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pre-AP Bio 5.2
- 2. Learning Objectives Identify factors that determine carrying capacity. Identify the limiting factors that depend on population density. Identify the limiting factors that do not depend on population density.
- 3. Limiting Factors Limiting factors determine the carrying capacity of an environment for a species. Density dependent Density independent
- 4. Density-Dependent Factors Density-dependent limiting factors operate strongly when population density reaches a certain level.
- 5. Competition • More individuals use up resources sooner. • Individuals may compete for food, water, space, sunlight, shelter, mates, territories.
- 7. Herbivore Effects Populations of herbivores and plants cycle up and down like populations of predators and prey.
- 8. Parasitism and Disease • Parasites and diseases can spread quickly through dense host populations. • Stress from overcrowding can lead to lower birth rates, higher death rates, and higher emigration rates.
- 9. Density-Independent Factors Density-independent limiting factors affect all populations regardless of population size and density.
- 10. Density-Independent Factors • Examples: hurricanes, droughts, floods, wildfires • Density-independent factors may actually vary with population density. Canyon Lake, TX
- 11. Controlling Invasive Species Density-independent measures? • Herbicides, mechanical removal Density-dependent measures? • Predation Expensive, temporary Another invasive species? Salvinia
- 12. Overview: Limits to Growth Density dependent Density independent Flood waters cover a field of wildflowers. Non-native snakes released into a wetland prey on native rodents. Flu virus spreads quickly in schools. Wildfires spread through a grassland.
Editor's Notes
- Read the lesson title aloud to students.
- Click to reveal each learning objective in turn. Read the objectives aloud or ask a volunteer to do so.
- Point out that scientists classify limiting factors into two groups: density-dependent factors and density-independent factors. Click to reveal circles and labels showing how the factors are grouped. Tell students that they will learn more about these groups of factors in the slides that follow. Ask: How might each of these factors increase the death rate in a population? Answer: Competition: Organisms may not have enough resources to survive; Predation: Organisms die when they are eaten; Parasitism and disease: Organisms are killed; Natural disaster and unusual weather: Organisms are killed or resources are diminished. Distribute the lesson worksheet and instruct students to create a Venn diagram comparing the two categories of limiting factors, density dependent and density independent, which they will learn about in the slides that follow.
- Tell students that density-dependent limiting factors operate strongly when population density—the number of organisms per unit area—reaches a certain level. Explain that these factors do not strongly affect small, scattered populations as much. Density-dependent limiting factors include competition, predation, herbivory, parasitism, disease, and stress from overcrowding. Note that some of these involve abiotic external factors and others involve biotic external factors.
- Tell students that when populations become crowded, individuals compete for food, water, space, sunlight, and other resources that are limited. Some individuals obtain enough to survive and reproduce. Others may obtain enough to live but not enough to raise offspring. Still others may starve or die from lack of shelter. Thus, competition for changing resource bases that are limited can lower birthrates, increase death rates, or both. Lead a short discussion guiding students to make their own conclusions about how competition can affect population growth. Remind students that four general factors affect population growth. Ask: What four factors affect population growth? Answer: birthrate, immigration, death rate, emigration Then, guide students to tie these factors to competition. Ask: How can competition affect the birthrate of a population? Answer: If competition results in individuals not obtaining enough resources to reproduce, the birthrate of the population may decrease. Ask: How can competition affect the death rate of a population? Answer: If individuals cannot obtain enough resources to survive, the death rate may increase. Ask: How can competition affect the rates of immigration and emigration? Answer: If there is not much competition for the resources in an ecosystem, individuals from other ecosystems may move in, increasing immigration rate. If competition for resources is severe, the rate of emigration may increase as individuals seek other ecosystems in which to live. Click to reveal the bullet points onscreen. Close the discussion by reiterating the following: Competition is a density-dependent limiting factor, because the more individuals in an area, the sooner they use up resources. Often, space and food are related. Many grazing animals compete for territories in which to breed and raise offspring. Individuals that can’t establish and defend a territory cannot breed. Competition can also occur among members of different species that attempt to use similar or overlapping resources that are limited. This type of competition is a major force behind evolutionary change.
