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 of factor. A kauri snail that spends the day under leaf litter is responding to light and dryness, which are abiotic, and is also avoiding predators, which are biotic.

Niche

An organism's ecological niche is the set of conditions and resources it uses and the role it plays in its community: what it eats, when and where it feeds, where it shelters and breeds, what it tolerates and what eats it. Two species cannot share exactly the same niche in the same place for long, because one would be the better competitor and the other would decline. The part of the niche an organism actually occupies, given competitors and predators, is narrower than the range of conditions it could tolerate alone.

Responses fix an organism to its niche. A plant whose seeds germinate only after a winter cold period places its seedlings in spring, when the soil is warm enough and the competition from adult plants is lowest. A nocturnal gecko feeds when its insect prey is active and when the temperature of its rock shelter is still tolerable, and it avoids the birds that hunt by day. The behaviour is part of how the species fits its niche, not an addition to it.

Selective advantage

A response gives a selective advantage when individuals that show it leave more surviving offspring than individuals that do not, and when the difference in the response is at least partly inherited. Over generations, the alleles that produce the response become more common. An explanation of any response therefore has two halves, and both are needed.

1. The proximate explanation says how the response occurs: the receptor, the signal (a hormone, a nerve impulse, a change in ion concentration) and the effector that carries it out.
2. The ultimate explanation says why the response exists: what it does for survival or reproduction in the organism's niche, and why individuals without it would do worse.

For example, the proximate explanation for a shoot bending towards a window is that auxin moves to the shaded side and makes those cells elongate faster. The ultimate explanation is that a plant whose leaves reach more light photosynthesises more, grows faster and sets more seed than a plant that does not bend.

Costs and trade-offs

A response is selected when its benefits outweigh its costs, and the costs are real. Migration exposes birds to storms and predators and uses a large share of their body fat. Territorial defence uses energy and risks injury. Night activity avoids daytime predators but means hunting in low light and, for an ectotherm, in the cold. Stating a cost next to the benefit, and explaining why the benefit usually outweighs it in that niche, makes an explanation much stronger than a list of advantages.

Linking ideas

Responses are rarely separate. A bird that migrates is responding to day length (orientation in time), finds its way by the sun and the magnetic field (orientation in space), and arrives to find breeding sites that are contested (intraspecific relationships) and food that other species are also eating (interspecific relationships). An answer that follows the same example through more than one of these is linking ideas, and it is the strongest way to show why a response gives an advantage.

The four groups of responses