Invasive species
Invasive alien species are non-native organisms that, when introduced to a new ecosystem, can cause significant harm to local biodiversity.
- Every ecosystem maintains a natural balance developed over thousands of years through ecological succession and co-evolution among species.
- Invasive alien species (IAS) are non-native organisms introduced, intentionally or accidentally, into an ecosystem where they do not naturally occur.
- These species can rapidly outcompete native organisms, disrupt food webs, alter habitats, and lead to the decline or extinction of native species.
While not all introduced species become invasive, those that do often share certain biological advantages such as rapid reproduction, broad diet, and lack of natural predators.
How Alien Species Arrive in New Ecosystems
Accidental introductions
- Through global trade: seeds, insects, or larvae hitchhiking on imported goods, packaging, or ship ballast water.
- Travel and tourism: seeds stuck to shoes or clothing; small animals in cargo shipments.
- Agricultural contamination: pests and weeds transported in crop soil or livestock feed.
- Transportation: ships, planes, and vehicles moving organisms across continents.
Spiders found in fruit shipments or insects carried in wooden pallets can become established in new environments if conditions are favorable.
Deliberate introductions
- Biological control: species introduced to control pests (e.g., cane toads in Australia).
- Aesthetic or commercial use: ornamental plants like Japanese knotweed or animals for aquaculture and farming.
- Recreation or hunting: non-native fish, birds, or mammals introduced for sport.
Japanese knotweed was introduced as an ornamental plant, but now spreads uncontrollably across Europe.
- Assuming that all alien species become invasive.
- In reality, only a small proportion of introduced species establish and spread enough to become ecologically disruptive.
Why Invasive Species Become Successful
- Lack of natural predators or diseases in the new ecosystem.
- Ability to reproduce quickly and in large numbers.
- Generalist diets can eat a wide range of food sources.
- High adaptability to new environmental conditions.
- Aggressive behavior or competitive superiority over native species.
- Invasive species behave like “ecological weeds”.
- Once established, they spread uncontrollably, displacing native flora and fauna
Impacts of Invasive Alien Species on Ecosystems
1. Competition for Limited Resources
- Invasive species often consume the same resources (food, water, nesting sites) as native species but more efficiently.
- This results in declining native populations due to resource scarcity.
The grey squirrel introduced to the UK from North America outcompetes the native red squirrel for food and habitat, leading to local red squirrel extinction.
2. Predation
- Some invasive species become new predators in ecosystems where native species lack defenses.
- Native prey species often fail to recognize or escape these predators.
Cane toads in Australia prey on small mammals, insects, and reptiles, and their toxic skin poisons native predators that attempt to eat them.
3. Spread of Diseases and Parasites
Parasitism
Parasitism is a symbiotic relationship where one organism (the parasite) benefits by living on or within another organism (the host) and obtaining nutrients from it, usually without immediately killing the host.
- Invasive species can carry pathogens or parasites that infect native organisms, often with devastating results.
- Native species usually lack immunity to these novel diseases.
The red-clawed signal crayfish (introduced to the UK) carries the crayfish plague, which has wiped out over 90% of the native white-clawed crayfish population.
4. Ecosystem Disruption
- Alter nutrient cycles, increase soil erosion, or reduce water quality.
- Some plants, like Japanese knotweed, destabilize riverbanks and outcompete native vegetation.
- Marine invasives like zebra mussels alter entire aquatic ecosystems by clogging pipes and filtering phytoplankton.
Cane Toads in Australia
- Introduced in 1935 to control sugarcane beetles.
- Native to Central and South America.
- Grow up to 25 cm and secrete bufotoxin, lethal to predators like snakes and crocodiles.
- With no natural predators, they spread rapidly across northern Australia.
- Feed on a wide range of insects and small vertebrates.
- Ecological impact:
- Decline of native predators poisoned by toads.
- Competition with native amphibians.
- Control measures:
- Fencing and trapping programs.
- Biological research on gene suppression and pheromone-based control.
- Public awareness campaigns to prevent further spread.
The Signal Crayfish in the UK
- North American species (Pacifastacus leniusculus).
- Introduced to the UK in the 1970s for aquaculture to replace declining native stocks.
Ecological Impacts
- Outcompetes native white-clawed crayfish (Austropotamobius pallipes) for food and shelter.
- Carries crayfish plague, a fungal disease fatal to native species.
- Causes bank erosion by burrowing into riverbeds.
- Reduces water quality and affects aquatic vegetation balance.
Management Strategy: The Southwest Crayfish Project
- Public awareness campaigns to prevent disease spread via boots and equipment.
- Captive breeding programs for native crayfish followed by reintroduction.
- Relocation of “at-risk” populations to isolated safe habitats.
- Legal protection under the UK Biodiversity Action Plan.
Factors Enabling Exponential Growth of Invasive Species
- Absence of natural predators or diseases in the new environment.
- High reproductive rate and rapid maturation.
- Broad ecological tolerance, ability to survive in varied climates or habitats.
- Generalist feeding habits allowing exploitation of multiple food sources.
- Human assistance (e.g., shipping routes, landscape alteration).
The cane toad’s ability to reproduce explosively and tolerate dry conditions enabled its exponential spread across northern Australia.
- Define an invasive alien species and explain how it differs from a non-native species.
- Describe three main ways invasive species affect local biodiversity.
- Using one local or global example, explain how invasive species can spread exponentially.
- Outline the management strategies used to control the red-clawed signal crayfish in the UK.
- Discuss why complete eradication of invasive species is often unrealistic.
- Explain how globalization has contributed to the rise of invasive species.
- Suggest methods communities can use to prevent accidental introductions of alien species.
