Evolution, extinction and biodiversity · System
When the environment changes: extinction
More than nine in ten of all the species that have ever lived are gone. Extinction is not a failure of the system — it is the system, and the question worth asking is what makes a species vulnerable to it.
Start here
Two species meet the same change. One survives it.
A woodland is cleared to half its size. The wood mice are fine within a few years. The dormice, living in the same wood, eating from the same trees, do not come back. Both faced exactly the same event.
What is most likely to decide which species survives a change?
How quickly it can produce a new generation with the variation the new conditions favour. A wood mouse breeds several times a year and eats almost anything; a dormouse has one small litter, eats a narrow range of foods and will not cross open ground. Natural selection needs generations and variation to work with, and the dormouse has less of both.
At the bench · four species, five pressures
Who survives what
1 combination tried
A species can be tough against one pressure and helpless against another.
The species
What happens
Brown rat
- DietAnything at all
- Breeding rateUp to five litters a year
- RangeEvery continent except Antarctica
- Genetic variationVery high
Hazel dormouse
- DietHazelnuts, flowers and insects, in a strict seasonal order
- Breeding rateOne small litter a year
- RangeFragments of southern English woodland
- Genetic variationLow, and falling as populations are cut off
Giant panda
- DietBamboo, almost exclusively
- Breeding rateOne cub every two years at best
- RangeMountain forest in central China
- Genetic variationLow
Herring gull
- DietFish, waste, chips, almost anything
- Breeding rateA clutch a year, long-lived
- RangeCoasts and increasingly towns across the northern hemisphere
- Genetic variationHigh
Half the habitat is cleared
The commonest cause of extinction today. What remains is also broken into fragments that populations cannot move between.
The climate warms by two degrees
Seasons shift, so food appears earlier or later than the species expects, and the suitable zone moves — usually polewards or uphill.
A predator arrives that was never here before
Introduced by people, usually. The local species has no evolved defence against it, because nothing like it has ever hunted them.
A disease sweeps through
Survival depends on whether anyone in the population happens to carry resistance — which is a question about genetic variation, not about strength.
The species is hunted or collected
Direct removal by people, for food, for trade or as a pest. The only pressure on this bench that can be switched off by a decision.
Population after fifty years
85% of the original population
Rats eat anything and live anywhere, including places people build. Half the woodland gone is barely an inconvenience — they move into the other half and into the town beside it.
Population after fifty years
80% of the original population
A generalist with a huge range simply shifts. There is nowhere it currently lives that it could not leave.
Population after fifty years
70% of the original population
High numbers, fast breeding and no fussiness. A new predator takes a lot of rats and the population replaces them within a season.
Population after fifty years
65% of the original population
A large, varied population almost certainly contains resistant individuals. Numbers crash and then recover from the survivors — which is natural selection doing exactly what the Natural selection lesson described.
Population after fifty years
75% of the original population
People have been trying to eliminate rats for centuries with poison, traps and dogs, and have never managed it anywhere they have not first eliminated the food and shelter.
Population after fifty years
15% of the original population
The worst case on the bench. Dormice will not cross open ground, so clearing half the wood does not halve the population — it strands the remainder in fragments too small to be viable, each one slowly losing variation.
Population after fifty years
30% of the original population
A warmer winter interrupts hibernation, which burns fat reserves the animal cannot replace, and shifting seasons break the strict order of foods it depends on.
Population after fifty years
45% of the original population
Nocturnal and arboreal, so it avoids some new predators — but with one litter a year it replaces losses very slowly.
Population after fifty years
35% of the original population
Small, isolated populations with low variation are unlikely to contain resistant individuals, and there is no route for survivors to recolonise from elsewhere.
Population after fifty years
80% of the original population
Not hunted, and legally protected. This is the one pressure it is safe from — which is why the population is still falling for the other four reasons.
Population after fifty years
20% of the original population
Bamboo forest cleared for farmland and roads, and pandas will not cross the gaps. The remaining populations are isolated from each other, which is a genetic problem as well as a spatial one.
Population after fifty years
25% of the original population
Bamboo is slow to move and much of the suitable zone would shift uphill beyond where the mountains stop. A specialist can only follow its food if the food can move.
Population after fifty years
70% of the original population
A large adult panda has essentially no natural predators. This is the one pressure it handles well, and it explains nothing about why the species is in trouble.
Population after fifty years
40% of the original population
Low genetic variation across a small isolated population is the textbook setup for a disease outbreak with no resistant survivors.
Population after fifty years
35% of the original population
Hunting was a serious historical cause and is now heavily suppressed by protection — an example of the one pressure a decision can actually remove.
Population after fifty years
80% of the original population
Gulls have responded to habitat loss by moving into towns, where the food is abundant and the roofs make excellent cliffs. Generalists convert a problem into an opportunity.
Population after fifty years
70% of the original population
A wide range and a varied diet mean a warming climate shifts where they are rather than whether they are.
