Attack of the alien invaders Pest plants and animals leave a frightening $1.7 trillion bill. Credit: Jason Jones Travel Photography / Getty Images They’re one of the most damaging environmental forces on Earth. They’ve colonised pretty much every place humans have set foot on the planet. Yet you might not even know they exist. We’re talking about alien species. Not little green extraterrestrials, but invasive plants and animals not native to an ecosystem and which become pests. They might be plants from South America, starfish from Africa, insects from Europe or birds from Asia. Yellow crazy ants, such as these attacking a gecko, are among thousands of invasive species causing ecological and economic havoc. Credit: Dinakarr, CC0, Wikimedia Commons