Abstract
A suite of factors may have contributed to declines in the tītī (sooty shearwater; Ardenna grisea) population in the New Zealand region since at least the 1960s. Recent estimation of the magnitude of most sources of non-natural mortality has presented the opportunity to quantitatively assess the relative importance of these factors. We fit a range of population dynamics models to a time-series of relative abundance data from 1976 until 2005, with the various sources of mortality being modelled at the appropriate part of the life-cycle. We present estimates of effects obtained from the best-fitting model and using model averaging. The best-fitting models explained much of the variation in the abundance index when survival and fecundity were linked to the Southern Oscillation Index, with strong decreases in adult survival, juvenile survival and fecundity being related to El Niño-Southern Oscillation (ENSO) events. Predation by introduced animals, harvesting by humans, and bycatch in fisheries also appear to have contributed to the population decline. It is envisioned that the best-fitting models will form the basis for quantitative assessments of competing management strategies. Our analysis suggests that sustainability of the New Zealand tītī population will be most influenced by climate, in particular by how climate change will affect the frequency and intensity of ENSO events in the future. Removal of the effects of both depredation by introduced predators and harvesting by humans is likely to have fewer benefits for the population than alleviating climate effects.
Highlights
Tītī are an abundant, medium-sized petrel that breed mainly on islands around southern New Zealand and South America during the Austral summer [1]
We develop a candidate set of population dynamics models which we fit, within a Bayesian framework, to data on the fluctuations of tītī abundance in southern New Zealand
The model that best fitted the data was S–1F0, with a Watanabe-Akaike Information Criterion (WAIC) weight of 0.49 (Table 3). This model includes a relationship between adult survival and Southern Oscillation Index (SOI) lagged by one year, and no lag between fecundity and SOI (Table 3)
Summary
Tītī (sooty shearwater; Ardenna grisea) are an abundant, medium-sized petrel that breed mainly on islands around southern New Zealand and South America during the Austral summer [1]. Most of the population migrates to the North Pacific outside the breeding season [2, 3]. They are the numerically dominant seabird species in the New Zealand region [1] and as a burrowing petrel have an important role in structuring the ecosystem on the islands on which they breed [4,5,6]. Several datasets indicate that populations have declined substantially over recent decades [7,8,9,10]. Sources of mortality that may have changed in recent decades, and could have contributed to declining populations, include depredation by introduced predators [13], bycatch in fisheries operations [14, 15], changing climatic conditions [8, 16, 17] and harvesting of chicks by humans [18]
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