Habitat-mediated population limitation in a colonial central-place forager: the sky is not the limit for the black-browed albatross

Ewan D Wakefield,Richard A Phillips,Jason Matthiopoulos

doi:10.1098/rspb.2013.2883

Abstract

Animal populations are frequently limited by the availability of food or of habitat. In central-place foragers, the cost of accessing these resources is distance-dependent rather than uniform in space. However, in seabirds, a widely studied exemplar of this paradigm, empirical population models have hitherto ignored this cost. In part, this is because non-independence among colonies makes it difficult to define population units. Here, we model the effects of both resource availability and accessibility on populations of a wide-ranging, pelagic seabird, the black-browed albatross Thalassarche melanophris. Adopting a multi-scale approach, we define regional populations objectively as spatial clusters of colonies. We consider two readily quantifiable proxies of resource availability: the extent of neritic waters (the preferred foraging habitat) and net primary production (NPP). We show that the size of regional albatross populations has a strong dependence, after weighting for accessibility, on habitat availability and to a lesser extent, NPP. Our results provide indirect support for the hypothesis that seabird populations are regulated from the bottom-up by food availability during the breeding season, and also suggest that the spatio-temporal predictability of food may be limiting. Moreover, we demonstrate a straightforward, widely applicable method for estimating resource limitation in populations of central-place foragers.

Highlights

A fundamental question posed by ecologists is: what factors regulate population growth? Often, food availability is assumed to be limiting [1]
Our results clearly indicate that the size of regional blackbrowed albatross populations (N) is limited by the availability of foraging resources
The most parsimonious model of N did not include P0, annual breeding success at Bird Island was correlated with January P0

Summary

Introduction

A fundamental question posed by ecologists is: what factors regulate population growth? Often, food availability is assumed to be limiting [1]. Population studies on colonial central-place foragers, such as seabirds, bats and hirundines, frequently make the implicit assumption that resources are accessible [4,5,6,7,8], with the potential consequence that an important limitation on population size is ignored. The need to understand their population dynamics has become more than academic, because many species are endangered by fisheries bycatch, over-fishing, invasive species and climate change [12,13,14] Such anthropogenic factors are additive to natural limits on population growth [13], so in order to understand their impacts, it is necessary to first quantify the processes that naturally limit seabird populations. Food availability is usually considered to be a key driver of population change [9,10], but parasites, disease and nesting habitat availability can have negative, density-dependent effects [15,16,17], whereas

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