Abstract
Spatial coherence between predators and prey has rarely been observed in pelagic marine ecosystems. We used measures of the environment, prey abundance, prey quality, and prey distribution to explain the observed distributions of three co-occurring predator species breeding on islands in the southeastern Bering Sea: black-legged kittiwakes (Rissa tridactyla), thick-billed murres (Uria lomvia), and northern fur seals (Callorhinus ursinus). Predictions of statistical models were tested using movement patterns obtained from satellite-tracked individual animals. With the most commonly used measures to quantify prey distributions - areal biomass, density, and numerical abundance - we were unable to find a spatial relationship between predators and their prey. We instead found that habitat use by all three predators was predicted most strongly by prey patch characteristics such as depth and local density within spatial aggregations. Additional prey patch characteristics and physical habitat also contributed significantly to characterizing predator patterns. Our results indicate that the small-scale prey patch characteristics are critical to how predators perceive the quality of their food supply and the mechanisms they use to exploit it, regardless of time of day, sampling year, or source colony. The three focal predator species had different constraints and employed different foraging strategies – a shallow diver that makes trips of moderate distance (kittiwakes), a deep diver that makes trip of short distances (murres), and a deep diver that makes extensive trips (fur seals). However, all three were similarly linked by patchiness of prey rather than by the distribution of overall biomass. This supports the hypothesis that patchiness may be critical for understanding predator-prey relationships in pelagic marine systems more generally.
Highlights
Predators and their prey must overlap in space and time for predators to survive [1,2]
The goal of this work was to use measures of the environment, prey abundance, prey quality, and prey distribution to identify the key features of prey suitability that predict the distributions of three co-occurring predator species breeding on islands in the southeastern Bering Sea: black-legged kittiwakes (Rissa tridactyla), thick-billed murres (Uria lomvia), and northern fur seals (Callorhinus ursinus)
We failed to predict at-sea predator densities when we characterized prey in terms of areal biomass density or numerical abundance; as a class, these prey variables predicted an insignificant fraction of the variance in predator distributions even when variance in these measures was included
Summary
Predators and their prey must overlap in space and time for predators to survive [1,2]. In marine pelagic systems, many studies have found weak or ephemeral spatial associations between predators and pelagic prey ([15,16,17,18] but see [19,20,21,22,23,24,25,26,27]) and even negative relationships [28], resulting in a large number of novel hypotheses to explain the divergence. Studies of marine predator-prey relationships most often examine the spatial relationship between the species of interest and the biomass or abundance of its prey spatially integrated or averaged in some way (e.g. g/m2 or individuals/m2 over some prescribed transect length) [30,31,32,33]. Patches occur over a gradient of sizes and densities, or predators do not choose prey based on patch size, correlations between predator and prey remain obscured
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