NZ NCEA Level 3 Biology

## What a response is A **response** is a change in an organism's physiology, growth or behaviour that follows a change in its surroundings. The change that sets it off is a **stimulus**, and a stimulus is useful to an organism only if it carries information about something that matters for survival or reproduction: where food is, whether a predator is near, whether the season is about to turn. Responses fall into four groups, set out in the table at the end. Plants respond mostly by growing or by changing the water pressure in particular cells, because they cannot move from place to place. Animals respond mostly by behaviour, using a nervous system and hormones to link receptors to muscles. Both groups also show responses that follow the clock and calendar rather than a stimulus that is present at that moment, so a response can be triggered by a cue that predicts a later change (shortening days predict winter) and not by the change itself. ## External environment The external environment has two parts, and a good explanation usually names both. * **Abiotic factors** are the non-living features: light intensity and day length, temperature, water availability, gravity, wind, tides, salinity, pH, and the Earth's magnetic field. * **Biotic factors** are the other living things: members of the same species (intraspecific) and members of other species (interspecific), which can be competitors, prey, predators, hosts, parasites or partners. A single response can answer both kinds…