Measuring Population Characteristics PDF

Measuring Population Characteristics It is usually impossible to get an exact count of the number of individuals in a population As a result, biologists have developed a variety of sampling techniques that estimate population size umm... 1 mouse. 2,3,4,5... For organisms that ar...

Measuring Population Characteristics It is usually impossible to get an exact count of the number of individuals in a population As a result, biologists have developed a variety of sampling techniques that estimate population size umm... 1 mouse. 2,3,4,5... For organisms that are stationary (plants) use of quadrats is standard method of sampling Quadrat - frame of a given size that is placed at random within a habitat Count is taken of population size within quadrat Used to extrapolate size of entire population Note: This method does not work very well for populations that are clumped ❏ quadrat sampled may fall in a clump (overestimation) ❏ quadrat sampled may fall between clumps (underestimation) For organisms that are mobile most practical sampling method is mark-recapture sampling ❶ Group of organisms is captured, marked in some way and released ❷ After allowing enough time for marked organisms to mix randomly into population, researchers capture a second sample of organisms ❸ Proportion of marked to unmarked organisms in second sample is used to estimate overall population size Rationale is: If you can measure proportion of marked individuals in second sample (i.e. 10 out of 100 = 10%) Equal to proportion of all marked organism in population (i.e. if 50 were marked, then that’s 10% of all individuals, so population size is 500) Following equation is used to estimate population size in mark-recapture sampling: # marked (M) # recaptured (m) Total population size (N) = Size of second sample (n) M m = N n Mn N= m For example: In October, 48 polar bears were captured and marked. Two months later, 30 polar bears are captured. Of these, 6 are bears that were marked. What is the best estimate of size of this population? 48 6 = N 30 (48)(30) N= (6) N = 240 Sampling predicts a polar bear population of 240 individuals Accuracy of mark-recapture sampling depends on following assumptions: ❏ all individuals in population have an equal chance of being captured ❏ during time between samples, ratio of marked to unmarked individuals is constant (i.e. marking does not affect chance of survival) ❏ during study period population size is constant