BIOLOGY · Stage 8 · Chapter 7: The Health of the Environment
Biology 7.6 Invasive species and ecosystems — Conservation Management
5 min read 7 parts Lesson 6 of 8
Lesson summary
7.6 Invasive species and ecosystems — Conservation Management — Alhussan International Academy — Biology (Stages 7–9) notes: Explain how invasive species change ecosystems and evaluate methods used to control them and to protect native species.Arrival — brown tree snakes reached …
- Why birds died
- Seed dispersal
- Interdependence
- Other pressures
- Prevention
- Physical removal
Section 1 of 7Brown tree snakes on Guam
- Arrival
- brown tree snakes reached the Pacific island of Guam hidden in crates from Papua New Guinea.
- Why birds died
- the birds had no defences against a nocturnal snake that climbed trees and ate adults, babies and eggs.
- In Papua New Guinea the snakes compete for food and are eaten by predators, so their population is in balance.
- On Guam they had a huge food supply, no competition and no predators.
- Outcome
- by 2012 most island birds were extinct and there were around 2 million snakes.
- When bird numbers fell the snakes ate lizards, so snake numbers stayed high.
Section 2 of 7Knock-on effects on the Guam food web
- Spiders
- many birds fed on insects and spiders, so when the birds disappeared the spiders lost their main predators and competitors.
- The spider population grew and spiders control the insect population, which prevents leaf-eating insects destroying all the plants.
- Seed dispersal
- over half of the tree seeds on Guam were dispersed by birds.
- With no birds the seeds fall under the parent trees, where they do not get enough light to grow.
- Eventually the forest may die out too.
- Interdependence
- it involves more than feeding relationships.
Section 3 of 7The conservation problem
- Conservation = protecting species and habitats and managing them so that they survive.
- Invasive species
- they are one of the biggest threats to biodiversity, particularly on islands where native species evolved with few predators or competitors.
- Rats, cats and goats introduced onto islands have caused many bird extinctions.
- Other pressures
- habitat destruction, pollution and climate change.
Section 4 of 7Methods of managing invasive species
- Prevention
- border and port checks, quarantine of imported animals and plants, cleaning of boats and fines for illegal release.
- Physical removal
- hunting, trapping or pulling up plants.
- It works if the population is small, and some islands have been cleared of rats entirely.
- Chemical control
- herbicides and poisons are quick but can harm native species and enter food chains.
- Biological control = controlling a pest using another species of organism.
- It can be cheap and long lasting but the control agent may attack native species.
Section 5 of 7Biological control of prickly pear
1Problem3 points
- prickly pear cactus grows naturally in India, South Africa and the Americas in balanced ecosystems.
- Introduced to Australia, nothing fed on it, it gradually took over and was declared a pest in 1910.
- By the 1920s over 24 million hectares were covered.
2Solution3 points
- in 1926 scientists introduced the Cactoblastis cactus moth, whose caterpillars feed inside the cactus in its native ecosystems.
- The moths reproduce quickly and eat a lot, so by 1933 the prickly pears had almost disappeared.
- A small population of cactus moths now keeps the few remaining cacti from becoming invasive again.
Section 6 of 7Protecting native species
- Protected areas
- national parks and nature reserves give habitats legal protection.
- Captive breeding
- zoos raise the numbers of rare animals, which may later be released.
- Seed banks
- they store seeds of rare plants at low temperatures so they can be regrown.
- Laws and treaties
- they limit hunting and trade in endangered species.
- Education
- it encourages local people to protect nature, especially when they benefit from tourism or sustainable resources.
Section 7 of 7Evaluating a management plan
| Method | Advantage | Disadvantage |
|---|---|---|
| Trapping and hunting | Targeted, no chemicals | Labour intensive; hard in large areas |
| Poison or herbicide | Covers large areas quickly | May kill non-target species |
| Biological control | Long-lasting, lower cost | Control agent may become a pest |
| Prevention | Cheapest overall | Needs constant checking |
- Criteria
- consider cost, effectiveness, effect on other species and the time taken.
- Monitoring
- successful programmes check that native populations recover and that the invader does not return.
- Island restoration
- removing introduced rats with bait stations lets seabird chicks survive.
- Bird droppings then add nutrients so plants grow better, which restores the food web if rats are kept out.
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