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Unformatted text preview: CHARACTERIZATION OF POPULATIONS Lab 4 4-1 Reminders ! Calculator Memory stick or floppy disk Key Concepts Sample vs. Population Distribution, Density, Cover, Dispersion, Frequency Quadrats, Transects In todays lab, you will be characterizing hypothetical populations as practice for next week when you do it for real on Waahila ridge. Because we are using hypothetical paper populations, we know the complete characteristics of those populations, i.e. the true population mean ( ), etc. We can then sample the population with different techniques and see how well our sample estimates the true population. I. CHARACTERIZATION OF POPULATIONS A population can be characterized based on four main properties: its distribution, density, percent cover, and dispersion. Distribution The distribution of a population describes where the population is located. The distribution is usually described as a specific area within which the population of interest is found. For example, Oahus Wallaby population (an Australian marsupial) is distributed within Kalihi valley. Density The density of a population refers to the number of individuals per unit area or sometimes per volume (as in some aquatic populations). Suppose you were planning a scuba diving trip to see sharks. The tour guide you are planning to hire guarantees that the density of sharks in his special dive location is 100. How many sharks will you see? You should explain to your tour guide that 100 is not a density and the number cannot be interpreted without an area or volume attached to it. If the density is 100 per 10,000 km 3 of water you arent going to see many sharks. On the other hand, if it was 100 sharks per 1000 m 3 , then you better request a dive cage! Relative density is the proportion of all organisms in an area that belong to a particular species of interest. = ) Sp. All ( ) X Sp. ( D D D r where D r is relative density and D is density. For example, suppose the lawn outside Dean Hall is comprised of three plant species (A, B, C). The density of each species is 3/m 2 , 50/m 2 , and 15/m 2 , respectively. The relative density of species C is calculated as follows: 22 . 15 50 3 15 = + + = r D Note that relative density is unitless because m 2 in the numerator and denominator cancel each other out. Percent Cover For an ecologist, counts of the number of individuals of each species in a given area do not always provide an adequate picture of what a community looks like. Let us say a community consists of a mixture of 20 Banyan trees and 10,000 Chinese violets (small herbs). What does this community look like? It depends on the physical size of each of the two species....
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