|
What is the idea of a climax environment, and why it is not stable? It seems that if a community is at it's climax, then it would finally be stable, and while it is in succession, it would be less stable, since it is changing. The concept of climax, the end point of succession, is generally considered a climatic climax. That is, climax occurs when a community that has reached a stable state under a particular set of climatic conditions (the complex interaction of temperature, wind, precipitation, solar radiation, etc.). If climatic changes occur continuously during succession, the community will continue through various successional stages and will not reach climax. Major changes in climate may be expected beyond the range of about 500 years (unless human influences cause more rapid changes, e.g., global climate change caused by the burning of fossil fuels). Thus, within that time period it is possible to have a stable climax community. In regard to whether stability is highest at climax or some pre-climax stage depends on how stability is defined. This term is defined differently by various ecologists, and thus is some view the climax community is the most stable; in other cases a pre-climax stage is most stable. Please review the various definitions of stability in the lecture slides. For the purpose of this course in general ecology, we will take the view the climax is the stable stage of a community.
The videos in the species slides to work on any computer, do I need something special for these? Unfortunately, the videos have a copyright (not mine) and connect be electronically transmitted. These are excellent examples of interaction, but are not essential. Your textbook and the interactive lesson on interactions also have excellent examples of interactions. "How is an ecotone quantified?" You must develop this answer based on your knowledge of what an ecotone is and would characterize this transition zones between communities. You may, e.g., start with a sampling transect (e.g., a 100-m sample) through the ecotone. What kinds of measurements would you take in order to determine whether you were in a transition zone between two communities?
The slides are not clear on what the two major types of succession are. Is it primary and secondary, or autogenic and allogenic? In the Ecology Notes book, it reads as if primary & secondary succession are a category under autogenic succession. If I am reading this right, then autogenic and allogenic would be the 2 categories this question is asking about. Is this correct?
Primary and secondary succession are two major types of succession. Autogenic and allogenic succession refer to the driving forces of succession within each of the major successional types. For example, in autogenic succession the organisms within the community alter the environment and community (interactions, trophic levels, etc.) to a level that facilitates the colonization of the community by organisms of the next successional stage. Allogenic factors drive succession from outside the community (e.g., fire, flood, etc.).
For the question "What ways are communities more than a collection of populations?" Do communities create coactions through trophic and spatial relations? Ecologists - and books on ecology - define communities in a variety of ways. Each definition differs somewhat; some are more comprehensive, while others are very simplistic. The common points among the definitions are that the community is defined and characterized by the nature of the interactions among populations and the habitat in which these interacting populations occur. The definition given by Ricklefs in the lecture is a very good basic definition of a community ("An association of interacting populations defined by the nature of their interaction or the place in which the live"). One caveat on Ricklefs' definition is that the "or" is often replace by "and."
The questions “In what ways can a species be considered dominant in a community? How would the selection of a dominant species influence the classification of a community?" are referring to Clements discrete theory, but I need a little more direction on why looking at a community for a dominant species agrees with Clements theory. A dominant species could be either a tree or undercover, so I am not clear on your examples from the slides (regarding the 2 plant communities and the continuum).
The concept of dominance in regard to the classification of a community is highly influenced by how dominance is defined and quantified. For example, if dominance is defined by size, physical appearance, abundance, or frequency, than one or more species may characterize a community. In contrast, if dominance were defined and quantified by the function a species has in the community, then the community may be classified differently. The boreal forest community is a good example. The dominant species are black spruce and white spruce, when dominance is defined by physiognomy and size. Thus, this community type is classified as a boreal (i.e., northern) forest. However, the organisms that control approximately 70% of the nutrients in the community are mosses. These organisms are the functional dominants. Thus, on this basis the boreal forest could be classified as a moss-lichen community. Other examples (see the lecture slides) include the elevational gradient from a beech to spruce fir forest.
Would a major cause of retrogressive succession be overgrazing and human activity causing desertification? Yes. Both of these can cause retrogressive succession. Fire, climatic changes, exotic species, off-road vehicle damage, etc. also can cause retrogression.
Where can I find the answer to "What are the principal stages of secondary succession as developed by Clements?" Your slides show Clements' Climax Types, but nothing regarding succession. Is Figure 13-6, Initial Floristic Composition what Clements developed?
Clements stages of succession and their driving forces are listed and defined in the section “Community Development – Stages of succession.” The stages are: nudation, migration, ecesis (pronounced: eekeesis), competition, reaction, and stabilization. Figure 13-6 does not use this terminology. Rather this figure shows the various major life forms that might occur along a successional sequence.
