See
http://www.districtgov.org/departmen...%201%20CIP.pdf, page 9 (under FENCES) for information on maintenance of the wildlife preserves. This document explains the annual maintenance budget, which sets your yearly maintenance assessment. It is not part of the amenities fees.
Preserves benefit everyone, not just the people who have property overlooking them; they provide a place for all the wild critters(who were here before us) to live without disturbing us, and help to maintain the water table.
The Florida Fish and Wildlife Conservation Commission monitors the preserves and tells what must be done to maintain them (including control of invasive species, which usually refers to plants, not animals). Be grateful that you live in a community and a state that care about our environment.