Symbiosis

1
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

2
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

3
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

4
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

5
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

6
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

7
Mutualism

• A biotic interaction in which both species gain
  benefit e.g. see p22 of monograph

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
8
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

9
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
10
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 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

11
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)

12
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