Ecology Notes Part #2 - I Demography 1 Demography is the...

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I) Demography 1) Demography is the science of measuring the important phenomena that collectively provide numerical descriptions of a population a The number of individuals within it b Their birth rates and death rates c Movements of individuals between populations of the same species d The change in the number of individuals through time 2) Ages, or Stages a When info is assembled about a population, this information is usually arranged in a manner that indicates how individuals of different ages (or stages) contribute to the population as a whole b These compilations of age- or stage- specific data are called life tables 3) Life Tables a There are two very different ways of generating life tables (i) Following cohorts of organisms This requires marking, or some other mechanism, in order to follow a single cohort through time (ii) Taking a “snapshot in time” of an entire population and assessing all cohorts (ages) that are present This requires being able to accurately age (or assign to stage) each individual captured and examined. b Life Table Data (i) Although life tables, when complete, can contain many columns of numbers, the data actually accumulated and entered by the biologist is really quite a small portion of the total: The number of individuals alive (or dying) at some age or during some time interval The production of offspring by individuals of each age c Compiling demographic statistics (i) Sampling (quadrats, line transects, etc.) (ii) Mark-recapture What is an individual? What is the geographic distribution of our study population? (iii) Even for one species, the survival curve can differ with environmental conditions Shape of curve Maximum age How might reproduction be affected?
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II) Population Sizes and Growth Rates Populations are collections of individuals! Individual birth rates and death rates, and the effect of competition/crowding on these rates, are acted on by natural selection and they evolve. a Populations simple “follow along.” (i) N 1 = N 0 R (ii) N t+1 = N t R (iii) N t = N 0 R t (iv) N t /N t+1 = 1/R b How population growth is changed by competition (density) 2) Competition a An interaction between individuals, brought about by a shared requirement for one or more resources that leads to a reduction in survival, growth, and/or reproduction of at least some of the competing individuals. b Usually, competition increases as density increases, though the relationship may not be linear! (In fact, it is almost never linear.) 3) Time lags a The earlier growth models assume that density of an organism affects its individuals “instantaneously” b But, what if there are lags? (i) A lag is a phenomenon that occurs when a factor causes a response in some system, but the response is delayed.
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