Integrated population modeling to assess demographic variation and contributions to population growth for endangered whooping cranes

Management efforts for endangered species are enhanced by knowledge of the causes and extent of demographic variation, but this information is often lacking even when abundance is well known. We developed an integrated population model to study the dynamics of the only remnant population of endangered whooping cranes (Grus americana) in North America, 1977-2013. Despite much conservation attention for this species, there has been little study on temporal variability in demography and how that variation contributes to population dynamics. Demographic rates were most variable for fledge rate and survival of the youngest age classes while adult survival was less variable. Population growth was influenced mainly by variation in fledge rate, breeding propensity and survival of hatch year and second year cranes while survival of older age classes had less influence. Fledge rates were correlated to the boreal snowshoe hare (Lepus americanus)-Canada lynx (Lynx canadensis) cycle and were lowest 2 years after increases in lynx population growth. In terms of management, mean annual fledge rates were similar between years with and without egg collection for captive rearing (0.523 vs 0.518), suggesting no influence of this practice on population-level productivity. Our findings add to the growing body of results showing that while population growth may be most sensitive to changes in adult survival in long-lived vertebrates, variation in breeding parameters often drives annual fluctuations in abundance. Maintaining adult whooping crane survival at currently high rates and increasing breeding output where possible may be effective management strategies for achieving conservation goals.