There has been an increase in the number of dead birds found on the beaches of the Great Lakes. These birds were poisoned by a bacterial toxin in the lake water. The birds do not ingest enough water to become sick directly from the toxin found in the lake water. Scientists think that the cause of the increasing bird deaths lies with an invasive species the zebra mussel. This freshwater organism was introduced into the Great Lakes accidentally by humans and has become well established in the Great Lakes. Zebra mussels filter out microscopic organisms, as well as the toxins found in the lake water. The toxins become concentrated in the zebra mussels, which are eaten by small fish called gobies, and the gobies are eaten by the birds. The concentration increases in each level of the food chain. It appears that the introduction of the zebra muscles into the Great Lakes has resulted in a new food chain that increases the concentration of the naturally occurring toxins and passes dangerous levels on to these top-level predators. This process is known as bioaccumulation. Why are the accidentally introduced zebra mussels referred to as an invasive species?