The effect of horses on sandy shore infauna

Beaches are under increasing pressure from anthropogenic threats, which include human trampling and 4WD crushing, but the impacts of horses on beaches are unknown. Horses on beaches may alter the structure of sandy shore food webs by trampling invertebrates and altering the physical properties of beaches. This study examines the impacts of horse trampling by measuring and indexing the abundance, diversity and assemblage composition of beach invertebrates at paired sites with and without horses at nine beaches (locations) across Victoria, Australia. Beach rugosity, slope, compaction and coverage of wrack were analysed because they may influence infauna, or be influenced by the presence of horses. Human and horse usage was indexed to determine the degree of usage at each site among locations. Horses were more common at sites where they were allowed and sites (horse versus no horse) within beaches were well matched, having similar slope, seaweed coverage, human and canine activity, but these attributes varied across locations. At sites with horse activity, there was less sand compaction in the upper beach (above the previous high tide) suggesting horses disrupt the sand matrix there. While invertebrates sampled by pitfall traps did not differ in richness or abundance between horse and no horse sites, the assemblage composition differed between sites, at seven of nine locations. Horse sites were associated with fewer isopods, a known key prey item of threatened shorebirds, and a group which is surface active on beaches. Amphipods were more common at horse sites, perhaps because the disrupted sand activated this group of burrowing invertebrates, which are also a major prey item of shorebirds. This study shows that horses disrupt the sand matrix on beaches and can alter assemblage structures of invertebrates at many beaches. The extent to which this alteration compromises the function of sandy shore food webs should be the subject of further, experimental study.