2.1 d - monitoring populations

• presence, absence or abundance of an indicator species can give information of environmental qualities such as presence of pollutants
• susceptible and favoured species can be used to monitor an ecosystem
• absence or reduced population indicates a species is susceptible to some factor in environment
• abundance or increased population indicated is favoured by the conditions
• the most popular way to measure the size of the population is called the Mark and Recapture technique
• the mark and recapture technique is used to estimate the population size of a species
• the mark and recapture method requires marking a number of individuals in a natural population, returning them to the population, and subsequently recapturing some of them as a basis for estimating the size of the population at the time of marking and released
• R

  marked recaptures
• T

  total in second sample
• M

  marked initially
• N

  total population size
• assumption 1

  during the marking and the recapture period, nothing has happened to upset the proportions of marked and unmarked individuals
• assumption 2

  the chance for each individual in the population to be caught are equal and constant for both the marking and recapture periods
• assumption 3

  sufficient time must be allowed between the initial marking period and the recapture period for all marked individuals to be randomly dispersed throughout the population
• assumption 4

  animals do not lose their marks which is an important factor for animals that shed or molt as they grow or respond to seasonal factors