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Biology 1001A
Tom Haffie

November 22, 2011 Lec 20: Population ecology • read text chapter 45 “intro”, 45.1, 45.2 • Population: group of individuals of the same species, living in a defined geographic area Characteristics of populations • Populations have many attributes that are not associated with individuals: – Allele frequencies, genotype frequencies – Population size (N) – Geographic range – Sex ratio – Age structure- pre reproductive vs. reproductive vs. post reproductive – Rate of growth • These are properties of populations, not individuals How to estimate population size? - Population size- easy to estimate population size of things that are not motile. - Easy to count a sample and then find the size of the total area size Mark-recapture method – capture & MARK an initial sample (M) – later, CAPTURE a second sample of animals (C) & see how many are marked RECAPTURES (R) – use this to estimate population size (N) – R/C = M/N – N = MC/R – Used to count motile populations – Principle here is that in the second sample, the portion that is already marked should be equal to the proportion of individuals that were caught the first time (M/N) Mark-recapture works if… • marked and unmarked animals equally likely to be captured • marks don’t affect survival • marks don’t wear off • time between two samples is long enough for marked animals to mix freely with unmarked animals, but not so long that population size changes Population density • density = N/area (or N/volume) • - Negative correlation between body size November 22, 2011 Why does density matter? • influences access to resources • Density too high: per capita growth rate (rate of increase per individual per unit time, r) may decrease
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