Long-Term Seed Dispersal within an Asymmetric Lizard-Plant Interaction

SIMPLE SUMMARY: For 24 years, we studied the interaction of the endemic Balearic lizard Podarcis lilfordi) and the dead horse arum (Helicodiceros muscivorus) on Aire Island, a small coastal islet of the Balearic Archipelago (Spain). From a small initial plant population, the frugivorous activity of lizards apparently resulted in an extraordinary increase in plant abundance over the entirety of Aire Island. The intensity of seed dispersal by lizards was higher in years with lower rainfall and, consequently, with lower availability of other food resources. Thus, the fruits of the dead horse arum were an alternative food strongly exploited during years of food shortage. ABSTRACT: During the last 24 years, the mutualistic interaction between the dead horse arum, Helicodiceros muscivorus, and the Balearic lizard, Podarcis lilfordi, was studied on Aire Island (Balearic Islands, Spain). From a small population of a hundred plants, the dead horse arum expanded extraordinarily throughout the island, reaching the highest known densities of the species and occupying areas of the island where it was not previously present. The current abundance of plants is a direct effect of the frugivorous activity of the Balearic lizard, which is the main, if not the only, effective seed disperser of the plant on Aire Island. However, abiotic factors predominated over biotic factors in driving abundance of plants. Over the years, plant densities varied significantly depending on the aridity of the island, with higher densities recorded in drier years. Lizards’ frugivorous activity and dispersal intensity was inversely correlated with annual rainfall. We found higher dispersal intensity in years with lower rainfall. We propose that the years of lower rainfall are those in which there is a lower prey availability. In such years, lizards compensate the shortage of other trophic resources with a more intense consumption of dead horse arum fruits. The mutualistic interaction is therefore asymmetric, since there is a greater influence of the frugivorous activity of the lizards on the plants than of the plants on lizards. It is, in short, a system chronically out of balance.