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Species diversity and feeding relationships.
- Variety of organisms that make up the community.
- Species richness and relative abundance.
- Two communities can have same species richness but different relative abundance.
- Compared using a diversity index.
Total number of different species in community.
Proportion each species represents of total individuals in community.
Shannon Diversity Index (H):
- H = -[(pA ln pA) + (pB ln pB) + (pC ln pC) + ...]
- Determining number and abundance of species is difficult.
- Molecular tools used to determine microbial diversity.
Species with a Large Impact:
Highly abundant or pivotal in community dynamics.
- Most abundant or have highest biomass.
- Powerful control over occurence and distribution of others.
- Most competitive for resources or fewest predators.
Total mass of individuals in population.
Introduced to new environment by humans, often lack predators or disease.
- Strong control on community by ecological roles, or niches.
- Not necessarily abundant in a community.
- Ecosystem Engineers
- Cause physical changes in environment affecting community structure.
- Some facilitate +'ve effects on survival and reproduction of others.
Bottom-Up and Top-Down Controls:
- Bottom-up model is unidirectional influence from low to high trophic levels.
- Presence or absence of mineral nutrients determines community structure.
- Top-down model(trophic cascade model) is coming from above trophic level.
- Predators control herbivores, which control primary producers.
- Event changing community, removing organisms, altering resource supplies.
- Climax community functioning as a superorganism debunked.
- No longer believed to be in equilibirium.
- Fire is significant disturbance in terrestrial ecosystems. (often necessary)
Community is constantly changing after buffeted by disturbances.
Intermediate Disturbance Hypothesis:
- Moderate disturbance fosters greater diversity than high or low disturbance.
- High disturbance exclude slow-growing species.
- Low disturbance allow dominant species to exclude less competitive species.
Sequence of community and ecosystem chang after a disturbance.
Occurs where no soil exists when succession begins.
Area where soil remains after a disturbance.
- Greatest impact on biological communities worldwide.
- Usually reduces species diversity.
- Prevent some important naturally occurring disturbances.