Biology Lecture No. 20: Population Ecology
(Monday, November 21, 2011)
RECALL (Practical Implications of Transmission Mode Hypothesis):
-By reducing the chances for indirect transmission of parasitic species in affected human areas, such
parasites will have to rely more on direct transmission, and therefore the evolution of less virulent
parasites will be promoted.
-A population can be defined as a group of individuals of the same species, living in a defined geographic
Characteristics Of Populations:
-Populations have many attributes that are not associated with individuals which include: allele and
genotype frequencies, population size (N), geographic range, sex ratio, age structure, and rate of
Estimating Population Size (N):
-Population size for non-moving organisms can be easily determined, however for such organisms that
freely move, the Mark-recapture method can be applied.
The Mark-Recapture Method:
-Capture and mark an initial sample (M).
-Later, capture a second sample of animals (C) and see how many are marked as recaptured (R).
-This method can be simplified by the formula R/C = M/N or when calculating population size, N = MC/R
Assumptions Of Mark-Recapture Method:
-Marked and unmarked animals are equally likely to be captured.
-Marks don’t affect survival.
-Marks don’t wear off or become unnoticeable.
-Time between two samples is long enough for marked animals to mix freely with unmarked animals,
but not too long that the population changes size (old individuals die, new individuals are born).
-Density for a population can be defined as the population over the area or volume of the