How is an ecotone quantified? I could not find any direct reference to this measurement, but am inferring from the notes that an investigator could make quantitative measurements of a dominant plant species within the physiognomy (e.g., spruce trees) and as you progress along an environmental or climatic gradient (e.g., elevation) you would see a decrease in its relative dominance, being replaced by the new dominant species (e.g., such as when you reach timberline). The ecotone would be the area of transition between the dominant species. This is a good analysis and a sufficient answer to the question. Once you have identified what an ecotone is and what characteristics you would observe in this transition zone between communities, than the measurements you suggested would follow. The principal technique would be to take the measurements you outline on a marked transect through the ecotone.
In what ways are communities more than a collection of populations? At first I thought that in communities the various populations obtain synergistic benefits through their interactions. Where they are "greater" than when found individually. However I do not believe that this would be always true, for instance in cases where the fundamental niches for two species overlap, in this case through competition one species could be more inhibited when associated in the community. Therefore it is through the nature and effects of the interactions of the communities that make them different than just a collection of populations, the book seems to contradict this though.
Ecologists - and books on ecology - define communities in a variety of ways. Each definition differs somewhat; some are more comprehensive, while others are very simplistic. The common points among the definitions are that the community is defined and characterized by the nature of the interactions among populations and the habitat in which these interacting populations occur. The definition given by Ricklefs in the lecture is a very good basic definition of a community ("An association of interacting populations defined by the nature of their interaction or the place in which the live"). One caveat on Ricklefs' definition is that the "or" is often replace by "and."
What are convergent communities and why do they develop? Convergent communities are instances where similar communities develop at different geographic locations due to similarities in climate. As for the second portion asking why they develop, the class notes make reference to the community as "responding to the environment as if it were an organism rather than a collection of organisms". While this makes sense to me on one hand, it does not seem like a sufficient answer, especially when there is large geographic distances separating the communities. I am still pondering how communities organize themselves, through time and associated interactions, it seems they "sort themselves" into the respective realized niches. I believe it was the textbook that talks about how depending on whether mountain ranges orientated themselves north-south (as in North America) verses east-west (as in Europe) effect community structure through inhibiting plant species migration along a latitudinal gradient (as in Europe). In this case I would think convergent community development would be inhibited. I was wondering if you point me in a better direction.
The principal variable for the distribution, structure and function of communities is climate (the complex interaction of factors such as temperature, wind, solar radiation, precipitation, etc.). Soil and biological factors are second and third, respectively, in regard to community type and development. Thus, similar community types often develop in response to the similar climates in geographically separate locations. Tropical alpine, Mediterranean communities, alpine, and shrub steppe are some examples of convergent communities. The factor of chance (in regard to the source of species) can be significant in determining the species composition of a community (as well as the mountain range barriers you mentioned). However, the environmental factors that comprise a location exert a strong selective force on the species in an area and over time the species form adaptations to both the environment the interactions with other species. Mediterranean communities around the world, e.g., are very similar in physiognomy, structure and function - but are compose of different species (e.g., Mediterranean communities in Australia and California exhibit the convergence at the community level, but are composed of completed different species).
What are the three major indexes by which an ecologist can describe the complexity of a community? What are some problems encountered with the quantification and interpretation of each of these indexes?
From the readings in the book I believe the indexes are: 1) Sequential Comparison Index (SCI) no. of individuals of same species/no. of individuals in sample 2) Shannon-Wiener Index (H') Likelihood that the next individual will be the same as the last 3) Simpson Index Likelihood that both of two individuals drawn will be of the same species I would think the first problem would be identifying your sample area since communities are dynamic and difficult to form arbitrary boundaries around, needed to define a study area. The second problem would be then in obtaining accurate field data on the species composition of the area, since they are observations based and biased by the individual collecting the data. As far as difficulties in interpretation. The last two indexes refer to probabilities, and from my limited experience, statistical information can be interpreted in many ways. Regardless however at some point I would hope that the data could be field validated, something again due to the dynamic nature of communities could be difficult. I am struggling here though.
There are many indexes used to quantify communities. However, the question refers to the following indexes: species evenness, species richness, and species diversity. Definitions and summary of problems with these indexes can be found under the last two slides of "Attributes of communities" in the lecture on communities. These indexes are commonly used to quantify the complexity of communities. Yet, the assumptions for these indexes and sampling techniques are sometimes problematic. For example, there is an assumption for "species richness" (the number of species in a community) that all species can be accurately counted. In many cases, particularly tropical rain forests, richness is dependent on projections from subsamples of the communities. Species evenness (the number of individuals per species) is also prone to problems because of the methods used to estimate the true population from subsamples. These quantitative indexes also do not account for the functional importance of different individuals and species in the communities. The Shannon-Wiener index for species diversity is also a common index, and also has similar problems of accurate sampling and the inability to quantify the functional importance of individuals and species in a community. For this introductory course in ecology, it is sufficient to name and define the indexes, and know some of the limitations. If you would like to study ecological sampling methods in greater detail, I would suggest the following reference. This is the principal reference ecological methods.
Krebs, C.J. 1999. Ecological methodology. Second edition. Addison-Welsey Educational Publishers / Benjamin-Cummings. |
