Characteristics of Populations page 4



Characteristics of Populations

Populations are groups of individuals of the same species living in a particular place at the same time.

Populations may differ in their number, patterns of living together, behavior, and organization. All of these factors are importance when considering the interactions of a population with its environment and other populations.


The number of individuals of a species living in a particular area is the density of that population.

Density is dependent upon many factors, such as danger of predators, the availability of food, water, light, heat, and nesting places. The density of a population increases when conditions are favorable to the population and decreases when they are not favorable. You might find 1,000 or more grass plants in a square meter of well-maintained lawn. In a semi-desert where water is limited, less than 10 grass plants might be found per square meter. The study of population density and factors that limit it are important in ecology.


The arrangement of the individuals of a population in a particular space is called the distribution. While population density tells us how many individuals exist in an area, distribution tells us how those individuals are arranged in that area. The patterns of distribution are random, uniform, and clumped.

In random distribution, organisms occur in no particular order. The appearance of the organism is random throughout the area without an overall pattern.

Organisms that are distributed in a uniform manner are spread evenly over an area. No one plant has more or less of the organisms than any other place.

Clumped organisms are concentrated at points throughout an area. Clusters may offer the population protection from enemies, wind, or cold.

Behavior of Populations

An important characteristic of animal populations is their behavior. Animal behavior consists of all movements that the animal makes: movements of motion, feeding, mating, nesting, and defense. Each animal population has its own particular set of behavior patterns.


The survival and well-being of an animal population is largely dependent upon the communication among the members of the population. Through specific methods of communication, members of populations indicate dominance, submission, the source of foods, the presence of danger, urge to mate, and the limits of territory. Even simple organisms have developed methods of communication.

Visual Displays

Animals may use visual displays to communicate. A visual display is the way the animal stands, moves, or shows body coloring. The cobra shows aggressive behavior when the snake arches its back, bares its fangs and displays the neck hood. Some species of animals have special adaptations for communicating their message. The porcupine fish turns into a ball of quills to tell an enemy to beware. Male birds are often brightly colored to attract female of their species and to draw the attention of enemies away from the female and young. The wood duck shows this type of coloration.


Communicating by Sound

Animals may also communicate by making sounds and sometimes combine their visual displays with sounds. Each call or cry carries its own special message. Even animals such as rabbits and giraffes, usually thought of as mute, may utter sounds when they are intensely afraid.

Use of Chemicals

Chemicals, including pheromones, may be secreted by some animals. These substances communicate territorial rights, fear, or a desire to mate. Chemicals can be produced and secreted by specialized glands or tissues and may be included in urine or saliva. The male lion marks his territory by spraying urine at points along the outer edge of his territory. The skunk is a well-known example of an animal that secretes chemicals to communicate.

Physical Contact

To convey concern, hunger, nurturing, play, or aggression animals may use physical contact. A young bird pecks at the parents’ bill to communicate hunger. Mother animals lick their young to communicate caring as well as to groom the offspring. A pet cat will rub up against a person to communicate pleasure. Giraffes use their long necks in struggles for dominance by slowly and forcibly knocking heads and necks. Animals seldom fight other members of their species to the death. Rather, they fight until one animal admits defeat by communicating signs of submission or by leaving the area.

Social Behavior and Organization

Another characteristic of populations is their social behavior. This is the interaction of animals among their own kind. These interactions depend upon communication between the individuals, and all contribute to the success of the population. Organized social interactions offer the advantages of cooperation in reproduction, caring for the young, obtaining food, and in defense. A herd of musk oxen form a circle for protection.

Social organization may be very simple of very complex. The simplest social behavior is herding, schooling, or flocking. In a school of fish, there is safety in numbers. Individuals are less likely to be captured by a predator while in a school than if alone.

Social organization also aids in reproduction and rearing. For the seal, meeting at the breeding area makes contact between male and female easier. Baboons live together for such advantages as grooming each other and caring for the young.

Review It

1) How do population density and population distribution differ?

