Describing Populations
What Are the Factors of Population Demographics?
Teacher Note: Connections
In this concept, students understand how studying population ecology helps us explain how populations change and remain stable. They learn how scientists quantify and model changes in population over short and long periods of time. To introduce this concept to students, share the video segment “What Is a Population?” with students and instruct them to make Frayer maps for the word population. Ask them to focus on how a population forms and changes over time. This strategy is found by searching for “Vocabulary Quadrants” on Science Techbook.
As students read and comprehend complex texts, view the videos, and complete the interactives, labs, and other Hands-On Activities, have them summarize and obtain scientific and technical information. Students will use this evidence to support their initial ideas on how to answer the Explain Question or their own question they generated during Engage. Have students record their evidence using “My Notebook.”
Populations
Ecologists study various levels of organization of the natural world. Defining these levels of organization helps clarify the scope of the ecologists’ studies. An ecosystemglossary term (opens in a new window) includes all the living and nonliving components of the environment in a specified area. A communityglossary term (opens in a new window) refers to a collection of interacting speciesglossary term (opens in a new window) or populations in an ecosystem. A populationglossary term (opens in a new window) is a group of individuals of a species within a community. A species is defined as a group of interbreeding organisms that can produce fertile offspring.
Population ecology focuses on all the members of a given species living in a given location. Studying populations gives ecologists information about the population size, density, and distribution of organisms.
Teacher Note: Misconception
Students may believe that population size can increase indefinitely. In fact, population size is limited by resource availability, predation, and many other factors.
Population Size and Density
The size of a population can be used to determine both the health of the given population as well as the health of the ecosystem. Changes in population size can be caused by natural or human-caused ecological and environmental processes. Determining the cause of an observed population fluctuation requires long-term studies of normal population fluctuations.
The size of a population varies with births, deaths, immigration, and emigration. The rate of increase of a population (r) is affected by factors such as the average number of offspring, the average life span, the age at which organisms reproduce, and survival of the young. Multiplying r by the current population size (N) is used to calculate the rate of a population’s growth (G). The equation can be written as G = rN.
Growing populations have a positive r value, hence G is also positive. Declining populations have a negative r value, hence G is negative. The value of r also changes based on the timeframe used in the calculation. Seasonal and other short-term changes in population size are common and natural. These can be due to fluctuations in temperature, disease cycles, food availability, and other factors. Long-term trends in populations indicate larger-scale ecosystem changes.
Population size is only one factor to consider when studying a population. Population densityglossary term (opens in a new window) is the number of individuals per unit area or volume. In human populations, urban areas have higher population densities than rural areas. Similarly, population densities vary based on the available resources in an area. Increased population density makes finding a mate easier but increases competitionglossary term (opens in a new window) for resources.
In cases where the population size is relatively small (such as on an island), population density can be measured directly. For larger populations, density must be estimated using sampling techniques. Sampling techniques vary based on the organism and location being studied. For example, counting the individual insects on a given tree and multiplying by the number of trees in the area will provide an estimate of the population density of a given insect population.
Population Distribution
Describing population distributions provides information about environmental and social factors affecting the population. Clumped distribution is common in situations where resources are unequally dispersed. Plants and other organisms may show clumped distribution around water sources or areas of increased nutrient concentration. Clumped distribution can also result from positive interactions between individuals. Clumped distributions, such as those seen in fish, can protect individuals in the group from predation and make it easier for individuals to find mates.
Uniform distribution results from negative interactions between individuals. In areas with a relatively even distribution of resources, plants with fewer competitors in their immediate vicinity have increased access to resources. These plants may produce chemicals to inhibit the growth of other plants. This produces a uniform distribution, as the plants do not grow well when they are too close to each other. Animals generally achieve uniform distribution through territorial behaviors rather than chemical inhibition.
Random distribution is less common in the natural world. This occurs when the relationship between individuals is neither positive nor negative. Individuals are independent of each other and resources are sufficient throughout the environment.
Population Demographics
Population ecologists study population size, density, and distribution to gain an understanding of the health of the population as well as how individuals in the population interact with each other and their surroundings. Demographics are statistical data that represent these factors. Scientists use census data to characterize the demographics of human populations. These data can then be used to make political and policy decisions. Ecologists can use population demographicsglossary term (opens in a new window) to develop action plans for conservation.
Teacher Note: Connections
In this item, students observe patterns in population changes and distributions. To extend this activity, students can use Discovery resources and other sources, such as the USDA, to research the honeybee population changes since 2006. Scientists disagree about the extent of the population changes and their potential impact, so students can take sides and cite evidence from their research in a class debate.
Honeybees in the wild and in honey-producing colonies have a distribution. However, honeybee populations are in commercial honey-producing colonies than in the wild.