- Tell students that the effects of predators on prey and the effects of herbivores on plants are important density-dependent population controls. One classic study focuses on the relationship between wolves, moose, and plants on Isle Royale, an island in Lake Superior. The graph shows that populations of wolves and moose fluctuate over time. Make sure students understand that two separate sets of data are plotted on the graph. Point out the left and right vertical axes, which are numbered in different increments. Explain that the left vertical axis and the blue line represent the wolf population; the right vertical axis and the red line represent the moose population. Ask: What general trends are shown in this graph? Answer: An increase in the wolf population is usually accompanied by a decrease in the moose population. A decrease in the wolf population is usually accompanied by an increase in the moose population. Use the graph to emphasize this cyclical nature of the predator-prey relationship: Explain that sometimes, the moose population on Isle Royale grows large enough that moose become easy prey for wolves. When wolves have plenty to eat, their population grows. As the wolf population grows, wolves begin to kill more moose than are born. This causes the moose death rate to rise higher than its birthrate, so the moose population falls. As the moose population drops, wolves begin to starve. Starvation raises the wolves’ death rate and lowers their birthrate, so the wolf population also falls. When only a few predators are left, the moose death rate drops, and the cycle may repeat. Click to reveal the black circle around the point representing the “CPV outbreak” on the graph. Explain that the population at this time was affected by an outbreak of canine parvovirus (CPV). Ask: Based on the graph, what effect did the canine virus outbreak have on the moose population? Sample answer: The large decrease in wolf population is probably due to the virus. With a smaller wolf population, the moose death rate dropped, leading to a much higher population after several years. Click to reveal the circle around the point showing wolf population growth around the year 2000. Ask: What might explain this spike in the wolf population? Sample answer: The large spike in moose population a few years before increased the amount of prey available, possibly increasing birth rate and decreasing death rate in the wolf population. Tie the concept of predator–prey relationships to humans: Explain that in some situations, human activity limits populations. For example, humans are major predators of codfish in New England. Fishing fleets, by catching more and more fish every year, have raised cod death rates so high that birthrates cannot keep up. As a result, the cod population has been dropping. The cod population can recover if we scale back fishing to lower the death rate sufficiently. Biologists are studying birthrates and the age structure of the cod population to determine how many fish can be taken without threatening the survival of the population.
- Tell students that herbivory can also contribute to changes in population size. From a plant’s perspective, herbivores are predators. So it isn’t surprising that populations of herbivores and plants cycle up and down, just like populations of predators and prey. On parts of Isle Royale, large, dense moose populations can eat so much balsam fir that the population of these favorite food plants drops. When this happens, moose may suffer from lack of food. Click to reveal the statement about herbivore and plant populations. Guide students to make inferences about the connections between herbivory and wolf populations on Isle Royale. Ask: If moose populations become very large, what will happen to wolf populations? What will happen to plant populations? Answer: Moose populations will, after several years, increase. Populations of plants that the moose feed on will decrease. Ask: If the plant populations decrease due to large moose populations, what will ultimately happen to the wolf population? Answer: Too many moose means too few plants. Moose populations will die off through increased predations and through starvation. With a smaller moose population, the wolf population will eventually decrease.
- Tell students that parasites and disease-causing organisms feed at the expense of their hosts, weakening the hosts and causing stress or death. The ticks on the hedgehog in the photo, for example, feed on their host’s blood and carry diseases. Parasitism and disease are density-dependent effects because the denser the host population, the more easily parasites can spread from one host to another. Click to reveal the first bullet point stating why disease is density dependent. Remind students of the a dramatic drop in the wolf population around 1980 due to an outbreak of CPV. Explain that at that time, a virus accidentally introduced to the island killed all but 13 wolves—and all but three females. This drop in the wolf population enabled moose populations to skyrocket to 2,400. Those densely packed moose then became infested with winter ticks that caused hair loss and weakness. Tell students that overcrowding can also lead to increased stress within a population. Explain that some species fight among themselves if overcrowded. Too much fighting can cause stress, which weakens the body’s ability to resist disease. In some species, overcrowding stress can cause females to neglect, kill, or even eat their own offspring. Thus, overcrowding can lower birthrates, raise death rates, or both. Stress can also increase rates of emigration. Click to reveal the summary statement about effects of stress.
