Chapter 33: Population Growth and Regulation

Scope of Ecology Ecology is the study of the interactions of organisms with other organisms and with the physical environment. The study of ecological interactions can be undertaken at many levels: the individual organism, populations, communities, ecosystems, and the biosphere. A population is all the members of the same species interacting with the environment at a particular locale.

A community consists of all the various populations in an area. An ecosystem is the community plus its nonliving habitat, including abiotic (nonliving) and biotic (living) components. The biosphere is the portion of the entire earth’s surface, including air, water, and land, where living things exist. Ecology describes the environment and tests models.

Ecological levels in a coral reef The study of ecology encompasses various levels, from the individual organism to the population, community, and ecosystem. The biosphere includes all the different ecosystems on planet Earth.

Community Composition and Diversity The composition of a community is a listing of populations present. The diversity of a community adds in the relative abundance of individuals. Ecologists have ideas about why populations assemble together in the same place at the same time.

The interactive model of community structure views the community as a stable assemblage that remains the same over time. The individualistic model views a community as a collection of species where each responds to its own requirements and tolerance factors. The interactive model of community structure predicts that the community remains stable because of homeostatic mechanisms. The individualistic model predicts that community compositions are not constant, and that the boundaries between communities will not be distinct from one another.

Two terrestrial communities Diversity of communities is described by the number of populations and their relative abundance, as witnessed by the plants and animals in this terrestrial community. A coniferous forest is shown here. Some of the mammals found here include moose, lynx, snowshoe hare, bear, and red fox.

This is a tropical rain forest This is a tropical rain forest. Some of the mammals found here include the monkey, sloth, anteater, kinkajou, jaguar, tapir, and bat.

Community Stability and Diversity A change in community composition over time is called ecological succession. Primary succession starts on areas devoid of soil and secondary succession starts with pioneer species in areas where there is already soil such as an abandoned field. Succession also occurs in aquatic communities.

Secondary succession in a forest In secondary succession in a large conifer plantation in central New York state, certain species are common to particular stages. However, the process of regrowth shows approximately the same stages as secondary succession in a former cornfield.

Models of Succession The climax-pattern model predicts that a particular area will always lead to a climax community characteristic for that area (i.e., tropical rain forest at the equator). The facilitation model says that each proceeding stage facilitates the development of the next stage. The inhibition model says that each preceding stage tries to prevent the arrival of the next stage.

The tolerance model says that plants from various stages try to colonize at the same time and chance arrival of seeds determines the outcome. The length of time it takes trees to develop gives the impression of a series of plant communities from the simple to the complex. These models are not mutually exclusive, and succession is probably a complex process.

Population Characteristics and Growth Population density is simply the number of individuals per unit area or volume. Distribution of these individuals can be uniform, random, or clumped. Most members of a population are clumped, as are the members of a human population.

Patterns of distribution within a population Members of a population may be distributed uniformly (Golden Eagle), random (moose), or clumped (human populations). The clumped pattern is the most common.

Abiotic factors such as water, temperature, and availability of organic nutrients often determine a population’s distribution. Biotic factors, such as the availability of food, or presence of disease, affect the distribution of populations. Limiting factors are those factors that determine whether an organisms lives in an area.

Patterns of Population Growth Each population has a particular pattern of growth. The per capita rate of increase is calculated by subtracting the number of deaths from the number of births and dividing by the number of individuals in the population. It is assumed that immigration and emigration are equal.

Every population has a biotic potential, the greatest possible per capita rate of increase under ideal circumstances. Two possible patterns of population growth are considered. Exponential growth results in a J-shaped curve because as the population increases in size so does the expected increase in new members.

Biotic potential Animal husbandry relies on biotic potential if a single female pig had her first litter at nine months, and produced two litters per year, each of which contained an average of four females (which in turn reproduced at the same rate), there would be 2,220 pigs by the end of three years.

Exponential growth Exponential growth results in a J-shaped curve because growth rate is positive. Notice the initial lag phase during which growth is slow because population size is small. During the exponential growth phase, growth is accelerating, and the population is exhibiting its biotic potential.

Environmental resistance occurs when most environments restrict growth, and exponential growth cannot continue indefinitely. Under these circumstances logistic growth occurs and an S-shaped growth curve results with four phases: lag, exponential growth, deceleration, and stable equilibrium. When the population reaches carrying capacity, the population stops growing because environmental resistance opposes biotic potential.

