Experimental evidence that the ecosystem effects of aquatic herbivory by moose and beaver may be contingent on water body type

Abstract

Summary Mammalian herbivores that consume both aquatic and terrestrial vegetation may have important but little understood effects on freshwater ecosystems. We assessed the effects of North American moose ( Alces americanus ) and/or beaver ( Castor canadensis ) on aquatic vegetation and abiotic conditions of three types of waterbodies. We established year‐round aquatic exclosures and reference plots in glacial lakes, dammed lakes and riverine ponds ( n = 3 in each case). Within plots, we monitored parameters including dissolved oxygen, light, plant diversity and species‐specific plant biomass. The effects of herbivory by beaver and moose appear to be contingent upon abiotic and plant community characteristics. Exposure to herbivory decreased biomass in riverine ponds and dammed lakes but not in glacial lakes and decreased species richness in riverine ponds and glacial lakes but not in dammed lakes. Changes in macrophyte species diversity correlated with exposure to herbivory only in glacial lakes. Disaggregating the effects of herbivory according to abiotic conditions and plant community characteristics revealed outcomes that are not detectable when results are aggregated. In some catchments, the response of the aquatic plant community to beaver and moose herbivory may substantially differ from the response of the surrounding terrestrial plant community. Five of the six waterbodies created by beavers (dammed lakes and riverine ponds) maintained >80% macrophyte cover, despite seasonal reduction in biomass by both moose and beaver. Herbivores appear to cause a short‐term reduction in plant biomass in dammed lakes, resulting in greater light availability, without depleting biomass over a number of years.