Biogeochemical cycles — EST 2 Biology Revision Notes & Topical Questions

An ecosystem is a community of organisms plus its abiotic environment. Energy enters through producers, and about 10% of the energy at one trophic level passes to the next; the rest is lost as heat, in waste and in movement. This is why food chains are short.

Scavengers feed on dead animals. Detritivores, such as earthworms, ingest dead organic matter. Saprotrophs, including many fungi and bacteria, release enzymes onto dead matter and absorb the smaller molecules after external digestion.

The water, carbon, nitrogen and phosphorus cycles each move elements between organisms and the environment.

In each case state the effect on both organisms to identify the interaction.

Innate behaviour is inherited; learned behaviour comes from experience. Biological clocks keep internal timing even without cues but are reset by light. Photoperiod triggers seasonal breeding and migration. In exam answers for explain questions, give cause-and-effect reasoning, because restating a fact does not score.

Worked questions follow in the exampleshabituation, fixed action patterns, operant conditioning, the waggle dance, kin selection and kinesis.

Carbon moves from the air into producers by photosynthesis, passes to consumers by feeding, and returns to the air as carbon dioxide through respiration by all organisms and decomposers.

Water moves between sea, air and land by evaporation, condensation and precipitation, and is taken up by plants and returned to the atmosphere by transpiration. In a multiple-choice paper, eliminate options that are true but do not answer the question, and watch for NOT and EXCEPT.

Practice linking trophic levels with recycling.

When you see a question about an interaction, write the effect on both species using plus, minus or no effect; a tick on a mammal is plus for the tick and minus for the mammal, which is parasitism. Also remember that behaviours such as the waggle dance, imprinting or habituation are examples of how information from the environment changes what animals do, and those that raise survival become more common.

Organisms respond to environmental changes to maintain homeostasis and improve survival, using behavioural and physiological mechanisms triggered by internal and external cues.

Organisms exchange information in response to internal changes and external cues, changing behaviour.

Communication and responses to information are vital to natural selection and evolution. Innate and learned behaviours that increase survival and reproductive success are favoured by fitness, so they influence the overall success of the population.

Organisms use energy to organise, grow, reproduce and maintain homeostasis. Balancing energy use against energy loss directly affects survival and reproductive success, and organisms use different strategies.

Energy transfer is shown by food chains, food webs and trophic pyramids. In a food chain, arrows show how chemical energy first captured by the primary producer, such as grass, passes to other organisms in the community.

Arrows point in the direction energy flows, from the eaten to the eater. Energy is lost at each step, so pyramids narrow upward.

Energy enters as light, passes along the chain and leaves as heat. Mineral nutrients are returned by decomposers and reused by producers. Both ideas are needed to explain how ecosystems persist.

Exam answers about behaviour should link a stimulus to a response and then to survival. For example, a rise in adrenaline raises heart rate and prepares muscles, so the animal can flee a predator and survive to reproduce. Behaviour that improves survival and reproduction is favoured by natural selection, so it becomes more common in the population.

For energy questions, say where the energy comes from and where it goes.

Strategies such as storing energy for a second year, as in sugar beet, or delaying breeding during migration, as in monarch butterflies, show that organisms balance the energy spent on growth, maintenance and reproduction. Each strategy improves the chance that offspring are produced at the best time.

A short summarycues trigger responses, responses affect survival, survival affects which alleles are passed on, and energy limits what each organism can do.