Avian-bryophyte interaction networks: Insights from an ornithological approach to bryophyte ecology

Matthew W. Chmielewski, Department of Biology and Center for Life in Extreme Environments, Portland State University, Portland, OR and Sarah M. Eppley, Department of Biology and Center For Life in Extreme Environments, Portland State University, Portland, OR

Background/Question/Methods
Animal dispersal is known to largely influence dispersal outcomes in tracheophytes. Animals can increase dispersal distance, direct dispersal to particular locations within a habitat, and influence germination probability. Despite the large volume of literature on this subject, our understanding the role that animals play in dispersing bryophytes (mosses, hornworts, and liverworts) remains limited. Reports that exist are limited to haphazard accounts across a variety of taxa, with little effort made to examine these relationships systematically. Birds are particularly promising vectors of bryophytes, given their association with bryophytes as a substrate for nesting material and arthropod foraging. Recent evidence suggests that some bryophyte distributions may be influenced by avian-mediated long-distance dispersal events, but the breadth of these interactions is currently unknown. We build on our previous work demonstrating that a variety of passerines harbor viable spores topically, by examining species-specific patterns of association in bipartite bryophyte-avian dispersal networks. We further describe a mechanistic approach aimed at determining topical retention time of spores on birds in order to model potential dispersal distances. Finally, we contextualize our findings within the larger scope of animal-bryophyte dispersal phenomena.
Results/Conclusions
We found that avian-bryophyte dispersal networks exhibit a level of specialization of association greater than to be expected by chance, with loose individual species-species dependencies. This diffuse dispersal network is similar in nature to what is seen in seed plant-animal dispersal relationships, where associations between plant seeds and suites of animals with similar traits rather than associations with specific species is the norm. We expect that future work may uncover co-dispersal of bryophytes on particular bird species. Clarifying these relationships will allow for better prediction of how avian diversity in a given location may impact bryophyte dispersal outcomes.
We generated preliminary topical retention time curves of bryophyte spores applied experimentally to bird surfaces. We retrieved spores from bird surfaces throughout our entire 15-day sampling period, with a large initial drop-off and high variance. While our sampling methods allow for examination of relative rather than absolute quantification, birds were capable of carrying dozens of spores on a single sampled location throughout the study period, suggesting that the entire available surface of individual birds may carry ecologically relevant spore loads within timeframes consistent with dispersal at landscape-continental scales. Our systematic approach to understanding the mechanisms of dispersal associations in this system highlight a larger field of animal dispersal of bryophytes that we propose merits further study.