Responses to the Environment: Study Guide (Biology 8.1)

Living things keep track of their surroundings

Unit 8 is about ecology: how organisms interact with each other and with their environment. It starts with the simplest interaction of all. Something changes, and the organism responds.

A stimulus is any change an organism can detect. It can come from outside (light, temperature, a predator’s smell) or from inside (low blood sugar, water loss, a rise in a hormone). The organism’s response can take two forms:

• A behavioural response is something the organism does: it moves, calls, hides, feeds or courts.
• A physiological response is a change inside the body: heart rate rises, a plant opens or closes its stomata, a plant starts to flower.

Responses are not random. Over many generations, natural selection has favoured responses that tend to keep the organism alive and help it reproduce in the conditions it usually meets.

Responses to the external and internal environment

Change detected	Example response	Type	How it helps
Length of the night changes with the season	many plants flower only at a certain time of year (photoperiodism)	physiological	flowers open when pollinators and good weather are likely
Light comes from one side	a shoot grows toward the light (phototropism)	physiological (growth)	leaves catch more light for photosynthesis
Day and night	some desert rodents feed only at night (nocturnal); many birds are active by day (diurnal)	behavioural	avoids heat and water loss, or matches when food and safety are best
A predator appears	heart rate and breathing rise, glucose is released into the blood (fight-or-flight)	physiological	muscles get fuel and oxygen for escape
Water loss from the body (internal)	the animal seeks water or a damp place	behavioural	restores water balance
A caterpillar chews a leaf	the plant makes defensive chemicals and releases airborne chemicals	physiological	deters the herbivore and can attract its predators

Notice the pattern in every row. Name the stimulus, name the response, then say how it raises survival or reproduction. That three-step chain is what exam questions reward. You do not need the cell-level mechanism behind any of these responses.

Photoperiodism: a closer look

In many plants, flowering is controlled by the length of the uninterrupted dark period. In a short-day plant, flowering happens only when the night is longer than a critical length. If a long night is interrupted by a short flash of light, the plant does not flower, even though the total darkness hardly changed. This tells you the plant is measuring continuous darkness, not daylight. Experiments like this are a good test of your control-setting skills (see Practice Question 4).

Taxis and kinesis: two ways to end up in the right place

Small animals often move toward good conditions or away from bad ones. They can do this in two different ways.

• Taxis is movement directed toward a stimulus (positive taxis) or away from it (negative taxis). The animal’s path points along the stimulus. Example: an animal that turns and moves straight away from a light source shows negative phototaxis.
• Kinesis is a change in the speed of movement or the rate of turning in response to how strong a stimulus is. The movement is not aimed anywhere. Example: an animal that moves fast and turns rarely in dry air, but slows down and turns often in damp air, will spend more time in damp places. Over time, the animals collect there.

How to tell them apart. A simple count at the end (“most animals were in the dark half”) cannot distinguish taxis from kinesis, because both give the same end result. You need to track individual paths: a straight, directed path suggests taxis; changes in speed or turning with no consistent direction suggest kinesis.

Innate and learned behaviour

• Innate behaviour is inherited. It is performed correctly the first time, without practice, by every normal member of the species. Examples: a spider spinning its first web; a newborn mammal suckling.
• Learned behaviour is changed by experience. Examples: young geese follow the first large moving object they see after hatching (imprinting); in many songbirds, young males learn their song by listening to adults; animals stop responding to a harmless, repeated stimulus, such as birds that ignore a scarecrow after a few days (habituation).

Many behaviours mix both: a songbird inherits the tendency to sing, but learns the details. What matters for this topic is that natural selection favours both innate and learned behaviours when they increase survival and reproductive success. Innate behaviour is reliable when there is no time to learn (escaping a predator on the first meeting). Learning lets an animal adjust to local conditions.

Communication: signals that change another organism’s behaviour

Organisms do not only respond to the physical world. They also exchange information with each other. A signal is a behaviour or structure that changes the behaviour of another organism.

