PhD thesis: Delving into the plant-animal double mutualisms and the importance of opportunistic vertebrates as pollinators in island ecosystems
Photo: (from left to right) Ruben Heleno (on the screen), Anna Traveset, Fran Fuster, Sandra Hervías and Luis Navarro
Esporles, October 13, 2020. Francisco Fuster has defended his doctoral thesis supervised by the doctors Anna Traveset from IMEDEA (CSIC-UIB), and Christopher Kaiser-Bunbury from University of Exeter, UK. The event took place on October 13 at the University of Balearic Islands.
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. This thesis aims to improve knowledge on the novel phenomenon of double mutualism, paying special attention on the importance of opportunistic nectarivores. Specifically, the thesis has three main research questions:
Where DMs are more frequent and which are the taxa mainly involved in them (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 We conducted a systematic review of published and unpublished data to provide geographical, taxonomical and ecological baseline information for future research on DMs.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.
How important opportunistic animals can be as double mutualists (chapters 2 - 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, lizards were differently involved in both pollination and seed dispersal. On E. fragilis, adult lizards were the most involved; but, on C. tricoccon female and juveniles were the main nectar consumers, and consequently pollinators, while males were the main fruit consumers and seed dispersers. 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.
How important nectarivore opportunist vertebrates can be relative to specialists and insects (chapter 4)? We measured and compared pollination effectiveness (PE) of vertebrate and insect pollinators of three plant species in the Seychelles archipelago (Thespesia populnea, Polyscias crassa and Syzygium wrightii), measuring quantity and quality components. Our study indicates that some Seychelles plant species depend on insects and specialised vertebrate pollinators, but also on opportunistic vertebrate nectar-feeders. The data on T. populnea and S. wrightii supported the hypothesis that specialist nectar feeders have higher PE than opportunistic nectar-feeding species; by contrast, the generalist geckos were more effective pollinators than specialist animals on P. crassa. 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.
Source: IMEDEA (UIB-CSIC)