BIOS 3010: ECOLOGY

Laboratory 4: Mark and Recapture Estimates of Population Size

Dr Stephen Malcolm

How do we count the numbers of animals in a population? This is a basic, but very important question in ecology, that can also be quite difficult to answer. For example, the insects illustrated above are the hoverfly, Eristalis arbustorum and the honeybee, Apis mellifera. The hover fly is thought to be a palatable, Batesian mimic of the honeybee which is protected from predators by a sting. The hoverfly may thus derive protection from its mimicry of the bee. Both insects are to be seen this month (September) foraging for nectar and pollen at flowers of aster (Aster prenanthoides, A. ericoides and A. novae-angliae) and fleabane (Erigeron philadelphicus). If we are interested in how mimicry might work we would be interested to know how many mimetic hoverflies and model bees are present to determine whether the hoverflies are (1) as abundant as bees, or (2) hoverflies are less abundant, or (3) hoverflies are more abundant.

If you are concerned about handling bees, other animals that are active at this time of year include grasshoppers, beetles and butterflies and you could count any of these groups using the methods described below.

Several methods are available and in this laboratory we will use a single, simple method known as the Lincoln- Petersen technique. This involves a single mark event and a single recapture event and the assumption that the population is closed (no emigration, immigration, death or birth). This is also known as either the Lincoln Index or Petersen Index, but it is not an index of relative population size, but an estimate of the number of individuals in the sample area (density).

A sample of individuals is captured, marked and released back into the population and allowed to mix naturally. This creates a proportion of marked individuals. Once the population has mixed the population is sampled a second time to determine the ratio of marked to unmarked individuals. From the size of the sample marked (n₁), the total number in the second sample (n₂), and the number of marked individuals recaptured in the second sample (m₂), the size of the population (N) is given by the following equation:

N = n₁· n₂/m₂

When populations are large.

When populations are small, the population size is estimated by:

(n₁ + 1)(n₂ + 1)

N = - 1

(m₂ + 1)

The approximate variance (s²) of this estimate is:

(n₁ + 1)(n₂ + 1)(n₁ – m₂)(n₂ – m₂)

s² =

(m₂ + 1)²(m₂ + 2)

The 95% and 99% confidence limits on the estimate are given by:

N ± 1.96(s) (s = √s²) 95%

N ± 2.58(s) (s = √s²) 99%

This estimate of population density is a measure of numbers at the time of release of the marked individuals, but not at the time of recapture. This is because mortality of both marked and unmarked individuals may occur, but if this mortality is equal it should not affect the estimate of population size.

Assumptions:

(1) All individuals must be equally catchable.

(2) Ratio of marked to total animals must not change between release and recapture.

(3) No loss of marks.

(4) No difference between mortality or movement of marked and unmarked individuals.

(5) Recapture sample must be an unbiased estimate of the ratio of marked to total animals.

Laboratory Exercise:

In this laboratory we would like you to choose one or more insect species (e.g. hoverflies and bees, or grasshoppers in different patches of vegetation), or butterflies foraging for nectar in different locations) and estimate their population sizes. To do so we would like you to ask a question that requires population estimates. For example, questions such as “are there more grasshoppers among legumes than grasses?” or “are there more bees than hoverflies foraging at aster and fleabane flowers?”, or “are there more Pieris butterflies foraging at goldenrod than at aster and fleabane?” are all appropriate; or you can make up your own question based on field observations.

Capture, mark and release your insects and then watch them to ensure that they disperse among the population. After 1 hour, recapture as many as you can and record the numbers marked and the numbers unmarked. Insects will be marked using Testors® enamel paint in a way that does not impede each animal’s subsequent movement or behavior.

Use these numbers to calculate the population size, variance of the estimate and the 95% and 99% confidence limits to the estimate. Lastly, in your laboratory write-up, use these numbers to answer the question that you posed in the field. What limitations do you think there were to your estimate and how would you improve the technique?