ECR feature: Axel Arango on diversity of the Seasonally Dry Forests in the Neotropics

Axel Arango is a PhD student at the Instituto de Ecología A.C. (INECOL) – Mexico. He is a macroecologist interested in untangling the patterns and processes shaping biodiversity worldwide. Here, Axel shares his work on the biodiversity patterns of the Neotropical Seasonally Dry Forests.

Axel Arango presenting his poster during the International Biogeography Society’s Humboldt 250 Meeting in 2019 at Quito, Ecuador.

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Institute. Instituto de Ecología A.C. (INECOL), Mexico

Academic life stage. PhD student

Major research themes. Evolutionary Macroecology, Phylogenetic ecology, Biogeography, Biodiversity

Current study system. My ongoing research focuses on untangling the patterns and processes shaping biodiversity worldwide, mostly using phylogenetic and macroevolutionary tools to unveil the mechanisms generating biodiversity. When I began my graduate research on these topics, I was attracted by the outstanding diversity and endemism of the Neotropical Seasonally Dry Forests and the lack of knowledge on the processes shaping their patterns despite being one of the most threatened biomes in the world due to human exploitation. This motivated me to answer some of the long-standing questions about this biome by taking advantage of recently developed and open-access databases. During this process, I realized that answering such biodiversity questions does require detailed information on several ecological and evolutionary aspects of the clades under study. Driven by this realization, I’m currently pursuing my PhD studying birds as a model system to explore and evaluate the diversification and biogeographic history across the Americas.

Recent paper in JBI. Arango, A., Villalobos, F., Prieto‐Torres, D. A., & Guevara, R. (2021). The phylogenetic diversity and structure of the seasonally dry forests in the Neotropics. Journal of Biogeography48(1), 176-186.

The remnants of a Dry Forest in Veracruz, Mexico.

Motivation behind this paper. This paper derived from my Master’s thesis at INECOL, which came to light when my supervisors and I got interested in describing large scale biodiversity patterns of neglected biomes, such as the Neotropical Seasonally Dry Forests (hereafter NSDFs). For more than two decades, the NSDFs had been described to present an inverse latitudinal diversity gradient, showing higher species richness away from the Equator. Several hypotheses had been posited trying to explain this phenomenon, arguing from different origin and colonization times for this biome to long-term climatic stability facilitating the diversification and expansion of these forests. However, only a few attempts tried to evaluate these hypotheses. By taking advantage of recently developed databases and exploring the climatic dynamics since the Pleistocene, we tested the geographic and evolutionary processes responsible for driving this uncommon distribution of diversity shown by the NSDFs.

Key methodologies. We used available data on woody plants distribution in the NSDFs ( and a recently published phylogeny for seed plants to estimate diversity and relatedness metrics. More than 4000 species and 800 assemblages of woody plants were used to evaluate the latitudinal gradient of the NSDFs and test if climatic stability since the Last Maximum Glacial was driving this pattern. We used different phylogenetic metrics such as phylogenetic diversity, which describes the amount of history contained within an assemblage, and the Net Relatedness Index (NRI) that describes the degree of phylogenetic relatedness (closely, distantly or randomly related) among species within an assemblage. These phylogenetic patterns can infer ecological (environmental filters, local extinction) and evolutionary (phylogenetic niche conservatism, diversification) processes that shaped the distribution and diversity of the NSDFs in the Neotropics. As such, our work was ingenious in the way it approached and evaluated the hypotheses on NSDFs diversity and distribution mainly by using comprehensive databases and state-of-the-art phylogenetic methods to solve a long-standing question on NSDF biodiversity patterns.

Unexpected challenges. Initially, we focused only on phylogenetic diversity measures to understand the latitudinal gradient of the woody plants associated with the NSDFs. However, after analyzing the results, a new question arose about what processes drove the reverse latitudinal gradient in these biomes. We decided to expand our study and test one of the most famous hypotheses explaining this phenomenon: the Pleistocene Arc Hypothesis (PAH). This hypothesis argues that the current disjunct distributions of the NSDFs were once connected during the Pleistocene’s cold and dry periods, which can be tested using climate data and phylogenetic structure metrics. However, a challenge we often face when working with these types of large datasets published by different researchers at different times is to match the taxonomy between both information sources, which might have been the most labor-intense process in this study.

A lonely Huaziche (Vachellia sp.), a common species of dry habitats in a Veracruz NSDF.

Major results. We found that the reverse latitudinal gradient showed by the NSDFs is not a result of the climatic stability since the Last Glacial Maximum, challenging one of the most prominent hypotheses previously suggested (i.e., PAH). Instead, it is likely the outcome of more intricate and deeper-time evolutionary processes. However, the idea of widespread clades in the past that subsequently fragmented into isolated lineages and diversified in situ remained valid by the evidence of several separate and unique regions composed by closely related species. Still, these isolation events and evolutionary processes may have occurred long before the Pleistocene and were perhaps associated with different timings of radiation and adaptations of the woody plants in this biome.

Next steps for this research. As part of an ongoing collaboration, we will evaluate if there’s a differential Diversification Rate at the proposed source areas of the NSDFs to argue that these places are the drivers of NSDF’s current diversity and distribution. We will also calculate measures that assess the species’ influence in ecosystem functioning (i.e., Functional Diversity) of these forests to discern how much they depart from the Phylogenetic Diversity. This approach could give us insights into the biogeographic history in these forests by explaining, for example, if its expansion was either mediated by Phylogenetic Niche Conservatism or ecological convergence.

If you could study any organism on Earth, what would it be? Dinosaurs – other than birds, of course! More specifically, all the extinct clades that inhabited the Earth millions of years ago. Extinct species can provide us with a more robust idea of processes building up biodiversity, and how cool it would be to look at all those different kinds of organisms forgotten by time?

Anything else to add? When I started working on this project, although I had experience in statistics and population ecology, I hadn’t had that much exposure to programming and only had a vague idea about community phylogenetics. Fortunately, with the help of my advisors (Fabricio Villalobos and Roger Guevara), I picked it up and built a robust project that was awarded by the International Biogeography Society at the Humboldt 250 Meeting in 2019. In hindsight, I think this combination of skills helps me better formulate and tackle broad questions about biodiversity.

Published by jbiogeography

Contributing to the growth and societal relevance of the discipline of biogeography through dissemination of biogeographical research.

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