2) List the three areas of population distribution and describe each one.

3) List and give an example of each form of communication within animal populations.

4) List the ways that social organization among animal populations helps ensure the survival of the populations. Give examples of how each way works to the benefit of the population.


Community Structure and Function

A given space is seldom occupied by a single population. Rather numerous species of plants and animals occupy an area. A community is made up of a number of populations that live together in the same area such as a pond, a forest, or a meadow. These populations depend on one another for food and shelter. The dynamic relationships in the community change with and adapt to changes within populations themselves.

Species Diversity and Dominance

Communities differ in species diversity. This means they differ in the number and kinds of populations that exist in the community. A desert community has different species than a coastal rainforest.

Usually a community has a few populations that are more important in determining the nature of the community than the others. These dominant species strongly influence the food supply and the environment of the other species in a forest; the large trees are usually dominant species. A common dominant tree in the eastern United States is the maple. In Pacific coast forests, dominant tree species are different. In northern California and southern Oregon, for example, coastal redwoods are often dominant. Large trees dominate forests because their canopy of leaves over the other population determines how much light is available to the other plants. Also, their leaves or needles make up much of the litter on the ground and therefore influence the make-up of the soil. The fallen leaves provide food for sod organisms that cause the leaves to decay. Their trunks and branches are the places where insects, birds, lichens, mosses, and fungi live. Some of them use the living tree for food.

Habitats and Niches

Every population in a community has its own special place to live within the community. This living place is known as its habitat. Some populations can live in a variety of habitats, whereas other can only survive within very limited conditions. For example, rats can live in cities or rural areas. Purple marsh crabs live only in salt water marshes. The habitat supplies all of the biotic and abiotic elements necessary for the survival of the population. The habitat can be considered the population’s address within the community.

Many plants and animals share the same general habitat. The lifestyle of an individual population in the habitat is known as the niche. Crickets and grasshoppers are closely related insects that live in the same habitat yet each occupies a different ecological niche. The grasshopper is very active during daylight. It can usually be found on a plant, its head pointing upward as it feeds on the plant parts. Although the cricket lives in the same field, it is quite different. During the day, the cricket hides under leaves or plant debris and is usually inactive. However, the cricket is active at night when it feeds, moves about, and makes sounds. The cricket and the grasshopper do not interfere with each other’s activities in the same habitat. In general, the niche of an organism is the total position and function of an individual to its environment.

Special Relationships Among Organisms

Living things continually interact with other living things. This interaction may be between individuals of the same species or between individuals of different species. The types of interactions differ among populations in a community.


The amount of food, light, space, minerals, and water that are available in a particular habitat is limited.

As a result organisms are in competition with one another for one or more of these factors. Populations with the most similar niches will have the most intense competition. Populations may compete directly leading to the extinction or adaptation of one of the populations. Many animals establish territories

4 within which they live and which they will defend against others of their species who try to intrude.

By staying in their own territories, competition and combat are lessened. In one area, five species of warblers live together in the same tree. You can see in Figure 1 below that each warbler has a specific niche, so they do not compete directly for food or space.

Figure 1: Five different species of warblers living in the same tree, but each with a different niche. From left to right the species are: Cape May, Yellow–Rumped, Black–Throated Green, Blackburnian and Bay–Breasted Warblers illustration from The

Birder’s Handbook , ed. Ehrlich, Paul and David Dobson


The relationship between predators, or hunters, and their kill, or prey, is called predation. Predators have developed characteristics that make it easier to capture their prey. Exceptionally keen eyesight has made it possible for some birds to fly high over the oceans, spot a tasty fish, and dive for their dinner.

Prey have also adapted to avoid capture. Some, such as the chameleon, use protective coloration to blend with their environment.

Predation is often density dependent. When the population of pretty becomes very dense, the individuals are often weaker due to competition within the population for common resources. The prey is then more easily captured by predators. Predation is a means of reducing an overly large population and controlling its size. It should be remembered that predation is one of the biotic factors of environmental resistance.

