A predatory waterbird as a vector of plant seeds and aquatic invertebrates

1. Omnivorous waterbirds play an important role in aquatic ecosystems as dispersal vectors via direct ingestion, transportation and egestion of plant and invertebrate propagules (i.e. endozoochory). Predatory birds also have the potential to disperse plants and invertebrates that were first carried internally or externally by their prey animals. However, the potential contribution of predatory waterbird species to propagule dispersal in aquatic ecosystems remains understudied. 2. We chose the grey heron Ardea cinerea (Ardeidae) to study the potential of predatory waterbirds to disperse propagules within and among aquatic ecosystems. We hypothesized that (1) herons disperse a wide variety of plant and invertebrate propagules, from different habitats, with different morphologies (i.e. dispersal syndromes), and including both native and alien species; (2) propagules are ingested with prey species that are primary dispersal vectors (i.e., herons are secondary dispersers); (3) heron pellets show a similar abundance and richness of propagules across their widespread range. We collected 73 regurgitated heron pellets containing undigestible remains from 12 locations across the United Kingdom and The Netherlands, and examined the taxonomic diversity of plant seeds, invertebrates and prey remains. 3. Pellets were dominated by mammal hairs (99% by volume), and bones confirmed the ingestion of small mammals (prevalence of 38%, e.g. water voles Arvicola amphibius), fish (14%), and birds or amphibians (6%). A total of 266 intact plant seeds were recovered from 71% of the pellets, representing 50 taxa from 17 plant families, including the alien Cotula coronopifolia. The cumulative number of plant species dispersed was lower at higher latitudes. Eight plant species recorded had not previously been recorded as dispersed via waterbirds, and only three species have an “endozoochorous dispersal syndrome”. Plant taxa were dominated by Caryophyllaceae, Cyperaceae, Juncaceae and Poaceae, with 24 species from the littoral zone (Ellenberg moisture values of 7-12) and 21 terrestrial species (Ellenberg moisture values of 4-6). Intact invertebrate propagules were found in 30% of the pellets, dominated by Cladocera (Daphniidae) and Bryozoa (including the alien Plumatella casmiana). 4. Our results demonstrate that grey herons disperse plant seeds and aquatic invertebrates widely in north-western Europe. Herons regurgitate pellets that contain plant and invertebrate propagules from both aquatic or terrestrial habitats, for which secondary dispersal via ingestion along with prey is the likely underlying mechanism (i.e. propagules either attached to or in the digestive systems of the various prey). Our findings showcase the potential of predatory waterbirds as vectors of plants and invertebrates, and how they may facilitate connectivity between freshwater and terrestrial habitats.Peer reviewe