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 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

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 MutualismSpecies 1Species 2 Ant & acaciaAnt – gains secure home, food supply Acacia – gains protection from predation Coral & AlgaeCoral – 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 digestedBacteria – gains protection, warmth, moisture & food LichenFungus – photosynthetic productsAlgae – 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  INTERACTIONEffect 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

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