Module 4: Community and Population Ecology

chapter 20 - 26

Ecosystem Resilience

Ability of an ecosystem to resist change and remain “unchanged” in the face of perturbations and/or the ability to recover following change

Disturbances

a force external to the community that is relatively discrete in time that changes resources or the physical environment

• normal and regularly occur

• human activity may influence the frequency and severity of disturbances (ex. forest fires)

  • climate change will increase the frequency and intensity of forest fires

• important facilitators of non-native plant invasions

Succession

how communities recover after a disturbance

• two types: primary and secondary

Community Succession

repeatable change in community composition through time following a disturbance

Primary Successsion

begins new communities, uninfluenced by prior communities

• newly exposed or newly formed land

• often starts from bedrock

• severe disturbance (rare)

  • lava, landslide

Secondary Succession

part of ecosystem remains after disturbance

• follows disturbance of pre-existing community

• often soil remains that may contain seeds from preexisting species

• less severe disturbance (more common)

  • fire, flood, severe winds

• influenced by pre-existing community

• occurs in oak and hickory forests

• annual plants grow → grass → shrubs → pines → oak and hickory

Pioneer species

first species that colonize an area after disturbance

• far dispersal distances to travel from nearby areas

• hardy, tolerant dormant stages (fire-resistant seeds)

• hardy, tolerant adult stages

• quick growth in high-light environments

• make environment more habitable

• replaced by later with shade-tolerant species

Species that colonize late in succession may have

low dispersal, slow growth, tolerant of low-light conditions

Climax community

when a steady state is reached (equilibrium)

• persists stably until a disturbance occurs

Facilitation

occurs during primary succession

• new bedrock can only by inhabited by few, tolerant species

  • species make environment more habitable

• primary succession

Inhabitation

any species can colonize a recently disturbed site and prevent others from establishing

• secondary succesion

• occurs when a species inhibits colonization by others

• starts with short-lived species to long-lived species

Tolerance

any species can colonize and have no influence on others ability to establish

• weaker competitors replaced by stronger competitors

• secondary succession

• Occurs when early-succession species neither inhibit nor facilitate later-succession species

Community dynamics and succession

highly complex

• rarely do communities follow one model completely

Intermediate disturbance hypothesis

biodiversity is highest when disturbances occur at an intermediate frequency

• low levels: more competitive organisms dominate pushing others to extinction

• high levels: all species or at risk of extinction

What is Intermediate disturbance hypothesis based on?

• disturbances have major effects on species richness

• interspecific competition, results in one species driving a competitor to extinction and becoming dominate

• moderate ecological scale disturbances present interspecific competition