Linking functional responses and trade-offs in habitat selection of a semi-nomadic waterbird

Background/Question/Methods

Functional responses in habitat selection can occur when animals must choose between habitats that provide contrasting resources (e.g., quality forage vs. safety from predators), resulting in habitat use that is dependent on habitat availability. The majority of studies have examined these trade-offs in large mammals living at high latitudes in the northern hemisphere. Little research has been done on habitat choices faced by nomadic animals that occupy landscapes with unpredictable changes in resource distributions. To understand limiting factors at the home-range scale, we investigated the forage-predation risk trade-off of the semi-nomadic Egyptian Goose (Alopochen aegyptiaca) by analysing its habitat selection and functional responses. Using 12 months of fine-scale telemetry data for 10 Egyptian Geese tagged in two contrasting landscapes within South Africa, we examined: (1) how habitat selection for habitat types varied across seasons, sites, and life-stages; (2) whether Egyptian Geese showed functional responses in habitat use; and (3) whether functional responses related to trade-offs between prioritising the use of habitats that provide either lower risk of predation or higher quality forage at the home-range scale. 

Results/Conclusions

Variation in habitat selection was best explained by season and site. Egyptian Geese showed marked selection for wetlands and agricultural habitats in both landscapes throughout the year, although the magnitudes of selection coefficients differed (wetlands: dry season > wet season; agricultural: wet season > dry season). Our results indicate that habitat selection is driven by changes in resource distribution and less so by life-stage requirements. Contrary to studies which emphasize the role of individual variation, Egyptian Geese showed high levels of synchrony in habitat choices across both landscapes. Egyptian Geese showed a negative functional response, with selection for both wetlands and agricultural habitats decreasing as their availability increased. Egyptian Geese face a trade-off between using wetlands, which provide a safer environment with lower quality forage, and agricultural land, which provides highly nutritious forage with higher levels of predation and human disturbance. Our results suggest that at the home-range scale, both forage optimisation and predation risk were limiting factors. Functional responses thus need to be considered across a range of environmental gradients in order to fully understand the fitness consequences of trade-offs experienced by nomadic animals.