Energy limitation is probably the most important determinate, but such factors as food, availability
Energy limitation is probably the most important determinate, but such factors as food, availability of shelter, and the amount of competition can also influence this.  A sigmoid-shaped graph results when this factor is included in a model of growth.  Such an equation is called a logistic growth equation.  FTP, name this quantity symbolized by a capital K, the maximum stable population size of a species that an environment could support over a period of time.
Energy limitation is probably the most important determinate, but such factors as food, availability of shelter, and the amount of competition can also influence this.  
Carrying Capacity