Key Area 3.5: Symbiosis

Symbiosis

The co-evolved, intimate and ecological relationship between members of two different species which live in direct contact with one another, often benefiting at least one individual involved. Dependency evolves over millions of years.

Parasitism

A symbiotic relationship between a parasite and host, in which only one species (the parasite) benefits, at the expense of the host.

Parasite activity

- The parasite derives its nutrition and energy from another organism. Exploited hosts are therefore harmed due to the loss of valuable resources to the parasite (eg. energy and/or materials).

- Parasites often have a very limited metabolism, so must stay in contact with a host and cannot survive outwith a host, (eg. tapeworms have no digestive systems as they live inside a host’s).

- Effective parasites don’t kill their hosts, ensuring a constant supply of materials and energy.

Parasites must be spread from host to host to survive, and and be transmitted by:

- Direct contact, where a parasite (eg. head/body lice) is passed by physical contact.

- Resistant stages, where parasite larvae and pupae survive adverse environmental conditions until a new host comes into contact with them (eg. cat flea larvae surviving up to 6 months; pupae 1 year)

- Vectors, where an intermediate organism transfers a parasite from one host to the next (eg. mosquitoes carry Plasmodium, which causes malaria, to organisms it feeds on).

Parasites and secondary hosts

- Some parasites require a secondary host to complete their lifecycles.

- Primary hosts act as the sites of sexual reproduction (eg. worm larvae forming cysts in slugs)

- Parasites have evolved to use secondary hosts, and usually live and feed inside their host (eg. slugs carrying parasites eaten by weasels, stoats, shrews)

Mutualism

An interdependent relationship in which both partner species benefit, in terms of nutrients, shelter, a service provided, etc.

Mutualism in which both species provide a service:

Sea anemones provides clownfish protection from predators, as they cannot tolerate its sting. Clownfish protect anemones from anemone-eating fish (eg. butterfly fish).

Mutualism in which an organism provides food in exchange for a service:

- Bees pollinate plants, aiding reproduction, and receive nectar and pollen from flower.

- “Cleaner” fish feed on dead skin and parasites on larger fish (eg. sharks, turtles) and receive food and protection. Larger fish are kept clean and are less prone to infection.

- Living together as lichen, algae photosynthesise and share food with fungi, while fungi shelter algae from harsh conditions.

Mutualism in which both organisms receive a resource:

Rhizobium bacteria live in the roots of legumes, fixing nitrogen for a plant’s protein synthesis. Plants provide carbohydrates for rhizobium, which they can feed on and use for growth and respiration.