Invasive species can be a real bother. These are plants or animals that, when they are accidentally or deliberately moved from one region to another, flourish to the extent of getting out of hand and becoming pests in their naturalized environment. They tend to reduce biodiversity, and can have adverse effects on human well-being. Much effort is devoted to controlling them after they are established, but a better understanding of why species become invasive offers the possibility of taking pre-emptive measures. In companion papers on pages 625 and 628 of this issue, Mitchell and Power and Torchin et al. illuminate the biology of inva-siveness. They report the results of surveying parasite loads of invasive plants and animals in their naturalized and native ranges. They find that parasitism is significantly reduced in organisms in the introduced range, so supporting the 'enemy release hypothesis'―the idea that species are more likely to become invasive when they are released from control by their natural enemies.
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