Estimation of Population Size Using Snails
The distribution and abundance of organisms are influenced by a number of physical
(temperature, humidity, rain, soil conditions, etc) and biological (species characteristics,
competition, predation, abundance of food, etc) factors. An important part of measuring
distribution and abundance is measuring the changes that occur in population size over space
and/or time. However, it usually is not possible to count every individual in a natural population
of animals. Ecologists generally have to rely on some kind of estimate of abundance or density.
For example, in some birds, the males are fairly conspicuous due to plumage, song or both, while
females may be hard to observe. In this situation, we could count the number of singing males of
a species of bird in a given community and then estimate the size of the breeding population by
assuming that for every male there is one female. Or, we could count the individuals in a sample
area and then extrapolate to the larger area in which the whole population lives. There are
several methods available for estimating population size, each having its own strengths and
weaknesses. Because of this, ecologists use at least two methods in any population study.
In this field lab you will estimate the
abundance of brown garden snails (Helix aspersa)
that live in some of the plantings around Hartnell’s
campus. You will use and compare 2 of 3 different
techniques for estimating population size:
enumeration, depletion, and mark and recapture.
You must examine the assumptions of each method
and determine their validity for this population.
Population size often depends on local environmental
factors, so be sure to make note of any factors that
might affect the snails.
© Copyright 2002-2003, Warren E. Savary and Luis A. Solorzano
Snail Biology
You will be estimating population size of the brown garden snail (Helix aspersa), an
oviparous (egg laying) snail of the family Helicidae. This snail is native to Europe and the
Mediterranean Region but was introduced by French immigrants into numerous areas including
California. It is a general feeder and will eat most anything in a garden. The brown garden snail
eats large holes in leaves and may totally consume seedlings.
A snail lays an average of 85 eggs in a nest 2.5 to 4 cm deep in the soil. Eggs hatch in 2
to 4 weeks depending on soil moisture and temperature. These conditions also determine the
number of times a snail oviposits, or lays eggs. Low temperatures (less than 12°C) and low
humidity inhibit the activity of the snail. During warm, damp weather, ovipositions may be as
frequent as once a month. When it is dry the snail will seal itself to an object or close off the
aperture of the shell with parchment-like material. The snails become active again when moist
conditions return.
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The population biology of snails makes them great subjects for studying population size.
They are easy to catch, and easy to mark (see below) and observe. Population sizes do not
change dramatically from day to day. Also, brown garden snails are abundant, so estimates of
their population size can be based on fairly large samples of individuals.
I. Enumeration
The easiest way to estimate population size is to count all of individuals that you catch
over several sampling periods. Usually, however, some individuals are present in a study area
and are not captured. So, the enumeration method provides a minimum estimate of population
size.
Assumptions:
1. The enumeration method assumes that all individuals were captured.
2. It is also assumed that the removal of the captured individuals did not attract more
individuals into the study area (no migration). If migration into the habitat occurs due to lower
density as the captured individuals are removed (even temporarily), this is termed a "vacuum"
effect.
3. This method also assumes that no individuals are born or die during the sampling
period.
II. Mark and Recapture
The mark and recapture method involves marking a number of individuals in a natural
population, returning them to that population, and then recapturing some of them. This provides
a basis for estimating the size of the population at the time of the marking and release. This
method is based on the principle that if a proportion of the population was marked in some way,
returned to the original population and then, after complete mixing, a 2nd sample was taken, the
number of marked individuals in the 2nd sample (R) would have the same ratio to the total
number in the 2nd sample (C) as the total of marked individuals originally released (M) would
have to the total population (P). That is:
R/C = M/P
Where R is the number of marked animals recaptured in the 2nd sample
C is the total number of animals captured in the 2nd sample
M is the number of animals marked and released in the 1st sample
P is the (as yet unknown) population size
Assumptions:
1. During the interval between the preliminary marking period and the subsequent
recapture period, nothing has happened to upset the ratio of marked to unmarked animals (that is,
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. That is,
marked individuals must not become either easier or more difficult to catch during the second
capture period compared to unmarked individuals.
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3. Sufficient time must be allowed between the initial marking period and the recapture
period for all marked individuals to be randomly dispersed throughout the population (so that
assumption 2 above is not violated). However, the time period must not be so long that
assumption 1 breaks down.
III. Depletion Method
A third method is called the depletion method of estimating population size. Depletion is
based on the assumption that with repeated sampling of a population over a short period of time,
a constant fraction of the remaining population will be taken in each sample. This fraction can be
estimated by depleting the population several times and doing a least squares linear regression of
the number taken in a given sample (Y) against the total taken in all previous samples (X). The
slope of the regression line is the estimate of the fraction of the remaining population taken with
each sample. The X intercept of the line is the estimate of the total population size.
Assumptions:
1. Population size does not change during the sampling period; that is, 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.
Materials and Methods
You will be using the enumeration and mark and recapture methods of population
estimation. Snails will be captured by picking them off of plants or the ground. We will collect
the snails in pans or buckets. Watch to be sure they are not escaping as you search for more
snails! The snails will be marked with nail polish, and then returned to the plants. Work in teams
of two or three. Each group will search one area with vegetation, such as Agapanthas or ivy.
Search your section carefully, but only do it once. Do not go back over an area that you've
already searched, even if you feel that you've gotten better at finding the snails. The purpose of
this is to standardize the capture effort, so that the probability of capture is similar among all
teams (how would it affect the estimates if some teams searched once, some twice, and some
three times?).
 For the enumeration method you will simply add up all of the snails that you capture
during both weeks of the study. Do not recount recaptured individuals.
 For the mark and recapture method of estimating population size all of the individuals
marked and released in the first week will be considered to be our initial marked sample
(M). Each lab group should use a different color to mark their snails. Our second sample
for the mark and recapture technique will be obtained during the second week of the
study.
When marking the snails, wipe the ventral (under) surface of the shell as clean and dry as
possible and put it in your bucket for air drying. Mark the snails with a large clearly visible dot
of nail polish in the color that represents your group. Allow the marks to dry thoroughly before
releasing them back into your study site. After marking the snails, estimate the size of the area
sampled. A meter stick or tape will be provided. Record the total number of snails collected by
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the lab groups and the area of campus that they were taken from. Also, record habitat
characteristics of your sampling location (e.g., shade, type of vegetation, in dirt, etc.).
Equipment:
Pans and buckets
Nail polish for marking
Meter tape
Clothing and shoes suitable
for getting dirty
Gloves (if you want)
Data Analysis
Description of your study site:
Area of study site:
Number of snails captured and marked on the first day:
Number of UNMARKED snails captured on the second day:
Number of MARKED snails captured on the second day:
Calculate the total number of snails that you captured in your study area – this is your
estimate of population size using the enumeration method. Divide your estimates of population
size by the area of vegetation that you sampled to get an estimate of population density.
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Calculate the mark and recapture estimate using numbers of snails captured from week to
week. The formula for the estimate is: P = (CM)/R (see above). Divide your estimates of
population size by the area of vegetation that you sampled to get an estimate of population
density.
Questions
1. What was the purpose of the laboratory study?
2. Did you have any predications (expectations) to test?
3. Which method do you think is more accurate? Why?
4. Are the estimated densities reasonable?
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5. How could you make a decision about this?
6. Did you violate any of the assumptions? Which ones?
7. Does violating the assumptions make any difference to your conclusions?
This lab was based on one conducted at Auburn University with snail biology information from
the University of Florida, IFAS. The photo of Helix aspersa is used with permission ©
Copyright 2002-2003, Warren E. Savary and Luis A. Solorzano.
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