Small seeds may be less vulnerable to rat (Rattus rattus) predation in Hawaiian forests

Background/Question/Methods Because most oceanic island ecosystems developed in the absence of terrestrial mammals, many island species are vulnerable to introduced mammalian predators.  Rats (Rattus spp.) are among the most widespread and significant invasive mammals affecting islands.  Rats may reduce recruitment through seed predation or enhance it by dispersing seeds when they consume fruits without destroying the seeds.  The features that determine whether island plants will suffer or benefit from the effects of rats are not well understood.  Vulnerability to rat predation was tested in Hawai‘i through field trials in mesic forests and laboratory trials (captive feeding) with the most common rat from the study sites (R. rattus).  In the field, fruits of 11 species were arranged individually in four treatments (no-vertebrate-access, small-vertebrate-access, open-access, and open-access-cage-control; N = 8 for each treatment) and fruit removal was recorded for up to 21 d.  Motion-sensing cameras were used on a subset of replicates to identify the animal responsible for fruit removal.  In the laboratory, feeding trials enabled the assessment of seed fate (predation vs. dispersal) of a wider array of seed sizes and plant species (45 species from 28 families). 

Results/Conclusions

Field trials revealed that 10 of the 11 species were removed by vertebrates, and the only animal photographed removing fruit was R. rattus.  A wide range of fruit and seed damage was recorded for the 45 species in the laboratory trials; most species had their seeds destroyed and were classified as vulnerable to rats.  Species that had low vulnerability to rat predation were those whose seeds were not eaten or were small enough to pass intact through their digestive system.  Combining data for all Hawaiian species assessed, we identified a size threshold for survival of seeds ingested by R. rattus.  Seeds that are consumed by R. rattus may be dispersed if their longest axis is <1.8 mm, whereas seeds above that threshold have a much greater chance of being destroyed.  This seed size threshold can be extended to island floras worldwide in order to better assess the vulnerability of plant communities to R. rattus.