Comparison of Schnabel and Lincoln-Peterson Population Estimates Using a Snail Population on Blackburn Fork Creek.

John Lewis, Tennessee Technological University, Cookeville, TN 38505

Abstract: This paper shows two different mark-recapture population estimators, the Lincoln-Peterson and the Schnabel. It indicates their differences and comes to the conclusion the Schnabel method is a more accurate way of determining the number of members in a population.

Key Words: Lincoln- Peterson, Schnabel, population estimate, mark-recapture, accuracy

Introduction

I intend to show through this study that mark-recapture techniques are a valuable tool to biologists. Although there are differences in the accuracy of the two estimates I will be comparing, one can get a good idea of the number in a population if enough samples are taken. A study by Collazo and Bonilla-Martinez (2001) showed that mark-resight estimates could be up to 38% higher than just doing regional counts. Mark-recapture methods may produce inaccurate population estimates, but could be employed to determine a change in a group’s size (Evans 2001). Shirakihara et. al. (2002) found their mark-recapture study with bottlenose dolphins to have a low variability, which adds to the credibility of these techniques. Population size can also be estimated by combining population density measurements with distribution maps (Lizcano et.al. 2002). Population viability analysis has become a common tool for biologists, but should not be used to determine minimum population sizes for an area (Reed et.al. 2002). Population viability analysis has become a common tool for biologists, but should not be used to determine minimum population sizes for an area (Reed et.al. 2002). Overall, mark-recapture estimates are a good tool. The objective of this study is to disprove the null hypothesis that there will be no difference between the Schnabel and Lincoln-Peterson population estimates in a snail population on Blackburn Fork Creek.

Methods and Materials

In this experiment I used hip waders to walk in the creek. I used a Surber-type square foot invertebrate sampler, a collection tray to count the sample in, and clear fingernail polish as my marking paint. I randomly placed the Surber sampler over an area of substrate in the stream with the net trailing downstream. I then disturbed the substrate within the frame, dislodging all gastropods, which flowed into the net. Next, all rocks and other materials were scrubbed to dislodge any tightly adhering snails (Bookhout 1996). The snails were placed in the collecting pan and dried. After drying, a thin dot of fingernail polish was placed on each specimen. The number of specimens marked was recorded on the data sheets (Bulow and Combs 1998). After the marks were allowed to dry, the snails were evenly redistributed throughout the sample area. These steps were repeated nine times in an effort to take a more accurate sample.

Results and Discussion

Data was analyzed using Programs for Ecological Methodology, 2^nd Edition (Krebs 1998). The following results were found:

PETERSEN METHOD OF POPULATION ESTIMATION

(Program PETERSEN, Version 5.1)

Input data: No. Marked at Time 1 = 16.

Total No. Caught at Time 2 = 19.

No. Marked and Recaptured at Time 2 = 3.

PETERSON ESTIMATED POPULATION SIZE = 84.00

SCHNABEL POPULATION ESTIMATE

(Program Schnabel, Version 5.1)

RAW DATA INPUT : NUMBER OF SAMPLES = 10

TIME Cap. Recap. NO. NEW MARKED

1 16. 0. 16.

2 19. 3. 16.

3 17. 3. 14.

4 16. 2. 14.

5 18. 1. 17.

6 18. 3. 15.

7 17. 4. 13.

8 20. 2. 18.

9 19. 2. 17.

10 16. 4. 12.

ORIGINAL SCHNABEL BINOMIAL ESTIMATE = 512.1

As one can see the Peterson method gave an extremely low estimate for the number of snails in Blackburn Fork. The Schnabel method gave a more realistic population estimate. This is because of the increased number of capture events in the Schnabel method. The accuracy of the estimate increased with the number of samples taken. Evans (2001) stated mark-recapture methods may produce inaccurate population estimates, but could maybe the Schnabel could be employed to determine a change in the size of this snail population. This type of estimate could be used in conjunction with a population viability estimate to determine a minimum population size for an area (Reed et.al. 2002). Densities found from this study could be used in combination with distribution maps to estimate other populations (Lizcano et.al. 2002). Adding additional credit to this type of study are Collazo and Bonilla-Martinez (2001) who have shown 38% higher estimates using mark-resight estimates. Credibility is also added by Shirakihara et. al. (2002)who found their mark-recapture study to have low variability. If done properly, the Schnabel mark-recapture method has a vast array of applications.

Conclusions

Although it takes more time to perform numerous capture events, the estimate one receives is more accurate as sample events increase. Biologists should use the Schnabel method over the Lincoln-Peterson in all circumstances. Time and budget should be the only constraint as to how many sample events are performed. Remember that accuracy is all important in making informed decisions as biologists.

Literature Cited

Bookhout, Theodore 1996. Estimating the Number of Animals in Wildlife Populations. Research and Management Techniques for Wildlife and Habitats 239-242.

Bulow, Frank J. and Combs, Daniel L. 1998. Animal Population Estimation. General Ecology Laboratory 69-71.

Collazo, J.A. and Bonilla-Martinez, G. Population size, survival, and movements of White-cheeked Pintails in eastern Puerto Rico. Caribbean Journal of Science 37:194-201.

Evans, Theodore A. 2001. Estimating relative decline in populations of subterranean termites due to baiting. Journal of Economic Entomology 94:1602-1609.

Lizcano, D.J., Pizarro, V., Cavelier, J., and Carmona, J. 2002. Geographic distribution and population size of the mountain tapir in Colombia. Journal of Biogeography 29:7-15.

Reed, J.M., Mills, L.S., Dunning, J.B., Menges, E.S., McKelvey, K.S., Frye, R., Beissinger, S.R., Anstett, M., and Miller, P. 2002. Emerging issues in population viability analysis. Conservation Biology 16:7-19.

Shirakihara, M., Shirakihara, K., Tomonaga, J., and Takatsuki, M. 2002. A resident population of Indo-Pacific bottlenose dolphins in Amakusa, western Kyushu, Japan. Marine Mammal Science 18:30-41.