Invasive alien species at Higher Level: The same statement as SL, with different examples: Guam's brown tree snake, Hawaii's birds, Japanese knotweed. At HL, expect to weigh management strategies, and the laws behind them, in a [7] or [9].
Practise this as you read
- Name the mechanism in every point: competition, predation or disease.
- Weigh each strategy: what it achieves and what can go wrong.
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Moved by people, then out of control: An alien species lives outside its natural range because people moved it. Most never spread. The ones that do, and harm native species, are invasive.
The points to remember
- An alien (non-native) species is one moved by people outside its natural range.
- It becomes invasive when it spreads fast and harms native species, ecosystems or people.
- Deliberate arrivals: hunting, farming, fur farms, pets, garden plants, pest control.
- Accidental arrivals: ballast water, stowaways in cargo, seeds in grain, hitchhikers on travellers.
- People bring them, the species do the damage: an indirect human impact.
Remember it as: People carry them in; the invader does the damage.
On purpose
- Grey squirrels: released in parks in England from 1876, as a curiosity.
- Japanese knotweed: sold to British gardens from the 1850s.
By accident
- Brown tree snakes: stowaways in military cargo reaching the Pacific island of Guam after 1945.
- Mosquitoes: carried to Hawaii in ships' water barrels in 1826.
Other routes to know: Trade, travel, the pet trade and farming move species every day: aquarium fish released into rivers, seeds stuck to boots, insects in timber.
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Most alien species die out or stay rare. An invader is different: its numbers grow faster and faster, an exponential increase.
The points to remember
- No natural predators, diseases or parasites in the new place: nothing keeps numbers down.
- Plenty of resources and few competitors, so food and space do not run out at first.
- They breed fast: many young, early breeding, several litters or broods a year.
- They are generalists: they eat many foods and tolerate a wide range of conditions.
- Native prey have no defences against a new predator, and native plants none against a new grazer.
- So numbers grow exponentially (a J-shaped curve) until limiting factors finally catch up.
Remember it as: No enemies, lots of food, fast breeding: boom.
No enemies
- Pine martens, their main predator, had been hunted almost out of England.
Food
- Greys digest acorns well, a big food supply red squirrels cannot use as easily.
Breeding
- Two litters a year, and they live happily in parks and gardens as well as woods.
Boom
- From a few released from 1876 to about 2.7 million today.
Why the growth stops: Exponential growth cannot go on for ever. Food runs short, disease spreads in crowded populations or predators adapt, so growth slows. By then the damage is usually done.
An invader lowers local biodiversity in three main ways. Learn one real example for each.
The points to remember
- Competition: the invader uses the same limited resources (food, space, nest sites) and wins.
- Predation: it eats native animals that have never met it and have no defence.
- Disease or parasites: it carries them in, often without harm to itself, and natives die.
- Each one lowers native populations; some natives die out locally, so biodiversity falls.
Remember it as: Compete, eat, infect.
Competition
- Zebra mussels in the Great Lakes filter out the plankton native mussels eat, and block pipes.
- They also cover native mussels' shells in thousands, so the natives starve.
Predation
- Brown tree snakes on Guam eat birds, eggs and lizards that evolved with no snakes.
- Ten of Guam's twelve native forest bird species are gone from the wild.
Disease
- Mosquitoes brought to Hawaii spread avian malaria.
- Many native honeycreepers now survive only on cold mountains, above the mosquitoes.
Name the resource: In a competition point, say what they compete for: food, space, light, water or nest sites.
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The damage spreads beyond the species an invader eats or beats. It passes along the food web, and can strip the ground bare.
The points to remember
- An invasive herbivore eats plants: fewer plant species and lower productivity.
- Native herbivores lose food through competition, so carnivores above them lose prey.
- Species diversity falls and the ecosystem may shift towards a tipping point.
- Eating seedlings and digging burrows can change succession, collapse riverbanks and muddy the water.
- Bare ground means soil erosion: no plant cover or roots binding the soil.
- Invasive plants can cause erosion too: hotter fires, heavy water use, changed soil nutrients.
- Some effects are positive: extra food for native predators.
An invasive animal
- Goats on the Galapagos island of Santiago stripped the plants.
- Giant tortoises lost food to them; bare slopes eroded.
- Over 79,000 goats were removed by 2006, and plants returned.
An invasive plant
- Gamba grass.
- It carries far more fuel, so fires burn up to eight times hotter and kill trees.
