3cEstimatingPopulationSizeDi

3cEstimatingPopulationSizeDi - Exercise 3C Estimating...

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Biology 6C 73 Exercise 3C Estimating Population Size & Distribution Parts of this lab adapted from General Ecology Labs , Dr. Chris Brown, Tennessee Technological University and Ecology on Campus , Dr. Robert Kingsolver, Bellarmine University. Introduction One of the goals of population ecologists is to explain patterns of species distribution and abundance. In today’s lab we will learn some methods for estimating population size and for determining the distribution of organisms. Measuring Abundance: Quadrats One of the first questions an ecologist asks about a population is, "How many individuals are here?" This question is trickier than it appears. First, defining an individual is easier for some organisms than others. In Canada geese, a "head count" of geese captured on the ground during their summer molt gives a clear indication of adult numbers, but should eggs be counted as members of the population or not? In plants, reproduction may occur sexually by seed, or asexually by offshoots that can remain connected to the parent plant. This reproductive strategy, called clonal reproduction, makes it difficult to say where one individual stops and the next one begins. Once the individual is defined, ecologists working with stationary organisms such as trees or corals can use spatial samples, called quadrats , to estimate the number of individuals in a larger area. Quadrats are small plots, of uniform shape and size, placed in randomly selected sites for sampling purposes. By counting the number of individuals within each sampling plot, we can see how the density of individuals changes from one part of the habitat to another. The word "quadrat" implies a rectangular shape, like a "quad" bounded by four campus buildings. Any shape will work, however, as long as quadrats are all alike and sized appropriately for the species under investigation. For creatures as small as barnacles, an ecologist may construct a sampling frame a few centimeters across, and simply drop it repeatedly along the rocky shore, counting numbers of individuals within the quadrat frame each time. For larger organisms such as trees, global positioning equipment and survey stakes may be needed to create quadrats of appropriate scale. The number of individuals counted within each quadrat is recorded and averaged. The mean ( x ) of all those quadrat counts yields the population density , expressed in numbers of individuals per quadrat area (barnacles per square meter, for example, or pine trees per hectare). Population size can then be estimated using the formula: N = (A/a) * n where: N = the estimated total population size A = the total study area a = the area of the quadrat n = the number of organisms per quadrat Note: this formula can be used with one quardat or an average of all the quadrats as long as the area (a) matches the number of organisms/quadrat (n).
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74 Exercise 3.C. Estimating Population Size & Distribution An alternative approach is to measure ecological density , expressed in numbers of individuals per resource unit (numbers of ticks per deer, for example, or numbers of maggots per apple).
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