Multiple Choice, Single Answer

When a plant or animal is carried to a region where it did not evolve, it usually fails to survive or remains rare. A small number of such species, however, spread rapidly, often because the predators, parasites and diseases that kept them in check at home are absent. These invasive species can crowd out native wildlife, damage crops and block waterways. Once an invader is widespread, removing it completely is rarely possible, and control can continue for decades at great expense. For this reason, many ecologists argue that the most effective strategy is to stop new species arriving in the first place, through border checks on cargo and luggage.