Lecture 16: Communities: Community Structure

Vocabulary practice flashcards covering community definition debates, spatial zonation and ecotones, biodiversity metrics, trophic structures, food webs, and trophic cascade mechanisms.

Community (basic definition)

The different species found in one place.

Interdependent communities

A historical perspective viewing the community as an "organism" that evolves as a distinct, co-evolving unit.

Independent communities

A historical perspective viewing a community as fully disassociated populations of species that share similar environmental needs and tolerances, but do not require each other to survive.

Community (modern view)

An association of interacting populations, with the interactions maintained via coevolution to varying degrees across space and time.

Zonation

Distinct, predictable spatial banding or transition patterns in the composition and distribution of species across an environmental or elevation gradient.

Ecotone

A region of rapid replacement of species along an ecological gradient, such as temperature, nutrients, or moisture availability.

Closed community

A community characterized by the clustering or overlapping of species distributions along an ecological gradient, resulting in distinct boundaries and sharp species transitions at ecotones.

Open community

A community structure where species are distributed independently with respect to one another along an ecological gradient, resulting in weak or nonexistent ecotones.

Species richness ($S$)

The total number of unique species found within a community; the simplest measure of community structure.

Shannon-Wiener diversity index ($H'$)

A metric quantifying biodiversity that accounts for both species richness and species evenness, calculated using the formula $H' = -\sum_{i=1}^{S} p_i \ln(p_i)$, where $p_i$ is the proportion of individuals of species $i$.

Pielou's evenness ($J'$)

A measurement of how equally individuals are distributed among species within a community, calculated using the formula $J' = \frac{H'}{\ln(S)}$.

Trophic level

A distinct position or feeding tier occupied by an organism within the food chain of consumer-resource feeding relationships.

Primary producers

Plants and other autotrophic (self-feeding, photosynthetic) organisms that generate biomass and form the base of the food chain.

Primary consumers

Organisms (herbivores and detritivores) that obtain their nutrition by consuming primary producers.

Secondary consumers

Carnivorous organisms that obtain their nutrition by consuming primary consumers.

Food web

The interconnected network of consumer and resource species interactions within an ecological community.

Omnivory

The feeding ecology where an organism feeds on resources across more than one trophic level.

Keystone consumer

A consumer species that maintains diversity and coexistence among resource species at lower trophic levels, frequently by preventing a competitively dominant prey species from monopolizing space or resources.

Top-down control

A regulatory mechanism where the abundance, biomass, or population size of a trophic level is controlled by the consumer level above it.

Bottom-up control

A regulatory mechanism where the abundance and biomass of a trophic level is determined by the rate of production or resource availability from the trophic level below it.

Trophic cascade

An ecological phenomenon where the indirect effects of consumer-resource interactions at one trophic level extend across and affect multiple other trophic levels.

Direct effect

An ecological interaction between two species that occurs without the involvement of any intermediate species.

Indirect effect

An ecological interaction between two species that is mediated through one or more intermediate species.

Density-mediated indirect effect

An indirect interaction between two species that occurs as a consequence of changes in the population density or abundance of an intermediate species.

Trait-mediated indirect effect

An indirect interaction between two species that occurs as a consequence of behavioral, physiological, or morphological trait shifts in an intermediate species rather than changes in its density.