Impacts of rodent eradication on seed predation and plant community biomass on a tropical atoll

Invasive rodent eradications are frequently undertaken to curb island
biodiversity loss. However, the breadth of rodents’ ecological impact,
even after eradication, is not always fully recognized. For example, the
most widespread invasive rodent, the black rat (Rattus rattus), while
omnivorous, eats predominantly seeds and fruit. Yet, the effects of seed
predation release after eradication on plant communities and ecological
functions are not well understood, posing a gap for island restoration. We
examined the role of seed predation release following black rat
eradication in changes to tree composition and above-ground biomass across
an islet network (Palmyra Atoll) in the Central Pacific. We conducted
repeated surveys of seed, juvenile, and adult tree biomass and survival in
permanent vegetation plots before and after the eradication of rats. We
observed a 95% reduction in seed predation for an introduced, previously
cultivated tree population (Cocos nucifera). Juvenile tree biomass of all
species increased 14-fold, with C. nucifera increasing the most,
suggesting that eradication increased this tree’s competitive advantage.
Indeed, based on stage-structured demographic models, rat eradication led
to a 10% increase in C. nucifera population growth rate. The effect of
invasive rodent seed predation varies considerably among the plant species
in a community and can shift competitive dynamics, sometimes in favor of
invasive plants. These bottom-up effects should be considered in
evaluating the costs and benefits of eradication.  Documenting
the variation in invasive rodent diet items, along with long-term surveys,
can help prioritize island eradications where restoration is most likely
to be successful.