Species Abundance

Species abundance is the number of individuals of each species in an area. It adds the information that species richness leaves out.Abundance can be recorded in different ways, depending on the organism:1. Counting individuals, for example the number of limpets in a quadrat or beetles in a pitfall trap.2. Percentage cover, for plants such as grasses and mosses where individuals are hard to tell apart. A quadrat divided into 100 small squares can be used, and the number of squares containing the species gives its percentage cover.3. Frequency, the percentage of quadrats in which a species is found.4. Abundance scales, such as the ACFOR scale (abundant, common, frequent, occasional, rare), which are quick but subjective.For mobile animals, population size can be estimated by mark-release-recapture.Abundance shows how evenly individuals are spread between species, which is called species evenness. A community in which all species have similar numbers is more diverse than one dominated by a single species.Example. Two ponds each contain 3 species of invertebrate and 60 animals in total.Pond A: 20 water boatmen, 20 mayfly nymphs, 20 snails.Pond B: 56 water boatmen, 2 mayfly nymphs, 2 snails.Both ponds have a species richness of 3, but pond A is more diverse because the individuals are spread evenly. In pond B the community is dominated by one species, and the loss of the rare species would be more likely. To combine richness and abundance in a single number, biologists use an index of diversity.