Your Questions
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What is the effect of herbivory on plant community diversity and species
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 leaves, woody
stems, fruits, and roots. 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 much of the American West, cattle grazing has significantly
altered native plant communities. Some of these ecosystems evolved
with grazing (e.g., by Pronghorn Antelope, American Bison). However,
following the arrival of European settlers, western landscapes were
subjected to huge increases in livestock numbers between the 1860s and
early 1900s. The damage brought about by these hundreds of thousands
of sheep and cattle can still be seen today.
In an ecosystem that evolved with grazers, it is possible for livestock
not to disrupt, but rather to sustain ecological processes. However,
the traditional season-long, free-range ranching method, where animals
are put out on an allotment and left there until the end of the season,
is the most devastating kind of grazing. This is because the animals
stick to places they like the best, so plants dont get a chance to recover.
In an arid landscape, these tend to be riparian areas, moister north-facing
slopes, and the like.
In recent years, weve learned how to more creatively manage livestock.
One practice is rest-rotation, where animals are rotated among multiple
divisions of an allotment during the course of a season and such that some
divisions lay unused for a full year's (or more) rest. In this system,
no one pasture is overused. A critical factor for this to work is
the time that plants are given to recover.
Effects of grazing on plant communities
With heavy grazing, plant species that are palatable to the animals decrease,
or may even disappear from an area, and those species that are unpalatable
increase. This arises because with grazing pressure, a plant is less
competitive -- this is especially the case when aggressive non-native species
are present.
Grazers have other environmental impacts. These include soil compaction,
which reduces plant vigor, and soil disturbance, which enhances runoff
and accelerates erosion. These soil changes can in turn have the
effect of creating a drier pasture. Cattle, and hay as supplemental
feed, often bring seeds in from other areas, thus being a vector for exotic
plant species. Cattle also cause nutrient loading to open sources
of water. On the other hand, grazers can be used as a tool for weed
management and for reducing fire fuels. Grazer effects on community
structure, altering the height and density of vegetation, can either enhance
or reduce habitat for other species such as ground-nesting birds or butterflies
requiring access to their host plants.
Recovery from overgrazing on very arid landscapes can take more than
10 years or decades. There are areas overgrazed in the late 1800s where
the impacts are still visible so there are certain conditions were damage
is considered permanent. However, in moist areas, such as in riparian
corridors, recovery can be seen within 3-5 years. In many such areas,
there may be recovery in terms of abundant plant biomass, but the community
structure and function will not have returned to a near-original state
if well-developed soil has been lost and exotic plant species have replaced
natives in the process.
In this lab, we will examine a set of fenced rangelands that have a
history of being grazed with varying intensities. We will compare
the plant communities growing within these areas in terms of species diversity
and abundance.
Your Assignment
This is a one-day lab involving four tasks:
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Formulate testable hypotheses and creating a robust experimental protocol.
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Select and mapping location of fenced rangelands and adjacent ungrazedareas
using a GPS strategy
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Quantify plant diversity and cover in sample areas -- field data collection
and computer lab analysis. Enter data into eBiome as appropriate.
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Write-up, discuss, and present results.
Objectives
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Understanding of field methodology involving multiple experimental plot
comparisons.
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Understanding the concepts of direct and indirect effects in structuring
communities.
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Introduction to the concept of measuring biomass and species diversity.
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Appreciation for the importance of herbivory in communities, and the impact
that altered abundance of a single species may play.
Key Skills
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Increased familiarity and confidence with the local ecosystem.
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Ability to conduct thorough ecological observations and collect data in
the field.
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Application of the concepts of species richness and alpha-diversity.
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Utilization of statistical analyses and graphical representations of data.
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Designed sets of mutually exclusive and testable hypotheses involving herbivory
and plant diversity and abundance using experimental and control plots.
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Familiarity with the measuring and interpreting plant biomass.
Timetable
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Total elapsed time to perform the experiment : One day
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Set-up, design, data collection, statistical tests, analysis, summary should
all be done in one day
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Total elapsed hands-on time : approximately Nine hours
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Layout of experiment, choosing variables, deciding on design = 1-2 hours
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Collecting data = 3 hours
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Statistical tests and data entry = 2 hours
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Writing lab report = 2 hours
Procedural Notes
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Appropriate field gear (hat, sunscreen, bug spray, food, water) are needed
for extended stay in desert terrain.
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Exercise depends on field component. Undertake in early mroning before
too hot. Lab component only affected by inclement weather if power outages
or surges require digital lab to be shutdown.
Materials Needed
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Computer and printer
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Software
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Excel or some other data base and statistical analysis package
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eBiome
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All requisite field materials - e.g.:
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Measuring tapes, meter sticks, flagging
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Plastic sample bags, marker pens
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Plant identification guides
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eBiome datasheets, journals
*Western Resource Office, The Nature Conservancy
All Materials Copyright © 2001 by T. Kittel, G. Kittel, J. Danoff-Burg,
and A. Hoylman
All Rights Reserved.
Rev 4/21/01 |
Exercise
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