Topic 11 - Community Ecology

INTERSPECIFIC INTERACTION Outcomes

Interaction	Effect (Sp. 1 / Sp. 2)	Description
Competition	- / -	Both species are harmed as they vie for the same limiting resources.
Consumption	+ / -	One species benefits by eating another (predation, herbivory, parasitism).
Mutualism	+ / +	Both species benefit from the interaction.
Commensalism	+ / 0	One species benefits while the other is unaffected.

Intraspecific Competition

• Competition within same species

• Drives density-dependent population regulation

Interspecific Competition

Competition between species for limiting resources

Can lead to Coevolution(When interacting species influence each others adaptations)

Forms of Competition

• Consumptive → resource depletion

• Interference (territorial/encounter) → direct interaction

• Preemptive → space occupation

• Overgrowth → shading/physical exclusion

• Chemical (allelopathy) → toxin release

Competitive Exclusion Principle

two species competing for the same limited resources cannot coexist at constant population sizes; one species will eventually outcompete and eliminate the other

Niche

The ecological ROLE of a species, including the range of resources it uses and the environmental conditions it tolerates.

Fundamental Niche

The full range of resources a species could theoretically use without constraints like competition.

Realized Niche

The actual range of resources a species uses when limited by factors like competition.

Symbiosis

An intimate, close-range interaction between species

Trophic Levels

The hierarchical levels in a food chain (e.g., primary producers, primary consumers, secondary consumers)

Trophic Cascade

A series of changes in species abundance across a food web, often triggered by the addition or removal of a top predator

Fishing can lead to trophic cascades in the ocean

Keystone Species

A species that has a disproportionately large effect on its community relative to its abundance

Succession

The gradual, directional change in a community's species composition over time following a disturbance

Succession Trends

Succession typically follows these general patterns:

1. Diversity increases over time.

2. Physical structure becomes more complex.

3. Pioneering species (fast-growing, short-lived) are eventually replaced by a climax community (stable, long-lived species).

   • Primary Succession: Starts from scratch on bare rock/lava with no soil.

   • Secondary Succession: Occurs after a disturbance where the soil remains intact (e.g., after a fire or storm).

Character Displacement

when traits evolve during

niche differentiation

Consumption Types: Predation

kills and consumes most or all of prey. Includes

carnivores (“meat eaters”) and seed predators (eat entire plant

embryo)

Consumption Types: Predation

onsumption of plants (herbivore = “plant eater”) often

specialize on specific plant parts rather than killing and

consuming entire organism

Parasitism

a close symbiosis, organism lives in or on a host.

Often not fatal unless parasite is similar in size to host.

Constitutive Defense

A defense that is

always present, even in

the absence of a

consumer

Inducible Defense

A defense that is produced after exposure to consumers: which saves energy

Mechanical defense

Thorns, spines, trichomes, silica,

thick exoskeletons or shells

Chemical defense

Toxins, smells, latex, etc.

Behavioural defense

Schooling in birds and fish, sounds, and many kinds of escape

Camouflage

Can evolve in both prey and predators to avoid being seen

Warning Colouration

Also called “aposematism”, bright colours warn of toxicity to avoid being consumed

Mimicry

Resembling other species can reduce chance of consumption

Mullerian mimicry

Two or more harmful or inedible species resemble one another

Batesian mimicry

A harmless or edible species resembles a harmful or inedible one

Obligate Mutualism

At least one species cannot persist (survive or reproduce) without their mutualist

Facultative

Either species can persist (survive and reproduce) without their mutualist

Pioneering species

the first species to appear. Adapted for life with disturbance (during secondary succesion)Climax community

Climax community

the “stable” community, comprised or long-lived species. Minor disturbances and species composition can continue, but long-lived species dominate.

Facilitation (succession)

Early species make the environment more favorable for later species, helping them establish.

Inhibition (succesion)

Early species make the environment less favorable for later species, preventing or slowing their growth.

Tolerance (succesion)

Early species have no impact on the establishment of later species.

General Trends During Succession

• Diversity increases: More species move in over time.

• Structural complexity: The physical environment becomes more layered (e.g., from grasses to shrubs to tall trees).

• Final Composition: The "end result" (Climax Community) is determined by a mix of Abiotic factors (climate/soil), Biotic interactions (competition), and Chance events (who happened to arrive first).

Primary vs secondary succesion

Primary succession occurs in lifeless areas where soil is absent, starting with pioneer species like lichens and mosses, while secondary succession takes place in environments that already have soil and some life, following disturbances like fires or floods.