8.5 Community Ecology
- Syllabus
- 2025
- Topic
- 8.5
- Level
- —
Learning objectives
8.5A—Describe the structure of a community according to its species composition and diversityDescribe the structure of a community according to its species composition and diversity.• The structure of a community is measured and described in terms of species composition and species diversity. Relevant equation: Simpson’s Diversity Index— Diversity Index = −∑ n N1 2 where n = total number of organisms of a particular species N = total number of organisms of all species8.5B—Explain how interactions within and among populations influence community structureExplain how interactions within and among populations influence community structure.• Communities are groups of interacting populations of different species that change over time based on the interactions between those populations.• Interactions among populations determine how they access energy and matter within a community.• Relationships among interacting populations can be characterized by positive and negative effects and can be modeled. Examples include predator/prey interactions, cooperation, trophic cascades, and niche partitioning.• Competition, predation, and symbioses, including parasitism, mutualism, and commensalism, can drive population dynamics.
Measuring Community Composition and Diversity
Community structure can be described by species composition—which species are present and their abundances—and species diversity, which summarizes how abundance is distributed across the species in the community.
\text{Simpson's Diversity Index}=1-\sum\left(\frac{n}{N}\right)^2
n = number of organisms of one species; N = total number of organisms of all species. Calculate (n/N)2 for every species, add those values, then subtract the sum from 1.
Illustrative calculation: a sample contains four species with 4, 3, 2, and 1 individuals, so N=10. The index is 1−[(4/10)2+(3/10)2+(2/10)2+(1/10)2]=1−0.30=0.70.
A larger index represents greater diversity under this measure because abundance is less concentrated in one species. A change in one species' abundance can therefore change both the composition and the calculated diversity of the community.
Species count alone does not fully describe community structure. Two communities can contain the same species but have different abundances and therefore different diversity-index values.
Interactions Shape Community Structure
A community is a group of interacting populations of different species. These interactions change access to energy and matter, so they alter population sizes and community structure over time.
| Interaction | Effect on population 1 / population 2 | How it can change dynamics |
|---|---|---|
| Competition | − / − | Both populations lose access to limited resources |
| Predation | + / − | Predator gains energy while prey survival decreases |
| Parasitism | + / − | Parasite benefits while the host is harmed |
| Mutualism | + / + | Both populations benefit |
| Commensalism | + / 0 | One benefits while the other has no measurable effect |
| Cooperation | + / + | Coordinated behavior can improve resource access, survival, or reproduction |
Niche partitioning reduces overlap in how populations use resources. By lowering direct competition, it can allow interacting species to persist together and changes how energy and matter are divided within the community.
Predator abundance changes → prey abundance or behavior changes → pressure on organisms at another trophic level changes → several populations shift. This indirect chain is a trophic cascade.
The signs describe effects on the interacting populations, not whether an interaction is morally good or bad. A direct effect on one population can also create indirect effects elsewhere in the food web.