Why are some areas of the world more biodiverse than others? In an effort to understand what factors contribute to the uneven distribution of species on Earth, an international collaboration of scientists has compiled the first-ever global assessment of fern diversity. Their findings, published in the Journal of Biogeography, draw on herbarium data, genetic data, and climatic data of approximately 800,000 ferns.
"The first thing we wanted to know is where are the centers of ferns' biodiversity, and second, why?" comments lead researcher Jacob S. Suissa, of Harvard University. "We wanted to understand the biogeographical patterns of fern diversity. Understanding these patterns in a major plant lineage like ferns, allows us to take a step towards understanding why there is an uneven distribution of species around the world."
The researchers say that this type of study with large amounts of data would not have been possible a few decades ago. "Natural history collections are the primary data for all biodiversity studies, and they are the backbone to this study," said co-author Michael A. Sundue, of the University of Vermont. "Scientists have been making collections and curating them for hundreds of years. But only recently, the digitization of these records has allowed us to harness their collective power."
"There has been a large effort over the last decade to digitize the impressive collection of specimens contributed by thousands of collectors and experts in the field and deposited in museums or natural history collections,” adds co-author Weston L. Testo, of the University of Gothenburg, Sweden. “For this study we used over 800,000 digitized occurrence records for nearly 8,000 fern species."
Their analysis determined that the majority of fern species (58%) occur in eight primarily mountainous hotspots: Greater Antilles, Mesoamerica, tropical Andes, Guianas, Southeastern Brazil, Madagascar, Malesia, and East Asia. These hotspots make up a mere 7% of the planet’s landmass.
Interestingly, the researchers also concluded that higher elevations demonstrate greater fern diversity, particularly from 2000-3000 meters in elevation. They explain that this species richness correlation to elevation is likely due to the presence of cloud forests that are ideal habitats for ferns.
The researchers hypothesize that species richness and diversification are at their highest in tropical mountains, where temperature seasonality is low year-round. "People tend to think of places such as the Amazon rainforest as biodiversity hotspots," said Suissa. "But for ferns, it is tropical and subtropical mountains that harbor a disproportionate number of rapidly diversifying species relative to the land area they occupy. Ferns may be speciating within these tropical mountain systems because of the variation in habitats that occur across elevational gradients. For instance, at the base of a tropical mountain, it is hot all year round, and at the summit it is perennially cold. Essentially, these dynamics create many different ecosystems within a small geographic space." The researchers hope to be able to test this hypothesis in the field in the years to come.