8.5 Community Ecology
Keywords
| English Term | 中文翻译 | Definition & Explanation |
|---|---|---|
| Community | 群落 | A group of populations of different species living close enough to interact with one another. |
| Species Diversity | 物种多样性 | The variety of different kinds of organisms that make up a community, including species richness and relative abundance. |
| Simpson's Diversity Index | 辛普森多样性指数 | A mathematical formula used to measure the biodiversity of a habitat. |
| Niche Partitioning | 生态位分化 | The process by which competing species use the environment differently in a way that helps them to coexist. |
| Trophic Cascade | 营养级联 | An ecological phenomenon triggered by the addition or removal of top predators, resulting in dramatic changes in ecosystem structure. |
| Symbiosis | 共生 | A close, prolonged physical relationship between individuals of two different species (includes mutualism, commensalism, and parasitism). |
1. Measuring Community Structure: Simpson's Diversity Index
A community is a collection of interacting populations of different species in a given area. The structure of a community is measured and described in two main ways:
- Species Composition (Richness): What species are present, and how many different species are there?
- Species Diversity: This takes into account both the number of different species AND their relative abundance (how evenly the individuals are distributed among those species).
Analogy: The Forest Diversity
Imagine Forest A has 99 Pine trees and 1 Oak tree. Forest B has 50 Pine trees and 50 Oak trees. Both have the same species richness (2 species). However, Forest B is much more diverse because it is more evenly balanced.
To quantify this mathematically, ecologists use Simpson's Diversity Index (SDI).
The Mathematics of SDI
- \(n\) = the total number of organisms of a particular species
- \(N\) = the total number of organisms of all species
- \(\sum\) = sum of the calculations for all species in the community
How to interpret the number:
- The term \((n/N)^2\) represents the probability that two individuals randomly selected from the community will belong to the same species.
- By subtracting that from 1, the final SDI value represents the probability that two individuals randomly selected will belong to different species.
- The closer the SDI is to 1, the higher the biodiversity. A highly diverse community is generally more resilient to environmental changes.
2. Interspecific Interactions
Communities change over time depending on how these populations interact. Interactions among populations determine how they access energy and matter, and these interactions can be categorized by their positive (\(+\)), negative (\(-\)), or neutral (\(0\)) effects on the species involved.
A. Competition ( \(-\) / \(-\) )
Competition occurs when individuals of different species compete for a resource that limits their growth and survival. Because both species expend energy fighting over the resource, it is a lose-lose situation.
- Niche Partitioning: To avoid direct competition and potential extinction, species often evolve to use the environment slightly differently. For example, several species of lizards might live in the same tree, but one species hunts exclusively in the canopy while another hunts on the trunk. This divides the resources and allows them to coexist.
B. Predation and Herbivory ( \(+\) / \(-\) )
One species benefits by feeding on the other, which is harmed or killed.
- Trophic Cascades: Predation doesn't just affect the prey; it ripples through the community. For example, reintroducing wolves (top predators) to Yellowstone National Park reduced the elk population. With fewer elk eating plants, the willow trees recovered, which in turn brought back beavers and songbirds. This top-down ripple effect is a trophic cascade.
C. Symbioses
Symbiosis describes two species living in direct, intimate contact.
- Mutualism (\(+\) / \(+\)): Both species benefit. (e.g., Bees getting nectar for food while pollinating the flowers).
- Commensalism (\(+\) / \(0\)): One species benefits, and the other is neither harmed nor helped. (e.g., Barnacles attaching to a whale to get a free ride to nutrient-rich waters).
- Parasitism (\(+\) / \(-\)): One organism (the parasite) derives its nourishment from another (the host), which is harmed in the process.
3. Cooperation and Access to Resources
While competition and predation drive many community dynamics, cooperation or coordination between organisms, populations, and species is equally critical.
Such positive interactions can result in enhanced movement of, or access to, matter and energy.
- Example: The mutualistic relationship between plant roots and mycorrhizal fungi. The fungi vastly increase the surface area for the plant to absorb water and minerals from the soil (access to matter), while the plant provides the fungi with energy-rich sugars produced during photosynthesis (access to energy). The entire forest community relies on these cooperative underground networks to thrive.
Quiz
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