11 - POPULATION BIOLOGY A POPULATION is defined as a group...

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POPULATION BIOLOGY A POPULATION is defined as a group of individuals of the same species in the same location at the same time. It is the basic unit of evolution. EVOLUTION, in its simplest form, can be defined as changes in gene frequencies in populations with time. Individuals cannot evolve – only populations can! Populations 4 9/10/07 1
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Some Attributes of Populations: Ecological : Size Distribution/Range Density – raw vs. ecological Dispersion Growth rate = recruitment minus loss Carrying capacity Density independent factors Density dependent factors Sex ratio Age structure Survivorship Reproductive “strategy” Resource needs Genetic : Gene frequencies Evolutionary history/future Populations 4 9/10/07 2
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Population Attributes (1): Size = the number of individuals in the population. It can be counted or estimated. Distribution (Range) = where the population resides. It occurs there because: 1) it got there, and 2) it can stand it (limits of tolerance) It will be more abundant in some places than in others. Density = the number of individuals per unit area or volume. Raw density = numbers per total area or volume Ecological density = numbers per area or volume of suitable habitat Dispersion = how individuals are spaced out in the landscape. There are only so many resources, and spacing helps to reduce competition: 1) Random (the most rare) 2) Uniform: competition for resources (creosote bushes in the desert); territoriality (American robin) 3) Contagious (clumped): clusters of resources; for reproduction; for Populations 4 9/10/07 3
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anti-predator behavior (blacktailed prairie dog); cooperation Capture/Recapture Method of estimating population size: Day 4: live trap 200 mice, 20 of which marked Can we calculate the total population? Our 2 nd trapping showed 10% of mice marked. 1 st day we marked 100 mice; therefore, those 100 mice must represent 10% of the total population of 1000 mice! N = population size M = number marked T = number captured 2 nd time w/some marked R = number recaptured that were marked N = M x T = 100 x 200 = 1000 R 20 This assumes: no trap shyness not trap happiness Populations 4 9/10/07 4
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Population Attributes (2): Recruitment rate natality plus immigration = #s added/time Loss rate mortality plus emigration = #s leaving/time Growth rate (r) recruitment – loss This is an exponential rate ( i.e., the part of the equation that is causing the population increase is also increasing). Exponential rates results in “J” curves. For simplicity’s sake, assume immigration = emigration; therefore, if b-d = +, the population is expanding if b-d = -, the population is shrinking if b-d = 0, “zero population growth” The Demographic Transition represents the human population increase during our industrial revolution. (More about this later.) Doubling Time is a useful way to look at growth rate. 70 (%) divided by the percent growth rate per
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11 - POPULATION BIOLOGY A POPULATION is defined as a group...

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