SAMPLING A MOBILE POPULATION: THE MARK-RECAPTURE METHOD

Transcription

1 INTRODUCTION SAMPLING A MOBILE POPULATION: THE METHOD Thus far, we have used sampling methods designed to measure the density and dispersion of sessile organisms (plants and galls). Obviously a different methodology is necessary to measure the abundance of organisms that can move around. A large number of techniques exist for sampling and estimating populations of mobile organisms. This exercise examines one method, the mark-recapture method, which is particularly well suited to the study of larger invertebrates and vertebrates. In this exercise, you will perform a simulation that will allow you to examine the effects of sampling effort on the final population estimate. You will also get the opportunity to estimate the population size of CMU students using a benign markrecapture technique. OBJECTIVES In today's lab you will estimate the population size of an unknown population. learn the four assumptions underlying the mark-recapture technique. calculate the likely error of your population estimate. use the mark-recapture technique to estimate the size of the student body at CMU. KEY WORDS Be able to define the following terms. marking period marked cohort immigration emigration sampling effort census population natality mortality marking effort Page 53

2 BACKGROUND Unlike plants, animals are often secretive and usually mobile. Consequently, simple counts or plot sampling techniques are useless for determining population size. Therefore, methods that involve marking individuals have been developed. The most common of these methods is the Lincoln-Peterson Method. Individuals are captured during a marking period and a mark is applied to their bodies. The marked individuals are then released. After a certain period of time (long enough to allow the animals to redistribute themselves randomly), individuals in the mobile population are again captured and counted. This second cohort of individuals is called the census population. Within this census population a certain number of individuals will carry the mark which was applied earlier (marked cohort) and others will be unmarked. It is important that you keep track of the number of marked and unmarked individuals in the census population. There are four very important assumptions that must be met in order for this technique to accurately estimate population size. 1. All individuals in the population must have an equal chance of being caught (no differences for age, sex, size, etc.). 2. There must be no input of new individuals to the population during the intervening time interval. Changes in the population due to natality (births) and immigration (outside animals entering the population) will dilute the marking effort 3. Marking must not alter an individual's behavior, make them more likely to emigrate or be preyed upon, or influence their likelihood of being recaptured. 4. Emigration (individuals leaving the population) and mortality (deaths) must be equal for marked and unmarked individuals. The ability to meet these assumptions requires a great deal of knowledge about the natural history of the species being studied. It is necessary to know such things as the reproductive history of the population, mortality patterns, seasonal patterns of activity and movement, biases in the capture of the animals, etc. It may turn out that the species in question violates some or all of the assumptions. If this happens, there are methods of estimating population size that take into account various factors that might be altering the numbers in the population. While we will not study these here it is important to know that they are available. Page 54

3 A large number of methods exist for sampling mobile populations. Birds may be sampled with mist nets, fish with trap nets, mice with Sherman live traps, and caribou by helicopter with a tranquilizer gun (Can you see Marlin Perkins now?). Regardless of the sampling method, the calculation for estimating population size is the same. One simply equates the proportion of marked individuals in the census population (m/p) to the proportion of marked individuals in the unknown population (M/P). Thus, our calculation formula is the following: M P = m p (eqn. 1) where: M = number of marked individuals P = unknown population size m = number of marked individuals recaptured (marked cohort) p = number of recaptured individuals, both marked and unmarked (census population) The size of M represents the marking effort and the size of p represents the sampling effort. The unknown population size (P) may be estimated by rearranged the above formula such that P = (Mp)/m. A BETTER ESTIMATOR The Schnabel method simply extends the Petersen method to more than 1 resample. The theory is exactly the same N 0 is estimated by the ratio of the number of marked animals released into the population to the estimated proportion of marks in the population. In fact, the Schnabel estimate of N 0 is simply a weighted average of s individual Petersen estimates, namely N 0 = Â( C t M t ) ÂR t (eqn 2) Page 55

4 where M t is the number of marked animals in the population just before the sample at time t is taken, C t is the number of animals in the sample at time t, and R t is the number of animals in the sample at time t that had a mark. The averaging over time periods provides a more precise estimate of the population size and allows you to evaluate if the assumptions have been violated. Not surprisingly, the assumptions for the Schnabel estimate are the same as those for the Petersen method. Example The following data are from sunfish marked and released in an Indiana lake. t C t R t Newly marked M t C t M t na Totals (S) Note that the cells in the M t column are the sum of the "new marked fish" from all previous time periods. Thus, with equation 2, the estimated N 0 is equal to 10740/24 = DIRECTIONS CMU STUDENTS Each laboratory section will participate in a population census of CMU students. We will use a modification of the mark-recapture technique. This modification (called the Schnabel Method) employs multiple marking periods and a marked cohort that is ever-increasing in size. The Tuesday morning section will begin marking students Page 56

5 during class change time. Tuesday afternoon and Thursday sections will mark and census students. The Friday morning section will only census students. Groups of 3-4 students should position themselves at strategic points around campus. Each group should approach traveling students and ask if they would be willing to participate in the four-day study. Interested students should be marked by attaching red flagging tape to their book bags or coats. Mark as many students as possible and record the number marked. Also, record the number of intercepted students already wearing a piece of red flagging tape. The results from this mark-recapture exercise will be tabulated and returned to each section the following week. You will be expected to discuss the population estimate and the degree to which the four assumptions were met. DISCUSSION QUESTIONS 1. Why are emigration and mortality acceptable in a sample population as long as the rate is the same for marked and unmarked individuals? 2. How different are the mean populations estimates and standard deviations for the various sampling and marking efforts? Explain. 3. Which sampling/marking effort gave you the most accurate results (i.e., smallest %Error)? Why? 4. What are some of the practical constraints when using a high sampling effort in an actual field study? 5. How would a biologist determine how many organisms to mark in order to generate a reliable estimate of P? Page 57

6 CMU STUDENT CENSUS Lab section: Campus location: newly marked (M) number unmarked number recaptured (r) Page 58