The study of any species requires an estimate of their density in different habitats. Scientists use many different methods of measuring density, depending on the size, abundance, behaviour and habitat of the species. Sometimes a line transect and quadrats are used to measure the abundance and cover of different floral species in a grassland reserve. Other methods to measure relative density include:
  1. Aerial photographs (eg. whales, herds of moose or other hoofed mammals during migration, flamingos)
  2. Capture-recapture (or mark and release)
  3. Traps (eg. Elliot traps for small mammals, pit-fall traps for reptiles, light traps for insects, harp traps for bats)
  4. Number of fecal pellets
  5. Vocalisation frequency
  6. Pelt records
  7. Catch per unit fishing effort
  8. Number of artifacts (nests, pupal cases, burrows etc.)
  9. Questionnaires (of hunters and trappers for example)
  10. Cover (of plants)
  11. Feeding capacity (amount of bait taken)
  12. Roadside counts
Each method has it’s advantages and disadvantages and is most valubale when used as an adjunct to more direct methods.This article from the CSIRO, “Estimating whale numbers in the Southern Ocean” explains how scientists go about estimating the abundance and population structure of minke whales.
This activity, Counting Smarties, from Maths by Email, models the “Mark and Recapture” or “Tag and Release” methods using smarties.
Reference: Charles J. Krebs (1985) “Ecology – The Experimental Analysis of Distribution and Abundance” Harper International, New York.