ECOLOGY

by Purwaning Rohmah/BIO off A 2015 UM

MARK-RECAPTURE METHOD

Ecologists are often interested in understanding population dynamics.  That is, how populations change over time in terms of genetic make up and number of individuals.  The size of any population is dependent on many things.  Population size and genetic make-up (gene pool) may change due to immigration/emigration, drift, and factors influencing births and deaths.

Estimating population size is a fundamental objective of many animal monitoring programmes. Capture–recapture methods are often used to estimate population size from repeated sampling of uniquely marked animals, but capturing and marking animals can be cost prohibitive and affect animal behaviours, which can bias population estimates.

How do ecologists estimate the size of any population of interest?  This can be a perplexing issue, especially if the population is quite large or cryptic.  It would be inefficient in terms of time and effort to count each and every individual, so ecologists estimate population size using several methods.  The methods used vary depending mainly on the organism of interest.  Plants are sessile, whereas most animals are mobile organisms.  A line-transect estimate may be useful for enumeration of sessile organisms, but not so for mobile organisms, or ones that are difficult to observe.

Mark-recapture is a useful method for determining the abundance of many animal populations.  This method is often used to estimate population size from repeated sampling of uniquely marked animals, but capturing and marking animals can be cost prohibitive and affect animal behaviours, which can bias population estimates. This method involves capturing a sample of animals and marking them in some way—for instance, using tags, bands, paint, or other body markings, as shown below. Then, the marked animals are released back into the environment and allowed to mix with the rest of the population. Later, a new sample is collected. This new sample will include some individuals that are marked—recaptures—and some individuals that are unmarked. Using the ratio of marked to unmarked individuals, scientists can estimate how many individuals are in the total population. Other names for this method, or closely related methods, include capture-recapture, capture-mark-recapture, mark-recapture, sight-resight, mark-release-recapture, multiple systems estimation, band recovery, the Petersen method, and the Lincoln method.

The example of mark-recapture method:

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2. Let’s say we want to find the size of a deer population. Suppose that we capture 80 deer, tag them, and release them back into the forest. After some time has passed—allowing the marked deer to thoroughly mix with the rest of the population—we come back and capture another 100 deer. Out of these deer, we find that 20 are already marked.

If 20 out of 100 deer are marked, this would suggest that marked deer—which we know are 80 in number—make up 20% of the population. Using this information, we can formulate the following relationship:

Alternative approaches to determine population size include electronic tracking of animals tagged with radio transmitters and use of data from commercial fishing and trapping operations.nd round, inflating population estimates. Alternatively, the same animals may preferentially be retrapped—especially if a food reward is offered—resulting in an underestimate of population size. Also, some species may be harmed by the marking technique, reducing their survival. The approach also assumes that animals don’t die, get born, leave, or enter the population during the period of the study.