GENERAL LAB INTRODUCTION

The dispersion of individuals in a population describes their spacing relative to each other.  Different species, and frequently different populations of the same species, can exhibit drastically different population dispersions.  The way individuals are distributed in a population can frequently provide information critical to understanding the life history traits of the population and/or about the environment of that species’ habitat.  Generally dispersion can follow one of three basic patterns:  random, uniform (also called evenly spaced or hyperdispersed), or clumped (also called aggregated or contagious).   Notably, movements within a population are referred to as dispersal while movements between populations are referred to as emigration or immigration.  Both affect population dispersion.

Population dispersion is commonly quantified by population ecologists.  With mobile organisms (i.e. most animals), this requires intensive sampling, often involving mark-recapture, and identification of such parameters as “home ranges”.   As such, quantifying the population dispersion of motile species is beyond the scope of our lab.  Instead, we will focus on determining the dispersion pattern of plants (which don’t move much!).  We will look at several terrestrial weed species (now called wildflowers!) commonly found in the lawn habitats of the FIU campus.  We will use a site that has recently been cleared of vegetation (that is, a construction area—which should not be hard to find on the FIU campus!) and is an environment open to colonization.  In addition, we will contrast the dispersion of two populations of the wetland plant Eleocharis:  One in the marsh on the western edge of Hennington Pond, the other in the marsh on the southern and western edge of the “FIU Gym Pond”.  Analyses of population dispersion patterns usually follow a standard method in which the observed patterns are compared to predictions of random dispersion.  The most common of these random dispersions is the Poisson Distribution.  Departures from the predicted pattern will suggest that the population under study exhibits either a uniform or clumped dispersion pattern.

PRE-FIELD LAB INSTRUCTIONS

1.  Generate several testable hypotheses as a class that you can test with today’s exercise.

2.  Discuss how to keep track of the 2 different kinds of dispersion data.  Set up field data sheets for your terrestrial and wetland samplings.

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 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.

FIELD LAB INSTRUCTIONS

We will first visit the terrestrial “recolonization” site.  Sampling today will involve 1m x 1m quadrats that are gridded into 100 plots each 10 cm x 10 cm.  Randomly locate your group’s quadrat and identify at least 4 different weed species found in your quadrat (your TA will have plant keys along if you need them).  Record data for each species by counting the number of individuals of each species in each of your 100 grids.  Keep track of which grid you are counting (e.g. by using numbers to label columns and letters to label rows, then identifying each grid with a number-letter designation).  This will allow you to look at the distribution of individuals of each species in space, if you choose to do so.

Next two stops are the wetlands of Hennington Pond and the FIU Gym Pond.  Each group should quantify the dispersion of Eleocharis in both settings, using the same 1m X 1m quadrat and recording your data as the number of individual stems in each grid.  Be sure to keep track of which grid you are counting, and be very careful when placing quadrats down so that you don’t bend stems or have stems wind up in the wrong grid cell. 

POST-FIELD LAB INSTRUCTIONS

            Upon returning to the lab, all you now need to do is compile your field data into a dispersion table that you will then give to your TA.  Your TA will combine the dispersion data for all groups in your lab, and get this combined dataset back to you in time to either write your report or to complete the worksheet below.  Your dispersion table should be similar to the table on p. 131 of your lab manual, except that you will actually calculate the Poisson Relative Frequency on your own time, using the combined dataset that your TA will give you.  Your dispersion table should have columns for:  1) a generic # of individuals per grid cell, 2) # of grid cells of species #1, 3) # grid cells of species #2, 3) # grid cells of species #3, etc.  Be sure to clearly denote what species you sampled.

Answer/address all of the following questions on your own paper.  In some cases, this will require computer printouts of spreadsheets, graphics, or statistical output.

1.       Generate a spreadsheet of your length and age data for Gambusia holbrooki collected in the 3 aquatic habitats that you sampled.   Use age classes similar to those shown in Table 1. Include the fecundity estimates shown on Table 1.

2.      Generate a life table for each of the 3 different “populations” of G. holbrooki.  What kind of life tables are these?  What did you hypothesize about similarities/differences in population structure for fish from these 3 different habitats?  Do your life tables support your hypothesis?

3.      Now use the Musculium partumeium data shown in Tables 1 and 2 to generate life tables for both the Fall 1981 and Spring 1982 cohorts of this freshwater clam.  What kind of life tables are these?

4.      What is the age to sexual maturity for these two aquatic species?

5.      Generate spreadsheets of your lab’s dispersion data for your terrestrial plant species and the two populations of Eleocharis  sp. that you sampled.  Expand these to include calculations of Poisson Distribution parameters.   How are each of these species dispersed?  Do your spatial data on distributions of individuals support your calculations in these conclusions?  Do your conclusions support your hypotheses about population dispersion in these species?