According to the Department for the Environment, Food and Rural Affairs (DEFRA), non-native invasive species can prey on, compete with and displace native wildlife. The can spread disease and block waterways. They can completely alter the balance of ecology in a water system.
Fish populations can alter, and they can block up waterways which mean boat users have to take different routes. The financial costs can run into millions of pounds.
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Steve McClaren says he's "very disappointed" by Derby County's decision to terminate his contract.
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Derby County fans have been tweeting their reaction to the news that Steve McClaren has been sacked from his position as head coach.