Signal type	Examples	What it achieves
Visual	bright warning colours on a poisonous frog; colourful flowers; a courtship display; firefly flashes	warns predators; attracts pollinators; attracts mates
Audible (sound)	bird song; alarm calls when a predator appears	defends territory and attracts mates; warns others
Tactile (touch)	grooming in primates; honeybees following a dancer in the dark hive	builds social bonds; passes on food information
Electrical	weak electric pulses made by some fish	finding mates and recognising others in murky water
Chemical	scent marks at a territory boundary; ant trail chemicals; airborne chemicals from damaged plants	marks territory; leads others to food; summons the herbivore’s enemies

Animals use signals to show dominance, to find food (a honeybee’s waggle dance tells nest-mates the direction and distance of a food source), to establish territory and to reproduce successfully (courtship and mating displays). Plants signal too: flowers advertise nectar to pollinators, and some plants under attack release chemicals that attract predators or parasites of the herbivore.

You do not need the detailed workings of any of these systems. Focus on the consequence: a signal changes the receiver’s behaviour, and that can change who survives and who reproduces.

Behaviour, fitness and natural selection

Fitness is an organism’s success in surviving and passing on its alleles, measured by the number of offspring that survive to reproduce. Behaviour affects fitness directly:

• A male whose song attracts more females fathers more offspring. If the song is partly heritable, it becomes more common. This is differential reproductive success caused by signalling.
• An animal that responds quickly to an alarm call is more likely to escape.
• Parents that feed and protect their young raise more surviving offspring.

Cooperation

Cooperative behaviour is behaviour that benefits other individuals as well as (or sometimes instead of) the actor. Examples:

• Pack hunting lets a group bring down prey that a single animal could not.
• Herds, flocks and schools: each individual has a lower chance of being the one caught, and more eyes watch for predators.
• Colonies of social insects such as bees, ants and termites share food, defence and care of young.
• Kin selection: helping close relatives can spread alleles you share with them, even if it costs you. An alarm call that draws a predator’s attention can still be favoured if the animals it saves are relatives.

In general, cooperative behaviour tends to raise the fitness of the individual and the survival of the population. Worked example 2 tests this with numbers.

Worked example 1: designing and analysing a choice-chamber experiment

Question. A student tests whether freshwater flatworms prefer dark or light conditions. She covers one half of a shallow dish with black card, leaving the other half lit by a lamp held above it. She places 40 flatworms along the centre line and counts how many are in each half after 15 minutes. As a control, she sets up a second dish in the same way but covers both halves.

Dish	Covered half (dark)	Uncovered half	Total
Experimental (one half lit)	29	11	40
Control (both halves covered)	22 (half A)	18 (half B)	40

(a) Identify the independent and dependent variables and three controlled variables. (b) Justify the control dish. (c) Use chi-square to test the null hypothesis for each dish. (d) Can she say the flatworms show taxis?

(a) Variables.

1. Independent variable: light condition (lit or dark half of the dish).
2. Dependent variable: number of flatworms in each half after 15 minutes.
3. Controlled variables (any three): water temperature, water depth, the source and size of the flatworms, starting position (centre line), time before counting, and the dish shape and material. A heat filter or distant lamp keeps the lit half from also being warmer.

(b) Why the control dish? If the flatworms gathered in one half for a reason unrelated to light (a slope in the bench, a draught, a vibration from one side), the experimental dish would mislead. The control dish has the same apparatus with no light difference. If animals spread evenly there, any uneven spread in the experimental dish can be put down to light.

(c) Chi-square.

1. Null hypothesis: the flatworms have no preference, so they are equally likely to be in either half. Expected count = 40 ÷ 2 = 20 per half.
2. Experimental dish: χ² = (29 − 20)² ÷ 20 + (11 − 20)² ÷ 20 = 81 ÷ 20 + 81 ÷ 20 = 4.05 + 4.05 = 8.1.
3. Degrees of freedom = 2 categories − 1 = 1. Critical value at p = 0.05 is 3.84.
4. 8.1 > 3.84, so reject the null hypothesis. The uneven spread (72.5% in the dark half) is unlikely to be chance alone.
5. Control dish: χ² = (22 − 20)² ÷ 20 + (18 − 20)² ÷ 20 = 0.2 + 0.2 = 0.4. 0.4 < 3.84, so fail to reject: no evidence of a side bias in the apparatus.

