Interactions in ecosystems

Density-independent factors effects are independent of the population density

Density-dependent factors effects change as population changes

Organisms interact with their environment.There are biotic (density-dependent) and abiotic (density-independent) factors in an ecosystem.

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Interactions can be between or within species.

The type of interaction influences the intensity asmembers of the same species often need the same resources but members of different species often need slightly different resources

Interactions between members of different species

Interspecific

Interactions between members of the same species

Intraspecific

Population growth graph

SCHOLAR - Advanced Higher Biology : Density dependent and density independent factors

PopulationsPopulations show changes over time but the numbers will be relatively stable over long periods of time.

Predation

A predator is an organism that consumes partor all of another organism.

Should this definition challenge our understanding of what makes a predator?

Scholar activity predator/prey population cycles.

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Predator-prey cycles

Homeostatic control of populations

Because of the carrying capacity of an environment and the factors involved in controlling population numbers, the system operates like a negative feedback control system.

Predator - Prey Interaction

Predator and prey numbers will be interconnected.

As one varies so will the other, the predator numbers usually tracks the prey numbers.

The number of predators usually shows a similar pattern to prey but lags behind.

This is because it takes time for populations to change.

Predators and diversity

Predators can help diversity by keeping in checkother species that might otherwise reduce the diversity.

e.g. Canada otters sea urchin

Prey defences against predators

Batesian mimicryOrganism that is edible has same colouration as a harmful organism.

Warning (aposematic) colourationOrganism has markings advertising it to be poisonousor unpleasent to eat.

Camouflage (crypsis and disruptive)Organism blends in with its background

Mullerian mimicryOrganisms that are inedible or poisonous have evolved the same or similar warning signals.

crypsisdisruptive

warning

Mullerian

Batesian

Grazing

If plant meristems are removed they may not be ableto grow.Plants with very low growing meristems (basal meristems) plantain, grasses and dandelions tolerate grazing well.

Grazers move from one ‘victim’ to another.(victim?)Grazers can have a significant effect on diversity inecosystems.

Grazing of dominant species e.g. grasses canallow other less dominant plant species the opportunity to grow.

Unselective grazers and selective grazers and how they affect biodiversity?

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Over grazing can lead to a loss in diversity as the plants without basal meristems can be eliminated.

Competition

Exploitation competition is when two species use the same resource and the survival chances of both isreduced.

Interference competition is where organisms of differentspecies actually fight over a resource.

Competition will occur when there is a common needfor a resource in an ecosystem.Competition can be either intraspecific or interspecific

Ecological niches

Scholar activity Competition between Paramecium.

Competitive exclusion principleWhen two species have identical niches they will compete and the most successful will drive the other to extinction.

The role an organism occupies in an environment andhow it interacts with the biotic and abiotic factors.

Niches

Scholar activity on seashore niches.

Resources are divided between the competitors,Resource partitioning.

Realised niche

Fundamental niche

An organism not experiencing competition will be able to make use of all its resources.

An organism will experience competition and these are the resources the species will actually be able to make use of.

The effect of exotic species

Examples include rhododendrons, grey squirrels,mink and New Zealand flatworm.

An exotic species is a species introduced to an ecosystem in which it would not normally be found.Can be deliberate or accidental.

Survival of weak competitors

An example of this is when the mass extinctionof the dinosaurs gave the mammals a chanceto fill the niches left behind.

Competition often effects populations differently.If environmental conditions change and the dominant organism is no longer best adaptedto survive and dies out, this can give weaker competitors a chance to increase in number.