You will hear part of an ecology lecture about invasive plants and animals.
First, you have some time to look at questions 31 to 40.
Now listen carefully and answer questions 31 to 40.
Today's topic is invasive species.
I'll begin by clarifying the definition.
An invasive species is a plant or animal introduced into an environment through human activity which then causes harm there.
Not every non-native species is invasive.
Many survive in a new region without creating serious problems.
In the most severe cases, an invasive organism can transform an environment almost completely.
It may prey on native wildlife or bring a disease that local species have never encountered and cannot resist.
It may also reproduce rapidly and consume resources that native species depend on.
Biological invasions are among the major causes of damage to an ecosystem and can even threaten its continued survival.
How do these organisms reach new places?
Human activity is by far the main cause, either accidentally or deliberately.
Someone returning from a holiday abroad may unknowingly carry plant seeds, on clothing or footwear.
Insects and other organisms can also travel within cargo or on the outside of ships, then escape after arrival.
People sometimes transport a species intentionally.
It might be released to control agricultural pests or imported to keep as pets.
Difficulties arise if these organisms enter the wild and reproduce or spread in ways nobody anticipated.
A well-known Australian example is the toad Rhinella marina.
It was brought from Hawaii in 1935 as a biological control.
The plan was for the toads to eat grey-backed beetles that were destroying sugar crops on plantations.
Scientists initially released only a small number in Queensland, but other states soon copied the program.
Within two years, 62,000 young toads had entered the wild.
The toads failed to protect the plantations, yet they multiplied quickly and spread across northern Australia.
They are poisonous throughout their life cycle, so animals that eat them can die.
Their large population also leaves less food available for native frogs.
The United Kingdom provides several further examples and has more than 3,000 invasive species.
Japanese knotweed was admired by 19th-century gardeners as an ornamental plant.
Once it escaped into the countryside, however, it spread aggressively and proved extremely difficult to remove.
Rhododendrons were introduced even earlier and now grow in many British parks and woods.
Their dense growth blocks so much light that native wildflowers cannot survive underneath.
Grey squirrels were brought from North America to private estates during the 19th century.
They are now widespread in both woodland and towns.
These squirrels compete with the smaller native red squirrel for the same food.
More seriously, grey squirrels carry a virus which is fatal to red squirrels.
As a result, red squirrel numbers have fallen dramatically, and only a few populations remain.
So, what can ecologists do about invasive species?
The first priority is to study the behaviour of a newly arrived organism.
Continued monitoring reveals whether it is beginning to damage its new surroundings and how its population is changing.
A national database is an effective way to record and track invasive species.
Relevant agencies can add observations, share information and coordinate any action required.
Members of the public are valuable in this work too.
They can be encouraged to photograph a suspected harmful species to help identification, then report where and when it was seen.
People should not touch or attempt to destroy the organism themselves.
Together, careful monitoring, shared records and public reporting can support a faster response when a harmful new species appears.
