressure. Following a fire, large numbers of lodgepole pine seeds germinate. The young seedlings form dense thickets of young trees. As the stand ages, many small trees die and the remaining trees grow larger as the population density drops (figure 17.7).

FIGURE 17.7. Changes in Population Density

(a) This population of lodgepole pine seedlings consists of a large number of individuals very close to one another, (b) As the trees grow, many of the weaker trees will die, the distance between individuals will increase, and the population density will be reduced.

17.1. CONCEPT REVIEW

1. Describe two ways that gene flow occurs.

2. Give an example of a population with a high number of prereproductive individuals and another with a high number of reproductive individuals.

3. Describe two situations that can lead to a clumped distribution of organisms.

4. How is population pressure related to population density?