Interdependence and competition in communities

Ecology • Adaptations, interdependence and competition

Flashcards

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Community - definition

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A community is a group of two or more populations of different species living together in the same area and interacting with each other .

Key concepts

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Community and population

A population is the total number of organisms of the same species in a given area; a community is two or more populations of different species living together and interacting. Ecologists study these interactions because they determine how energy and materials flow through an ecosystem and how species coexist or exclude one another. A clear definition of community and population provides the framework for understanding interdependence and competition .

Interdependence

Interdependence means that species depend on others for survival functions such as food, cleaning, pollination and habitat structure. Cause → effect: removal or severe reduction of one species reduces resources or services for others, which causes population declines or unexpected community shifts. Stable communities show much interdependence, with predator and prey numbers linked in cycles and feedback loops .

Competition (interspecific and intraspecific)

Competition is the contest for limited resources. Intraspecific competition occurs between individuals of the same species and increases when a population grows, causing slower growth, lower reproductive success and survival. Interspecific competition occurs between different species and causes resource partitioning, local displacement or extinction if one species outcompetes another. Competition acts as a driver for natural selection and evolution because better-adapted individuals gain access to resources and reproduce more successfully .

Factors organisms compete for (limiting factors)

Organisms compete for both biotic and abiotic factors. Biotic factors include food, mates, territory and hosts. Abiotic limiting factors include light, water, mineral nutrients, oxygen (dissolved in aquatic systems), temperature and space. Cause → effect: reduction in any limiting factor reduces the growth rate, reproduction and survival of organisms dependent on it, intensifying competition and altering community composition. Many species evolve dispersal or niche differences to reduce direct competition (for example, seed dispersal away from the parent plant) .

Examples and consequences

Plant example: when a canopy tree dies, increased light causes seedlings to grow faster and compete for canopy space; the fastest-growing seedling reaches the canopy and alters light availability for others. Animal example: lions and hyenas compete for prey; changes in prey availability cause predator numbers to rise then fall with a time lag. Invasive species example: introduction of a larger or disease-tolerant species can outcompete natives, leading to native decline and wide community impacts (e.g., grey squirrels replacing red squirrels) fileciteturn0file3turn0file6.

Stable communities and predator–prey cycles

Stable communities show regular feedback between trophic levels: an increase in prey leads to an increase in predators (after a lag), which then reduces prey numbers; reduced prey causes predator numbers to fall, allowing prey to recover. Cause → effect: these cycles help maintain balance but change when one species is removed or abiotic conditions shift, producing knock-on effects that are difficult to predict fileciteturn0file5turn0file12.

Key notes

Important points to keep in mind

A population refers to one species; a community contains multiple interacting populations .

Interdependence causes knock-on effects when any species or service changes; stable communities show balanced interdependence and cycling between predators and prey .

Competition exists within species (intraspecific) and between species (interspecific); both types reduce access to limited resources and affect survival and reproduction .

Limiting factors are resources that restrict growth or distribution; light, water, nutrients, space and oxygen commonly limit organisms in many habitats .

Invasive species can outcompete natives by size, fecundity or disease resistance, producing long-term community change .

Adaptations increase competitive success; dispersal or niche differentiation reduces competition between offspring and parents or between species .

Cause → effect reasoning improves explanations: state the limiting factor change, then describe how resource availability alters competition and the resulting population effect .