Defaunated and invaded insular tropical rainforests will not recover alone: Recruitment limitation factors disentangled by hierarchical models of spontaneous and assisted regeneration

Abstract

Abstract Most tropical forests are now severely degraded and their ability to recover is highly dependent on frugivores which ensure seed dispersal for most woody plants. The global collapse of large vertebrates therefore raises major concerns about tropical forest succession, but few field studies have been conducted to disentangle recruitment limitations during disrupted succession. This study took place on Réunion (Mascarenes) where all large native frugivores have been extinct since human colonisation in 1665 and where multiple invasions threaten native ecosystems. We set up 20 experimental blocks on a lava flow dated back to 1800, in plant‐impoverished post‐defaunation vegetation bordered by old‐growth forests. We assessed fecundity, seed dispersal and seedling recruitment of the complete fleshy‐fruited plant community and used Bayesian analyses to disentangle the impact of multiple factors on these key processes. In the same blocks, we sowed four native trees assumed to be disperserless to test their capacity to establish, controlling for two additional post‐dispersal limitations (seed predation and competition with invasive plants). On the flow, small‐seeded native plants were fairly dispersed but did not recruit, probably due to strong competition with invasive plants; the few native species that recruited somehow were mostly medium‐seeded plants that were still dispersed; large‐seeded plants were absent from seed rain (which shows that invasive frugivores did not replace extinct ones) and subsequently from spontaneous recruitment. Instead, some alien plants, notably the tiny‐seeded highly dispersed Clidemia hirta and the medium‐seeded Psidium cattleianum largely dominated seedling recruitment. Native plants recruited better at the forest margin, including some large‐seeded species nearby mother trees. Sown large‐seeded species were able to emerge and survive in all plots whatever the treatment, which demonstrates that dispersal loss was the primary cause of regeneration failure on the flow. Synthesis. The strong modulation of the establishment capacity of native plants by seed mass shows that invasive plants win by forfeit of large‐seeded plants after native frugivores loss. Our study emphasises the fundamental role of dispersal loss and competition with invasive plants in the disruption of ecological succession, as well as the urgency of restoring seed dispersal and strengthening biosecurity regulations.