Modelling the effect of El Nino on the persistence of small populations: The Galapagos penguin as a case study

Small populations are vulnerable to long-term declines, even where short-term censuses indicate increasing trends in numbers. Census data for the Galapagos penguin (Spheniscus mendiculus) collected between 1970 and 2004 provide evidence that despite year-to-year population increases detected in most of the annual censuses, the strong El Nino events of 1982-83 and 1997-98 were followed by population declines of more than 60% from which the species has yet to recover. Such large declines raise concerns about the future viability of the species because the frequency and severity of El Nino events are predicted to increase. We used the simulation software VORTEX to evaluate the potential effects of El Nino on the risk of extinction of the Galapagos penguin population and its four constituent subpopulations. Weak and strong El Nino events were treated as catastrophes, with varying frequencies, which simulated past, current and future effects on the penguin population. The "Current El Nino" scenario, based on the frequency of El Nino events recorded in the Galapagos between 1965 and 2004, indicated an approximately 30% probability of extinction within the next 100 years for the penguin population. More ominously, the species may be at a greater risk if the frequency of strong El Nino episodes increases only marginally. A probability of extinction greater than 80% was predicted when the current frequency (5%) of strong El Nino events was doubled (to 10%). The probabilities of extinctions were higher for each subpopulation treated individually, ranging from 34% for Isabela and Fernandina, 64% for Bartolome-Santiago to 78% for the smallest subpopulation on Floreana. Sensitivity analyses identified survival of penguins during El Nino events and sex ratio as influential parameters. The estimates of extinction risk may be conservative as other threats associated. with increased human activities on the islands may further compromise species persistence. (c) 2007 Elsevier Ltd. All rights reserved.