Unique Evolutionary Histories in Marginal Habitats of the Atlantic Forest Uncovered

The Atlantic Forest, a rich mosaic of habitats located along the coast of Brazil, has been the subject of numerous studies due to its incredible biodiversity. As a biodiversity hotspot, this region is home to many plants and animals, many of which are found nowhere else in the world. The rainforest at the heart of the Atlantic Forest is typically the focus of most research, while its marginal habitats, such as cloud forest, semi-deciduous forest, and restinga, have received far less attention. However, a recent study by Massante & Gastauer published in the Annals of Botany sheds light on the unique evolutionary histories of these marginal habitats and challenges the common belief that they are merely subsets of the rainforest.

The study examined the angiosperm tree diversity patterns along elevation gradients in the southeast Atlantic Forest, focusing on the lesser-studied marginal habitats. To understand these patterns, the researchers calculated various phylogenetic indices, such as mean pairwise phylogenetic distance (MPD), mean nearest taxon distance (MNTD), phylogenetic endemism (PE), and taxonomic and phylogenetic beta diversity (BD and PBD). These indices were then related to elevation and environmental variables.

Surprisingly, the results showed that communities in wetter and colder forests, such as cloud forests, have basal phylogenetic overdispersion and short phylogenetic distances towards the tips. In contrast, communities associated with water deficit and salinity, like the coastal restinga, exhibited basal phylogenetic clustering and no phylogenetic structure towards the tips. This suggests that the marginal habitats have different evolutionary histories than their rainforest counterparts.

Despite its high species richness, one exciting finding was the low phylogenetic endemism in the rainforest. In contrast, cloud and semi-deciduous forests showed unusually high phylogenetic endemism. This indicates that these marginal habitats may be home to a unique set of species that have evolved independently from those in the rainforest.

Furthermore, the study found that beta diversity and phylogenetic beta diversity between most habitat types are driven by the turnover of species and lineages, except for restinga. This further emphasizes the distinctiveness of these marginal habitats and the need to study them separately from the rainforest.

These findings challenge the previous notion that all marginal habitats in the Atlantic Forest are simply subsets of the rainforest. In their article Massante & Gastauer write:

[W]e show that the evolutionary history and environmental specificity of these habitat types are important factors for maintaining biodiversity, both originating new species in mountains and conserving old species in restinga and semi-deciduous forest. We suggest that these marginal habitat types ultimately act as ‘equilibrium zones for biodiversity’. Even though they have been considered similar in structure and function (Scarano, 2002), their contribution to the Atlantic Forest diversity goes far beyond. They regulate the overall species richness and evolutionary diversity of the Atlantic Forest.

Massante & Gastauer 2023

The research by Massante & Gastauer adds invaluable evolutionary insights into our understanding of the Atlantic Forest. Rezende and colleagues add in their commentary:

Massante and Gastauer’s new findings show us that the evolutionary history of the Atlantic Forest biome can only be understood in the light of its marginal habitats. However, there is still much to discover about the origin and the evolution of this hotspot, and how such information can be crafted into effective conservation strategies. Only by knowing a biome’s ‘full’ evolutionary history, and not just that of its main habitat, is it possible to create conservation strategies targeting not just species richness, but the underlying evolutionary dynamics producing that richness as well.

Rezende et al. 2023

READ THE ARTICLE

Massante, J.C. and Gastauer, M. (2023) “Evolutionary history of marginal habitats regulates the diversity of tree communities in the Atlantic Forest,” Annals of Botany, 131(2), pp. 261–274. Available at: https://doi.org/10.1093/aob/mcac111.