Changing environmental spectra influence age-structured populations: increasing ENSO frequency could diminish variance and extinction risk in long-lived seabirds
As global climate changes, there is increasing need to understand how changes in the frequencies of environmental variability affect populations. Age-structured populations have recently been shown to filter specific frequencies of environmental variability, favoring generational frequencies, and very low frequencies, a phenomenon known as cohort resonance. However, there has been little exploration of how changes in the spectra of environmental signals will affect the stability and persistence of age-structured populations. To examine this issue, we analyzed a likely example to show how changes in the frequency of an influential climate phenomenon, the El Niño-Southern Oscillation (ENSO), could affect a marine bird population. We used a density-dependent, age-structured population model to calculate the transfer function (i.e., the frequency-dependent sensitivity) of Brandt’s cormorant (Phalacrocorax penicillatus), a representative marine bird species known to be influenced by ENSO. We then assessed how the population would be affected by ENSO forcing that was doubled and halved in frequency. The transfer function indicated this population is most sensitive to variance at low frequencies, but does not exhibit the sensitivity to generational frequencies (cohort resonance) observed in shorter-lived species. Doubling the frequency of ENSO unexpectedly resulted in higher mean adult population abundance, lower variance, and lower probability of extinction, compared to forcing with the historical or reduced ENSO frequency. Our results illustrate how long-lived species with environmentally driven variability in recruitment, including many species of marine birds and fish, may respond in counterintuitive ways to anticipated changes in environmental variability.
KeywordsClimate change El Niño-Southern Oscillation Pacific California Current Cohort resonance Population dynamics
We thank the US Fish and Wildlife Service and Farallon National Wildlife Refuge; Pete Warzybok and the many Point Blue Farallon biologists, interns, and volunteers who collected data over the years; and the Farallon Patrol skippers for their long-term support of wildlife research on the Farallon Islands.
AES was supported by a National Science Foundation Graduate Research Fellowship grant no. 1148897, University of California, Davis Graduate Group in Ecology Fellowship, Selma Herr Fund, and Jastro Shields Research Fellowship. Funders for Point Blue’s Farallon Research Program include Baker Trust, Marisla Foundation, Campini Foundation, Kimball Foundation, Mead Foundation, and individual donors. This is Point Blue contribution number 2054.
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