Seabird guano inputs increase impacts from introduced mammals on the native plants and animals of an oceanic island.

Seabirds create fluxes of nutrients from marine to terrestrial ecosystems that influence the food webs of small islands. We investigated how guano inputs shape terrestrial food webs by comparing species of selected plant and animal species in a red-footed booby colony in Mona Island (Puerto Rico, Caribbean Sea), to sites of the island lacking guano inputs. We quantified guano deposition and its relationship to plant biomass production, fecundity and density, as well as the activity of native and introduced animal species. In general, guano inputs increased the gross primary plant productivity, size, and fecundity by twofold. Guano inputs were also associated with twofold increases in density of Anole lizards, but also to increases in the activity of introduced pigs (> 500%), goats (> 30%), and cats (> 500%), which negatively impact native species. In particular, elevated pig and cat activity within the booby colony was correlated with lower activity of endemic ground lizards and of introduced rats. Our results also suggest that severe droughts associated with climate change exacerbate the negative effects that introduced species have on vegetation and reduce the positive effects of seabird guano inputs. Our findings underscore the importance of allochthonous guano inputs in subsidizing plant productivity and native and endemic species in small oceanic islands, but also in increasing the negative impacts of introduced mammals. Management and conservation efforts should focus on the exclusion (or eradication) of introduced mammals, particularly pigs and goats, from remnant seabird colonies in Mona Island.