- Bare, burnt ground then washes away in the wet-season rains.
Other routes from an invader to erosion
- A smaller leaf canopy lets rain hit the soil: more runoff carries it away.
- Changed soil organisms weaken the soil's structure, so it breaks up and erodes.
- Trampling kills roots and compacts the soil, so rain runs off instead of soaking in.
- A disease brought by the invader kills native plants, leaving soil bare.
- Farming an invasive crop with heavy irrigation can wash the topsoil away.
Link every point to erosion: In an erosion answer, finish each point with 'so the soil is exposed and eroded'. 'It damages the soil' is not enough, and removing the invader is not a cause of erosion.
Papers often give a fact file on an invader and ask how it affects the ecosystem, sometimes both positively and negatively. Read it fact by fact.
The points to remember
- Sort each fact into helps or harms native species, then write one linked point per fact.
- Positive: food for native predators, easing pressure on their other prey; eats another invader or a spreading plant.
- Negative: competes for food or nest sites; eats eggs or young; attacks natives; brings disease; natives may die out.
- Shared predators may switch to the invader (fewer natives eaten) or multiply on it (more natives eaten).
- Culling the invader can kill look-alike natives by mistake; publicity can bring conservation help.
- Say what it competes for: 'outcompetes native species' alone is not enough.
- Use only what the figure gives you, and end each point with the change in the native population.
Positive
- Food for native pelicans, cormorants and Murray cod.
Negative
- Uproots water plants, removing habitat for native fish and insects.
- Muddy water blocks light, so fewer water plants photosynthesise.
- Eats native fish eggs, so native fish numbers fall.
Two traps: 'Caught for sport' is not an effect on the ecosystem. And 'it outcompetes native fish' alone is too vague: say what for, such as food or space, or that a species may die out.
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Learn one local example of an invader and the strategy used against it. Stopping an invader early is cheaper than fighting it later.
The points to remember
- Prevention first: check cargo, treat ballast water, ban the sale of risky species.
- Early detection: find new arrivals while numbers are still small.
- Eradication: remove every one, by trapping, culling, pulling up or herbicide.
- Control: keep numbers low for good, by the same methods or biological control.
- Laws back this up, and native habitats are restored afterwards.
Remember it as: Stop it, spot it, remove it, keep it down.
Arrival
Japanese knotweed was sold to British gardens from the 1850s and dumped with garden waste.
Spread
A piece of root the size of a fingernail can grow a new plant. It shades out native plants and cracks paths and walls.
Strategy
The law: since 1981 it is an offence to cause it to grow in the wild. Control: herbicide for 3-5 years, or digging out.
New step
In 2010 a sap-sucking insect from Japan, a psyllid, was released as biological control.
Name it, date it, judge it: Give the species, the place, what was done and how well it worked. Old cases are real cases: say what happened, step by step, and why each step worked.
Biological control uses a natural enemy from the invader's home range. It can work brilliantly, or create a new invader.
The points to remember
- Biological control: releasing a natural enemy (herbivore, predator, disease) of the invader.
- Strength: it targets the invader, so fewer pesticides and herbicides: less pollution and bioaccumulation.
- Strength: cheap and lasting, because the enemy breeds by itself; some are useful to people too (goats give milk).
- Strength: it adds food-web links: food for native predators, and negative feedback keeps the invader low.
- Limit: it may eat native species (fewer plants, nest sites and refuges) or compete with native herbivores.
- Limit: with no predators it may grow exponentially into a pest, spread to farmland or carry disease.
- Limit: it is slow, weather or local predators may kill it, and it needs years of testing first.
It worked
- Giant salvinia choked Lake Moondarra in Queensland.
- A weevil that eats only salvinia was released in 1980; within about a year the lake was clear.
- The myxoma virus (1950) killed over 99% of the rabbits that had overgrazed farms, until rabbits evolved resistance.
It went wrong
- Cane toads (1935) failed to control cane beetles and spread across northern Australia.
- Their poison kills native predators such as quolls and goannas that try to eat them.
- No predator controls the toads, so they became a worse invader than the beetle.
The test before release: Today a control agent is tested for years to check that it eats only the target, the lesson of the cane toad.
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How this comes up: Paper 2, Section B (b): evaluate a strategy for managing invasive species [7]. Both sides, named examples, a conclusion.
Governments spend millions each year fighting invasive alien species.
Evaluate the use of biological control to manage invasive alien species, with reference to named examples.
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