Sampling Distribution and Abundance

It is almost never possible to count every organism in an area, so ecologists take samples and use them to estimate the whole population. Distribution is where organisms are found in a habitat; abundance is how many individuals of a species are present.Random sampling avoids bias: the area is divided into a grid and coordinates are chosen with a random number generator. Non-random sampling includes opportunistic (choosing convenient sites), stratified (sampling each distinct zone in proportion to its size) and systematic sampling (at regular intervals, for example along a transect, useful where conditions change across an area).Large samples and repeats make results more representative and reliable.Mobile animals are sampled with pitfall traps (ground invertebrates), sweep nets (long grass), pooters (small insects), Tullgren funnels (soil organisms), tree beating and kick sampling (streams).To estimate the size of a population of a mobile animal, capture-mark-release-recapture is used. A first sample is caught, marked harmlessly and released; later a second sample is caught and the number of marked individuals is counted:estimated population size = (number in first sample × number in second sample) ÷ number of marked individuals recapturedWorked example: 40 woodlice are marked and released. Later 50 are caught, of which 10 are marked. Population = (40 × 50) ÷ 10 = 200 woodlice.This assumes marks are not lost and do not affect survival, no migration, births or deaths occur between samples, and marked individuals mix evenly with the population.