Ecosystems and interdependence

An ecosystem is the interaction of a community of living organisms (biotic) with the non-living (abiotic) parts of their environment.

The levels of organisation run from a single organism upwards: an individual organism, a population of that species in a habitat, the community of all the populations living there, and the ecosystem — the community plus the physical surroundings it interacts with.

To survive and reproduce, organisms require a supply of materials from their surroundings and from the other living organisms there.

Two forces shape a community, and the importance of both is examinable. Competition is what happens when organisms need the same limited resource: the better adapted get more of it, survive and breed, which is natural selection operating inside the community. Interdependence is the opposite relationship — species relying on each other.

Within a community, each species depends on other species for food, shelter, pollination, seed dispersal and so on. If one species is removed it can affect the whole community — this is called interdependence.

A stable community is one where all the species and environmental factors are in balance, so that population sizes remain fairly constant. "Fairly constant" is the phrase to use: numbers still rise and fall, but around a steady level rather than climbing or crashing.

When you are given information about a habitat, two things are expected of you: suggest the factors organisms are competing for, and suggest how they are adapted to the conditions. Both are answered from the data in front of you rather than from memory, so read what the question actually describes — the climate, what else lives there, what is scarce.

You should also be able to extract and interpret information from charts, graphs and tables relating to how organisms interact within a community. Quote figures with units, and say what the number means for the population.

Abiotic means non-living. The abiotic factors which can affect a community are:

• light intensity • temperature • moisture levels • soil pH and mineral content • wind intensity and direction • carbon dioxide levels for plants • oxygen levels for aquatic animals.

Note the last two are each tied to a group: carbon dioxide matters to plants because of photosynthesis, and dissolved oxygen to aquatic animals because of respiration.

Explaining a change works the same way every time: the factor changes → one species is affected directly → the species that depend on it are affected next.

For example, if light intensity falls in a woodland, the plants photosynthesise less and grow more slowly, so there is less food for the herbivores that eat them, so herbivore numbers fall, so the predators that eat the herbivores have less food. Take the chain one step at a time and the marks follow.

Biotic means living. The biotic factors which can affect a community are:

• availability of food • new predators arriving • new pathogens • one species outcompeting another, so the numbers are no longer sufficient to breed.

That last one is worth reading carefully. A species does not have to be killed off to disappear — if it is outcompeted until too few individuals remain to find each other and breed, the population collapses anyway.

Organisms have features — adaptations — that enable them to survive in the conditions in which they normally live. These may be structural, behavioural or functional, and questions often ask you to say which kind you are describing.

• Structural: a physical feature of the body. Thick fur, a large surface area, spines instead of leaves. • Behavioural: something the organism does. Migrating, hibernating, hunting at night. • Functional: something about how the body works. Producing very concentrated urine, going into torpor, changing metabolic rate.

Some organisms live in environments that are very extreme — high temperature, high pressure, or high salt concentration. These organisms are called extremophiles. Bacteria living in deep sea vents are extremophiles, surviving temperatures and pressures that would destroy most cells, because their enzymes do not denature under those conditions.

Environmental changes affect the distribution of species in an ecosystem. The changes include temperature, availability of water, and the composition of atmospheric gases, and they may be seasonal, geographic or caused by human interaction.

Seasonal change moves species temporarily — birds migrate, and return. Geographic change means conditions differ from place to place, so a species is found in some areas and not others. Human interaction is the one that changes distribution permanently: a warming climate shifts the range a species can live in, and a species already at the edge of its range has nowhere left to move to.

Evaluating an impact means using the data given, saying which direction the distribution shifts and why, and noticing what else depends on that species.

Organisms in a community depend on one another, and two kinds of close relationship are named. The difference between them is simply who gains.

In parasitism, one organism — the parasite — lives on or in another and takes what it needs from it. The parasite benefits; the host is harmed, though usually not killed, because a parasite that kills its host quickly loses its home.

In mutualism, both organisms benefit. Neither is harmed, and in some cases neither could survive without the other.

Photosynthetic organisms are the producers of biomass for life on Earth. Feeding relationships within a community can be represented by food chains, and all food chains begin with a producer which synthesises molecules — usually a green plant or alga making glucose by photosynthesis.

Producers are eaten by primary consumers, which in turn may be eaten by secondary consumers and then tertiary consumers. Consumers that kill and eat other animals are predators, and those eaten are prey.

In a stable community the numbers of predators and prey rise and fall in cycles, and you should be able to interpret graphs that model them.

Read a predator-prey graph in this order. The prey peak comes first. More prey means more food for predators, so the predator numbers rise after it. More predators eat more prey, so the prey numbers then fall. With less food the predators fall too, which lets the prey recover — and the cycle repeats. The predator curve always lags behind the prey curve, and spotting which line peaks first is how you tell them apart when they are unlabelled.

Ecologists use quadrats and transects to determine the distribution and abundance of species.

A quadrat is a square frame placed on the ground; you count what is inside it, repeat at random positions, and use the mean to estimate abundance across the whole area. A transect is a line across a habitat, with quadrats placed at intervals along it — used when you want to see how a species changes across a gradient, such as from the shore inland.

Random placement matters for abundance, because choosing where to put the quadrat biases the result. A transect is deliberately not random, because there you are studying the change with distance rather than the average.