Title: Symbiosis
1Symbiosis
- 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
2Parasitism
- 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
3Parasite 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
4Pinworm
- 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
5Important 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
6Commensalism
- 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
7Mutualism
- 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
8More 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
9Cost, 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
10Effect 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
11Coral 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)
12Competitive 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