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| A new study sheds light on why some species seem to thrive nearly everywhere, while others are rare and have very limited ranges. Pictured is the boojum tree (Fouquieria columnaris), native only to a few desert regions in Mexico's Gulf of California. Photo Credit: Daniel Stolte |
Few ideas in science have been tested and confirmed as thoroughly as evolution by natural selection. 160 years ago, Charles Darwin proposed the theory of evolution by natural selection after observing organisms that had developed highly specialized traits to better survive or reproduce in their environments. Whether the same process can explain global patterns of biodiversity, however – why most species are restricted to certain environments while a few outliers seem to be found everywhere – remains largely uncertain.
"We still are not exactly sure why most species are confined to narrow ranges, while only a few thrive nearly everywhere," said Brian Enquist, professor in the University of Arizona Department of Ecology and Evolutionary Biology and senior author of a new study providing the strongest global evidence yet that abundant plant species became so dispersed over time because of their ability to tolerate diverse climates.
Enquist's team found that species capable of tolerating a broad range of climates also tend to become ecologically dominant – abundant, widespread and central to how ecosystems function. Published in the journal Proceedings of the National Academy of Sciences, the results reveal a general law of biodiversity that helps explain why some plants are rare while others dominate ecosystems across the planet.
While strolling through the Arizona desert, one might notice that some cacti have evolved highly specialized adaptations to thrive in a hostile environment, such as thick, water-storing stems, spines in the place of leaves, and a waxy outer coating. While such features allow them to endure scorching heat and drought, they restrict cacti to very specific environments. On the opposite end of the spectrum are species that range widely across the planet – plants that seem to survive hot, cold, wet and dry climates alike with little to no trouble.
"If you look at the incredible biodiversity on the planet, one thing that immediately pops out is that the vast majority of species are really rare, while just a small fraction dominates," Enquist said. "It's been a very big mystery why that is."
The team, which includes researchers in Europe and South America, set out to solve this mystery by analyzing plant dispersion data from all continents except Antarctica. They looked for underlying trends among plants making up large portions of biomass and searched for an explanation as to how and why certain plants have evolved to be so abundant.
"We looked at about 300,000 species of land plants spanning all the major continents across the globe – the largest compilation yet," Enquist said. "We found that tolerance to many different climates and a high abundance across the Earth go hand in hand."
The researchers report that dominant species, which represent roughly 1% of all tree species, account for as much as 60–70% of global tree abundance, effectively dominating entire ecosystems. Take Tecoma stans, for example. Commonly known as yellow bells or yellow elder, this plant is a dominant species native to the Americas and found in warm regions all around the globe, including the U of A campus, a nationally recognized arboretum.
"According to Darwin's theory and our findings, the species that are dominant today are not just lucky; they've been through the evolutionary ringer," said Enquist. "Through natural selection, they've gradually acquired traits that enable them to thrive, expand and persist in increasingly wider ranges of environments."
Dominant species are organisms that dominate the biomass and shape how energy and carbon move through the biosphere. Their sheer abundance impacts the structure and functioning of ecosystems, such as by altering the soil chemistry or providing habitats and food for animals. By testing one of Darwin's underexplored ideas, the study extends evolutionary theory into the realm of global ecology.
"We were surprised to find that some plant species seem to be everywhere, from the tropics to the temperate zones," said Gabriel Moulatlet, the paper's first author and a postdoctoral research associate. "We believe that their broad climatic tolerance makes them the 'ecological winners' that influence all other plants and animals in their ecosystems."
A climatic domino effect
Being able to handle a wide variety of climatic conditions and environments could be good news for dominant species, because they are more likely to continue to adapt and resist future human impacts and climate change than species with more narrow climatic tolerances, according to the paper. On the other hand, it could be catastrophic if one of these species faces demise.
"Still, some dominant species could face conditions they've never experienced before," Enquist said. "Without intervention, their decline could trigger large, ripple effects across ecosystems."
By identifying the biological rules behind abundance and rarity, the study offers a framework for forecasting biodiversity change – whether a given species will persist, spread or decline, and how this could impact conservation efforts. The team plans to do further research to better predict how certain plant species will be affected by the climate in the future and identify conservation efforts to ensure that these consequences aren't devastating.
"Our findings enable us to carry out fine-scale, quantitative assessments of biodiversity, mapping species' ranges and distributions without the enormous time and cost of traditional field surveys," Enquist said, "With these data, we can pinpoint biodiversity hotspots and identify where climate change will most threaten species in the future. This gives us an early warning system that helps us predict where intervention will be needed as the pace of climate change accelerates."
With conservation efforts increasingly moving toward on-the-ground interventions, parts of our planet have already begun to resemble living zoos or botanical gardens, as species are relocated to new habitats, Enquist said.
According to the authors, the findings suggest that many rare plant species, such as the iconic cacti of the Sonoran Desert, including the giant saguaro, could face heightened extinction risk as rising temperatures and prolonged drought push them beyond their climatic limits. Scientists are now working to identify when and where these thresholds might be crossed as the planet continues to warm.
"Our well-being depends on plants," said Moulatlet, "As climates change and deforestation and habitat loss accelerate around the globe, understanding how plant life has evolved and how it will respond in the future is essential."
Published in journal: Proceedings of the National Academy of Sciences
Authors: Gabriel M. Moulatlet, Cory Merow, Brian Maitner, Brad Boyle, Xiao Feng, Amy E. Frazier, Cesar Hinojo-Hinojo, Erica A. Newman, Patrick R. Roehrdanz, Lei Song, Fabricio Villalobos, Pablo A. Marquet, Jens-Christian Svenning, and Brian J. Enquist
Source/Credit: University of Arizona | Kylianne Chadwick
Reference Number: ebio112525_01