ECR Feature: Jacob Suissa on the diversity of tropical montane ferns

Jacob Suissa is a PhD candidate at Harvard University. His major research interest is in the evolution of plants, particularly ferns. Jacob shares his recent work that has utilised vast herbarium records to understand global diversity in montane fern assemblages.

Jacob in the Tepui’s in the Brazilian highlands.

Links. Webpage | Instagram

Institute. Harvard University

Academic life stage. PhD candidate

Research interests. Plant evolution.

Recent paper in JBI. Suissa J et al. (2021) Mountains, climate and niche heterogeneity explain global patterns of fern diversity.

Whether its collecting plants in the Peruvian Andes or the New England Berkshires, mountains have always fascinated me. Specifically, I am captivated by the way vegetation drastically changes as I ascend a mountain. Mountain ranges across Earth, however, are not all the same with respect to their diversity. Tropical mountains contain many more species than temperate mountains of similar or larger size. For instance, there are over 10,000 flowering plant species in the Talamanca Mountain range in tropical Central America, compared to only around 5,000 species in the temperate Rocky Mountains (the “Rockies”) of North America. This difference is even more striking when considering their size: the Rockies are 3,000 miles long with a maximum height of over 14,000ft, while the Talamanca’s are only 250 miles long with a maximum height of 12,500ft. Determining what drives this uneven distribution of species across tropical and temperate ecoregions is a major biogeographical question.

Visa of the Costa Rican continental divide along the Talamanca Mountain range. 

I study ferns, the second most diverse group of vascular plants, and they are a great study system for understanding these patterns of biodiversity. Contrary to the common depictions of ferns as ancient shade and water loving plants, the majority of fern diversity is actually relatively young (<100 my), and they occur in a variety of different ecosystems ranging from lowland desert outcrops to the high elevation Páramo (tropical alpine meadows). For over 100 years it has been suggested that, relative to temperate mountains and the lowland tropics, tropical mountains may contribute disproportionately to shaping the patterns of global fern biodiversity. Massive amounts of digitized herbarium specimen data exist (from decades of botanical collections) that could be used to explore these biogeographic patterns. However, to date, no such study has been conducted.

Pleopeltis from the Peruvian Andes

In our paper, we stand on the shoulders of past botanists by leveraging over 800,000 global occurrence records for nearly 8,000 fern species from the database of digitized herbarium records hosted on the Global Biodiversity Information Facility (GBIF). We integrated these occurrence data with genetic and climatic information to uncover how historical, ecological, and evolutionary processes contribute to patterns of global fern biodiversity. We first divided the Earth into a series of 1×1˚ grid cells and quantified a series of metrics within each cell including species richness, environmental variability, and lineage diversification. We discovered that the majority of fern species (58%) occur in eight principally montane hotspots that only cover a total of 7% of Earth’s land area. These spots include the Greater Antilles, Mesoamerica, the tropical Andes, Guianas, Southeastern Brazil, Madagascar, Malesia and East Asia. Importantly, within these hotspots we found that fern diversity is highest above 3500ft in elevation and peaks around 8,000ft in the tropics, which corresponds to cloud forests, the home of fern epiphytes. This recovered pattern echoes the importance of tropical mountains and cloud forests in contributing to fern biodiversity.

            Once we had characterized general trends in species richness, we turned our attention to understanding the ecological and evolutionary processes that generated these patterns. We incorporated climate, soil, and topological data and found a strong positive relationship between increased climatic space with species richness and diversification within our montane hotspots. These patterns suggest that ferns might be undergoing greater lineage diversification across ecological gradients within tropical montane ecosystems, compared to adjacent lowlands and temperate mountains. Lowland tropical rainforests like the Amazon are touted for their plant biodiversity; while they are diverse for certain organisms like woody trees, herbaceous understory fern species are quite depauperate. We think that this is because of the relatively low diversity of different ecosystems in these lowland tropical zones. Furthermore, unlike temperate mountains, a unique set of dynamics occurs in tropical mountains. First, in the tropics small amounts of elevational change leads to a high degree of climatic change, which creates a series of stacked ecosystems within a small geographic space. Second, there is very low temperature seasonality within each of these stacked ecosystems, meaning each habitat band is climatically stable throughout the year. The relationship between these two processes creates a diversity of seemingly allopatric or isolated habitats in a relatively narrow geographic space, and we suspect that these processes facilitate greater diversification of ferns across elevational gradients in tropical mountains, relative to adjacent lowlands and temperate mountains.

Descending Cerro Bruster (Panama) with bags full of ferns.

Our findings are not only important for deepening our understanding of global patterns of fern biodiversity, but also for contextualizing biodiverse regions for conservation. With the discovery of fine-scale patterns of geographical and elevational diversity in a major group of land plants, we can pinpoint where on Earth the important centers of biodiversity are. Given that tropical montane plant communities are at a disproportionately higher risk of climate-induced extinction, knowing that these sites are biodiversity hotspots can help make decisions on conservation efforts in the face of anthropogenic and climate-mediated habitat destruction.

Published by jbiogeography


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