BIOTIC INTERACTIONS

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BIOTIC INTERACTIONS

• 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

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Density Dependence/ Independence

• 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.

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

• Predation is a force for natural selection
• Selection pressure
• Causes co-evolution of predator/prey
• Predator gets faster, stronger, hunt
  co-operatively 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
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Predation

• 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 - angler fish
• camouflage - lion, preying mantis
• Prey evolve to avoid predation

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Avoiding 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

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Avoiding Predation

• 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

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Avoiding Predation

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

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Predator Prey Interactions
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Predator prey Interactions

• 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

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Competition

• 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

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Competition

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

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Galapagos Finch- fortis

• 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.

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Two types of Competition

• 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

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Niche

• A niche is an organisms 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

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Competitive Exclusion Principle

• 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,)

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B competes more strongly
A competes more strongly
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B competes more strongly
A competes more strongly
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Fundamental Realised Niche

• 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

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Resource Partitioning

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

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Resource Partitioning

• 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

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Importance of Niche overlap

• 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

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Alien Species

• 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)

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Alien species prey on defenceless animals

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

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Alien species grow unchecked

• 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

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Alien species grow unchecked

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

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Alien species steal Niches

• 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

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Resource Partitioning

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

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Symbiosis

• 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

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Parasitism

• 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

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Parasite transmission

• 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

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Pinworm

• 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

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Important aspects of host- parasite interactions

• 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

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Commensalism

• 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

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Mutualism

• A biotic interaction in which both species gain
  benefit

Mutualism Species 1 Species 2
Ant acacia Ant gains secure home, food supply Acacia gains protection from predation
Coral Algae Coral gains carbohydrate from photosynthesis Algae protection and mineral nutrients
Mycorrhizae plants Mycorrhiza gains photosynthetic product Plant improved mineral and water absorption

Ruminant herbivore bacteria Ruminant - gets its food digested Bacteria gains protection, warmth, moisture food
Lichen Fungus photosynthetic products Algae gains water, minerals and structural support
Rhizobium and legumes Rhizobium gains photosynthetic product Plant gains nitrate for protein synthesis
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More on Rhizobium

• 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

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Cost, Benefits Consequences

• INTERACTION Effect on Population Density
• Predation
• Parasitism
• Commensalism
• Mutualism
• Competition

Predator increases, prey decreases Parasite
increases, host decreases Commensal increases,
host density is unaffected Both species in
mutualism increase Both species in competition
decrease
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Effect of External factors

• 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

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Coral Bleaching

• 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)

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Competitive Exclusion

• 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

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Essay

• Compare parasitic, commensalistic and mutualistic
  interactions, using neamed examples. (15)

• Parasitism
• Definition (1)
• Ecto/endo (1) 1
• Obligate/ facultative (1) 1
• Effect on host/ parasite (energy) (1)
• Evolutionary pressures (1)
• Life cycle vs host (1) 1
• Max. 7

• Commenalism
• Definition (1)
• Examples (1)
• Diffiulty relating clear examples (1)
• Benefits analysis (energy) (1)
• Max. 3

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Keystone species

• 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