Avian Dispersal Networks, Metacommunity Structure, and Bryophyte Community Assemblages

Spatial processes have a profound influence on the structure and function of community assemblages. The dispersal of organisms from their place of origin to the location in which they live out their reproductive life is particularly important for plant communities, which generally cannot adjust their location post-germination. Connection between communities at a landscape scale can also influence species persistence, local and regional diversity, and functional turnover at the metacommunity scale. Animals have been shown to disproportionately deposit propagules in particular microsites in many plant species, facilitating the arrival of plants to appropriate niche-space. Birds are particularly notable seed dispersers, given their ability to fly long distances and their behavioral inclination toward using specific microsites within their habitat for foraging and nest building. Despite the known influence of animal behavior on plant dispersal outcomes, little work has been done to investigate the role of animals in dispersing bryophyte (moss, hornwort, liverwort) propagules. In order to examine how birds may affect bryophyte dispersal, I conducted two studies focused on understanding how bird species identity and behavior influence the bryophyte propagules they carry. In addition, I conducted a study to understand how metacommunity structure across a landscape can be influenced by focal spatial scale. In the first study I examined how bird species and foraging behavior impact the topical load of bryophyte spores found on bird surfaces. In order to determine this, I captured passerine birds in mist nets and swabbed them for spores. I found that spores were more abundant on passerine tails than legs, and that overall spore load was higher on larger birds. Thrushes in particular carried more spores than other groups overall. Bark and foliage foraging birds had more spores on their tails than ground foraging birds. From these samples I was able to germinate 242 individual bryophytes, demonstrating that carried spores were readily viable. In the second study, I examined species-specific relationships between bryophytes and the birds carrying them. Swabs from captured birds were grown in the lab and bryophyte species were determined genetically. I used a bipartite network approach to determine the level of specialization of associations within the overall network, as well as how specialized the avian associations of individual bryophyte species were. I then used the phylogenetic distance of bryophytes found on individual bird species in order to assess how specialized the assemblages on a given bird species were compared with a null, random model. I found that bryophyte associations with birds were nonrandom, and that the extent to which those associations were specialized differed by bird foraging behavior. In addition, I found that the diversity of propagules on bird surfaces was significantly nonrandom, with the exception of those bryophytes found on Spotted Towhees. In the final study, I examined the metacommunity structure of bryophytes at both patch and landscape scales across a relict landscape of Valdivian forest in North-Central Chile. This landscape consists of distinct natural patches of forest maintained by coastal fog deposition, surrounding by a dry matrix inhospitable to patch-resident bryophytes. I used quadrats to sample bryophyte species abundance at the base and at breast height of ten trees in each patch, in 20 patches across the landscape. I found that when considering the whole park as one metacommunity, the bryophyte community exhibited a Gleasonian structure, in which individual species turnover was idiosyncratic. Considering assemblages from both heights separately, a Clemenstian pattern was observed, suggesting that within each height compartment, turnover of species tended to happen together. Treating each patch as a metacommunity of individual community trees resulted in a wide variety of metacommunity structures across the park that did not reflect either longitude or latitude. Low canopy cover and small DBH resulted in structures reflecting random species loss. Underlying Shannon diversity did not explain differences in the observed structures. This dissertation provides the first evidence that passerine birds carry bryophyte propagules, and that their individual species use of habitat and foraging behaviors are likely to influence the number and diversity of the bryophytes they are dispersing. This has implications for understanding disjunct species and genetic distributions observed in bryophytes that to date have lacked an explanatory mechanism for long distance directed dispersal. In addition, understanding how avian behavior may disperse propagules at a local to regional scale may provide better insight into the trajectory of bryophyte recruitment on impacted landscapes. I also found that assignation of metacommunity structure is sensitive to spatial scale in bryophytes. Together, these findings increase our understanding of the role that spatial processes play in forming bryophyte communities