What are the impacts of introducing a non-native species to an ecosystem?
In some instances, the introduced species may not survive. If there is no ecological niche for the species to fill or the species cannot adapt to fill a different ecological niche, the species will likely go extinct relatively quickly at the local level.
However, if the species is a generalist, or a species able to thrive in a variety of environments and consume many food sources, that species will likely do well. If the ecosystem has reached its stable state, this means that the invasive species will have to replace a native species. No two species can share the same ecological niche, thus one will be better adapted and survive. If the invasive species is better adapted, it will out compete the native species.
If the species reproduces quickly, it is also more likely to thrive in a new ecosystem. If it can reproduce and grow faster than its competitor, it will eventually out compete that species.
Typically, invasive species harm an ecosystem. For example, the Burmese python is found in the US but it isn't supposed to be here. These snakes were likely released by humans and were pets at one time. The environment is suitable for them and they have adapted to the area.
Introducing a new species can also introduce any diseases that species has. These new diseases can spread to other native species and negatively affect them.