• Three Categories of interactions
– Predation
– Competition
– Symbioses
• As a result organisms evolve (change):
– develop to maximise the benefit of their interaction (minimise the disadvantage)
• Intraspecific
– Between individuals of the same species
• Interspecific
– Between individuals of different species

• Density dependent – the severity of the effect increases as the population size increases.
• Density independent – the severity of the effect is the same irrespective of the population density.

Density Dependent
Predation
Food
Water
Disease
Space
Density Independent
Any abiotic factor e.g.
Temperature
Light pH

• Predation is a force for natural selection
– Selection pressure
• Causes co-evolution of predator/prey
– Predator gets faster, stronger, hunt cooperatively etc.
– Prey gets faster, form herds, modify behaviour/ physical characteristics
• Grazing is classified as a predation?!
– Grazing promotes biodiversity by selectively reducing dominant (more frequently encountered) species
VIDEO

• Predators are adapted to enhance their success:
– have highly evolved senses
• sight – eagles/ cats
• smell – anteaters, pigs
• Infra red (rattlesnake), hearing – owls
• echo location – bats, electrical – sharks, platypus
– Predators can cooperate (lions, army ants, chimpanzees)
• Allow exploitation of resources beyond the capability of a single individual of the species
– Predators can also use:
• mimicry
• camouflage -
angler fish lion, preying mantis
– Prey evolve to avoid predation

• Prey can use:
– behavioural adaptations
• hiding (fish on coral reefs)
• running away (antelope from lion, seal from killer whale)
• mobbing (kittiwake on gulls)
• herding (musk ox)
• distraction displays (plover broken wing, butterfly eyes on tail)
– Active defence
• fight back (water buffalo/ gnu mothers)
– Camouflage (crypsis)
• animal is coloured to merge into background
• e.g. stonefish, chameleon, stick insect

• Prey can use:
– Aposematic (warning) colouration
• Animals advertise their toxicity
• wasps & bees yellow & black
– Mimicry – one organism resembles another
• Batesian mimicry
» a harmless species mimics a toxic one
– Hoverfly looks like a wasp
– need more wasps than hoverflies otherwise predators learn yellow
& black is not toxic. (but see below)
• Mullerian mimicry
– two or more aposematically coloured species develop similar warning colouration
– e.g. bees & wasps
– warning signal is greatly reinforced by such large numbers showing the same warning

– Mechanical & chemical defences
• plants contain toxins, grow spines/ thorns
• Animals secrete toxins/ bitter taste/ slimes
(slugs, frogs), grow armour (pangolin), spines
(hedgehog)

• Cyclical oscillations in predator population reflects cyclical oscillations in prey population
• Carrying capacity = population which can be supported by the ecosystem
• As snowshoe hare population increases
– carrying capacity (lynx) of ecosystem increases - more food available
– Lynx population increases
– hare population eventually exceeds carrying capacity of the ecosystem
(food, space run out)
– population (hare) crashes
– lynx population no longer has sufficient food resource
– consequently lynx population crashes
• Grass (hare food) population would peak BEFORE the hare population
• FIRST in food chain peaks FIRST in cycle
• NB the predator DOES NOT usually control prey population, it is a species’ food supply which controls its population size

• Resources are limited (e.g space, food, water)
– The ability of organisms to gain resources will determine their success.
• As the density of population increases competition becomes more severe
– Some organisms are more effective at securing resources
– Those are successful, survive and reproduce
– Less successful organisms perish
• Competition causes natural selection
– Species change (evolve) to reduce competition

• Intraspecific competition is more severe than interspecific because the same species compete directly for exactly the same resource.

• Fortis eats seeds.
• During drought big tough seeds are all that are available to eat
• Big beaks make this easier
• Prior to drought average beak was 10,68mm long and 9.42 mm deep
• After a drought period, average beak length 11.07mm long and 9.96mm deep
• Competition for food caused nearly 6% change in beak shape in one year.

