Dissecting a biodiversity hotspot

By Danilo Neves

The Mata Atlântica is renowned worldwide for being one of the biodiversity hotspots for conservation prioritisation. Less known facts are that (i) the hotspot status is specifically referring to its core habitat, the rain forests, and that (ii) the Mata Atlântica also houses a complex mosaic of ecosystems that are associated with five key factors: frost, surface rockiness, soil waterlogging, drought and salinity (Figure 1). These factors limit the occurrence of most rain forest tree species in the Mata Atlântica, thus giving rise to distinct vegetation types, referred to as marginal habitats: high elevation forests, rock outcrop dwarf-forests, riverine forests, semideciduous forests and restinga (coastal white-sand woodlands).

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The rain forest (left) is placed by Scarano (2009) as the ’core’ expression of the Mata Atlântica, where deep shade plays the chief role as a limiting factor for competing plants. The semideciduous forests (right) replace rain forests where seasonal rainfall regimes bring regular periods of drought. Credits: Kyle Dexter (rain forest in Itatiaia National Park, southeastern Brazil) and Marco Aurélio Fontes (semideciduous forest patch in Luminárias, Minas Gerais state, southeastern Brazil).

Previous studies have argued that these marginal habitats are mainly comprised of rain forest tree species that can tolerate the harshest extremes of the limiting factors (listed above). Our recent study, however, showed that these limiting factors lead to floristically distinct tree communities, thus indicating that the marginal habitats are not simply a nested subset of the more diverse Mata Atlântica rain forest (Figure 2). We also show that marginal habitats receive considerably low protection, despite harbouring almost half of the 7,099 endemic plant species to the Mata Atlântica biodiversity hotspot. These 3,160 endemic plant species are not found anywhere else in the world, including in the rain forests of the Mata Atlântica.

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Decomposition of the pairwise floristic dissimilarity of rain forest and marginal habitat sites of the Mata Atlântica (e.g. bullets in the Araucaria-dominated triangle represent pairwise dissimilarities between each of the 193 Araucaria­-dominated sites and all the 328 rain forest sites; i.e. 63,304 pairwise dissimilarity values). Numbers represent the mean turnover (%) and nestedness (%) components of the Jaccard dissimilarity for each marginal habitat.

Our study also emphasizes that the restinga is strikingly distinct both floristically and environmentally, thus suggesting the need for further investigation (Figure 3). If restingas are indeed a distinct phytogeographical region, instead of an extension of rain forests into saline white-sand environments, they may be much more threatened than assumed based upon classifications that places these two habitats together. Restinga has suffered massive fragmentation due to high human occupation in coastal areas and a rapidly developing tourism industry.

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The restinga (coastal white-sand woodlands) replaces rain forest where the stress due to water deficit is increased by a sandy substrate with high salinity, and by salt sprays coming directly from the ocean. Credits: Eloina Matos (restinga in Bahia state, northeastern Brazil).

 To find out more, read our article here