For over 20 years, Yukon-Charley Rivers National Preserve has monitored up to twelve wolf packs that routinely utilize Preserve lands. It is the second longest wolf study in Alaska. The Preserve chose to study wolves in part because they are a good indicator of the health of the ecosystem. Wolves depend on healthy populations of large ungulate prey (such as moose and caribou), which in turn respond to vegetation, weather, and other environmental factors. In addition, one of the Preserve's mandates is to maintain opportunities for subsistence hunting and trapping within the boundaries of the Preserve. Monitoring the harvest of all large mammals in the preserve, including wolves, also helps the National Park Service fulfill its responsibilities as mandated by Congress.