"People in Albany and New York City are using “green roofs” to improve the environment. A green roof can be added to many buildings that have large, flat roofs. Green roofs have three parts: a protective layer to separate plant roots from the roof of the building, an absorptive layer to catch and hold rainwater, and a layer of plants. Often, green roofs use Sedum, a short, desert plant, because it is efficient at storing water in its leaves and can withstand the colder climate.
A green roof saves energy, reduces carbon dioxide in the atmosphere, and prevents rainwater and melting snow from overloading sewer systems. It can also protect the roof of a building from damage. However, green roofs can be expensive to install, and require care and maintenance.

Most varieties of Sedum are not native plants in Albany or New York City. State one reason why it may be dangerous to introduce a new species to an established ecosystem"