Non-native species have caused ecological harm in many ecosystems, but they can also play beneficial roles when native species are missing. We tested the roles of two non-native mammals in the forests of the island of Guam, which, due to invasive brown treesnakes (Boiga irregularis), are devoid of native birds and bats, the island’s main seed dispersers. Feral pigs (Sus scrofa) are abundant in Guam’s forests, but rats (Rattus spp.) are uncommon in forests due to predation by snakes. We conducted feeding trials with captive rats and pigs in Guam to determine their impact on seeds. We fed over 20 species of fruits to rats and pigs, recorded their interactions with these fruits, and tested germination of gut-passed and handled seeds.
Results/Conclusions
Rats destroyed most of the seeds that they were fed. Pigs swallowed and passed seeds from mostly fleshy, many-seeded pioneer species. Both rats and pigs ate higher proportions of sweet, fleshy, invasive fruits, compared to native fruits. Both consistently cleaned the flesh off of larger-seeded fruit species, resulting in improved germination for these seeds. While pigs did successfully pass more seeds than rats, this was species- and size-dependent, and pigs still destroyed or ignored a large number of seeds. These species-specific effects of non-native species need to be considered as plans develop to control invasive snakes and ungulates in order to restore Guam's forests. Further, as novel ecosystems become the norm globally, we are pressed to better understand the mixed roles of native and non-native species in these altered systems.