Exercise Contents:
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| • | 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 ForestExercise 5: Fragmentation and Community Evenness
J. Danoff-Burg
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Your Questions
BackgroundA community is composed of a group of species that are found in the same general type of habitat. Communities have certain characteristics, such as physical structure (vertical, horizontal and species composition), species diversity (richness and relative abundance), trophic interactions (food chains and food webs), and functions (nutrient cycling and decomposition) that can be used to distinguish different habitats. Species diversity is our emphasis for this lab and has three main aspects - alpha, beta, and gamma diversity. Alpha diversity is obtained when we characterize a single local community in terms of the richness (number of species) and the abundance (number of individuals per species) of the species that are locally present. Alpha diversity is conducted using richness counts and by using one of several diversity indices such as the Shannon-Wiener or the Simpson diversity indices. Beta diversity is the comparison between local communities. Gamma diversity is when we compare diversity indices between different ecosystems or biomes. The latter diversity type will not be systematically explored during the SEE-U course, and beta diversity will only be touched on next. Our main focus is learning about the alpha diversity indices and doing some rough comparisons between sites locally. One of the important aspects of community ecology is the comparison of beta diversity between two or more communities. This type of comparison is done by calculating a coefficient of similarity or beta diversity value and can be done either qualitatively or quantitatively. The values for similarity range from 0%, when no species are common to 100% when both communities have exactly the same compositon. In many ecological studies, a value of greater than 50% indiscates a high similarity between communities. When more than two communities are compared, another type of analysis, multivariate, is used. There are two main types of multivariate analysis. One is classification, in which groups are formed based on the presence or absence of certain species. The other is ordination in which groups are formed based on a mathematical algorithm. In both types, the results need to be interpreted, using the ecological knowledge of the scientists of where and how the samples were collected. Alpha diversity indices will be our focus today. We will conduct Shannon-Wiener and Simpson diversity indices for our communities. These indices are discussed in the introductory text for Module 5 and won't be discussed further here. Be certain to read the introductory text to better understand the strengths and weaknesses of these indices. Another important manner of characterizing communities is to look at community evenness. These relative abundance values have serious consequences for the health of the community and thus the ecosystem. Because there is a limitation to the number of individual species that can occupy a given area of land, the most abundant species will be present in greater numbers in less even communities than in more even communities. Consequently, less even communities will be dominanted by a single species, often a species that is non-native or exotic (more on this in Module 14 when we talk about exotic species). Communities that are dominated by a single species are thought to be less efficient in a variety of ecosystem features such as decomposition, nutrient cycling, and carbon sequestration, among others. As such, these ecosystems are thought to be less healthy. As we will have likely already mentioned and will certainly discuss further during Module 13, fragmentation is one of the most important threats to the Mata Atlantica and to most other ecosystems within Brazil. Fragmentation along with invasive species may be responsible for structuring the ecosystems within the remnants of this most imperiled biome. Fragmentation brings with it significant abiotic changes as well as increasing the probability of introducing exotic species that may take over an ecosystem. The abiotic changes may by themselves make the edges less habitable for species that live deep in the forest interior. These species are also called deep forest endemic species because they are only found there and are usually the species that are the first to disappear or become threatened when an area is fragmented. Today, we will compare three areas that are of different levels of fragmentation to determine whether there is a continual decrease in evenness and in alpha diversity index values with increasing levels of fragmentation. Our initial hypothesis is that evenness, the Shannon-Wiener, and the Simpson diversity indices will decrease as we go from a forest interior, to the interior of a narrow (around 30 m) strip forest, to a forest edge.
Your AssignmentThis is a one-day lab involving four tasks:
 Objectives
Key Skills
Timetable
Procedural Notes
Materials Needed
All Materials Copyright © 2002 by J. Danoff-Burg
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Community evenness looks at the number of each species present and then visually compares the relative abundance of all species found in a given area. Areas that are more even will have many species equally abundant. In contrast, areas that are less even will have one or very few species that are superdominant, with all other species present only in very low numbers.