IB Environmental systems and societies 3.2B Invasive Species Notes

Invasive Species: How Non-Natives Overrun Ecosystems

1. Invasive alien species (IAS) are non-native organisms that spread and cause harm in an ecosystem where they do not naturally occur.
2. They can outcompete native organisms, disrupt food webs, and drive native species toward decline or extinction.
3. Native species are vulnerable because they never co-evolved with the newcomer and lack defences against it.

How Alien Species Arrive: Accidental or Deliberate

1. Accidental introductions
   1. Global trade moves seeds, insects, and larvae in cargo, packaging, and ship ballast water.
   2. Travel and tourism carry seeds stuck to shoes, clothing, and luggage.
   3. Agricultural contamination brings pests and weeds in crop soil or livestock feed.
2. Deliberate introductions
   1. Biological control releases a species to control a pest, such as cane toads in Australia.
   2. Ornamental and commercial use brings in plants like Japanese knotweed and animals for aquaculture.
   3. Recreation and hunting stock non-native fish, birds, or mammals for sport.

Why Invasive Species Succeed: No Predators, Fast Breeding

1. They face no natural predators or diseases in the new ecosystem.
2. They reproduce quickly and in large numbers.
3. Their generalist diet lets them eat many different food sources.
4. A broad ecological tolerance lets them survive in varied climates and habitats.
5. They often out-compete native species that fill the same generalist niche.

Impacts on Ecosystems: Four Main Ways Natives Suffer

1. Competition for resources
   1. Invasives use the same food, water, and nesting sites as natives, but more efficiently.
   2. Native populations then decline through resource scarcity.
2. Predation
   1. Invasives can become new predators where native prey lack defences.
   2. Native prey often fail to recognise or escape the newcomer.
3. Disease and parasites
   1. Invasives can carry pathogens or parasites that infect native organisms.
   2. Natives usually lack immunity to these new diseases.
4. Ecosystem disruption and hybridization
   1. Invasives can alter nutrient cycles, speed up soil erosion, and lower water quality.
   2. Zebra mussels reshape aquatic systems by clogging pipes and filtering out phytoplankton.
   3. Breeding with close native relatives can cause hybridization that dilutes the native gene pool.

Named Examples You Can Cite for Marks

1. Grey squirrel (UK): out-competes the native red squirrel for food and habitat.
2. Signal crayfish (UK): carries crayfish plague that has killed over 90% of native white-clawed crayfish.
3. Nile perch (Lake Victoria): a predator that wiped out hundreds of native cichlid species.
4. Burmese python (Florida Everglades): preys on native mammals and birds with no natural check.
5. Brown tree snake (Guam): drove several native bird species to extinction.
6. Water hyacinth: chokes lakes and rivers, blocking light and lowering oxygen for aquatic life.

Case study

Cane Toads in Australia

• Introduced in 1935 to control sugarcane beetles.
• Native to Central and South America.
• They secrete bufotoxin, which is lethal to native predators like snakes and crocodiles.
• With no natural predators, they spread rapidly across northern Australia.
• Ecological impact:
  • Native predators declined after being poisoned by the toads.
  • Native amphibians faced new competition.
• Control measures:
  • Fencing and trapping programmes.
  • Public awareness campaigns to slow the spread.

What Drives Exponential Growth

1. No predators or diseases keep numbers in check in the new range.
2. A high reproductive rate and fast maturation add many offspring each generation.
3. A generalist niche lets them exploit many food sources and habitats.
4. Human assistance through shipping and landscape change keeps opening new ground.
5. Together these give a positive feedback loop where a bigger population produces even faster spread.

Why Eradication Is So Hard

1. Prevention through biosecurity and border checks is cheaper and easier than removal.
2. Once a population is large and widespread, full eradication is usually unrealistic.
3. Managers instead aim to contain and control numbers to protect native species.