(d) Taxis? Not yet. The counts show the flatworms end up in the dark, but both taxis and kinesis would give that result (Figure 1). To claim negative phototaxis she would need to track individual paths and show that animals move directly away from the light. Also, one trial of 40 animals supports the conclusion for these conditions; repeating with fresh animals would make it stronger.

Worked example 2: does cooperation pay?

Question. Field biologists followed packs of a fictional pack-hunting carnivore. Each pack made about 10 hunts a week, and the average kill weighed 30 kg. They also counted the pups that survived to one year.

Pack size (adults)	Hunt success rate	Pups surviving to 1 year (per pack)
2	20%	1.0
4	50%	3.2
6	70%	4.4
8	80%	4.8

(a) Calculate the food available per pack member each week for each pack size. (b) Calculate surviving pups per adult. (c) Explain what the data suggest about cooperative hunting and fitness, and state one limitation.

(a) Food per member.

1. Kills per week = 10 hunts × success rate. Pack of 2: 10 × 0.20 = 2 kills.
2. Food per week = kills × 30 kg. Pack of 2: 2 × 30 = 60 kg.
3. Food per member = 60 ÷ 2 = 30 kg.
4. The same steps for the others: pack of 4: 5 kills, 150 kg, 37.5 kg each; pack of 6: 7 kills, 210 kg, 35 kg each; pack of 8: 8 kills, 240 kg, 30 kg each.

(b) Pups per adult. 1.0 ÷ 2 = 0.50; 3.2 ÷ 4 = 0.80; 4.4 ÷ 6 ≈ 0.73; 4.8 ÷ 8 = 0.60.

(c) Interpretation. Moving from 2 to 4 adults raises food per member by 25% (30 to 37.5 kg) and pups per adult from 0.50 to 0.80. Cooperation lets the pack catch prey more reliably, so each individual eats better and raises more young: cooperation increases individual fitness. More surviving pups also help the population persist. Above about 4 to 6 adults, the gains level off: each extra hunter adds less success but another mouth to feed, so food per member falls back to 30 kg in a pack of 8. Larger packs still raise the most pups in total (4.8), perhaps because more adults guard the den.

Limitation. This is an observational study. Larger packs might also hold better territories with more prey, a confounding variable. The data show a correlation between pack size and success, which supports, but does not prove, that cooperation causes it.

Common misconceptions

• “Animals behave in a certain way because they want to help the species.” Behaviour spreads when it raises the fitness of the individuals (or their relatives) that perform it. Population survival is a consequence, not a goal.
• “Taxis and kinesis are the same because animals end up in the same place.” Taxis is directed; kinesis changes speed or turning. You need path data to tell them apart.
• “Learned behaviour is not affected by natural selection.” The ability to learn, and what an animal is able to learn, is inherited and selected.
• “Plants cannot respond to their environment.” Plants respond with growth, flowering and chemical changes, and they can signal to other organisms.
• “A control is just the condition with nothing added.” A good control keeps everything the same except the factor being tested, so it rules out other explanations (such as a side bias in a choice chamber).
• “Rejecting the null hypothesis proves my hypothesis.” It shows the result is unlikely to be chance alone. It supports the hypothesis for those conditions.
• “I need to learn the hormone and nerve pathways for each behaviour.” Specific mechanisms are outside the scope of the exam. Link stimulus, response and fitness instead.

Where this leads

Responses to the environment feed directly into energy use: endotherms and ectotherms respond to temperature in very different ways, and reproduction depends on energy available. That is the next topic, Energy Flow Through Ecosystems. Before moving on, test yourself with the practice questions, then use the revision notes and the checklist.