The tropics contain the overwhelming majority of Earth's biodiversity: their terrestrial, freshwater and marine ecosystems hold more than three-quarters of all species, including almost all shallow-water corals and over 90% of terrestrial birds However, tropical ecosystems are also subject to pervasive and interacting stressors, such as deforestation, overfishing and climate change, and they are set within a socio-economic context that includes growing pressure from an increasingly globalized world, larger and more affluent tropical populations, and weak governance and response capacities Concerted local, national and international actions are urgently required to prevent a collapse of tropical biodiversity

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The future of hyperdiverse tropical ecosystems

The American tropics (the Neotropics) are the most species-rich realm on Earth, and for centuries, scientists have attempted to understand the origins and evolution of their biodiversity. It is now clear that different regions and taxonomic groups have responded differently to geological and climatic changes. However, we still lack a basic understanding of how Neotropical biodiversity was assembled over evolutionary timescales. Here we infer the timing and origin of the living biota in all major Neotropical regions by performing a cross-taxonomic biogeographic analysis based on 4,450 species from six major clades across the tree of life (angiosperms, birds, ferns, frogs, mammals, and squamates), and integrate >1.3 million species occurrences with large-scale phylogenies. We report an unprecedented level of biotic interchange among all Neotropical regions, totaling 4,525 dispersal events. About half of these events involved transitions between major environmental types, with a predominant directionality from forested to open biomes. For all taxonomic groups surveyed here, Amazonia is the primary source of Neotropical diversity, providing >2,800 lineages to other regions. Most of these dispersal events were to Mesoamerica (∼1,500 lineages), followed by dispersals into open regions of northern South America and the Cerrado and Chaco biomes. Biotic interchange has taken place for >60 million years and generally increased toward the present. The total amount of time lineages spend in a region appears to be the strongest predictor of migration events. These results demonstrate the complex origin of tropical ecosystems and the key role of biotic interchange for the assembly of regional biotas.

https://www.pnas.org/content/pnas/115/23/6034.full.pdf

Amazonia is the primary source of Neotropical biodiversity.

Summary
Oaks (Quercus, Fagaceae) are the dominant tree genus of North America in species number and biomass, and Mexico is a global center of oak diversity. Understanding the origins of oak diversity is key to understanding biodiversity of northern temperate forests.
A phylogenetic study of biogeography, niche evolution and diversification patterns in Quercus was performed using 300 samples, 146 species. Next-generation sequencing data were generated using the restriction-site associated DNA (RAD-seq) method. A time-calibrated maximum likelihood phylogeny was inferred and analyzed with bioclimatic, soils, and leaf habit data to reconstruct the biogeographic and evolutionary history of the American oaks.
Our highly resolved phylogeny demonstrates sympatric parallel diversification in climatic niche, leaf habit, and diversification rates. The two major American oak clades arose in what is now the boreal zone and radiated, in parallel, from eastern North America into Mexico and Central America.
Oaks adapted rapidly to niche transitions. The Mexican oaks are particularly numerous, not because Mexico is a center of origin, but because of high rates of lineage diversification associated with high rates of evolution along moisture gradients and between the evergreen and deciduous leaf habits. Sympatric parallel diversification in the oaks has shaped the diversity of North American forests.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.14773

Sympatric parallel diversification of major oak clades in the Americas and the origins of Mexican species diversity

The Caatinga Domain (CD) in northeastern Brazil harbors the largest and most continuous expanse of the seasonally dry tropical forest and woodland biome (SDTFW) in the New World. Phytogeographical data collected over the past 10 years support previous hypotheses that recognized two major biotas in Caatinga SDTFW: the Crystalline Caatinga, mostly associated with medium to highly fertile soils in the wide Sertaneja Depression; and the Sedimentary Caatinga, mostly associated with poor sandy soils derived from patchy sedimentary surfaces. A third floristic set is represented by tall Caatinga forests. The CD is the richest SDTFW area in the New World, with 3150 species in 930 genera and 152 families of flowering plants. About 23% of the species and 31 genera are endemic to the CD. We performed phylogenetic meta-analyses to estimate times of divergence and ancestral areas for SDTFW lineages, which indicated that plant diversity in the Caatinga arose mostly by in situ speciation following Mid to Late Miocene vicariance events with two major SDTFW nuclei: (1) the northwestern Caribbean dry coast of Colombia and Venezuela; and (2) the southwestern South American dry forests of southern Bolivia and northwestern Argentina. Phylogenetic analyses also uncovered unexpected patterns of recent radiations, with evolutionarily new species and incomplete lineage sorting that sharply contrast with the most common phylogenetic patterns found in SDTFW clades. Recent, mostly Pleistocene, ecological speciation better explains the emergence of distinct biotas on sandy and karstic surfaces.

Diversity and Evolution of Flowering Plants of the Caatinga Domain

Knowledge about the biogeographic affinities of the world’s tropical forests helps to better understand regional differences in forest structure, diversity, composition, and dynamics. Such understanding will enable anticipation of region-specific responses to global environmental change. Modern phylogenies, in combination with broad coverage of species inventory data, now allow for global biogeographic analyses that take species evolutionary distance into account. Here we present a classification of the world’s tropical forests based on their phylogenetic similarity. We identify five principal floristic regions and their floristic relationships: (i) Indo-Pacific, (ii) Subtropical, (iii) African, (iv) American, and (v) Dry forests. Our results do not support the traditional neo- versus paleotropical forest division but instead separate the combined American and African forests from their Indo-Pacific counterparts. We also find indications for the existence of a global dry forest region, with representatives in America, Africa, Madagascar, and India. Additionally, a northern-hemisphere Subtropical forest region was identified with representatives in Asia and America, providing support for a link between Asian and American northern-hemisphere forests.

/pdf/phylogenetic-classification-of-the-world-s-tropical-forests-jjcq7yweiu.pdf

Phylogenetic classification of the world's tropical forests

Seasonally dry tropical forests are distributed across Latin America and the Caribbean and are highly threatened, with less than 10% of their original extent remaining in many countries. Using 835 inventories covering 4660 species of woody plants, we show marked floristic turnover among inventories and regions, which may be higher than in other neotropical biomes, such as savanna. Such high floristic turnover indicates that numerous conservation areas across many countries will be needed to protect the full diversity of tropical dry forests. Our results provide a scientific framework within which national decision-makers can contextualize the floristic significance of their dry forest at a regional and continental scale.

/pdf/plant-diversity-patterns-in-neotropical-dry-forests-and-2j6nh4wncp.pdf

Alma Rosa Olvera-Luna

Papers

Plant diversity patterns in neotropical dry forests and their conservation implications