Snail Data Paper (Final Version WORD)

8/8/2019 Snail Data Paper (Final Version WORD)

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Michael Lambert September, 12, 2010

Ecology Lab Paper One final version BIOL. 3060

Using Freshwater Snails to Test the Accurateness of the Population Estimation

Techniques Enumeration, Mark and Recapture, and Depletion

Abstract

The purpose of our study was to determine if there were any differences

between three population density estimate methods, and if so, which method was

the most accurate. The methods used were enumeration, mark & recapture, and

depletion. The site where the experiment took place was a small section of

Choctafaula Creek in Macon County, Alabama. The species of snail we studied

was Elimia flava. Our hypothesis was that all three methods would yield equally

accurate findings. The null hypothesis was that the separate methods would yield

different results. The final results according to the enumeration technique were76 snails per sqr. m; the mark and recapture method yielded 141 snails per sqr.

m; and the depletion method resulted in 55 E. flavaper sqr. m. These results

contradicted our hypothesis since the three techniques yielded different findings.

Introduction

The purpose of our study was to use three different techniques regarding

population density and to determine which was the most accurate. Population

estimation is important because measur[ing] changes in population size overspace and time... [helps researchers] to understand how different physical or

biological factors influence the distribution or abundance of species (BIOL 3060

Teaching Staff, 2009). Population estimations may be used by researchers to

determine the effects that human actions have on the environment. The three

techniques we used to determine population density were enumeration, mark &

recapture, and depletion.

The enumeration method only counts the number of individuals in any

specific study area. The assumptions for enumeration were:

(1) that all individuals were captured

(2) that the removal of the captured individuals did not attract more individualsinto the vacated study area (i.e., no migration)...

(3) The method also assumes that no individuals are born or die during the

period over which sampling is done (BIOL 3060 Teaching Staff, 2009).

The mark & recapture method tags all individuals captured in a population,

releases them back into the study area and recaptures them at a later date. The

assumptions for the mark & recapture technique were:


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1) ... No new individuals were born or immigrated into the population and

none died or emigrated.

2) All individuals are equally likely to be caught within each capture period.

3) Marked individuals must not become either easier or more difficult to catch

during the second capture period compared to unmarked individuals.

4) Sufficient time... [permitted] all marked individuals to be randomly

dispersed throughout the population (BIOL 3060 Teaching Staff, 2009).

The depletion method assumes that a specific fraction of a population will

decrease at a constant rate over a measured time interval. The assumptions for

the depletion technique were:

1) ... No births, deaths or migration occur.

2) The probability of capture is the same for each individual in the population.

3) The probability of capture does not change from one sample to the next

(BIOL 3060 Teaching Staff, 2009).

We examined different methods of population estimation with the intent tofind out which one was the most accurate.

Our working hypothesis was that all three techniques used would give

similar results. If the final results were significantly different from each other then

our hypothesis would be disproved.

Methods

The study site was a small creek about seven meters across. It was only a

few centimeters deep in most places and had many small rocks jutting out the

surface of the water. On either end of the study area the water got much deeper,

down to about one meter. Both banks were sandy and most plant life started

about a meter away from the waters edge. A bridge was directly above part of

the study area which cast a large shadow over about half of it. Near one of the

banks, just outside of the bridges shadows reach, was a patch of aquatic

grasses that was about four meters long and two meters wide. On the other side

of the creek was a very small mound of sand that made a tiny island. Several

logs were partially submerged just outside the study area.

The animal whose population we measured was the Elimia flava.This is an aquatic snail that lives in freshwater streams and creeks. The E. flava

was appropriate for this study because of the ease of capture and marking. They

only live for a few years. They travel slowly, covering only a few centimeters a

day, and stay in the study area over the duration of the experiment. This makes

them good subjects for the mark & recapture method.


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To begin the experiment, we took measurements of the study area,

including air and water temperatures, water depth and clarity, along with the

velocity of the current and the area of the study area. These measurements are

provided in the following table.

