Black Rock Forest

Exercise 5: Deer Herbivory And Plant Diversity
Module 5: Community Ecology: An Introduction to the Players and How to Measure Communities

Your Questions

1. What is the effect of deer herbivory on plant diversity and abundance?

Background

A variety of factors act to structure communities — factors as varied as weather fluctuations and natural disasters, to exotic species introductions, to the natural tension between producers and consumers. In the later case, these tensions reveal themselves in the form of predation, competition, parasitism, and in the case of this laboratory, herbivory. A herbivore is an animal that feeds primarily on plant materials such as grasses and leaves. In some cases, although not always, this may kill the plant. Whether or not the plant is killed, however, herbivory has the potential to dramatically alter plant and animal communities.

Throughout eastern deciduous forests, white-tailed deer (Odocoileus virginianus) have increased dramatically over the past hundred years. This population increase can be attributed to a variety of factors, including altered landscapes and land-use patterns by humans as well as the removal of several limiting agents — namely wolves and cougers. The chief limiting agents throughout eastern North America is now human hunting (which is allowed in Black Rock Forest), although hunting has declined in many regions and mortality caused by hunting is not always great enough to offset the high birth rates in healthy deer populations.

How has the increase in deer densities altered plant (and therefore, indirectly animal) communities? Like all species, deer have feeding preferences for particular plant species. Removal of these species or inhibition of their growth alters the competitive dynamics among surviving plant individuals of both the same and different species. In this lab we will examine a series of long-term deer exclosures — fenced areas from which deer have been excluded — and compare the plant communities growing within these exclosures to adjacent communities growing outside the exclosures and under the pressures of deer herbivory.

Your Assignment

This is a one day lab involving four tasks:

1. Formulating testable hypotheses and creating a robust experimental protocol. (1-2 hrs)
2. Locating and mapping the deer exclosures using a GPS strategy, and selecting adjacent control plots.
3. Identifying plant species in the exclosure and control plots.
4. Quantifying plant biomass in the exclosure and control plots. (3 hrs)
5. Quantifying species diversity (species richness and alpha diversity) in the exclosure and control plots. (2 hrs)
6. Write-up and discuss results (2 hrs).

Objectives

1. Understanding of field methodology involving control and experimental plot comparisons.
2. Understanding the concepts of direct and indirect effects in structuring communities.
3. Introduction to the concept of measuring biomass.
4. Appreciation for the importance of herbivory in communities, and the impact that altered abundance of a single species may play.

Key Skills

1. Increased familiarity and confidence with the local ecosystem.
2. Ability to conduct thorough ecological observations and collect data in the field.
3. Application of the concepts of species richness and alpha-diversity.
4. Utilization of statistical analyses and graphical representations of data.
5. Designed sets of mutually exclusive and testable hypotheses involving herbivory and plant diversity and abundance using experimental and control plots.
6. Familiarity with the measuring and interpreting plant biomass.

Timetable

1. Total elapsed time to perform the experiment : One day
   • Set-up, design, data collection, statistical tests, analysis, summary should all be done in one day
2. Total elapsed hands-on time : approximately Nine hours
   • Layout of experiment, choosing variables, deciding on design = 1-2 hours
   • Collecting data = 3 hours
   • Statistical tests and data entry = 2 hours
   • Writing lab report = 2 hours

Procedural Notes

1. General clickable photos of the two plots will be available online that the students will use as data repositories
   • Each photo will display locations of 100+ sampling areas delineated as red points
   • Clicking on each of the red circles yields a pop-up window that contains forms in which the student can enter data relevant to that sampling area
   • The results will be stored in a database that will be downloaded by the students in Excel formatting
   • Students will be able to control the size and composition of their databases by selecting the sampling areas to be included in the Excel file
   • Photos of each sampling area will also be included — students can view these all at once in a fly-through, or individually by launching pop-up windows
   • Once data for a sampling area is entered into the database, the red point that represents this area on the map will change to a blue point
   • Each of the pop-up windows will be password protected so students will not be able to tamper with others’ data
   • One of the goals of this endeavor is to establish databases over a number of years so that students will be able to use them in longitudinal analyses
2. Could also accumulate photos from other comparable systems around the country (there are quite a few others) that the students could compare among sites to evaluate the universality of these conclusions
3. Web site with a streaming video tour through these other sites where there are comparable deer-exclosure sites around the country, using either QuickTime or IP/TV, to demonstrate the extreme differences that occur when deer are excluded around the country
   • This would help to introduce a regional scale into the project
4. Could also construct a 3-D photo reconstruction of the plots from the neighboring plots above them so that students could actually compile the tree heights of all plants in the plot from the video
   • This would have to be done well ahead of time
   • Should also include a scale for the students to use to calibrate heights

Materials Needed

1. Computer and printer
   • Color printer is greatly preferred
   • Need Excel or some other data base and statistical analysis package
2. All requisite field materials (ropes, measuring tape, meter sticks, quadrat borders, baggies, tree and annual plant identification guides, etc.)