Amphibian Exposure to Aquatic Herbicides: Ecological Interactions with Invasive Plant Management

Abstract

Loss and degradation of wetland habitats are primary factors in amphibian declines. Wetland restoration may involve the use of aquatic herbicides to remove invasive plants, yet the impacts of aquatic herbicide tank-mixes on wetland fauna have rarely been considered. A paucity of data exists for native amphibian ecology, leading to data gaps for assessing risks of herbicide use to listed amphibians like Oregon spotted frogs (Rana pretiosa). To address those data gaps, a weekly field survey was conducted in Oregon spotted frog habitats during the aquatic weed management season, followed by ecologically-relevant toxicity tests. Detailed information is presented on life histories of Pacific Northwest amphibian species. Results suggest that metamorphic and post-metamorphic anurans and larval salamanders are most at risk of exposure to aquatic weed management. Laboratory toxicity tests were conducted using environmentally-relevant exposure scenarios with aquatic herbicide tank mixes. Exposure rates were estimated for expected concentrations in 2 cm of water after direct over-spray. Anurans were exposed for 96-h, then reared in clean water for 2 mo to assess latent effects. Multiple endpoints were collected, including behavior, body condition, feeding rates, and liver condition. Juvenile Oregon spotted frogs were exposed to imazapyr + Agri-Dex tank mixes at high and low application rates for control of reed canarygrass (Phalaris arundinacea). No significant differences were observed for any endpoint. Metamorphosing northern red-legged frogs (Rana aurora) were exposed to a triclopyr + Competitor tank mix at a labeled rate for control of purple loosestrife (Lythrum salicaria). Metamorphs exposed to the triclopyr tank-mix were stressed during the 96-h exposure, and completion of metamorphosis was delayed by 1 d. Finally, because fear of predation is known to increase mortality of larval amphibians exposed to pesticides and aquatic predators, a proof-of-concept toxicity test incorporating terrestrial predator-prey ecology was developed. Northern red-legged frogs, housed in two different clean-water substrates, were exposed to the visual cue of garter snake (Thamnophis sirtalis) presence. Baseline behavior metrics and sample sizes that would be required for exposures with chemicals were determined. Frogs moved more and used more movements associated with evasion when exposed to snakes.