Biotic and Abiotic Interactions
Predation
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Patterns of predator prey interaction
– Stable coexistence
– Cyclical variations
– Erratic swings
– Extinction of prey species
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Important factors to consider
– Carrying capacity of the habitat
– Reproduction rate of prey
– Reproduction rate of predator
– Degree of flexibility of predator to switch prey
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Abiotic - Non living factors
– Temperature
– Light intensity
– Soil pH
– Availability of water
– Availability of minerals
Biotic – living factors
– Competition
– Predation
– disease
Density dependent and density independent factors
Role of Predators in maintaining diversity
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By removing prey who are strong competitors, weaker competitors can
survive
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Reduce effect of competitive exclusion
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Examples where removing predators has collapsed an ecosystem
– Otters and the sea kelp forests
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Density dependent factors
– Effect increases of population density increases
– Population level off, then decrease
– Example - availability of food
Density independent factors
– Effect independent of the size of the population
– Example – forest fire
Biotic Interactions
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Interspecific interactions
– Between individuals of different species
Intraspecific interactions
– Between individuals of the same species
Biotic Interactions
• Predation
• Grazing
• competition
The ecosystem benefits from
– Increasing species diversity
– Increasing stability
– The ability to adapt to environmental change
Prey defenses to predators
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Camouflage
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Mimicry,
Warning (aposematic)colouration
– Red, black and yellow are aposematic colours
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Batesian,
Mullerian
Grazing
• A grazer is defined as
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Niche
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Any species that moves from one victim to another, feeding on part
of each victim without actually killing it outright
• E.g. eating a shoot or a limb
• Can increase or decrease species diversity depending on grazing
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pressure.
Grazing of Vegetation
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Plant species with basal meristems can be grazed without suffering
mortality
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Plant species with aerial meristems can be eliminated by grazing when
the aerial part is removed.
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Grassland is often referred to as a plagioclimax, where a diversion
deflected succession off course to reach the natural climax community
e.g. fire, grazing, mowing
Competition
• Exploitation competition
– Consumer significantly reduces the resource
• Interference competition
– One species prevents individuals from another from using a resource
• Competitive interactions can:
– Short term – variations in abundance and distribution
– Long term – evolutionary adaptations to improve competitive ability
Styles” of Competition
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Exploitation competition
– occurs when individuals use the same limiting resource or resources,
thus depleting the amount available to others.
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Interference competition
– occurs when individuals interfere with the foraging, survival, or
reproduction of others
– directly prevent their physical establishment in a portion of a habitat.
The sum of
– An organism’s adaptations
– The resources it needs
– The lifestyle to which it is fitted
Gause’s principle of competitive exclusion
– When cultured together Paramecium aurelia has a competitive
advantage over P. caudatum for gaining food.
– Fundamental niche
– Realised niche
Introduction of “exotic species”
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Rhododendron ponticum - Scotland
– Dense canopy
– Leaves contain toxins
Nile Perch – Lake Victoria
– Carnivourous fish eating species
Rabbits and Prickly Pear cactus – Australia
Hedgehogs – Outer Hebrides
Rats – New Zealand
Importance of survival of weaker species
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Species diversity is essential
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E.g. extinction of the dinosaur and the emergence of mammals
If environmental conditions change,
– The dominant species may no longer be the best adapted and die
out
– This gives the weaker competitors who are better adapted a chance
to increase in number
Essay Question – 15 marks
Give an account of interactions between predators and their
prey