Megafrugivores as fading shadows of the past: extant frugivores and the abiotic environment as the most important determinants of the distribution of palms in Madagascar

Abstract

The extinction of all Madagascar's megafrugivores ca 1000 years ago, may have left its signature on the current distribution of vertebrate‐dispersed plants across the island, due to the loss of effective seed dispersal. In this study, we dissect the roles of extinct and extant frugivore distributions, abiotic variables, human impact and spatial predictors on the compositional turnover, or beta‐diversity, of palm (Arecaceae) species and their dispersal‐related traits across 40 assemblages in Madagascar. Variation partitioning showed that palm beta‐diversity is mostly shaped by the distribution of extant frugivores (eight lemur, three bird, two rodent and one bat species) and the abiotic environment (e.g. forest cover, slope and temperature), and to a lesser extent by the distribution of extinct megafrugivores (several giant lemur and elephant bird species). However, the contribution of these variables differed between dry western assemblages and wet eastern assemblages, with a more prominent role, albeit still small, of extinct megafrugivores in the west. These results suggest that palm distributions in the dry west of Madagascar, where megafrugivores were probably most abundant in the past, still show signatures of past interactions. With a fourth‐corner analysis we observed that the distribution of palm species with relatively large fruits and seeds was negatively associated with frugivore richness of both past and present communities and home range sizes of extant mammalian frugivores. This suggests that palm species with relatively large fruits tend to occur in places with fewer, small‐ranged mammalian frugivores, which may indicate dysfunctional seed dispersal. Nevertheless, our results also indicate that several wide‐ranging bird species with high dispersal ability (large hand‐wing index) that also occasionally feed on fruits may compensate for this potential dispersal loss. Our study sheds new light on dispersal anachronisms in Madagascar, and how defaunation and past species interactions may underlie current plant distributions.