What factors determine cyclic amplitude in the snowshoe hare (Lepusamericanus) cycle?

Charles Krebs, John Bryant, Knut Kielland, Mark O'Donoghue, Frank Doyle, Suzanne Carriere, Donna DiFolco, Nathan Berg, Rudy Boonstra, Stan Boutin, Alice Kenney, Donald Reid, Karin Bodony, Judy Putera, Henry Timm, Toby Burke, Julie Maier, Howard Golden