Exercise Contents:
 |
|
| • | Your Questions |
| • | Background |
| • | Your Assignment |
| • | Methodology |
| • | Objectives |
| • | Key Skills |
| • | Timetable |
| • | Materials Needed |
Exercise Contents: |
|
| • | Your Questions |
| • | Background |
| • | Your Assignment |
| • | Methodology |
| • | Objectives |
| • | Key Skills |
| • | Timetable |
| • | Materials Needed |
Atlantic Forest
Exercise 5: Herbivory and Plant Diversity
T. Kittel, G. Kittel,* J. Danoff-Burg, and A. Hoylman
|
Your Questions
 BackgroundA 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 1860’s and early 1900’s. Similar impacts of grazing have occured in São Paulo. This was accelerated during the middle of last century, in the early 1930s and 1940s, when large tracts were given to friends and family of powerful state governmental officials as patronage. Most of the recipients of these gifts promptly cleared the forest and either planted large monocultures (single species crops) or grazed large herds of cattle. Many of these gifts are now recognized as illegal and much of the land is being converted to other uses, including more sustainable small farming and conversion back to recovering Atlantic Forest habitat. Nonetheless, the damage brought about by these hundreds of thousands of sheep and cattle can still be seen today in this area. In an ecosystem that evolved with grazers, it is possible for livestock not to disrupt, but rather to sustain ecological processes. (Because all of São Paulo has not been a grassland in recent millenia, this long-term evolutionary equilibrium is therefore not possible here if the goal is to recover a native vegetation community.) 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 don’t 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, we’ve 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 communitiesWith 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 in the Southwestern U.S. that were overgrazed in the late 1800’s 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 AssignmentThis is a one-day lab involving four tasks:
Objectives
Key Skills
Timetable
Procedural Notes
Materials Needed
*Western Resource Office, Colorado, The Nature Conservancy All Materials Copyright © 2001 by T. Kittel, G. Kittel, J. Danoff-Burg,
and A. Hoylman
Rev 4/21/01, 5/25/02 |
