Plant communities at the periphery of the Atlantic rain forest: Rare-species bias and its risks for conservation

Initiatives that establish species rarity as an indicator of conservation priority might be biased if they disregard important evolutionary and adaptive processes taking place in lower diversity communities and ecotones. Conservation policies regarding the Atlantic forest strongly emphasize the core formation (i.e. the rainforest stricto sensu) rather than the marginal habitats (e.g. restingas, swamps, and high altitude campos) and species that are rare/endemic. To discuss this issue I revisit a hypothesis I have forwarded in 2002 that postulates that plant colonization of habitats marginal to the Atlantic rain forests of the State of Rio de Janeiro was largely related to terrestrial nurse plants that originally, in the rainforest habitat, were canopy plants such as epiphytes or hemi-epiphytes. Adaptations to water and nutrient restrictions, typical of life in the canopy, granted success to such plants upon migration to sandy, swampy or rocky substrates in neighbouring areas. Many such species, then, behaved as nurse plants and favoured colonization of these more extreme habitats by a number of other rainforest species. I now review recent evidence that corroborate this hypothesis, while examining the nature of such nurse plants. In all marginal habitats, nurse plants are often highly abundant locally and have high ecophysiological vigour, while both widespread and endemic species are found among them. Thus, nursing effect, local abundance, and ecophysiological performance are not related to species geographic distribution or to their spectrum of habitat preference. Paradoxically, several abundant nurse plant species have low Darwinian fitness. These studies provoke two reflections. First, the Atlantic forest sensu lato, i.e. the core formation plus the peripheral ones, should be treated collectively as a biodiversity hotspot, rather than the core rainforest formation alone. Second, widespread or common species play important functional roles in such marginal habitats and, despite their ubiquity, ecologically they might be less fit than rare/endemic ones at the local level due, for instance, to current constraints to sexual reproduction. Thus, they should also be targeted as conservation priorities.