Logistic growth Logistic growth results in an S-shaped growth curve because environmental resistance causes the population size to level off and be in a steady state. The stable equilibrium phase occurs at the carrying capacity, the number of individuals the habitat can support indefinitely.

Survivorship Populations are made up of individuals of different ages. Populations tend to have one of three types of survivorship curves, depending on whether most individuals live out the normal life span (type I), die at a constant rate regardless of age (type II), or die early (type III). Much can be learned about the life history of a species through its survivorship curve.

Survivorship curves Humans have a type I survivorship curve: the individual usually lives a normal life span, and then death is increasingly expected. Hydras have a type II curve in which the chances of surviving are the same for any particular age. Oysters have a type III curve: most deaths occur during the free-swimming larva stage, but oysters that survive to adulthood usually life a normal life span.

Human Population Growth The human population is expanding exponentially. The doubling time is the length of time it takes for a population to double, currently estimated at 53 years. Only when birthrate equals death rate will there be zero population growth.

More-Developed Versus Less-Developed Countries Most of the expected increase in human population will occur in certain less-developed countries (LDCs) of Africa, Asia, and Latin America. Doubling time in more-developed countries (MDCs) is about 100 years because a decrease in death rate due to medical advances was followed by a decrease in birth rates.

Standard of living People in the more-developed countries have a high standard of living and will contribute the least to world population growth, while people in the less-developed countries have a low standard of living and will contribute the most to population growth.

World population growth The graph shows world population growth to 1998 with estimates to 2150.

The relationship between decreased death rate followed by a slower birth rate is called demographic transition. Despite introduction of medical care, LDCs still have twice the MDC growth rate. Support for family planning, social progress, and delayed childbearing could help prevent an expected increase in population size.

Age Distributions Many MDCs have a stable age structure, but most LDCs have a youthful profile—a large proportion of the population is younger than age of 15. This means their populations will expand greatly in the near future. Zero population growth or replacement reproduction does not occur when each couple has only two children because there is momentum from younger women entering reproductive years.

Age-structure diagram for MDCs The age-structure diagram for more-developed countries shows they are approaching stabilization.

Age-structure diagram for LDCs The age-structure diagram of less-developed countries shows their populations will expand rapidly due to their age distributions.

Regulation of Population Growth Two life history patterns exist in populations. Opportunistic populations have a short lifespan, small stature, and produce many offspring to take advantage of new resources. Equilibrium species live longer, are larger, and produce fewer young but have greater parental care; they hold population size near carrying capacity.

Life history patterns Dandelions are an opportunistic species with these characteristics: small individuals, short life span, fast to mature, many offspring, and little care of offspring. Bears are an equilibrium species with these characteristics: large individuals, long life span, slow to mature, few offspring, and much parental care of offspring. Often the distinctions between these two possible life history patterns are not as clear-cut as they may seem.

Population growth is limited by both density-independent factors (e. g Population growth is limited by both density-independent factors (e.g., weather) and density-dependent factors (predation, competition, and resource availability). Density-independent factors operate regardless of population density. Density-dependent factors increase in intensity as population size increases.

Competition Competition occurs when two species try to use a resource that is in limited supply. According to the competitive exclusion principle, no two species can occupy the same ecological niche at the same time when resources are limiting. Resource partitioning occurs when resources are partitioned between two or more species. An organism’s ecological niche is the role the organism plays in the community, including its habitat. The niche includes the resources an organism uses to meet its energy, nutrient, and survival demands.

Competition between two populations of Paramecium When grown alone in pure culture (top two graphs). Paramecium caudatum and Paramecium aurelia exhibit sigmoidal growth. When the two species are grown together in mixed culture (bottom graph), P. aurelia is the better competitor, and P. caudatum dies out.

Competition between two species of barnacles Barnacles competing on the Scottish coast is an example of ongoing competition that restricts populations. Competition prevents two species of barnacles from occupying as much of the intertidal zone as possible. Both exist in the area of competition between Chthamalus and Balanus. Above this area, only Chthamalus survives, and below it only Balanus survives.

Predation Predation occurs when one living organism, the predator, feeds on another, the prey. Predators include lions, whales that filter feed, parasites that draw blood from hosts, and herbivores that eat grass, trees, and shrubs.