- Tell students that density-independent limiting factors affect all populations regardless of population size and density. Environmental change, including unusual weather such as hurricanes, droughts, or floods, and natural disasters such as wildfires, can act as density-independent limiting factors. In response to such factors, a population may “crash.” After the crash, the population may build up again quickly, or it may stay low for some time.
- Explain that events such as storms can nearly extinguish local populations of some species. For example, thrips, aphids, and other leaf-eating insects can be washed out by a heavy rainstorm. Waves whipped up by hurricanes can devastate shallow coral reefs. Extremes of cold or hot weather also can take their toll, no matter how sparse or dense a population is. More prolonged environmental changes, such as severe drought, can devastate populations. These kinds of environmental changes can thus affect ecosystem stability. Tell students that the photo shows dead fish rotting on a receding shoreline due to drought conditions at Canyon Lake, Texas. Ask: Why is drought a density-independent factor? Answer: It can affect populations no matter how large or small they are. Ask students to make inferences about the impact of the drought on a variety of populations in this ecosystem. For example, a population of water plants might become overcrowded as a result of a decrease in the water level of the river. Or, plants along the riverbank might dry out and die, limiting nesting places for some birds. Have volunteers discuss their inferences with the class. Point out that sometimes, the effects of so-called density-independent factors can vary with population density. On Isle Royale, for example, the moose population grew exponentially for a time after the wolf population crashed. Then, a bitterly cold winter with very heavy snowfall covered the plants on which moose feed, making it difficult for all those moose to move around to find food. Because emigration wasn’t possible for this island population, many moose died from starvation. The effect of bad weather on this large, dense population were greater than it would have been on a small population. In a smaller population, there would have been less competition, so individual moose would have had more food available. This situation shows that it is sometimes difficult to say that a limiting factor acts only in a density-independent way. Click to reveal the bullet point about the effects of density on density-independent factors. Ask: Reconsider the drought scenario. Is it possible for drought to act as a density-dependent factor? Sample answer: A large population might be affected by a drought more than a much smaller population due to competition for any water available.
- Remind students about Salvinia, a major invasive species along the Gulf Coast. Ask students what kinds of limiting factors might help control Salvinia populations. Encourage students to share their ideas. Click to reveal the bullet points as you discuss some of the ways that researchers have been looking for ways to apply limiting factors to control Salvinia and the potential problems of these measures. Explain that artificial density-independent control measures—such as herbicides and mechanical removal—offer only temporary solutions and are very expensive. So ecologists tried to identify density-dependent limiting factors that control Salvinia in its natural habitats. Studies in South America uncovered several insects that feed on Salvinia. Further studies showed that one particular species of weevil feeds only on Salvinia. That’s important, because some efforts to use one exotic organism to control another ended up introducing another destructive invasive species. When no Salvinia are available, these weevils starve to death before they feed on another plant. In some parts of East Texas, weevils raised in nurseries and released quickly became established and spread naturally. In several places where Salvinia had caused problems, the weevils significantly reduced Salvinia populations.
- Have volunteers come to the board to draw lines matching examples with the correct category of limiting factors. Click to reveal correct pairings.
- Allow students to work with a partner following the presentation to compare Venn diagrams. Then, ask for volunteers to share the items they wrote in the different segments of the diagram. Worksheet Answers: Density-dependent only: operate when population density reaches a certain level; competition; predation; parasites and disease; stress from overcrowding Density-independent only: affect all populations regardless of size and density; drought; storms; fires Both: may be used in the fight against invasive species; some environmental changes can have both density-dependent and density-independent effects; both can affect birthrates, death rates, immigration, and emigration.