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Population Limiting Factors (Density Dependent)
1.00_ECOLOGY_BIO 1 INTRO Population Limiting Factors (Density Dependent) Slides
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Populations To define a population you need to know the type of individual, the time and the place e.g. all the buttercups on the sports field of our school in February, 2006.
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KEY FEATURES OF POPULATIONS
Population size Is the number of individuals in a population. Has an important effect on the ability of the population to survive. A small population is more likely to become extinct: in the case of random events or natural disaster due to inbreeding where the population is more genetically alike. Recessive traits are more likely to appear. with reduced variability it is harder to adapt to changes.
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Limiting Factor A limiting factor is something that keeps the size of a population down. Limiting factors can depend on the density of individuals in the population or not.
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Population Regulation
Population size is regulated by factors that limit population growth. Density independent factors – regardless of population density, these factors are the same for all individuals. Density dependent factors – the effects of these factors are influenced by population density.
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Limiting Factors Limiting factors determine the carrying capacity of an environment for a species. 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. Density dependent Density independent
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Density-Dependent Limiting Factors
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Population Limiting Factors
Population Ecology: Density-dependent factors Population Limiting Factors Density-dependent factors Any factor in the environment that depends on the number of members in a population per unit area Usually biotic factors These include Predation Disease Parasites Competition
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Density-Dependent Factors
Density-dependent limiting factors operate strongly when population density reaches a certain level. 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.
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Density-dependent limiting factors are affected by the number of individuals in a given area.
Predation Competition Parasitism and disease
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Competition More individuals use up resources sooner.
Individuals may compete for food, water, space, sunlight, shelter, mates, territories. 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.
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Competition Crowded populations mean the individuals have to compete for food, water, space, sunlight, or anything else necessary for survival and reproduction Some may have enough to survive, but can’t raise offspring Some may die of starvation The more individuals in an area, the faster resources are used up Could be between individuals of the same or different species
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Predation Predator-prey relationships tend to fluctuate (go up and down) If there is a lot of prey, the predator can eat more, increasing the number of predators If the prey decreases too much in population, there won’t be enough food for predators and so the population of predators decreases
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Predator–Prey Relationships
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.
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Herbivory (Predation of Plants)
Plant and herbivore populations fluctuate just like the predator and prey populations
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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. 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.
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Parasitism and Disease
Parasites and disease causing organisms feed at the expense of the host This can weaken or kill the host The larger the population the easier it is for the parasite or disease to travel through hosts
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Stress from Overcrowding
Some species will fight amongst each other if they are overcrowded This fighting can cause stress leaving individuals susceptible to disease In a few species, overcrowding can cause females to neglect or kill their own offspring
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Chapter 5 Part 3 - Density-Dependent Limiting Factors
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