Laboratory #2: Estimating Animal Population Size:
Grasshoppers
Introduction
Knowing the size or density (size / unit area) of a population is the first stage in many
ecological questions. The most direct way to determine population size is to count all the individuals,
but for most populations, a complete count is not possible. Fortunately, several methods are available
that estimate population size after sampling a portion of the total population. In this lab we will use the
capture-recapture method to estimate population size in two habitats.
Methods
Knowing your organisms
In this lab you may encounter up to 4 different types of grasshoppers in three different families.
To begin, each pair of students should spend about 10 minutes simply capturing grasshoppers and
identifying them with respect to the type of grasshopper and sex (see Appendix for an identification
guide). Check all your identifications with one of the instructors before proceeding to the next step.
Estimating population size using capture-recapture
Capture-recapture methods are some of the most commonly used methods to estimate
population size. There are many variations of the capture-recapture technique. We will use the earliest
and simplest, the Lincoln-Peterson Index (so called because it was derived independently by Peterson
in 1894 for estimating fish populations and by Lincoln in 1930 to estimate duck population sizes from
hunters’ returns of leg bands).
All capture-recapture methods are based on establishing a population of known size using
marked individuals. The number of recaptures can then be used to estimate the “catchability” of the
population as a whole – that is, the fraction of the population that is captured by sampling. In this way,
the total population size can be estimated by extrapolation. The Lincoln-Peterson Index is the simplest
of the mark-recapture methods because it is based on only two captures; the mark and the recapture.
Initially, a sample of the population is captured, marked and released back into the population, thus
establishing a known population of marked individuals. A second catch is then made. The proportion
of marked animals recaptured in the second sample equals the catchability of the population:
C=
r
M
(1)
Where C defines the “catchability” of a population, M = the number of animals marked and released,
and r = the number of marked animals re-captured. Finally, total population size, N, can be estimated
by dividing the number of individuals captured in the second sample, s, by the catchability.
N=
s
C
(2)
The above method assumes the following:
1. The marked individuals become randomly mingled with the rest of the population.
2. Losses or gains to the population due to deaths, births, immigration, or emigration are
negligible.
3. All individuals are equally likely to be caught. That is, being captured once does not affect the
probability of an individual being caught again.
4. Marking does not affect the individuals.
5. Samples are taken randomly.
Procedure for the Lincoln-Peterson Capture-Recapture
The capture-recapture experiment will be carried out on 2 plots: a circular plot and a donutshaped plot which surrounds it (Figure 1). This design will allow us to estimate the densities in both
plots as well as to test the assumption that immigration and emigration are negligible. Grasshoppers
captured in the inner circle will be marked with one color and grasshoppers captured in the outer circle
will be given a different color. The class will divide into pairs
with one student capturing grasshoppers and the other marking
them and recording data. Change jobs at least once during the
lab so you’ll both get a chance to catch the grasshoppers. Each
time you catch a grasshopper, record the following on your data
sheets:
X
X/Sqrt(2)
1. Location: In the inner or outer circle; spend
approximately equal times in both areas.
2. Species and sex (see Identification Guide).
Mark each grasshopper with a dot of paint on the pronotum
Figure 1: Plot design for sampling
(front-upperside of thorax). Use different colors for the inner
circle and outer circle. For the recapture period, mark grasshoppers with a different color so that we
don’t “double count” grasshoppers. Turn in your data at the end of the lab so that it can be tabulated
and handed out to the class. You can use the procedure outlined below to calculate the LincolnPeterson estimate of population size.
Estimating population size with a 95% confidence interval
1. The simplest Lincoln-Peterson estimate of population size (N) is given in equation 2. A better
estimate that corrects for bias due to small population size is:
N=
s1
M
(3)
N ±1.96 (4)
r1
2. The 95% confidence interval is:
3. Where (the standard deviation) is:
M r 2 s1sr r 1 r 2 r2
(5)
Estimating the 95% confidence intervals allows you to make a conservative estimate of whether
populations estimated in the different habitats are equally abundant. A 95% confidence interval
means that you can be 95% certain that the true population size lies between the upper and
lower bounds that you calculate. If the intervals between days overlap, the differences are not
significant; if they do not overlap there is a significant difference.
References
Borror, D.J., D.M. DeLong, and C.A. Triplehorn. 1997. An Introduction to the Study of Insects. 6th
edition. Saunders College Publishing. 800pp.
Bronson, W.S. 1943. The grasshopper book. Harcourt, Brace and Company, New York.
Caughley, G. 1977. Analysis of vertebrate populations. John Wiley and Sons, New York, N.Y.
Southwood, T. R. E., and P. A. Henderseon. 2000. Ecological methods, 3rd edition. Blackwell, Oxford.
Appendix: Identifying Grasshoppers
You are most likely to encounter the following groups of grasshoppers which are in three different
families:
•
Family Acrididae (Short-horned Grasshoppers):
o Sub-family Oedipodinae (Band-winged grasshoppers)
o Sub-family Melanoplinae (Spur-throated grasshoppers)
o Sub-family Gomphocerinae and Acridinae (Tooth-Legged / Slant-Faced Grasshoppers)
•
Family Tetrigidae (Pygmy Grasshoppers)
•
Family Tettigoniidae (Long-horned Grasshoppers or Katydids)
The distinctive characteristics are pointed out for each group below.
1. Family Acrididae (Short Horned Grasshoppers):
1.1.Family-Level Characters
a) Pronotum not prolonged back over abdomen
b) Wings usually well-developed
c) Tarsi 3-segmented
d) Short antennae
1.2.Subfamily: Oedipodinae (Band-winged grasshoppers, adult characters):
a) Large, "flying" grasshoppers with multi-colored banded "wings" (see below).
b) Distinctive longitudinal keel on their pronotum (first segment of the thorax, just behind
the head) (see below).
c) Face is vertical or nearly so.
d) Sometimes they make crackling noises when they fly.
1.3.Sub-Family: Melanoplinae (Spur-throated grasshoppers). Note: This group contains many
common species including most of the pests in the grasshopper group. Most species we will
encounter are in the genus Melanoplus.
a) Distinctive spine or spur on their prosternum (e.g., an Adam's-apple-like structure
between their two front legs - similar to the Yellow Locust in drawing).
b) Pronotum is flat on the back and broadly rounded at the rear end.
c) Face is usually vertical.
1.4.Sub-Family: Gomphocerinae and Acdidinae (Tooth-Legged or Slant-Faced
grasshoppers). A species that you may encounter is Chortippus curtipennis, a specialist of
grasses.
a) Similar to the spur-throated grasshoppers but with the face slanting backward
b) Males with a row of pegs on inner surface of hind femur.
c) Less abundant than other Acrididae
2. Family: Tetrigidae (Pygmy Grasshoppers). Note: Pygmy grasshoppers overwinter as adults
and are most often encountered in spring and early summer
2.1.The main character distinguishing pygmy grasshoppers is their pronotum which extends
back over the abdomen and is pointed at the back.
3. Family Tettigoniidae (Long-horned Grasshoppers or Katydids)
3.1.Long antennae and an extended ovipositor.
4. Distinguishing Male and Female Grasshoppers