Beavers and lilies: Selective herbivory and adaptive foraging behaviour

Abstract

1. With the global population of beavers (Castor spp.) increasing, and reintroductions widespread, it is crucial to be able to predict potential impacts on flora and fauna based on defined foraging behaviours. 2. Nymphaea alba (white water lily) is regularly consumed by beavers and provides a model system to test selective foraging behaviour and quantify potential impacts on aquatic resources in standing-water habitats. 3. Using biometric relationships within N. alba pads, we accurately reconstructed the size and weight of consumed pads, demonstrating that beavers (Castor fiber) selected pads that were significantly larger and heavier than unselected pads. By selecting larger leaves, beavers may also avoid chemical defences associated with anthocyanin pigments that dominate in smaller leaves. Grazing was concentrated in shallow depths (55.7 ± 10.7 cm) close to the shore (2.95 ± 0.62 m) relative to ungrazed plots (100.5 ± 9.2 cm; 4.79 ± 0.68 m). The level of selectivity was unchanged with increasing distance from a central feeding place. 4. Beavers removed 24-50% of pads within grazed areas, but relative to the whole N. alba leaf pad resource, the impact of this foraging was low (0.38-1.23% loss). Plant species diversity was unaffected by foraging, and there was no evidence of indirect effects on non-targeted N. alba pads or flowers. 5. When foraging in the aquatic environment, beavers are highly selective and can have a minor effect on food resources whilst feeding optimally and opportunistically. Since beavers demonstrate adaptive foraging strategies depending on their foraging environment, this knowledge should be incorporated into future decisions on further reintroduction or habitat restoration programmes.