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Biology Basics: Population Ecology

Population ecology is the branch of biology that studies the structures of populations and how they change. The unique thing about population ecologists is that they study the relationships within ecosystems by studying the properties of populations rather than individuals:

  • Population size is the total number of individuals in the population.

  • Population density refers to how many organisms occupy a specific area.

  • Dispersion describes the distribution of a population throughout a certain area. Populations disperse in three main ways:

    • Clumped dispersion: Organisms form clusters, with few in between.

    • Uniform dispersion: Organisms spread evenly throughout an area.

    • Random dispersion: Organisms scatter randomly throughout an area so that one place in the area is as good as any other for finding the organism.

  • Age structure refers to the distribution of organisms of different ages in the population. Age-structure diagrams, like the one in this figure, show the number of individuals in each age group in a population at a particular time.

    Age-structure diagrams break down age groups in populations.
    Age-structure diagrams break down age groups in populations.

To assess the properties of populations, ecologists conduct surveys. Depending on the size and mobility of the organisms, ecologists use the following survey methods:

  • Total counts count every member of the population.

  • Sampling methods examine small samples of the population as representatives of the larger population. Two commonly used methods of sampling are

    • Quadrat method: Ecologists mark off small areas of known size within a larger area (usually, they place the quadrats randomly within the larger area) and then survey the organisms within the quadrat. They use an average of the information from all the quadrats to represent the larger population.

    • Capture, mark, and release: Ecologists capture and tag a sample of individuals from the larger population. Then, a short time later, they capture another random sample and count how many tagged individuals are in the second capture. The proportion of marked individuals in the second capture equals the proportion of the total number of tagged individuals (from the first capture) to the number of individuals in the entire population.

  1. Question one refers to the following scenario:

    You are an ecologist studying the recovery of California condors in the area around the Grand Canyon. You want to conduct a survey to determine the current population size. You catch 7 condors, tag them, and release them. Two months later, you catch 10 condors. Only 1 condor of your second catch is tagged. What is your estimate of the condor population? What sampling method are you using?

    Questions 2 - 4 refer to the following scenario:

    You’re an ecologist who wants to estimate the size of the bull trout population in the Flathead Basin in Montana. You catch 10 bull trout, tag them, and release them. One month later, you catch 300 fish, of which only 1 is a tagged bull trout.

  2. What’s your estimate of the total population size of bull trout in the Flathead Basin?

    1. 1 bull trout

    2. 300 bull trout

    3. 3,000 bull trout

    4. 30,000 bull trout

  3. What’s the name of the method you used to estimate the bull trout population?

    1. Total count

    2. Quadrat method

    3. Capture, mark, and release

  4. Why is the method you used a good choice for this particular population? In your explanation, be sure to mention why this method is a better choice than the other two methods.

The following are the answers to the practice questions.

  1. Your second catch contains 1 tagged condor out of 10, or 1/10, which equals 0.1. The proportion of tagged condors in the second catch equals the proportion of all tagged condors to the entire population.

    You originally tagged 7 condors, so 7 condors/X total condors = 0.1 (X is the unknown number of total condors). If you solve for total condors by multiplying both sides of the equation by X and then dividing both sides of the equation by 0.1, you get 7/0.1 = 70. The total number of condors in the area around the Grand Canyon as determined by the capture, mark, and release method is 70.

  2. The answer is 3. 3,000 bull trout.

    In your second catch, only 1 of 300 fish is a tagged bull trout, which is a proportion of 1/300, or 0.003. That means your original catch of 10 bull trout should represent 0.003 of the total population: 0.003 = 10/N. To solve for N, rearrange the equation to N = 10/0.003 = 3,000. (To rearrange the equation, you multiply both sides of the equation by N so it cancels on the right and then divide both sides of the equation by 0.003 so it cancels on the left.)

  3. The answer is 3. Capture, mark, and release.

  4. Capture, mark, and release is a good method for fish populations because fish are very mobile and because they live in the water, where they’re hard to survey.

    A total count would be almost impossible; you’d have to either catch all the fish in the area and then return them to the basin, which would probably destroy the fish habitat in the process, or you’d have to figure out some way to do underwater observations on a moving population!

    Likewise, the quadrat method still requires a full count of each quadrat, which would be very difficult underwater and with mobile organisms.

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