There are many factors that limit production and decrease surplus of wildlife populations such as:
Disease: The factor is a main contributor to limit wildlife populations, each species have their own kind of disease, for example avian cholera can be lethal for ducks etc.
Predator: When people hunt they in a way compete with predators, sometimes limiting the prey species for predators, this can limit the wildlife for both the prey species as well as the predators, when there will be less prey species the predators will also suffer the effect.
Pollution: As every species on the globe is sharing the bad affects of pollution, wildlife is no exception from that.
Hunting: Hunting is a tool sometimes used to maintain and limit wildlife, there are seasons for various species to control and limit the population..