Organisms and Their Environment

Habitat, population, community and ecosystem

Ecology questions usually begin with definitions, and these four words are easy to mix up. Learn them in order of size: habitat, population, community, ecosystem.

A habitat is the place where an organism lives - a pond, a hedgerow, the underside of a stone. A population is a group of organisms of one species, living in the same area, at the same time.

A community is all of the populations of different species in an ecosystem: every plant, animal and microorganism living together in one place. An ecosystem is a unit containing the community of organisms and their environment, interacting together - the living organisms plus the non-living surroundings such as light, water, temperature, soil and minerals.

• Habitat = the place where an organism lives.
• Population = one species, in the same area, at the same time.
• Community = all the populations of different species in an ecosystem.
• Ecosystem = the community of organisms plus their environment, interacting together.

Energy flow through an ecosystem

The Sun is the principal source of energy input to biological systems. Light energy from the Sun is captured by green plants and other producers and converted into chemical energy by photosynthesis.

That chemical energy is stored in the organic nutrients the plant makes. When an animal eats the plant, energy is transferred to the animal; when a carnivore eats that animal, energy is transferred again. Energy therefore flows through the living world in one direction: Sun -> producer -> consumer -> consumer.

Energy is not recycled the way nutrients are. At every step some energy is transferred to the environment, mainly as heat. Energy that leaves a food chain as heat can no longer be used by living organisms, so the flow is one-way and the total energy available shrinks at each stage.

Food chains, food webs and trophic levels

A food chain shows the transfer of energy from one organism to the next, beginning with a producer. A producer is an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis. A consumer gets its energy by feeding on other organisms.

Consumers are named by their position: primary (eats the producer), secondary, tertiary and quaternary. A herbivore is an animal that gets its energy by eating plants; a carnivore eats other animals; an omnivore eats both. A decomposer gets its energy from dead or waste organic material.

A food web is a network of interconnected food chains. In a real habitat most organisms eat more than one kind of food, so the chains join up. A trophic level is the position of an organism in a food chain, food web or ecological pyramid: producers, primary consumers, secondary consumers, tertiary consumers and quaternary consumers.

Pyramids of numbers, biomass and energy

An ecological pyramid shows the trophic levels of a food chain as a stack of bars. A pyramid of numbers shows how many organisms are at each level; a pyramid of biomass shows the mass of living material at each level; a pyramid of energy shows the energy transferred to each level.

A pyramid of numbers can be misleading, because one large producer can support many consumers: a single oak tree supports thousands of insects, giving a top-heavy, inverted shape. A pyramid of biomass is more useful because it compares the actual mass of living material and takes account of the size of the organisms. A pyramid of energy is the most reliable of all, because it measures the energy actually available at each level, it is never inverted, and it accounts for the energy lost between levels.

Only about 10 per cent of the energy is transferred from one trophic level to the next. The rest is lost: most of it as heat in respiration, and also in movement, in excretory products such as urea, and in parts of organisms that are not eaten, such as bones, roots and fur.

This loss explains why food chains usually have fewer than five trophic levels: after four or five transfers there is too little energy left to support another population. It also explains why it is more energy efficient for humans to eat crop plants than to eat livestock fed on those crops - eating the crops directly avoids the loss at the extra trophic level.

• Pyramid of numbers - a count of organisms; it can be inverted.
• Pyramid of biomass - the mass of living material; it takes account of the size of the organisms.
• Pyramid of energy - the energy at each level; it is never inverted and is the most reliable.
• About 10 per cent of the energy is transferred at each step; the rest is lost as heat, in movement, in excretory products and in uneaten parts.

The carbon cycle

Carbon is constantly recycled between the atmosphere, living organisms and the ground. Carbon enters the living world from the atmosphere as carbon dioxide, and the same carbon dioxide is eventually returned to the atmosphere.

Photosynthesis removes carbon dioxide from the air and turns it into organic compounds in plants. Feeding passes that carbon along food chains, as animals eat plants and are themselves eaten. Respiration in plants, animals and microorganisms releases carbon dioxide back into the atmosphere.

When organisms die, decomposers break down the dead material and waste, releasing carbon dioxide as they respire. Over millions of years, some dead organisms are buried and slowly converted into fossil fuels. When humans combust (burn) those fossil fuels, the stored carbon is released back into the atmosphere as carbon dioxide.

What happens when a species is removed from a food web

The populations in a food web are interdependent: each one depends on the others for food, and each is controlled by the others as a predator. Removing a species therefore has a knock-on effect that spreads through the whole community.

If a top predator is removed, the population of the species it fed on usually increases, because there is less predation. That larger population then eats more of the organisms below it, so their numbers decrease. Those organisms may in turn stop controlling the population beneath them, and so on down the food web.

The effect is not always in one direction. Removing a species also removes a food source for whatever fed on it, so those populations may fall. Humans cause these changes when they overharvest a food species or introduce a foreign species to a habitat.

• The prey of the removed species: population increases, because there is less predation.
• The organisms eaten by that prey: population decreases, because there is more grazing or predation.
• The species that fed on the removed species: population decreases, because a food source has been lost.
• The whole community changes, because a food web is a network of interdependent populations.

Investigating a habitat: quadrats and transects

You cannot usually count every organism in a habitat, so you sample it. A quadrat is a square frame, often 0.5 m x 0.5 m, placed on the ground; you count the individuals of the chosen species inside it. To avoid bias, the positions must be chosen randomly - divide the area into a grid and use a random number generator to pick the coordinates of each quadrat.

Place many quadrats, count the individuals in each, and work out the mean number per quadrat. Then scale up: estimated population = mean number per quadrat x (total area of the habitat / area of one quadrat). Repeating the sampling and using the mean reduces the effect of chance.

A transect is a line across the habitat with quadrats placed at regular intervals along it. It shows how the distribution of a species changes from one part of the habitat to another - for example from the shore of a pond to the middle, or from the shade of a tree to open ground. Quadrats estimate population size; transects show distribution.