Chapter 21 - Populations and Communities

Section 1 - Living Things and the Environment


  All the living things, or organisms, and nonliving things that interact in a particular area make up an ecosystem. Organisms live in a specific place within an ecosystem. An organism obtains food, water, shelter, and other things it needs to live, grow, and reproduce from its surroundings. Some organisms make their own food in a process called photosynthesis. Other living things depend on plants and algae for food. The place where an organism lives and that provides the things the organism needs is called its habitat.

  An organism interacts with both living and nonliving parts of its habitat. The living parts of an ecosystem are called biotic factors. The nonliving parts of an ecosystem are called abiotic factors. Abiotic factors include water, sunlight, oxygen, temperature, and soil.

  A species is a group of organisms that are physically similar and can mate with each other and produce offspring that can also mate and reproduce. All the members of one species in a particular area are referred to as a population. All the different populations that live together in an area make up a community. The smallest level of organization is a single organism, which belongs to a population that includes other members of its species. The population belongs to a community of different species. The community and abiotic factors together form an ecosystem.

  The study of how living things interact with each other and with their environment is called ecology. Ecologists, scientists who study ecology, look at how all the biotic and abiotic factors in an ecosystem are related. They study how organisms react to changes in their environment. Living things constantly interact with their surroundings, responding to changes in the conditions around them.




Section 2 - Studying Populations


  Some methods of determining the size of a population are direct and indirect observations, sampling, and mark-and-recapture studies. Direct observation involves counting the members of a population one by one. Indirect observation involves counting tracks, nests, or other signs rather than the organisms themselves.

  Many times, it is not possible to count every member of a population because the population is too large or spread out or the members are hard to find. Instead, ecologists make an estimate of the population. An estimate is an approximation of a number, based on reasonable assumptions. One type of estimating involves counting the number of organisms in a small area (a sample), and then multiplying to find the number in a larger area.

  Ecologists sometimes use a technique called mark and recapture. Some animals are first captured, marked, and released into the environment. Then another group is captured. The ecologists count the marked animals in this group. Using a mathematical formula, the ecologists can estimate the total population of those animals in the area.

  Populations can change in size when new members enter the population or when members leave the population. The major way in which new individuals are added to a population is through the birth of new offspring. The birth rate of a population is the number of births in a population in a certain amount of time. The major way that individuals leave a population is by dying. The death rate is the number of deaths in a population in a certain amount of time. If the birth rate is greater than the death rate, the population will generally increase in size. If the death rate is greater than the birth rate, the population size will generally decrease. The size of a population can also change when individuals move into or out of the population. Immigration means moving into a population. Emigration means leaving a population. Graphs are useful to show changes in the size of a population over time.

  One way to state the size of a population is in terms of population density-the number of individuals in a specific area. Population density is calculated by dividing the number of individuals in the population by the total area. The result tells how many individuals there are per unit area.

  A limiting factor is an environmental factor that prevents a population from increasing. Some limiting factors for populations are food and water, space, and weather conditions. The largest population that an environment can support is called the carrying capacity




Section 3 - Interactions Among Living Things


  Every organism has some unique characteristics that enable it to live in its environment. In response to their environment, species evolve, or change over time. The changes that make organisms better suited to their environment become common in that species by a process called natural selection. Individuals whose characteristics are best suited for their environment tend to survive and produce offspring. The offspring inherit those characteristics and also live to reproduce. Individuals that are poorly suited to the environment are less likely to survive and reproduce. The poorly suited characteristics may disappear from the population over time. The results of natural selection are adaptations, the behaviors and physical characteristics of species that allow them to live successfully in their environment.

  Every organism has a variety of adaptations that are suited to its specific living conditions. These adaptations create a unique role for the organism in its ecosystem. An organism's particular role in its habitat, or how it makes its living, is called its niche. A niche includes the type of food the organism eats, how it obtains this food, which other species use it as food, when and how the organism reproduces, and the physical conditions it requires to survive.

  Some adaptations involve how organisms interact. There are three major types of interactions among organisms: competition, predation, and symbiosis. Competition is the struggle between organisms to survive as they attempt to use the same limited resource. Predation is an interaction in which one organism kills and eats another organism. The organism that does the killing is the predator. The organism that is killed is the prey. Predators have adaptations that help them catch and kill their prey. Prey organisms have adaptations that help them avoid being caught and eaten. Predation can have a major effect on the size of a population.

  Symbiosis is a close relationship between two species that benefits at least one of the species. The three types of symbiotic relationships are mutualism, commensalism, and parasitism. Mutualism is a relationship in which both species benefit. Commensalism is a relationship in which one species benefits and the other species is neither helped nor harmed. Parasitism involves one organism living on or inside another organism and harming it. The organism that benefits is called a parasite, and the organism it lives on or in is called a host.




Section 4 - Changes in Communities


  Fires, floods, volcanoes, hurricanes, and other natural disasters can change communities in a short period of time. Even without a disaster, communities change. The series of predictable changes that occur in a community over time is called succession.

  Primary succession is the series of changes that occur in an area where no soil or organisms exist. The area might be a new island formed by the eruption of an undersea volcano or an area uncovered by a melting sheet of ice. When the land is first exposed, there is no soil. The first species to populate the area are called pioneer species. Pioneer species are usually lichens and mosses, which can grow on bare rocks. As they grow, the lichens and mosses help break up the rocks to form soil. When these organisms die, they provide nutrients that enrich the developing soil. Over time, seeds of plants land in the new soil and begin to grow. The specific plants that grow depend on the biome of the area. In time, as the soil grows older and richer, a mature forest may develop.

  Secondary succession is the series of changes that occur in an area where the ecosystem has been disturbed, but where soil and organisms still exist. Natural disturbances include fires, hurricanes, and tornadoes. Human activities, such as farming, logging, or mining, also may disturb an ecosystem. Unlike primary succession, secondary succession occurs in a place where an ecosystem currently exists. Secondary succession occurs more rapidly than primary succession. The particular plant species that appear and then are replaced in the process of succession depend on the biome.




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