Deer in this ecosystem?

Let's say a deer for whatever reason moved into a wetland ecosystem and in this ecosystem there is streams and swampy deltas where they meet the ocean, The water here is brackish (flatulates between salty and fresh water) and contains a wide variety if marsh grasses. In this ecosystem ocean dwelling fishes use this area as a nursery for their young. There are also many insects in this ecosystem...so how might the deer evolve to be better suited for this ecosystem?Deer in this ecosystem?
They already have. Google ';Florida Key deer.';