Are whooping cranes destined for extinction? Climate change imperils recruitment and population growth

Abstract

Identifying climatic drivers of an animal population's vital rates and locating where they operate steers conservation efforts to optimize species recovery. The population growth of endangered whooping cranes (Grus americana) hinges on juvenile recruitment. Therefore, we identify climatic drivers (solar activity [sunspots] and weather) of whooping crane recruitment throughout the species' life cycle (breeding, migration, wintering). Our method uses a repeated cross-validated absolute shrinkage and selection operator approach to identify drivers of recruitment. We model effects of climate change on those drivers to predict whooping crane population growth given alternative scenarios of climate change and solar activity. Years with fewer sunspots indicated greater recruitment. Increased precipitation during autumn migration signified less recruitment. On the breeding grounds, fewer days below freezing during winter and more precipitation during breeding suggested less recruitment. We predicted whooping crane recruitment and population growth may fall below long-term averages during all solar cycles when atmospheric CO₂ concentration increases, as expected, to 500 ppm by 2050. Species recovery during a typical solar cycle with 500 ppm may require eight times longer than conditions without climate change and the chance of population decline increases to 31%. Although this whooping crane population is growing and may appear secure, long-term threats imposed by climate change and increased solar activity may jeopardize its persistence. Weather on the breeding grounds likely affects recruitment through hydrological processes and predation risk, whereas precipitation during autumn migration may influence juvenile mortality. Mitigating threats or abating climate change should occur within ≈30 years or this wild population of whooping cranes may begin declining.

Keywords: LASSO; atmospheric CO2; boreal pond; decadal cycle; groundwater; precipitation; reproduction; solar activity; sunspots.

Figure 1

Migratory route of whooping cranes in North America. The breeding grounds are on and around Wood Buffalo National Park, Canada, and the wintering grounds are on and around Aransas National Wildlife Refuge, Texas, USA

Figure 2

The solar cycle as indicated by sunspot number has varied over the last three centuries (1712–2007). The Dalton minimum occurred during 1811–1821 and modern grand maximum occurred during 1954–1964. We estimated the typical cycle as a sixth‐order polynomial of the number of years since a cycle began

Figure 3

Predicted whooping crane population growth for two atmospheric CO₂ concentration scenarios during a typical solar cycle (horizontal line indicates 0% population growth, λ = 1)

Figure 4

Predicted juvenile recruitment (ratio of hatch‐year [HY] to after‐hatch‐year [AHY] birds) of whooping cranes during an 11‐year solar cycle for two atmospheric CO₂ concentration scenarios. Top dotted line represents long‐term mean recruitment (0.137) and bottom dotted line represents required recruitment (0.103) needed to maintain a stable population (λ ≥ 1) given the long‐term mean annual survival of 90.6%. Points represent mean predictions and vertical bars 95% confidence intervals

Figure 5

Predicted whooping crane population growth during an 11‐year solar cycle for two atmospheric CO₂ concentration scenarios. Top dotted line represents long‐term mean population growth of 3.5%. Points represent mean predictions and vertical bars 95% confidence intervals. Only a solar cycle similar to the Dalton minimum with ≤400 ppm CO₂ will likely maintain population growth at or above the long‐term mean of 3.5%

Figure 6

Predicted whooping crane survival needed to maintain 0% population growth and 3.5% population growth (long‐term mean) during an 11‐year solar cycle for two atmospheric CO₂ concentration scenarios. Dotted line represents long‐term mean annual survival of 90.6%. Points represent mean predictions and vertical bars 95% confidence intervals