• Exploitation competition - occurs indirectly through a common, limiting resource, which acts as an intermediate. For example the use of the resource(s) depletes the amount available to others, or they compete for space.
• e.g grey/ red squirrel food;
• e.g.
• Interference competition - occurs directly between individuals via aggression etc. when the individuals interfere with foraging, survival, reproduction of others, or by directly preventing their physical establishment in a portion of the habitat.
• e.g ant & Rattan & herbivores
• e.g ant, acacia & giraffe

• A niche is an organism’s position within an ecosystem described in terms of abiotic and biotic interactions:
• abiotic interactions (i.e. mineral needs/ pH tolerance, moisture/temperature range)
– The larger the range of physical conditions tolerated, the wider the niche and more widespread the organism is
• biotic interactions (i.e. position in the food chain, diversity of food sources exploited, diversity of species which exploit it as a food source)
– The greater diversity of these interactions the more widespread the organism
• Within an ECOSYSTEM no two organisms can occupy the same niche

• Two organisms cannot coexist sharing the same niche in an ecosystem.
• They will compete, one will be more successful and the second will become extinct.
– Experimentally demonstrated using
Paramecia species (Gause,)

B competes more strongly
A competes more strongly

B competes more strongly
A competes more strongly

• The fundamental niche is the entire range of abiotic & biotic parameters an organism can survive within.
– Fundamental niches can overlap
• Realised niche is the actual range of parameters within which the species occurs.
– Realised niche can be smaller than the fundamental niche
– Realised niches cannot overlap
• Species cannot share exactly the same resources
• Competition would lead to the exclusion of one of the two species occupying the same niche
– Adaptations of species is such that they are best suited to a subset of their fundamental niche parameters
• e.g. barnacle zonation on the shore

• To reduce competition between organisms with overlapping niches, species adapt and diverge to become specialised for a smaller realised niche

• Resource partitioning:
– e.g. Cormorant/ Shag
• Cormorant nests high on cliffs or broad ledges
• Shag –nests on shallow ledges, low on cliffs
• Cormorant – feeds on mixed diet no sand eels/ sprats
• Shag – Eats mostly sand eels/ sprats

• Within a population some individuals are adapted to living at the extremes of the niche
– i.e. they are adapted to conditions slightly different to those currently found in the ecosystem
– such organisms will survive, albeit less successfully, in the overlap of niches
• This variability within a population is vital for allowing a species to survive change
• These weaker individuals may have traits ideally suited to the new conditions

• Realised niches cannot overlap
• Indigenous species are adapted to exploit niches within their home ecosystem, and resist competition from other indigenous species
• A new species (alien, exotic or introduced) may:
– Prey on other species in the ecosystem, not adapted for defence against their predation
– Compete more effectively for resources, ousting an indigenous species from a niche
– Be immune from natural biological control mechanisms so grow unchecked
• Introducing species, particularly to islands can cause grave harm to the established species (extinction )

• Examples
– Hawaiian Islands
• Hawaii’s endemic moths destroyed by introduced parasitic wasps
• Hawaii’s plants threatened by seed & fruit predation by rats
• Hawaiian native snails threatened by introduced snails
– Hebrides
• Hedgehogs eat eggs of ground nesting birds

• Australia
• Cacti are not native to Australia
• Prickly pear (S. America) grows unchecked, not natural predator
• Native plant species are ousted (no space)
•1925, Cactoblastis (moth), lays its eggs specifically on the cactus and larvae burrow in causing bacterial infection
•Good biological control

• Australia
– Rabbit rapid reproduction & poor control by predators
– Population explosions occur
– Eat grass
– Myxomatosis introduced as biological control in 1950’s
– Similar explosions seen with mice

• Example
– Hawaiian Islands
• Ants are not native to Hawaii
• Their introduction has led to loss of endemic flightless fly which previously filled the niche
• now occupied by ant

– e.g. Shore birds (beak length)
– e.g herbivores on African plains (Giraffe, Elephant &
Antelope
)

Symbiosis = living together
Two species form a close relationship
They co-evolve to maximise the benefits from their interactions (parasitism only one species benefits)
Three types of symbioses:
Parasitism
Commensalism
Mutualism

The symbiont (the parasite) benefits, the host (parasitised) loses
Two forms of parasitism:
Ectoparasite – live externally on the host e.g. ticks & fleas, leeches,
Endoparasite – live inside the host e.g. malaria, tapeworm, hookworm, most gut bacteria are not parasites

