Abstract
Freshwater environments are fragmented and heterogeneous in space and time. Long term persistence thus necessitates at least occasional dispersal of aquatic organisms to locate suitable habitats. However, the insubstantial movements of many require zoochory –hitchhiking a ride with more mobile animals. We review evidence for zoochory of freshwater bryozoans, a group that provides an excellent model for addressing this issue. The feasibility of long distance transport by waterbirds of bryozoan propagules (statoblasts) is evaluated in relation to statoblast resistance to extreme conditions and waterbird gut retention times, flight durations and distances. We highlight genetic evidence for colonization following waterbird-mediated transport. The consequences of zoochory for biodiversity are manifold. Taxa that release statoblasts show lower levels of genetic differentiation, genetic divergence and haplotype diversity than those whose statoblasts are retained in situ (hence less available for zoochory). Zoochory may also disseminate pathogens and parasites when infected host stages are transported. Such co-dispersal may explain some disease distributions and is supported by viability of infected statoblasts. Zoochory can also be expected to influence local and regional population and community dynamics, food web structure and stability, and organismal distributions and abundances. Finally, zoochory may influence host-parasite coevolution and disease dynamics across the landscape with the benefits to parasites depending on their life history (e.g. simple vs. complex life cycles, generalists vs. specialists). Our synthesis highlights the complex ecological and evolutionary impacts of zoochory of freshwater organisms and raises questions for future research.
Highlights
Freshwater environments are patchy in space and time and surrounded by an inhospitable landscape
Bird species with heavier gizzards and shorter ceca had a lower incidence and abundance of statoblasts in the lower gut. These results suggest that statoblast dispersal is more likely in birds with lighter gizzards and that longer ceca will entail a longer passage time which, in turn, may favor long distance dispersal
There is limited evidence that endozoochory is more common than ectozoochory and that some waterbird species are more important as vectors, sample sizes were low and analyses based on pooling statoblasts and cladoceran ephippia
Summary
Freshwater environments are patchy in space and time and surrounded by an inhospitable landscape. This collective evidence suggests that a diversity of birds could mediate both local and long distance dispersal. There is limited evidence that endozoochory is more common than ectozoochory (but viability was not tested) and that some waterbird species are more important as vectors, sample sizes were low and analyses based on pooling statoblasts and cladoceran ephippia
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