Relationships between organismsIB MYP Biology: Revision notes

Organisms in an ecosystem interact in many ways.

• Predation: a predator kills and eats its prey, for example a lion hunting a zebra.
• Competition: organisms compete for the same limited resource, such as food, water, light, space or mates. Intraspecific competition is between members of the same species (two lions fighting over a kill). Interspecific competition is between different species (lions and hyenas competing for prey).

In some relationships two different species live closely together.

• Mutualism: both organisms benefit. Examples: nitrogen-fixing bacteria in the root nodules of bean plants (bacteria make nitrogen compounds for the plant; the plant gives sugars), mycorrhiza (fungus on tree roots helps the tree take in water and mineral ions; the tree gives sugars), and the clownfish and sea anemone (the anemone protects the fish and the fish keeps the anemone clean).
• Commensalism: one organism benefits and the other is neither helped nor harmed. Example: an orchid growing on the branch of a tree, getting light.

In parasitism one organism, the parasite, benefits and the other, the host, is harmed.

• Tapeworm: lives in the human intestine. Hooks and suckers attach it to the gut wall and it absorbs digested food, so the host loses nutrients and weight. It is spread by undercooked meat containing the tapeworm.
• Head lice: live on the scalp and feed on blood. They spread by head-to-head contact.

A parasite that lives inside the host (tapeworm) is an endoparasite; one that lives on the outside (head lice) is an ectoparasite.

Malaria is caused by a parasite called Plasmodium, which lives in the blood. It is spread by a vector: the female Anopheles mosquito. A vector is an organism that carries a parasite from one host to another without being harmed itself.

1. The mosquito bites an infected person and takes up parasites with the blood.
2. The parasites develop and reproduce inside the mosquito.
3. The mosquito bites a healthy person and passes the parasites into the blood in its saliva.
4. The parasites go to the liver, then infect and reproduce in red blood cells, which burst. This causes repeated fevers.

Malaria can be controlled by breaking the life cycle.

• Reduce mosquito numbers: drain standing water where mosquitoes breed, or spray insecticide.
• Prevent bites: sleep under insecticide-treated bed nets, use repellent, fit screens on windows.
• Treat patients with anti-malarial drugs so they do not infect more mosquitoes.

Each method has drawbacks, such as cost, insecticide resistance and harm to other organisms or habitats.