Week1

Week2

Environmental Mon. Tues. Wed. Thur. Fri. Mon. Tues. Wed. Thur. Fri.

Air temp. 29 34 28 31 30.5 C 27 27 33 30 30.5 C

Water temp. 24 25 25.5 25 24.5 C 22 24 23 24 23 C

clarity before .15m 0.19 Same 0.19 17cm 0.166 Same 17.5

clarity after .15m 0.19 0.19 17cm 0.166 17.5

Depth (m) 0.15 0.19 0.13 0.14 0.19 0.17 0.166 0.18 0.145 17.5Velocity

(sec/m) 0.213 0.225 0.25 0.25 0.25 0.286 0.25Study site 100 76 49 23 109

area (sqr. m)

Simple measurements taken around the study area

Once the basic measurements of the study area were taken, we began

collecting E. flava. The way we captured the snails was very simple. We waded

around in the water and picked them off of the small rocks there. We placed them

in buckets and then pooled them all together to tag them. This continued until

200 person minutes had been spent collecting snails.While some people gathered the snails, others tagged them. The captured

E. flavawere dried with a towel and placed in a shallow pan to finish air drying.

Once the snails were mostly dry, a small dab of highly visible paint was applied to

the ventral surface (underside) of their shells. They were once more set aside so

the paint would dry. Afterwards, they were placed in another large bucket with

creek water in it. After each harvest, the E. flavawere brought back to the lab to

be counted.

This entire procedure was conducted Monday through Friday for one week,

each day using a different color. At the end of the week all E. flavawere releasedinto the same area. They were scattered randomly and given the weekend to

disperse. The following monday we began to recapture them, which used the

same procedures as the original capture week. At the end of the second week

the snails were again released into the same area of the creek.

To calculate enumeration, we summed the total snails and divided the total

by the area. This formula was:


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(5,497 snails / 72 m^2 = 76.35 snails per m^2)

The mark & recapture method compared the number of snails captured

and then released in week one to the number recaptured in week two. The

formula used was:

(Total captured in week 2 (recaptures and new captures) x Total captured in week

one/ recaptured)

This gave numerical results of:

(3,816 x 2,697/1,016 = 10,130)

(10,130 / 72 m^2 = 140.7 snails per m^2)

Results

According to the enumeration method, there are roughly 76 snails per sqr.m in the study area. According to the mark & recapture method, there are about

141 E. flavaper sqr. m in the study area, and according to the depletion method,

there are about 55 E. flavaper sqr. m in the study area.

Here is a regression line showing the expected population of E. flavain the

study area. Where the lines would intersect with the x-axis are the expected

populations of E. flava.


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Chart showing the depletion of snail population over a period of two weeks

DiscussionWe fulfilled our objective of using and comparing three separate population

estimation techniques. The results of our experiment contradicted our hypothesis.

Because there is no way to guarantee that all individuals were captured,

the enumeration method is subject to possible error. Because the study area was

searched by multiple groups of students, the same rocks were searched multiple

times. This could have led to an influx of additional E. flavabeing found and thus

increasing the enumeration total. A way to improve this would be to have a larger

study area. This would make it easier to assign certain portions of the study area

to specific groups of students.Due to the new captures in week two, it can be concluded that additional

snails immigrated into the study area, thus, violating the first assumption of the

mark & recapture method. Isolating the study area from outside populations could

eliminate this potential error.


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Due to immigration, the first assumption of the depletion method was

violated. This could have overestimated the final population count. Again,

isolating the study area could potentially eliminate this problem.

Though the three methods yielded different results, mark & recapture

appeared to be the most accurate because only one of its assumptions was

violated.


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Literature Cited

BIOL 3060 Teaching Staff. 2009. Principles of Ecology. Auburn University.

Documents

Snail Data Paper (Final Version WORD)