How might exotic species be beneficial?

When exotic species are introduced into an ecosystem they may not have natural predators available. This means that the species will grow rapidly without predation and will stifle other natural species in the ecosystem. An example of this would be zebra mussels in land locked lakes. The mussels were transported by boats and they don't have any predators in the lakes. Their population keeps increasing and they are using all the resources but have nothing to control the population. All the other organisms in the system are then negatively impacted.