Ecosystem responses to the argentine ant invasioneffects on vertebrates - respuestas del ecosistema ante la invasión de la hormiga argentina: efectos en vertebrados

Resumen

ABSTRACT Biological invasions are one of the main drivers of biodiversity loss. Certain effects of invasive species are commonly overlooked and potentially compromise the structure and function of ecosystems in unpredictable ways. This thesis takes advantage of the vast knowledge of the invasive Argentine ant, "Linepithema humile", to focus on subtle and indirect effects on the recipient ecosystems, specifically on native vertebrates (amphibians, birds) in Doñana National Park (southern Spain). This tiny intruder is notorious for displacing the native ant community in ecosystems it has successfully invaded. Therefore, this thesis assesses whether the effects of potential prey depletion scale to higher trophic levels. Although the Argentine ant lacks recognized weapons, it is well known for its aggressiveness when outcompeting native ants. I evaluated whether native vertebrates at their most vulnerable stages are susceptible to being attacked or disturbed by the invasive ant and if they suffer from lethal or sublethal effects that hamper their development. These issues were addressed through field sampling and monitoring, field and laboratory experiments, and analyses of chemical, isotopic, histological, physiological, and behavioral parameters. My results reveal the Argentine ant invasion has negative impacts on native vertebrates in Doñana at different levels. For example, the native amphibians studied here have altered their diet, by shifting to non-ant preys in infested areas. Additionally, vertebrates in their early stages of development showed poor body condition under experimental (juvenile amphibians) and field (chicks) conditions when they were fed a diet supplemented with Argentine ants or raised in invaded areas. Furthermore, both myrmecophagous species, such as the natterjack toad, Epidalea calamita, and non-ant predators, such as the great tit, Parus major, modified their habitat use in invaded compared to uninvaded areas, although for different reasons (foraging and breeding, respectively). Finally, I demonstrate, for the first time in the literature, that the invasive Argentine ant has a powerful venom—iridomyrmecin—that is able to paralyze and kill native vertebrates, specifically juvenile amphibians. This unexpected finding deserves special attention, as it may play a key role in the previously demonstrated negative effects/invasiveness of this species. Overall, this thesis reveals overlooked indirect and subtle effects of an invasive species at different ecosystem levels, but also yields novel information about the mechanisms underlying these effects.