Non-native mammals are weak candidates to substitute ecological function of native avian seed-dispersers in an island ecosystem

Although prominent examples exist of non-native species causing substantial ecological harm, many have neutral or positive effects, including filling surrogate roles once performed by extinct native organisms. We tested the ecological roles of two non-native mammals as seed dispersers or seed predators in Guåhan, which, due to invasive brown treesnakes (Boiga irregularis), is devoid of native seed dispersers – birds and bats. We conducted feeding trials with captive rats (Rattus spp.), which are present but uncommon due to predation by snakes, and pigs (Sus scrofa), which are abundant. We examined if and how they interacted with common forest fruits. We then compared how any gut-passed or handled seeds germinated compared to seeds left in whole fruit or depulped seeds.
Rats and pigs interacted with most of the fruits and seeds (>80%) that they were fed. Of those, most seeds were destroyed – 78% for rats and 90% for pigs, across both native and non-native plant species. Compared to se...