Organisms depend on one another

1. Organisms in a community obtain food, shelter and other resources through relationships with other species.
2. Flowering plants may depend on animals for pollination, while some animals depend on those plants for nectar or pollen.
3. Fruit-eating animals can disperse seeds, allowing plants to establish away from the parent plant.
4. Trees can provide nesting sites and shelter, so removing a tree can affect organisms that do not eat the tree itself.
5. A change in one population can therefore affect other populations both directly and through a sequence of linked changes.
6. Interdependence does not mean that every interaction benefits both organisms: some relationships benefit one organism while harming another.

Competition occurs for limited resources

1. Organisms compete when they need the same resource and there is not enough for all of them.
2. Plants compete for light, water, mineral ions and space.
   1. A taller plant may shade a shorter plant, reducing the light available for photosynthesis in the shorter plant.
   2. Roots from neighbouring plants can draw water and mineral ions from the same part of the soil.
3. Animals compete for food, water, mates and space that provides food, shelter or breeding sites.
4. Competition can occur between members of the same species or between members of different species.
   1. Individuals of the same species often have very similar needs and can compete for mates as well as other resources.
   2. Members of different species may compete for a shared food source or nesting site, but they do not normally compete for the same mates.
5. Competition can reduce growth, survival or reproductive success because an organism obtains less of a resource it needs.
6. Competition does not require fighting or direct contact, as one plant can remove water before another plant’s roots absorb it.

Predators and prey affect each other

1. A predator kills and eats another animal, which is its prey.
2. The predator gains food, while the prey individual is killed.
3. If prey become more abundant, predators may obtain more food, allowing more predators to survive and reproduce.
4. Predator numbers usually respond after a time delay, because growth and reproduction take time.
5. A larger predator population may remove more prey, contributing to a later decrease in prey numbers.
6. As prey become scarce, predators may have less food and their population can subsequently decrease.
7. Reduced predation can then allow prey numbers to recover, producing linked fluctuations under suitable conditions.
8. Real populations do not always follow neat repeating cycles because food supplies, disease, weather and other predators also affect them.

Mutualism benefits both partners

1. In a mutualistic relationship, each species gains a benefit from its association with the other.
2. A bee obtains nectar as a food source from a flower, and pollen carried on the bee can be transferred to another flower of the same species.
3. The plant benefits because pollen transfer can enable fertilisation, leading to seed production.
4. The two benefits must be identified separately: the bee receives food and the plant gains assistance with reproduction.
5. Another example is the association between some plant roots and fungi, where the plant supplies sugars and the fungus helps the plant obtain water and mineral ions.
6. If one partner declines, the other may lose that benefit, although the effect depends on whether alternative partners are available.

Parasitism benefits one partner

1. A parasite lives on or in another organism, called its host, and gains resources while harming the host.
2. A tapeworm living in an animal’s intestine absorbs nutrients from digested food, leaving less available to the host.
3. A flea feeds on a mammal’s blood, gaining nutrients while the host loses blood and may suffer irritation.
4. The host may grow more slowly, reproduce less successfully or become more vulnerable to disease because of the parasite’s effects.
5. A parasite usually exploits a living host over a period of time rather than immediately killing and eating it as a predator does.
6. A parasite can still cause severe illness or death, so parasitism should not be described as harmless to the host.

Linked populations can remain stable

1. A relatively stable community can retain its range of species while population sizes fluctuate within limits.
2. Stability does not require the number of every species to remain exactly constant.
3. Changes in predation, competition or beneficial partnerships can alter survival and reproduction, so a disturbance can spread through the community.
4. Predictions should follow the relationships given and recognise that an organism may have several food sources or several interacting partners.