Heading for a cliff: Can dramatic declines in elk recruitment cause lagged population declines?

Background/Question/Methods

Many ungulate species experience highly variable recruitment rates from year to year, often due to environmental conditions or increased predation on young. The restoration and range expansion of large predators (namely wolves and grizzly bears) in the Western United States over the last 15 years has coincided with widespread and pronounced declines in elk calf productivity (i.e., the rate of recruitment of calves to yearlings). As recruitment has declined, the age structures of these populations have become markedly older. Given the maximum lifespan of many of elk is 15 to 20 years, there is a perception among managers and the public that populations with declining calf productivity will soon reach a demographic “cliff,” in which the oldest cohorts of animals born immediately prior to predator restoration will begin to senesce (in terms of both survival and reproduction), triggering rapid population declines. We used an age-structured demographic model to evaluate the timing and magnitude of reduced population productivity on the short- and long-term dynamics of an elk population across a range of assumptions about senescence, harvest and source of recruitment decline (fertility rate, early calf mortality, late calf mortality). We highlight the general phenomenon of declining calf productivity with an example from a well-studied population (Clark’s Fork herd) in Northwestern Wyoming.

Results/Conclusions

Overall, our results indicate that the age at which reproductive and survival senescence begins and the relative magnitude of senescence can impact population growth rates but do not produce lagged population declines. Lagged population declines are unlikely under most conditions, except when adult survival compensates for lost calf productivity. In the Clark’s Fork herd, declines in recruitment on the order seen in this herd (10 calves per 100 cows in early winter) without noticble declines in adult population size are unlikely without high levels of survival compensation. We review the empirical and theoretical evidence for survival compensation following declines in fertility or calf survival rates. Finally, we make recommendations about future research needs on ungulates in areas where large predators have recently been restored.