Ubiquitous on the small, rugged islands of British Columbia’s central coast, North American river otters bridge the marine-terrestrial interface by foraging in the sea and living on land. Their patchy land use includes extensive, concentrated, and clearly circumscribed activity centers (ACs), where they modify terrestrial shoreline habitats by adding marine nutrients (feces, urine, and food waste), and causing physical disturbances (trampling/digging soil and vegetation, moving leaf litter, etc.). Through these activities, river otters may be important drivers of ecosystem structure and biodiversity on coastal islands, but the extent of their influence and the mechanisms by which they affect other terrestrial taxa is not well known. We examined structural and chemical properties of soil, leaf litter, plants, and invertebrate communities on twelve river otter ACs, as well as in areas immediately adjacent to, and 10 m offsite from, the ACs. The objectives of our study were to determine whether – and how – invertebrate abundance and diversity are affected by river otter activities (nutrient additions and/or physical disturbances), and whether the effects are taxon or trophic-level dependent.
ACs had compacted soils, shallow leaf litter, open tree canopies, short vegetation, and reduced plant cover compared to adjacent and offsite areas. Adjacent areas had lower plant diversity than offsite, and moderately compacted soil, but plants were tall and there was a thick leaf litter layer with a high marine detritus component. Offsite areas had greater plant diversity, loose soils, a shallower leaf litter layer, and greater tree canopy cover. We collected over 7,100 invertebrates from leaf litter and beat sheet samples. Mites made up nearly 60% of the total sample, followed by springtails (~22%), and spiders (~12%). Invertebrate abundance was generally lower on ACs than other areas. Herbivorous invertebrates were most abundant and diverse in adjacent areas, where foliage is nutrient-enriched; herbivores are scarce in AC samples, where there is a marked loss of vegetation. Similarly, larger detritivores were most abundant in the deep and enriched leaf litter of the adjacent areas. Visual surveys of web-spinning spiders, and spiders collected from litter samples, indicated reduced abundance on ACs, likely due to the absence of vegetative structure, litter, and associated prey. Small decomposers (mites, springtails) were less affected by structural AC modifications. River otters are drivers of invertebrate communities on islands: changes in otter distrubutions (e.g., due to loss of marine prey) could have important implications for coastal ecosystems.