Plant-lizard interactions are still poorly studied, despite lizards are known to interact with flowering plants in many parts of the world. They are commonly reported on islands although the number of documented interactions has also increased in mainland, mostly in isolated environments. In this study, we first performed a global review to explore whether lizard-flower and lizard double mutualistic interactions in continents occur in environments similar to those of islands. Then, we aimed at explaining the factors driving the current distribution of such interactions worldwide. To do this, we considered four environmental factors (latitude, altitude, rainfall and temperature), and phylogeny and body size of lizards that may influence flower visitation. Furthermore, we investigated for the first time the functional role (legitimate visit versus florivory) of lizards in plant reproduction and the conditions under which each type of interaction occurs. Finally, we explored the factors influencing the distribution of lizard double mutualisms worldwide. We recorded a total of 452 lizard-flower interactions (ca 3.4% and 0.1% of the extant lizard and plant species, respectively). Lizard-flower interactions were more frequently recorded on islands (79%) regardless of phylogeny and lizard body size, whereas in mainland the number of interactions increased with altitude. Our analyses also revealed that only 20% of all interactions confirmed pollination effectiveness and a strong association of the type of interaction with environmental factors and species traits. Regarding the distribution of lizard double mutualisms, we found a positive effect of island and rainfall, but a decrease in their occurrence with latitude, altitude, temperature and body size. We predict that mutualistic plant-lizard interactions will be increasingly documented, especially in isolated environments (both on islands and continents), which will help us to better understand the biological patterns of this phenomenon and the mechanisms underlying them.

PREMISE OF THE STUDY:The characteristic scarcity of insects on remote oceanic islands has driven nonflower-specialized vertebrates to broaden their trophic niches and explore floral resources. From our previous studies in the Galápagos, we know that native insectivorous and frugivorous birds visit a wide range of entomophilous flowers and can also act as effective pollinators. Here, we tested whether opportunistic Galápagos birds show any preference for specific floral traits, and if so, this preference differs from that of insects. METHODS:Sixteen floral morphology and nectar traits of 26 native species were studied, as well as the frequency with which they are visited by birds and insects. Nonmetric multidimensional scaling (NMDS) was used to evaluate the distribution of flower traits values along two main dimensions and measure the similarity between the plants visited mostly by birds versus those by insects.KEY RESULTS: NMDS of floral traits resulted in two species groups: (1) bell-shaped, white flowers with wider corollas at nectary level and higher nectar volume, associated with high bird visitation rates; and (2) bowl and tubular-shaped flowers with narrower corollas at nectary level and lower nectar volume, associated with high insect visitation rates. CONCLUSIONS:Despite the divergence in floral trait preferences between opportunistic Galápagos birds and insects, bird-visited flowers display mixed traits not fitting the classical ornithophilous syndrome. This finding is compatible with the existence of a transitional or bet-hedging phenotype between insect and bird visitors and underscores the importance of coevolution and floral diversification in nonspecialized plant-visitor interactions.

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