Delving into the plant-animal double mutualisms and the importance of opportunistic vertebrates as pollinators in island ecosystems

Abstract

Double mutualism (DM) is a recently observed and considered phenomenon in ecology, in which a same animal species pollinates and disperses a same plant species. Very few DMs have still been reported, and little is known about their occurrence and species involved. Generalist and opportunist animal species, which have broad diet niches and sometimes act as efficient pollinators and seed dispersers, might be particularly good candidates to stablish double interactions, as they more likely may consume nectar, pollen and fruits of a same plant. Still little information exists, however, about their effectiveness compared to specialised species. In Chapter 1 we conducted a systematic review of published and unpublished data to provide geographical, taxonomical and ecological baseline information for future research on DMs. Most DMs occur in tropical and island ecosystems, involving mainly opportunistic vertebrate nectar consumers and generalist plants. Moreover, nearly 30 % of the species involved in DMs are threatened. High prevalence of DMs on islands and their vulnerability suggest that many DMs are also threatened with still unknown consequences for the maintenance of community composition and ecosystem functioning. In order to explore the importance of opportunistic animals as double mutualists, in Chapter 2 and 3, we analysed the role of a nectar-opportunist lizard (Podarcis lilfordi) who visits the flowers and consumes the fruits of the plants Ephedra fragilis and Cneorum tricoccon. Given that conspecific individuals may have different potential roles in pollination and seed dispersal, and, thus may have different consequences for plant success, intraspecific variation was integrated in the observational work and analyses. Our results show that P. lilfordi acted as an effective double mutualist, however, adult female and male lizards and juveniles were differently involved in both pollination and seed dispersal. Our results support the hypothesis that intraspecific differences in mutualistic species constitute important differences which may result in different mutualistic roles. Thus, studies performed at the species level might mask the real function of intraspecific groups and/or individuals on mutualistic interactions. In order to analyse and compare the importance of opportunistic nectarivore vertebrates compared to specialists and insects, in Chapter 4, we measured the quantity and quality components of pollination effectiveness (PE) of vertebrate and insect pollinators of three plant species in the Seychelles archipelago (Thespesia populnea, Polyscias crassa and Syzygium wrightii). Our study indicates that some Seychelles plant species depend on insects and specialised vertebrate pollinators, but also on opportunistic vertebrate nectar-feeders. Furthermore, we showed that PE of a particular pollinator species varies depending on the plant species, regardless of its feeding behaviour. Results also highlight the vulnerability of pollination interactions to invasive alien species, as the PE of vertebrate pollinators was compromised by the presence of the invasive yellow crazy ant, Anoplolepis gracilipes, which primarily reduced their flower visitation rates. This thesis contributes with a wide picture of DMs around the World, and with some analysed examples. It also underlines that measuring effectiveness of interactions and their components is important and need to be integrated in mutualistic studies, as well as focus such studies at intraspecific level, otherwise the real function of each intraspecific group is masked. We hope that this thesis is the beginning of a change in the way of perceiving and studying mutualisms and, in general, ecological interactions.