Legacy of the lost and pressure of the present: Malagasy plant seeds retain megafauna dispersal signatures but downsize under human footprint

Abstract

Past megafaunal extinctions and ongoing declines of remaining large-bodied frugivores limit the dispersal of large-seeded plants, contributing to their (local) demise, thereby downsizing plant seeds across assemblages. Here, we test this hypothesis by integrating trait and distribution data for 2,852 endozoochorous plant species, 48 extant and 15 extinct frugivore species across 361 assemblages on Madagascar. Using structural equation models, we show that mega-seeded plants - with seeds that are too large to be swallowed by any extant Malagasy frugivore - have larger seeds in assemblages where past megafrugivores were larger, highlighting the legacy of past interactions on persisting mega-seeded plants. Furthermore, human footprint reduces seed sizes directly and indirectly by downsizing of extant frugivore in the assemblages. Such human-driven seed downsizing can lead to erosions of important ecosystem functions such as tropical forest carbon storage.