Slide1
Does greater species diversity lead to greater stability in ecosystems?
Viewpoint: Yes, greater species diversity does lead to greater stability in ecosystems.
Viewpoint: No, ecosystem stability may provide a foundation upon which diversity can thrive, but increased species diversity does not confer ecosystem stability.
Slide 2
Does greater species diversity lead to greater stability in ecosystems?
Conclusions
the question of what makes system stable remains largely unanswered
Doak et al (1998) Diversity correlated with stability of biomass simply due to emergent statistical properties. The relationship would arise even in the absence of any ecological relationships due to the probablistic outcomes of “averaging” the independent biomass fluctuations of numerous species
Slide 3
Does greater species diversity lead to greater stability in ecosystems?
Conclusions
The Current Thinking: Controversy is not over, but most lean toward this:
• Diversity gives rise to stability (dynamic) BUT diversity is not the driving force of the relationship
• The cause is the increased ability of more diverse communities to exhibit differential responses to change
• communities are structured by weak interactions, which prevents large
fluctuations in the face of perturbation. As # spp decreases, the average interaction strength increases, and the probability of de-stabilization increases..
slide 4
Does greater species diversity lead to greater stability in ecosystems?
Conclusions
1. Diversity enhances community stability and decreases stability of populations of individual species (supports May’s (1973) model results).
2. Diversity had stronger effect on RESISTANCE than on RESILIENCE
3. Why the diversity-stability relationship?
a) species vary in their resistance to perturbation
b) species compete for resources – when one declines due to perturbation another compensates by increasing (competitive release)
slide 5
Biotic Succession
tEcological succession
predicable and orderly changes in the composition or structure of an ecological community
Succession may be initiated either by formation of new, unoccupied habitat (e.g., a lava flow or a severe landslide) or by some form of disturbance (e.g. fire, severe windthrow, logging) of an existing community
Slide6
primary succession
process that begins in areas where no soil is initially present
secondary succession
process that begins in areas where soil is already present
slide 7
Natural vegetation of a particular location evolves in a sequence of steps involving different plant communities.
The evolutionary process is known as plant succession.
Plant succession usually begins with a fairly simple community known as a pioneer community.
Slide 8
About mount st Helen
Mount St. Helens the day before the 1980 eruption, which removed much of the northern face of the mountain, leaving a large crater
sLide 9
The entire process of succession is given as follows:
1.Nudation: It can be defined as the development of bare area without any form of life. Under this we have:
Topographic
Climatic
Biotic
2. Invasion: Successful establishment of species in a bare area. Under it we have:
Migration (dispersal)
Ecesis (establishment)
Aggregation (colonization)
3. Competition and coaction.

4. Reaction stabilization.
Slide 10
Types Of Succession
Based on the different criteria ecological succession may be of the following types:
1. Primary Succession: If an area in any of the basic environments is colonized by organisms for the first time, the succession is called Primary Succession.
2. Secondary Succession: If an area under colonization is cleared by whatsoever agency of the previous plants it is called Secondary Succession.
3. Autogenic Succession: After the succession has begun in most of the cases, it is the community itself which as a result of its reactions with the environments, modifies its own environment and thus causing its own replacement by new communities. This course of succession is known as Autogenic Succession. (drying up of a pond)
Slide 11
Types Of Succession
4. Allogenic Succession: in some cases replacement of one community by another is largely due to forces other than the effects of communities on the environment. This is called Allogenic succession and it may occur in a highly disturbed or eroded area, or in ponds where nutrients and pollutants enter from outside and modify the environment and in turn the communities.
5. Autotrophic Succession: It is characterized by the early and continued dominance of autotrophic organisms like green plants. It begins in a predominantly inorganic environment and the energy flow is maintained indefinitely. There is a gradual increase in organic matter content supported by the energy flow. (eutrophication)
6. Heterotrophic Succession: It is characterized by the early dominance of heterotrophs, such as bacteria, actinomycetes, fungi and animals. It begins in a predominantly organic environment and there is a progressive decline in the energy content.
Slide 12
7. Retrogressive Succession: It means a return to simpler and less dense or even impoverished form of community from an advanced or climax community. In most cases the causes are Allogenic.
8. Induced Succession: Activities such as overgrazing, frequent scraping, shifting cultivation or industrial pollution may cause deterioration of an ecosystem. Agricultural practices are retrogression of a stable to a young state of man’s deliberate action.
9. Cyclic Succession: It is of local occurrence within a large community. Here cyclic refers to repeated occurrence of certain stages of succession whenever there is an open condition created within a large community.
Slide 13
The Major Points:
1. The species living in a particular place gradually change over time as does the physical and chemical environment within that area.
2. Succession takes place because through the processes of living, growing and reproducing, organisms interact with and affect the environment within an area, gradually changing it.
Each species is adapted to thrive and compete best against other species under a very specific set of environmental conditions. If these conditions change, then the existing species will be outcompeted by a different set of species which are better adapted to the new conditions
Slide 14
4. The most often quoted examples of succession deal with plant succession. It is worth remembering that as plant communities change, so will the associated micro-organism, fungus and animal species. Succession involves the whole community, not just the plants.
5. Change in the plant species present in an area is one of the driving forces behind changes in animal species. This is because each plant species will have associated animal species which feed on it. The presence of these herbivore species will then dictate which particular carnivores are present.
The structure or ‘architecture’ of the plant communities will also influence the animal species which can live in the microhabitats provided by the plants.
Slide 15
The Major Points:
7. Changes in plant species also alter the fungal species present because many fungi are associated with particular plants.
8. Succession is directional. Different stages in a particular habitat succession can usually be accurately predicted.
9. These stages, characterised by the presence of different communities, are known as seral stages.
Slide 16
10. Communities change gradually from one seral stage to another. The stages are not totally distinct from each other and one will tend to merge gradually into another, finally ending up with a ‘climax’ community.
11. Succession will not go any further than the climax community. This is the final stage.
This does not however, imply that there will be no further change. When large organisms in the climax community, such as trees, die and fall down, then new openings are created in which secondary succession will occur.
12. Many thousands of different species might be involved in the community changes taking place over the course of a succession. For example, in the succession from freshwater to climax woodland.

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