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Symbiosis

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.

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Symbiosis

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  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

  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

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