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Studies from seed plants have shown that animal dispersal fundamentally alters the success of plant dispersal, shaping community composition through time. Our understanding of this phenomenon in spore plants is comparatively limited. Though little is known about species-specific dispersal relationships between passerine birds and bryophytes, birds are particularly attractive as a potential bryophyte dispersal vector given their highly vagile nature as well as their association with bryophytes when foraging and building nests. We captured birds in Gifford Pinchot National Forest to sample their legs and tails for bryophyte propagules. We found 24 bryophyte species across 34 bird species. We examined the level of interaction specificity: (i) within the overall network to assess community level patterns; and (ii) at the plant species level to determine the effect of bird behaviour on network structure. We found that avian–bryophyte associations are constrained within the network, with species-specific and foraging guild effects on the variety of bryophytes found on bird species. Our findings suggest that diffuse bird–bryophyte dispersal networks are likely to be common in habitats where birds readily encounter bryophytes and that further work aimed at understanding individual bird–bryophyte species relationships may prove valuable in determining nuance within this newly described dispersal mechanism.

Contains species name definitions, a table of foraging strategies adapted from the EltonTraits 1.0 database, trees used to calculate Faith's PD, cover of bryophytes in the local environment, and a heatmap of bipartite relationships.

The evolution of sustained plant–animal interactions depends critically upon genetic variation in the fitness benefits from the interaction. Genetic analyses of such interactions are limited to a few model systems, in part because genetic variation may be absent or the interacting species may be experimentally intractable. Here, we examine the role of sperm-dispersing microarthropods in shaping reproduction and genetic variation in mosses. We established experimental mesocosms with known moss genotypes and inferred the parents of progeny from mesocosms with and without microarthropods, using a pooled sequencing approach. Moss reproductive rates increased fivefold in the presence of microarthropods, relative to control mesocosms. Furthermore, the presence of microarthropods increased the total number of reproducing moss genotypes, and changed the rank-order of fitness of male and female moss genotypes. The genotypes that reproduced most often did not produce sporophytes that produced the most spores, highlighting the challenge of defining fitness in mosses. Nevertheless, these results demonstrate that microarthropods provide a fitness benefit for mosses, and highlight the potential for biotic dispersal agents to alter fitness among moss genotypes in this plant–animal interaction.

Supplementary Figure 1 A) OTC warming experiment on Juan Carlos Point, King George Island, Antarctica. B) Dissected archegonia of female Polytrichastrum alpinum from control plot. C) The dominant terrestrial plant cover (including Polytrichastrum alpinum and Sanionia georgicouncinata), on King George Island.

Model parameters and exploratory figures examining bird dispersal of bryophytes