Transmission is: vertical (mother to baby – HIV, rubella) horizontal (amongst members of species) direct close contact – cold, measles sexual contact – HIV, syphilis indirect contact – polio, cholera (through water) vector contact – malaria, sleeping sickness
Parasites develop ingenuous strategies to transfer between host
Often complex multistage , multihost life cycles involved

Human gut parasite
Eggs transferred into mouth (oro-faecal transmission)
Develop and grow in small intestine
Warm, moist, good food supply
Once mature females fill with eggs
Migrate to anal region
In evening/sleep, migrate out of anus, lay eggs perianally (around anus)
Secretion causes irritation/ redness of perianal region (pruritus ani)
Host scratches irritation
Poor hygiene allows transfer of egg into mouth

Parasites adapt to improve effectiveness of parasitism
Obligate parasites – must live as a parasite
Facultative parasites – can live as parasites when host is alive, but switch to saprophytes once host dies
Hosts adapt to counter parasitism immune system preening behaviour plants produce defensive chemicals, galls develop to seal off parasite from rest of host
Escalation of “war” leads to specificity in host/ parasite relationships e.g. smallpox virus, fleas

• A biotic interaction between two species
– one species benefits, the other is UNAFFECTED
– Difficult to find clear examples
– Lichen on a tree is possibly one case
– Where carriage is provided e.g. hermit crab & anemone, energy is expended in transporting the anemone,
• But hermit crab appears to benefit because it actively replaces the anemone when removed – likely mutualism
– In the nitrogen cycle, Nitrobacter depends on
Nitrosomonas for its nitrite
• The two species otherwise live entirely independently in the soil

• A biotic interaction in which both species gain benefit
Mutualism Species 1 Species 2
Ant & acacia
Coral & Algae
Mycorrhizae & plants
Ruminant herbivore & bacteria
Lichen
Rhizobium and
Ant – gains secure home, food supply
Coral – gains carbohydrate from photosynthesis
Mycorrhiza – gains photosynthetic product
Ruminant - gets its food digested
Acacia – gains protection from predation
Algae – protection and mineral nutrients
Plant – improved mineral and water absorption
Bacteria – gains protection, warmth, moisture & food
Fungus – photosynthetic products
Rhizobium – gains
Algae – gains water, minerals and structural support
Plant – gains nitrate for

• Rhizobium responsible for N fixation in nodules on roots of legumes
– Nodules form as a result of interaction between bacteria and root hair cells
– 90% of fixed nitrogen passes to plant
– plant gives carbohydrate to bacteroids
– Enzyme involved is NITROGENASE
– Rhizobium produces NITROGENASE
– However nitrogenase is poisoned by OXYGEN
• The PLANT produces a protein which binds the oxygen and prevents NITROGENASE being poisoned
• leghaemoglobin traps oxygen

• INTERACTION Effect on Population Density
• Predation
Predator increases, prey decreases
• Parasitism
• Commensalism
• Mutualism
• Competition
Parasite increases, host decreases
Commensal increases, host density is unaffected
Both species in mutualism increase
Both species in competition decrease

• Quantitatively, the outcome of a species interaction is determined by:
– Biotic factors e.g. disease, food availability
– Abiotic factors e.g. temperature, water availability
• If there is a pre-existing stress, negative interactions are more damaging.
• Humans further complicate the interaction by using medicines, fertilisers, pesticides & herbicides to alter the consequences of species interaction between ourselves and our crops

• Coral is dying in a number of areas around the world
– bleaching – when coral dies it turns white
– death is due to loss of algal mutualism
– this due to increase in sea temperatures
(1ºC)

• In closed conditions
– Competition between two species will lead to the exclusion of one of the species
– The triumphant species will ultimately depend on the conditions within the system
• In real ecosystems, competition may lead to the exclusion of a species through most of its range
– Local conditions may allow pockets of reduced density to survive, because they are better suited to these local conditions
– Should conditions change to favour the outcompeted species these pockets are sources from which the species can migrate and colonise its former range

• Compare parasitic, commensalistic and
 mutualistic interactions, using neamed

 Definition




(1) +1
 Effect on host/ parasite (energy)
(1)
 Evolutionary pressures (1)
 Life cycle vs host
 Max. 7

• A keystone species is one whose removal will have an extremely detrimental effect on the community
– e.g. The removal of sea otter from californian kelp forest