Download Symbioses

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
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
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
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
Important aspects of hostparasite 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
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
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
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
Cost, Benefits & Consequences
INTERACTION
Effect on Population Density
Predation
Predator increases, prey decreases
Parasitism
Parasite increases, host decreases
Commensalism
Commensal increases, host density is unaffected
Mutualism
Both species in mutualism increase
Competition
Both species in competition decrease
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
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)
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