Wintering Swan Geese maximize energy intake through substrate foraging depth when feeding on buried Vallisneria natans tubers

Yan Chen,Anthony D Fox,Lei Cao,Yong Zhang,Willem F De Boer

doi:10.1186/s40657-019-0145-x

Abstract

BackgroundForaging theory predicts that animals select patches that offer the highest net rate of energy gain. Hence, prey distribution patterns and spatiotemporal heterogeneity play important roles in determining animal feeding patch selection. For waterfowl foraging on buried aquatic plant tubers, the distribution and biomass of these plant organs vary with depth in the substrate. Since excavation costs also increase with depth, the energy intake of the animals foraging on these plants is highly sediment depth dependent.MethodsHere, using observations of Swan Geese (Anser cygnoides) foraging on Vallisneria natans tubers, we test our hypothesis that geese feeding on tubers buried at intermediate sediment depth maximize their daily energy intake because of the interaction between tuber size and abundance with depth. To do this, we measured the distribution patterns of buried Vallisneria tubers under both undisturbed conditions and post-exploitation by geese (i.e. giving-up conditions). We investigated the relationship between tuber size and burial depth, and total tuber biomass within each sediment layer in undisturbed and exploited plots. Finally, we compared modelled Swan Goose daily energy intake feeding on Vallisneria tubers buried at different sediment layers (1–10, 11–20 and 21–30 cm below the surface).ResultsDry weight of Vallisneria tubers linearly increased with burial depth, while average total dry weight density of tubers showed a unimodal relationship, peaking at intermediate levels. Not surprisingly, Swan Geese foraged most intensively on tubers buried at intermediate sediment depths, where they maximize their daily energy intake. Our results support our hypothesis that Swan Geese feeding on tubers at intermediate depths maximize their daily energy intake.ConclusionsOur study is the first to quantify foraging strategies of Swan Geese during the wintering period, emphasizing the importance of plant traits on foraging selection of belowground foragers.

Highlights

Foraging theory predicts that animals select patches that offer the highest net rate of energy gain
Diet analysis Diet analysis showed that all 100 samples taken from December to January comprised 100% Vallisneria tuber fragments and those in February 98%, confirming that geese almost exclusively fed on Vallisneria tubers at these sites during these months
A quadratic regression model captured the relationship between mean tuber dry weight density (g/m2) and tuber burial depth well, with highest tuber

Summary

Introduction

Foraging theory predicts that animals select patches that offer the highest net rate of energy gain. Prey or plant distribution patterns and their heterogeneity play an important role in determining densities of foraging predators and herbivores (the aggregative response, e.g., Rowcliffe et al 1999). For a given tuber size, the chance of germinating and sprouting to the surface successfully decreases with burial depth (Klaassen and Nolet 2007; Hamberg et al 2017). For this reason, we would predict that plants balance the costs of such an avoidance strategy amongst deeply buried tubers by increasing their energy content to enhance the likelihood of successful germination and growth to the surface (Jokela et al 2000). Larger tubers are often found at deepest levels in the sediment, because costs of germination, shoot initiation and penetration to the surface increase with increasing burial depth (Santamaria and Rodriguez-Girones 2002)

Methods

Results

Discussion

Conclusion