Atlantic Forest

Exercise 4: Small Mammal Trapping, Population Size Estimation, and Population Viability Analysis
Module 4: Growth and Competition: Population and Metapopulation Dynamics

Your Questions

1. How can the size of an animal population be measured?
2. What influences the growth of a population?
3. What methods are used by conservation biologists to predict the dynamics of a population?

Background

To manage a species, whether common or rare, we need to have a firm understanding of the factors that influence population size. Ecologists achieve this by monitoring population size, understanding the factors limiting the population, and using mathematical models to predict the future dynamics of a population.

A population is a collection of individuals from the same species that occupies some specified area. Populations change in abundance due to intraspecific interactions (interactions between individuals of the same species-competition for example), interspecific interactions (interactions between individuals of different species-predation and herbivory for example), and environmental factors such as temperature, rainfall, and nutrient availability. Populations also undergo random fluctuations — this is especially important for small populations, who are particularly susceptible to these fluctuations. As a result, small populations are more likely to go extinct due just to chance occurrences than are large populations.

Changes in population size can also be thought of as density independent (changes occur independent of the size of the population — for example weather-induced population declines) or density dependent (changes are a function of the size of the population itself — for example when a population gets so large that it reduces the availability of a needed resource, and thus the population size reduces). In a density-dependent situation, mortality, reproduction and other factors usually change concurrently with changes in population density, such that as population density becomes more extreme (high or low) a greater proportion of the population is more strongly or weakly influenced.

Ideally, a measure of the size of a population would be obtained by counting all the individuals, with the knowledge that none are missed in such a count. For many organisms, however, estimates of population size by direct counts are impractical as the animals are either too mobile, too secretive, or too numerous. For mobile or secretive animals, for example, it is difficult to obtain direct estimates of population density, even in very small areas. However, animal population estimates can also be made using a variety of indirect techniques such as counting signs of animals (e.g. tracks, feces, vocalizations) and by estimates derived from observing a subset of animals and extrapolating to estimate the size of the entire population. Transect estimates and mark-recapture techniques are examples of the latter. The mark, release, and recapture (MRR) technique can provide a robust estimate of population size by marking a segment of the population on one occasion and then resampling the population at a later time (often more than one occasion) to quantify numbers of previously marked plus new unmarked animals. The ratio of marked and unmarked animals is used to estimate total population size.

Many variables interact to determine the size of a population and its persistence. Often the only way to gain insight is to develop a mathematical model of the population and use it to perform a Population Viability Analysis, or PVA. PVA’s are models that relate a dependent variable (in our case, population size) to the independent variables that influence it (for example, mortality, harvesting, and weather). A PVA estimates the probability of a population persisting in an environment. It is a form of risk analysis in which one is interested not only in the average size of a population at some time in the future, but perhaps more importantly in the range of possible future values. A robust PVA incorporates density dependent and density independent events, as well as the role of random fluctuations.

Your Assignment

Your assignment involves the following tasks:

1. Bait and set small mammal traps at designated points in the area around our accomodations at Intervales State Park, checking the traps twice in the morning over 2 days. Animals are marked and released.(3 hrs am day 1 to mark captured mammals and 3 hrs am day to recapture mammals)
2. To understand the elements of PVA through a coin-flipping exercise and to perform a PVA using computer software to address questions concerning management of an endangered species. (3 hrs day 1 afternoon).
3. Write-up and discuss results. (2 hr day 2 afternoon)

Objectives

1. Understand the factors that regulate population size.
2. Knowledge of the different types of population growth, how they differ, and when they tend to occur in nature.
3. Understand the theoretical underpinnings involved in population size estimates.
4. Recognize the flaws and assumptions inherent in estimation methods and how these flaws could impact conservation biology methods.
5. Understand the basis of population viability estimation.

Key Skills

1. Dexterity with trapping mammals and other organisms with a similar lifestyle.
2. Ability to extrapolate trapping data that the students collect to other organisms with different lifestyles.
3. Familiarity with several of the main population size estimation methods and ability to use them on a given set of data.
4. Ability to predict population growth.
5. Ability to perform a simple PVA.

Timetable

1. Total elapsed time to perform the experiment : Two days
   • Set-up, field work, data analysis and population estimation, and report should all be done in two days (because of the lag time in the mammal recapture event)
2. Total elapsed hands-on time : approximately Nine hours
   • Layout of experiment, designing excel formulae, researching alternative species = 1.5 hours
   • First day of field work = 2.5 hours
   • Second day of field work = 1.5 hours
   • Analysis and extrapolation to other species and methods = 1 hours
   • Writing oral presentation and presenting it = 2.5 hours

Procedural Notes

1. The TAs would have to initially set the baited mammal traps one day ahead of time so that the students could just go out and mark those that were in the traps, release them, reset the traps, and then repeat it the next day
   • Baited traps would be preferable over unbaited traps in that it introduces some experimental error (good for analyses of flaws of methods), but also ensures that more mammals will be captured (good for data analyses)
2. The PVA program (shareware written by James Gibbs in Gibbs et al. Problem-solving in Conservation Biology and Wildlife Management) should be downloaded in advance from the Bandigraph website. It is a web-based program so it will run on any platform.
3. If the Bandigraph program is not available, the Conserving Earth's Biodiversity CD by Edward Wilson and Dan Perlman also has some rudimentary PVA models that can be used in its place. The interpretation of these models is a bit complicated so your instructor and TAs will assist in this.
4. Writing the excel formulae alone could take about a half an hour before the students could finish things
5. The laboratory page written by Dr. Matt Gompper and edited by J. Danoff-Burg is also a useful reference for this activity. Several copies of this page should be printed off before leaving for Intervales.

Materials Needed

1. Computer and printer
   • Need Excel or some other data base and statistical analysis package
   • Need Bandigraph program for PVA.
2. Field materials: mammal traps, baits, quick-drying waterproof paint, field guides for species identification.