Alien species cause a number of impacts in the ecosystems they live in, ranging from hardly detectable to dramatic change. Measuring such impacts is not always straightforward. Furthermore, a wide range of methods and variables has been used to describe these impacts, making comparison between species and habitats difficult.
The management of invasive species is made more complex when more than one invasive species is present in a given ecosystem at the same time. Several invasive species may have a greater effect than one species does alone and there can be synergistic effects that result in a cascade of impacts and a large change in the way the ecosystem functions.