Predator-Prey Population Dynamics Predator-prey interactions between two species are influenced by environmental factors. Cycling of population densities may occur, as in the case of the Canadian lynx and hare; predators kill off prey and then the predator population declines when food is in short supply. Predator-prey systems are not usually simple two-species systems.

Predator-prey interaction: lynx and snowshoe hare The Canadian lynx (Lynx canadensis) is a solitary predator. A long, strong forelimb with sharp claws grabs its main prey, the snowshoe hare (Lepus americanus). The number of pelts received yearly by the Hudson Bay Company for almost 100 years shows a pattern of ten-year cycles in population densities. The snowshoe hare population reaches a peak abundance before that of the lynx by a year or more.

Prey Defenses Coevolution occurs when two species adapt to selective pressures of each other. Prey defenses against predation take many forms: camouflage, use of fright, and warning coloration are three prey defense mechanisms.

Antipredator defenses On the left is an example of concealment. Flounders can take on the same coloration as their surroundings. In the middle is an example of fright. The South American lantern fly has a large false head that resembles that of an alligator. This may frighten a predator into thinking it is facing a dangerous animal. On the right is warning coloration. The skin secretions of dart-poison frogs are so poisonous that they were used by natives to make their arrows instant lethal weapons. The coloration of these frogs warns others, such as birds, to beware.

Mimicry Mimicry occurs when one species resembles another that possesses an antipredator defense. Batesian mimicry occurs when one species has the warning coloration but lacks the antipredator defense of the species it mimics. Müllerian mimicry occurs when two species with the same warning coloration both have defenses.

Mimicry Flies of the family Syrphidae are called flower flies because they are likely to be found on flowers, where they drink nectar and eat pollen. Some species mimic a wasp, which is protected from predation by its sting.

Symbiosis Symbiosis refers to close interactions between members of two populations. Three types of symbiosis occur: parasitism, commensalism, and mutualism. Symbiotic associations do not necessarily fall neatly into these three categoties.

Parasitism In the symbiotic relationship called parasitism, the parasite benefits and the host is harmed. Parasites derive nourishment from their host and the effect can be mild or fatal to the host. Many parasites use a secondary host to disperse or complete their stages of development, as is the case in the life cycle of a deer tick.

Commensalism In commensalism, one species benefits and the other is neither benefited nor harmed. Often a host provides a home or transportation for another species. For example, barnacles attach to backs of whales, remoras attach to sharks, clown fishes live within the tentacles of sea anemones, and cattle egrets eat insects off large grazing mammals.

Egret symbiosis Cattle egrets eat insects off and around various animals, such as this African cape buffalo.

Mutualism In mutualism, both members benefit. Lichens have traditionally been regarded as mutualistic but experiments suggest that the fungus may be parasitic on the algae. The bullhorn acacia tree provides a home for the ant Pseudomyrmex ferruginea, in swollen acacia thorns. Ants feed from nectaries at base of leaves and also eat Beltian bodies at leaf tips.

In return ants protect this tree from herbivores. Cleaning symbiosis involves crustaceans, fish, and birds that act as cleaners of a variety of vertebrate clients. In some cases, the cleaners may exploit the situation and feed on host tissues, but cleaning appears to improve the fitness of the client.

Cleaning symbiosis Cleaners remove parasites from their clients. Here a cleaner wrasse is entering the mouth of a spotted sweetlip.

Chapter Summary Ecology is the study of the interactions of organisms with each other and the physical environment. Ecology includes the organism, the population, the community, the ecosystem, and the biosphere. Communities are assemblages of interacting populations that differ in composition and diversity.

Environmental and biotic factors influence community composition and diversity. Ecological succession is a change in species composition and community structure over time. Population size depends upon births, deaths, immigration, and emigration. Exponential and logistic patterns of population growth have been developed.

Mortality rates within a population can be illustrated by a survivorship curve. Life history patterns range from one in which many young receive little care to one in which few young receive much care. The human population is still growing exponentially, and how long this can continue is not known.

Factors that affect population size are classified as density-independent and density-dependent. Competition often leads to resource partitioning, which reduces competition between species. Predation often reduces prey population density, which in turn can lead to a reduction in predator population density. Symbiotic relationships include parasitism, commensalism, and mutualism.