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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2015
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Massey University
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Abstract
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.
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Keywords
Rockhopper penguins, Eastern rockhopper penguin, Eudyptes chrysocome filholi, Campbell Island, New Zealand, Penguin populations, New Zealand, Predation effects, Bird population decline