Feral horses (Equus caballus) increase suspended sediment in subalpine streams

Abstract

Feral horses preferentially utilise aquatic landscape elements. Damage to stream banks by feral horses potentially degrades water quality and affects aquatic ecosystems. Despite the global predominance of horse activity in or near water, there is poor understanding of in-stream impacts. Motion-sensing cameras were used near subalpine streams to quantify frequency and behaviour of horses interacting with streams. Deployed turbidity loggers were used to indicate the presence of suspended sediment from interactions and to assess long-term (3 year) seasonal suspended sediment regime at sites with very small and large amounts of horse activity. In summer, 99.9% of large grazer interactions with streams were horses and we estimate 130 horse interactions per kilometre of stream per day, resulting in stream turbidity increases up to 100 NTU. Long-term turbidity at sites with greater riparian damage from horses was significantly greater than at control sites. Turbidity in horse-affected streams peaked at 50 times the national turbidity guideline and, in summer, averaged eight times the guideline. Suspended sediment leads to a high risk of loss of aquatic diversity and impairment of ecosystem function. Linear relationships between turbidity and riparian damage suggest that any level of horse presence affects subalpine waterways.