Species solve the problem of staying alive in two very different ways. Some flood the world with offspring and abandon them. Others have one at a time and look after it for years. Neither is better — each suits a different kind of environment, and each produces a recognisable shape on a survivorship graph.
📚 What you need to know
r-strategists produce many offspring with little or no parental care, and suit unstable environments.
K-strategists produce few offspring with heavy parental care, and suit stable environments.
r-strategists colonise new or disturbed habitats first; K-strategists dominate mature ones.
A survivorship curve plots the number of survivors from a starting group against age.
Type I: most survive to old age. Type II: constant death rate. Type III: huge early losses.
Human activities such as climate change, habitat destruction and pollution disrupt life cycles.
Two strategies, two environments
The logic is simple once you see it. In an unpredictable place — a puddle, a fresh landslide, a ploughed field — the winning move is to breed fast and get lots of offspring out before conditions change. In a crowded, stable place, offspring have to compete from birth, so it pays to produce a few well-prepared ones instead.
Treat this as a sliding scale rather than two boxes. A rabbit is more r than an elephant but far more K than a housefly.
Feature
r-strategist
K-strategist
Number of offspring
Very many
Few
Parental care
Little or none
High, often for years
Lifespan
Short
Long
Population pattern
Fluctuates rapidly; often a J-curve
Stable near carrying capacity; an S-curve
Best suited to
Unstable, unpredictable habitats
Stable, mature habitats
Role in succession
Early colonisers of bare ground
Dominant in the later, mature stages
Succession link worth remembering: after a fire or a landslide, r-strategists arrive first because they get there fast and breed fast. As conditions become stable and competition increases, K-strategists take over.
Survivorship curves
A survivorship curve answers one question: out of a group born at the same time, how many are still alive at each age? The y-axis is usually a log scale, which is what makes the three shapes so easy to tell apart.
Read the steep part of each curve. Where the line falls fastest is the stage of life at which most individuals are dying.
Curve
Shape
What it means
Examples
Type I
Flat, then a sharp fall at the end
Very low death rate when young; most individuals reach old age
Humans, elephants, large mammals
Type II
Straight diagonal line
The same proportion dies in every age band
Many small birds, some lizards, hydra
Type III
Steep drop, then almost flat
Enormous losses among the young; the few that survive live a long time
Oysters, most fish, oak trees
How humans disrupt life cycles
Life cycles are usually timed to the seasons. Anything that shifts the timing, or removes the place a species needs at one stage of its life, causes trouble — even if the species itself is untouched.
Human impact
Effect on the life cycle
Example
Climate change
Warmer springs shift flowering, hatching and migration, so events that used to line up no longer match.
Caterpillars hatch earlier, so migrating birds arrive after the food peak has passed and raise fewer chicks.
Habitat destruction
Removes the specific site a species needs for one stage of its life.
Developed beaches leave sea turtles nowhere to nest; drained wetlands leave frogs nowhere to breed.
Pollution
Damages health directly or removes the food supply.
Pesticides reduce insect numbers, cutting food for birds and bats and harming bee foraging and reproduction.
The idea examiners look for: many life cycles are synchronised with the seasons. Climate change shifts different species by different amounts, which breaks the synchrony — and that mismatch, not the temperature itself, is what causes the population to fall.
Worked examples
WORKED EXAMPLE
Reading a survivorship curve
A survivorship curve for a fish species starts at 10 000 eggs. Only 40 individuals are alive at 10% of maximum lifespan, but 25 are still alive at 80%. Identify the curve type and explain what it tells you about the species’ strategy. [4]
Step 1: describe the shape10 000 down to 40 is a huge early loss, then almost flat to 25Step 2: name the typeType IIIStep 3: link to strategyhuge numbers of offspring with no parental care, so most die as eggs or youngan r-strategist: survival depends on numbers, not on carequote the numbers from the question — that is where the description marks are
WORKED EXAMPLE
Explaining a timing mismatch
Explain how warmer springs could reduce the breeding success of a migratory bird, even though the birds themselves are not harmed by the warmth. [4]
Step 1: what the bird depends onit times its migration to arrive when caterpillar numbers peakStep 2: what warming changeswarmer springs make caterpillars hatch earlier than usualStep 3: why the bird cannot followmigration is triggered by day length, which does not change with temperatureStep 4: the consequencethe chicks hatch after the food peak, so fewer survive and breeding success fallsthe mark is for the mismatch in timing, not for “it gets too hot”
💡 Exam tip
Give at least two contrasting features when comparing r and K strategies, and use the word “whereas”.
Link the strategy to the environment: unstable for r, stable for K. That link is usually a mark.
Check the y-axis before describing a survivorship curve — a log scale changes what a straight line means.
Type I with K-strategists, Type III with r-strategists. Say the pairing explicitly.
For human impact questions, name the impact, the life-cycle stage affected and the population consequence.
Have one named example ready for each of climate change, habitat loss and pollution.
⚠ Common mix-up
Thinking K-strategists are always big animals. Oak trees are K-strategists too; the strategy is about offspring, not size.
Calling r-strategists badly adapted. They are extremely well adapted — to unpredictable conditions.
Mixing up Type I and Type III. Type I stays high for most of life; Type III collapses immediately.
Describing a log axis as if it were linear. Equal drops on the graph mean equal proportions, not equal numbers.
Saying climate change harms species only by overheating them. Broken timing is often the bigger problem.
Treating r and K as two fixed boxes. It is a continuum, and species sit at different points along it.
Up next: Energy and Biomass in Ecosystems — having covered who lives where and how many there are, the next topic follows the energy that keeps them all going.
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