Biotic Factors Lab

Mark-Release-Recapture

The measurement of the number of individuals in a natural population is difficult to obtain. The more individuals within a natural unit that we can get under our control, the better the estimated values. Sometimes direct enumeration works well and is all that is needed. Bison on a certain patch of prairie and giraffes on a given patch of veldt are good examples of this. The individuals are large, can be observed easily because they are conspicuous against their surroundings. Long-legged wading-fowl and many plants also offer the same benefit. However, many field biologists want to use common indicator species to address specific questions. In other words, they need animals which are common enough to lend themselves to regular, adequate sampling, and are useful in experimental models when addressing specific questions. Most of these animals are usually not large or conspicuous against their surroundings.

The counts can be estimated using a technique called "Mark-Release-Recapture". Peterson working with schooling fish and Lincoln working with waterfowl discovered that random samples of a population could be taken, marked and released (M) to the population. Then a second random sample taken from the population would lead to a proportion of marked (m) and released animals to unmarked animals (n) captured and this would lead to a good approximation of the population size (P).

P = Mn/m

The simplest form of this sampling technique is called the Lincoln-Peterson Index.

ni = total number of individuals captured during a particular sampling episode. i = the trapping session

ai = total number of marked animals released after a given trapping session within a field of study.

r = number of marked animals recaptured during a given session of sampling. i = the last session in which the animal was captured, or recaptured and j = the sampling session presently at hand

Pi = the estimated value of the entire population for a given period using the mark-release-recapture data

Sj = the number of new initial captures in a sample

Pi = ai n2 / r 12

The index predicts backward to the day of the previous capture. It does not predict the size of the population on the day the last sample is taken.

There are several assumptions that the biologist must make for this system to work:

• Consistency is necessary. If everything is completed the same way every time, it becomes a constant affect overall.
• There must be a neutral marking system that will not affect the animals making them categorically different from unmarked animals when returned to the field. Depending on the animal there are a number of marking systems possible, paints and dyes, tags, bands, toe-clipping, ear-notching, etc.
• You must select a marker that will not become lost or indistinguishable. Laboratory tests and a literature search should provide evidence that a marking system is adequate.
• Marked individuals must mix in randomly with the other members of the population. The concentration of markers upon release must become "homogenized" by the total field population and the proportion of markers in later samples will be a random sample of the total field population.
• There can be no birth or immigration because this will dilute the marked subset by increasing the unmarked individuals.
• All individuals must be equally susceptible to sampling regardless of mark status.
• All indivudals must be equally susceptible to sampling regardless of status within the field population.
• No death or emigration because it may cause an uneven effect on the proportion of marked to unmarked individuals.

LAB

1. Place your traps in a grid with each trap 20 paces from the nearest at dusk. Bait the traps with an apple slice and bird seed.

2. Check traps at sunrise.

3. Mark each deer mouse using the ear notch technique.

4. Release each deer mouse at the trap caught.

5. Repeat this process for three days.

6. Using the equations above, calculate the population size for the deer mice in the area surveyed.