RAPID: Population viability below the quasi-extinction threshold

Tufts University, Medford MA

Investigators

Abstract

From time to time, formerly widespread species have declined to extinction. For example, passenger pigeons were once abundant throughout North America, but no longer exist. We often know about the factors that cause a widespread species to decline (in the case of the passenger pigeon, the original cause was hunting), but we know much less about what causes some populations to recover, and others not to do so, once that original cause of declines is gone. Monarch butterfly populations have declined dramatically in North America, and now western populations may be approaching extinction. In the 1980?s, millions of monarch butterflies overwintered on the California coast, and bred during the summer west of the Rocky Mountains. By the 2000?s, the overwintering population had declined to ~200,000-300,000 butterflies. In 2018, the population dropped to ~30,000 butterflies. This RAPID project will use the unexpected, rapid decline of western monarch butterflies to test theories of extinction risk. We will evaluate what changes in this suddenly-small population, through a combination of carefully controlled experiments and sightings reported by volunteers looking for monarch butterflies throughout the west, especially in California. We will use this knowledge to help guide management and recovery of this iconic butterfly population. The research will test the hypothesis that vital rates, like survival and reproduction, decline as populations become very small, perhaps because individual animals have difficulty finding mates, or cannot protect themselves from predators as well as they could in groups. If very small populations decline due to these "Allee effects", there is a fleeting window of time in which to study population dynamics before local population extinction. Because we know so little about suddenly-small populations, ecologists typically base conservation plans on predicting declines to a ?quasi-extinction threshold?, in other words, a population size below which the species is unlikely to be able to recover. Based upon estimates of survival on overwintering grounds and reproduction in early spring, earlier work warned that 30,000 butterflies may be a quasi-extinction threshold for western monarchs. This project will (1) Test whether winter survival and spring reproduction are now lower than they were in the past; (2) Calculate rates of summer range expansion, and compare 2019 to years in the recent past, to evaluate potential reductions of survival, reproduction, or movement at other life stages; (3) Combine results of (1) & (2) with existing data to model dynamics of this migratory population through its full annual cycle. This model will be used to calculate annual population growth rates in small and large populations, and also to evaluate the potential of different management strategies to help western monarch butterflies recover from quasi-extinction. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.