Factors affecting the population dynamics of eastern rockhopper penguins (Eudyptes chrysocome filholi) on Campbell Island, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Ecology, Massey University, Palmerston North, New Zealand
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The population dynamics of seabirds may be influenced by ‘top-down’ predation effects, or from the ‘bottom-up’ by environmental limitations on food availability. Southern Rockhopper Penguin (Eudyptes chrysocome) populations have declined hugely at multiple sites across their circumpolar, sub-Antarctic range in recent decades, resulting in an extinction risk of ‘Vulnerable’. They are a small-bodied penguin, adapted to exploit seasonally abundant, pelagic prey by being migratory and having prolonged fasting periods onshore and foraging offshore during breeding. Mysteriously, like other Eudyptes penguins they lay an extremely dimorphic two-egg clutch in which the first-laid egg is smaller and less successful, and rarely fledge two chicks. The world’s largest population (c. 620,000 pairs) of the Eastern sub-species (E. c. filholi) of Rockhopper Penguin formerly bred at Campbell Island, New Zealand. Prior to the current work, the only previous intensive research at this site in the mid-1980s revealed the population had declined by a startling 94% since the early 1940s as local sea-surface temperatures (SSTs) increased. It was hypothesized that climate change had reduced the availability of the penguins’ preferred zooplankton prey, so that the Campbell population’s decline was caused by its unusual reliance on a fish-based diet which resulted in low body masses and demographic rates. I examined this hypothesis of bottom-up population regulation by estimating the population growth rate from 1984–2012 and assessing links between SST, diet quality, chick-provisioning rates, chick and adult body masses, egg masses, reproductive success, and first-year and adult survival rates. Additionally, I considered whether top-down predation effects were causing the recent, unusually rapid population decline in a relatively small, fragmented colony. I assessed the population growth rate using digital imagery analysis of current and historic colony photographs, dietary trophic level using stable isotope analysis of blood samples, and chick-provisioning and adult survival rates using an automated gateway to record the colony-sea transit times of individual transponder-tagged penguins breeding at closely monitored nest sites. I estimated the total Campbell Island population size to be 33,239 breeding pairs in 2012, a 21.8% decrease from an adjusted estimate of 42,528 pairs in 1984. However, the recent decline occurred before 1996 with most colonies stable or growing thereafter under cooler SSTs and more abundant prey. My results supported the previous assertion that the overall population size declined during periods of warm SST and that a zooplankton-based diet was a higher quality diet, resulting in heavier adults and chicks. However, the primary determinant of chick growth was how often they were fed, and the rigid division of labour between parents during early-chick rearing reduced potential chick-provisioning rates, especially from males in a poor-food year. The ongoing, localized decline of my study population was caused by high rates of predation on adults by sea lions, as well as on eggs by skuas. I did not find support for overlap between migration and egg-formation underlying eggsize dimorphism. This study emphasizes the constraints that a species’ genotype and ecological context place on the degree of behavioural plasticity it exhibits when faced with environmental variability. My results highlight the vulnerability of Eastern Rockhopper Penguins, and likely other Eudyptes penguins, to the more variable and warmer environment and less predictable food supply in a future under climate change.
Rockhopper penguins, Eastern rockhopper penguin, Eudyptes chrysocome filholi, Campbell Island, New Zealand, Penguin populations, New Zealand, Predation effects, Bird population decline