A niche is basically an organism's role or job within its ecosystem. It is also the interaction of that organism within its environment, with other species in that ecosystem, its usage of resources and any function it fulfills therein. An environmental issue that involves niche is the effects of an invasive or exotic species accidentally or purposefully introduced into an ecosystem. An example of this is purple loosestrife, a non-native plant, which was introduced into the United States and is slowly taking over and killing off native plants. This in turn, means less food for insects that depend on native plants for nutrition. It is crowding out native plants and taking over their niche. Another example is when zebra mussels were acccidentally introduced to the Great Lakes from the Caspian Sea in ballast water from ships coming from that area. These mussels have taken over the niche of the native shellfish, robbing the native shellfish of their food supply, causing our native shellfish to die off. The zebra mussels on the other hand are overpopulating because they have no natural predators and are causing extensive damage to waterways, pipes and structures all over North American waterways. Sometimes just clearing land in an ecosystem, thus removing native plants to grow crops can devastate an ecosystem. An example of this was the American Dust Bowl of the 1930's. Native prairie grasses were removed to raise food crops and drought conditions set in. The crops were less adapted to these dry conditions than native plants were because the native plants root system could keep the soil in place during dry spells. The crops eventually died and the topsoil which is a precious resource was then easily eroded due to wind and was carried away, creating devastating effects on the ecosystem. The important niche that native grasses filled couldn't be replaced by crops. For an ecosystem to be sustainable each organism has a niche it occupies to keep that ecosystem healthy.