Symbiotic Relationships

Symbiosis usually means living together in a close association. Forms of symbiosis differ among populations in a community.


When more than one species live together in a close association that is helpful to both species, the relationship is known as mutualism. The oxpecker bird and the rhinoceros exhibit this relationship. The oxpecker receives protection and obtains food from the ticks and other pests infesting the rhino’s skin. The rhino receives cleaning and early warning of approaching dangers. Algae and fungi live together in a mutualistic relationship in lichens. The algae are able to produce food by photosynthesis. The fungi obtain water and minerals and provide attachment for the lichen.

Oxpecker bird on a rhinoceros



Wasp larvae on a caterpillar

Parasitism is a relationship in which one organism, the parasite, spends much or all of its life living in or on another organism, the host. The parasite is dependent upon the host for food. The parasite benefits from the relationship and the host is always harmed. Parasites may bring about the death of their host, but most often they only weaken their host. Wasp lay their eggs on a caterpillar. As the larvae hatch, they feast on the caterpillar. Parasitism is a very common symbiotic relationship. Human parasites include tapeworms, some types of roundworms, lice, ticks, mites, and leeches.


Remora fish and shark

Some organisms live together so that one organism benefits by the relationship while the other organism is neither helped nor harmed. This type of relationship is known as commensalisms. An example of commensalisms is the relationship between the shark and small remora fish.

The remoras attach themselves to the shark as it swims through the water. the shark. The shark is not harmed by the remora, whereas the remora is definitely helped.

Review It


Define dominant species and give an example.


Define and give an example of the following interactions: parasitism, predation, commensalisms, competition, and mutualism.


Explain the difference between habitat and niche.

Population Dynamics: How Populations Change

Populations can change in many ways. They can get larger or smaller. The areas they occupy can increase or decrease. The role of male to females can vary. Changes in a population occur in response to some change in the factors that influence that population.

Biotic Potential and Environmental Resistance

Every kind of organism produces more offspring than are necessary for that species to survive. The maximum reproduction rate of a population, under ideal conditions, is called biotic potential. Biotic potential assumes that all of the young all live long enough to reproduce. To reach its biotic potential a population must have all of the food or raw materials, water, and space it needs to survive. It also needs an ideal climate and the absence of competitors and disease. The age at which an individual is able to reproduce and the number of young born at any one time influence the biotic potential.

Dramatic examples of biotic potential can be calculated. If a pair of houseflies produced one generation every two weeks, they could have 391,000,000 descendents at the end of one year. Some bacteria divide about every 20 minutes. At this rate, 72 generations could be produced in 24 hours. The number of individuals produced would be enough to cover the entire surface of the earth to a depth of over 20 centimeters.

Populations do not reach their biotic potential. Many factors influence the growth rate of a population.

Environmental resistance is the name given to the collection of factors that reduce the growth rate of a

6 population. The advantage of a high biotic potential, such as that of the housefly, is to counteract the effects of environmental resistance.

Biotic and Abiotic Factors That Influence Population Growth

Factors that influence population growth are either biotic or abiotic. Biotic factors are living organism or their characteristics. Examples of biotic factors that influence a population are: the presence of predators (or hunters), the food supply, the density of the population itself, the biotic potential of the species, and disease. Abiotic factors are nonliving physical elements in the environment. Abiotic factors include temperature, humidity, water, light, soil, slope, air, and wind.

In the case of the housefly, one reason that it does not reach its biotic potential is that its predators are numerous. Not only is it an important food supply for other insects and birds, but it is so annoying to humans that the population is reduced by fly swatters, electronic zappers, and other means.

Density Dependent and Density Independent Factors

Carrying Capacity

1.) Define biotic potential and explain why it can be useful to living organisms.

2.) List the abiotic and biotic factors of environmental resistance.

3.) Define density-dependent factor and density-independent factor give an example of each.

4.) Under normal conditions what is the result of the interaction of biotic potential and environmental resistance?