The 'few winners and many losers' paradigm revisited: Emerging prospects for tropical forest biodiversity

Despite its relevance to contemporary biodiversity conservation, the ecological mechanisms controlling nonrandom species replacements and biotic homogenization of native biotas remain poorly understood. Here, we advocate for the widespread occurrence of a pattern of winner-loser species turnover exhibited in tropical forest floras: the replacement of old-growth tree species by a small set of pioneer or successional tree species across edge-dominated habitats in hyper-fragmented landscapes. A growing body of evidence on biotic reassembly in human-degraded forest sites suggests that winner-loser replacements are often advanced by native rather than exotic plant species, leading to taxonomic homogenization at multiple spatial scales (i.e. from local to regional assemblages). This process does not rely on biological invasions and associated shifts in geographic ranges of nonindigenous species. Moreover, this form of biodiversity erosion is not limited to restricted-range and endemic species but can include any species of many ecological groups that are apparently intolerant to highly modified habitat conditions, such as desiccated/illuminated forest edges and fire-degraded forest fragments persisting in open-habitat matrices. This unidirectional pattern of species turnover is expected to set in motion devastating cascading effects onto higher trophic levels that will eventually disrupt the structure of the entire forest ecosystem. Our framework not only reaffirms the winner-loser replacement paradigm, but also contends that the proliferation of disturbance-adapted native organisms has a pivotal role in sealing the fate of tropical biodiversity in particular contexts.