How can exotic species affect populations of native species?
Invasive species that are successful typically outcompete the native species to the detriment of the native species. Invasive species are usually generalists and/or adaptable and they often thrive in their new environment. Any species in competition with them will likely suffer.
This may mean that the invasive sp. consumes more of a resource, such as sunlight, prey, and so forth. The invasive sp. could also outcompete other species by growing faster than native species and spreading faster, taking up more valuable space and leaving less for the native sp.
There are multiple ways invasive sp. can negatively affect native species' populations, but the effects are typically negative.