Chapter 16 – Community Structure

What is an ecological community?

A set of plant and animal species that occupy an area and interact directly or indirectly.

What questions are central to ecological community studies?

What controls species abundance?
How do species interact?
How do communities change over time?
How do communities interact across landscapes?

What are the two ways communities can be defined?

Broad: All species in an area.
Restricted: A subset like a plant or bird community.

What attributes define a community?

Species number, relative abundance, interactions, and physical structure.

What is species richness (S)?

The total number of species in the community.

What is relative abundance?

The percentage each species contributes to the total number of individuals.

What is a rank-abundance diagram?

A plot showing each species’ relative abundance against its rank.

What is species evenness?

How evenly individuals are distributed among species.

What does a gradual slope in a rank-abundance curve indicate?

High species evenness.

What does Simpson’s index (D) measure?

Species diversity based on richness and evenness.

What is the range of Simpson’s Index (D)?

0 to 1 — lower values = higher diversity.

What is Simpson’s reciprocal index (1/D)?

A diversity measure where higher values = greater diversity.

What is the Shannon-Weiner index (H)?

A diversity measure that increases with species richness and evenness.

What value does Shannon index have when only one species exists?

H = 0.

What are dominant species?

Species that predominate due to number, size, or competitive ability.

What is a keystone species?

A species whose impact on community structure is disproportionate to its abundance.

Example of keystone species: Sea otter. Why?

Sea otters eat sea urchins, maintaining kelp forest communities.

What is a food chain?

A simple diagram of energy flow from prey to predator.

Why are food webs more realistic than food chains?

They represent complex feeding relationships with many linkages.

What are basal species?

Species that are consumed but feed on none (e.g., primary producers).

What are intermediate species?

Species that eat others and are eaten themselves.

What are top predators?

Species that prey on others but are not preyed upon.

What are trophic levels?

Feeding groups like producers, herbivores, carnivores.

What are guilds?

Groups of species using a common resource similarly (e.g., seed-eating birds).

What is a functional type?

Grouping species by common responses to the environment.

In terrestrial communities, what mainly defines physical structure?

Vegetation growth forms.

What is the canopy layer?

Upper forest layer where most photosynthesis occurs.

What is the understory?

Layer beneath the canopy that grows only if light is sufficient.

What happens on the forest floor?

Decomposition and nutrient cycling.

What determines vertical structure in aquatic communities?

Light penetration.

What is the photic zone?

Upper water layer where light supports photosynthesis.

What is the aphotic zone?

Deep water layer without light.

What is the benthic zone?

Bottom layer where decomposition is most active.

What is zonation?

Spatial change in community structure across a landscape.

What causes zonation in salt marshes?

Microtopography, water depth, oxygenation, salinity.

What determines intertidal zonation?

Tidal action.

What is the organismic concept of communities?

Communities are integrated units with narrow boundaries and coevolved interactions.

Who proposed the organismic concept?

Frederic Clements.

What is the individualistic (continuum) concept?

Species distributions are independent and based on individual tolerances.

Who proposed the individualistic concept?

H.A. Gleason.

Why are community boundaries difficult to define?

Transitions between communities are often gradual.

What does Sorensen’s Coefficient measure?

Similarity between communities based on presence/absence.

What is the formula for Sorensen’s Coefficient (CC)?

CC = 2c / (s1 + s2)

What does a CC of 1 mean?

The two communities have identical species.

What does Percent Similarity (PS) measure?

Similarity between communities based on relative abundance.

What is the range of PS?

0 to 100%.

What does a PS of 100 represent?

Communities have identical relative abundances.

What is a similarity matrix?

A table comparing multiple communities’ similarity values.

What does high similarity among communities mean?

They share similar species composition or abundance patterns.

What landscape communities were least similar in the example?

Upland habitats vs. wetlands (4%–22% similarity).