They cannot move and space themselves out to avoid competition. Shrubs in arid environments appear to be regularly spaced. Measurements produced conflicting results and arguments over sampling techniques and spatial statistics. Estimating population size of aminals Generally, the bigger the animal, the lower the population density. Animals hide and move around.
Heterogeneous environments makes extrapolation from one area to another difficult.
Mark-Recapture Method Catch and mark a known number of the population Release the marked animals, give them time to mingle Resample the population Ratio of marked to unmarked gives population size estimate. Counting Birds Birds are generally more visible and more vocal than most other animals. Small army of skilled amateur birders. Transects don't usually give absolute counts. Allows relative comparison of bird population changes in space and time.