Population after fifty years
75% of the original population
Large, aggressive, colonial and quite capable of driving off most things. Not a species that struggles with predators.
Population after fifty years
60% of the original population
Dense colonies spread disease efficiently, which pulls the score down — but the population is large and varied enough to contain resistance.
Population after fifty years
65% of the original population
Legally protected in the UK and widely disliked; culls are local and the population is not limited by them.
What makes a species vulnerable
Four properties, and the same four every time.
Risk factor one
Specialist diet or habitat
A species that needs one food or one habitat has nothing to fall back on when that thing changes. Generalists survive changes that wipe out specialists in the same place.
Risk factor two
Slow reproduction
Natural selection works through generations. A species producing one offspring every two years gets very few attempts before conditions change again.
Risk factor three
Small or fragmented range
A species in one valley can be wiped out by one event. Fragmenting a range also stops populations mixing, so each fragment loses variation separately.
Risk factor four
Low genetic variation
The most important and the least visible. Selection can only work with variation that already exists — a population that has lost it has nothing to select from.
Key fact
A species becomes extinct when the environment changes faster than it can adapt, or in ways its existing variation cannot cope with. Specialists, slow breeders, species with small ranges and populations with little genetic variation are the most vulnerable. Extinction is permanent.
Think again
“Extinction is unnatural — it only happens because of us.”
Over ninety-nine per cent of all species that have ever existed are extinct, and almost all died out long before there were any humans. There have been five mass extinctions in the fossil record; the largest, at the end of the Permian, removed something like nine tenths of marine species, and the most famous ended the non-bird dinosaurs sixty-six million years ago after an asteroid impact. Species go extinct in ordinary times too, at a slow background rate, simply because environments change. None of that lets anybody off. What is different now is the rate: current extinction rates are estimated at tens to hundreds of times the background rate, driven by habitat loss, introduced species, hunting and a changing climate. The argument for acting is not that extinction is unnatural — it is that we have made it happen far faster than it otherwise would, which is a claim you can check with numbers.
“If a species goes extinct, another one just takes its place.”
Something usually does move into the space, eventually, and eventually is doing a great deal of work in that sentence. After each mass extinction the recovery of diversity took millions of years — not centuries. In the meantime the ecosystem runs without whatever the missing species was doing, and as Disturbing a food web showed, the effects reach species that had no obvious connection to it. There is also the matter of what is lost: an extinct species takes with it a genetic combination that took millions of years to assemble and that nothing else has. A great many of our medicines began as compounds found in living things — aspirin from willow bark, penicillin from a mould, the anticancer drug vincristine from the Madagascar periwinkle — and the ones in a species that dies out unstudied are not recoverable. De-extinction projects exist and are technically interesting; none has restored a functioning population of a lost species, and none is a reason to be relaxed about losing one.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Read the risk
Which of these makes a species most likely to become extinct when its environment changes?
Rung 2 · The one that catches people
Is extinction a natural process?
Rung 3 · Explain the difference
A brown rat and a hazel dormouse live in the same wood, and half of it is cleared. Explain why the rat population recovers and the dormouse population may not, referring to at least three of the four risk factors.
Rung 4 · Take it somewhere new
A conservation team can do one of three things for an endangered woodland species: increase the size of the reserve, plant hedgerows linking the reserve to another population twenty miles away, or start a captive breeding programme. Argue for one, using what you know about the risk factors, and say what the other two would and would not fix.
Key note
Environments change, and a species survives only if its existing variation allows some individuals to cope and it can produce new generations fast enough. Specialists, slow breeders, small ranges and low genetic variation all raise the risk. Extinction has happened throughout Earth's history, including five mass extinctions; what is different now is how fast it is happening and why.
Going further
Kakapo are large flightless parrots from New Zealand, and every one of their vulnerabilities is a feature that used to be an advantage. New Zealand had no land mammals, so there was nothing to run from: flight was an unnecessary expense and the kakapo gave it up, freezing when threatened instead — a superb defence against a hunting eagle that looks for movement, and a fatal one against a stoat that hunts by smell. They breed only in years when a particular tree fruits heavily, which can be two to four years apart, and lay very few eggs. Every one of those traits was well fitted to New Zealand before humans arrived with rats, cats and stoats. The population fell to 51 birds. Every living kakapo is now individually named, radio-tagged and monitored, and the population is slowly climbing — which tells you both that recovery is possible and roughly what it costs.
Before this lesson
Connects to
At GCSE this becomes
- Causes of extinction, the fossil record as evidence, and the biological and economic arguments for maintaining biodiversity.
Where to next
Ask Mr Badmus AI
Want to work out which pressures a species you know would survive?
The four species are real and their traits are accurately described; the fifty-year population figures are illustrative outcomes for a single pressure applied in isolation, not projections. Real extinctions almost always involve several pressures at once, which is one reason they are hard to predict.
Lesson content © MrBadmusAI.