Ecology Lab, PCB 3043L
A
population is defined as the individuals of the same species living in a given
region, or habitat. Ecologists
frequently employ a variety of different methods to study the structure
of populations. Key parameters
of interest when describing population structure include total number of
individuals in a population, the age distribution of individuals, probabilities
of survival (or mortality), and rates of fecundity.
Life history tables, or life tables, is one method of quantifying population structure that
addresses all of these parameters. Life
tables provide age-specific information
on survival and fecundity rates for a particular population.
This kind of population structure analysis can be used:
1) to quantify the age structure of a population; 2) to estimate an
optimal age of sexual maturity, and; 3) to calculate the intrinsic rate of
growth used in the logistic growth equation.
There are two kinds of life tables which are generated from two very different types of life history data collected from a population. The first type, called a dynamic life table or a cohort life table or a horizontal life table, involves following a group of individuals from birth to death. This generally requires resampling of marked individuals (although repeated and representative sampling of a non-moving population may suffice). Furthermore, since individuals must be followed from birth to death, this technique does not work well for long-lived individuals (unless you, the researcher, have a great deal of time on your hands!). The second type of life history table, called a static life table or a time-specific life table or a vertical life table, is based on sampling a population and obtaining information on individuals of all ages at a single point in time. In this case, it is clearly important that you have a large enough sample to be representative of your population, so that you can be sure that the age distribution in your sample resembles that of the entire population. Often, ecologists doing the intensive one-time sampling of a population that is required for this second type of life table analysis devise non-destructive methods, so that their large sampling effort does not affect the dynamics of the population being studied.
The
field portion of today’s lab will involve working in three different aquatic
settings on campus: 1) in the
shallow wetland region of Hennington Pond; 2) in the deeper lake areas of
Hennington Pond, and; 3) in OE pond. In
this lab you will also be using already published data on the structure of a
population. The objectives are:
1) to generate life history tables for two aquatic species.
The first, Gambusia holbrooki,
is a small live-bearing fish which occurs throughout south Florida and the
Everglades, and populates our campus ponds.
The second, Musculium partumeium,
is a freshwater clam found in Virginia; 2) to compare and contrast the different
types of life history tables you generate for M.
partumeium and G. holbrooki; 2) to
compare and contrast life tables for G.holbrooki
taken from 3 different habitats (but from only 2 different ponds); 3) to use
your life tables to determine how many different species of G. holbrooki
you actually sampled, and; 4) to estimate
the optimal age of sexual maturity for both aquatic species.
1.
Generate several testable hypotheses as a class that you can test with
today’s exercise (Hint: the
objectives, above, may help with this).
2.
Discuss how to keep track of the 2 different kinds of population data.
Set up field data sheets for your G.
holbrooki sampling.
3.
Divide into groups and work as teams in the field.
Work should be divided up so that all team members get to experience each
aspect of the exercise. In other
words, don’t make one person record data for the entire lab exercise!
4.
Be sure that you have all field sampling equipment that you will need.
Read below and make a list before you leave the lab.
5.
All field teams should participate in sampling all 3 aquatic habitats.
After sampling, return to the lab and your TA will pool data from all
teams to generate larger datasets for each habitat.
Use these complete datasets for your analysis.
We
will go out to the shallow wetlands at the west end of Hennington Pond, to the
deep areas at the east end of Hennington Pond, and to OE Lake and dipnet to
collect sample specimens of G. holbrooki.
For each collected specimen we will determine its standard length (in mm)
non-destructively, so that we may release the fish after sampling (handle them
carefully!). The age of each fish
(in days) will be calculated using the relationship:
Age
= 8*(Stnd Length)-68 [r2=0.702,
p=0.001]
In addition to this
age:length relationship, past researchers have also generated data on the
size-specific fecundity of G. holbrooki.
These data can be transformed to age-specific fecundity and used in a
life table analysis of the population(s) we sampled.
Age classes for use in the life tables have been predetermined and
provided to you in Table 1, which may be downloaded or printed from the Ecology
Class web page.
Your
data for the freshwater clam, Musculium partumeium, were collected in the early 1980’s in a
small stream near the University of Virginia campus.
These data were published by Hornbach and Childers (1986)—a copy of
this paper is available outside ECS 262 or by contacting your TA.
They collected age-specific data on 4 cohorts of the freshwater clam M.
partumeium over about 16 months. Population
and fecundity data for two of these cohorts, one beginning in the Fall of 1981
and the other in the spring of 1982, have been provided in Table 2, which may be downloaded or
printed from the Ecology Class web page.. From
this data you can generate a life history table for EACH cohort.
Hornbach, D. J., and D. L.
Childers. 1986.
Life-history variation in a stream population of Musculium
partumeium (Bivalvia:Pisidiidae). Journal
of the North American Benthological Society
5:263-271.