Factors influencing seed predation in rainforest tree communities of subtropical Australia: Fragmentation, seed size and shifts in vertebrate assemblages

Abstract

Rainforest fragmentation has large impacts on ground-active vertebrates, including granivorous mammals (e.g., rodents), and birds. We tested for differences in community-wide patterns of interference with seeds of rainforest trees, between fragments and continuous forest. We also assessed whether these differences were associated with changes in the seed predator communities. Seeds of 20 locally-common tree species were deployed at experimental stations in 12 rainforest sites (six continuous forest and six fragments) across an agricultural landscape in subtropical Australia, where vertebrates rarely cache seeds. Eleven species had small seeds (<10 mm), nine had larger seeds. We recorded seeds' fates after five days. Concurrently, we used automated motion-sensitive cameras to record seed predators' frequencies, and their interactions with seeds. Across all sites and species, 47% of seeds disappeared (varying among species from 17% to 76%), while 6% were partly damaged. Overall rates of disappearance and partial damage did not differ between habitats. However, seeds of five tree species were significantly more heavily predated, and those of five species less predated, in fragments than in continuous forest (10 species not differing significantly). Among-site variation in seed predator species composition was closely associated (r = 0.96) with the multispecies pattern of seed predation, while both differed significantly between continuous forest and fragments. We conclude that rainforest fragmentation strongly influenced patterns of seed mortality, as an indirect consequence of the vertebrate seed predators' responses to habitat fragmentation. In turn, this could induce both increases and decreases in tree species' regeneration rates, and changes in future tree species composition.