Metabolic costs drive seasonal habitat shifts

Posted on December 16, 2014

Most foraging studies have assumed that the metabolic costs of finding food are fixed, despite the fact that animals often switch seasonally between different types of habitat. For example, herbivorous waterfowl such as ducks, geese and swans often move seasonally between streams and adjacent pastures, and other species cycle between river and lake habitats. To examine how habitat shifts by waterfowl relate to feeding conditions, mute swans (Cygnus olor) were studied between March and September on the River Frome, in Dorset, U.K.. Swans fed on pasture grasses (e.g., rye grass, bent grass) in winter and spring, and on water crowfoot (Ranunculus penicillatus) in the river in summer and autumn. Samples of aquatic and pasture plants were collected each month and swan feeding rates were estimated from experimental data. Metabolic data were obtained from published sources. Using an optimal foraging model, the net amount of energy gained by swans was calculated on a monthly basis. The quality (energy content) of food didn’t vary seasonally, and although the biomass of water crowfoot fluctuated, the amount that was eaten by swans stayed relatively constant. In contrast, the metabolic costs of foraging fell by 73% between March and September. This was linked to a substantial drop in mean water velocity, and hence the energy that swans needed to allocate to swimming. The foraging model was run with different assumptions regarding food quality, food quantity and foraging costs. In all cases where foraging costs were assumed to vary seasonally, the model correctly predicted a habitat shift to river feeding in March-April, but when costs were assumed to remain constant, a habitat switch wasn’t predicted. So swans delayed the shift to the river until the net energy gain from foraging outweighed the metabolic cost of feeding in flowing waters. Therefore, seasonal changes in the metabolic costs of foraging, as opposed to the quality or quantity of food, can drive shifts in the habitats used by aquatic species.

Reference: Wood, K.A. et al. 2013. Go with the flow: water velocity regulates herbivore foraging decisions in river catchments. Oikos 122, 1720–1729.