Some regions of the planet are populated by thousands of species of animals and plants. In other areas, life is scarce or practically non-existent. This great variability is explained in many cases by conditions as easily understandable as temperature, altitude or insularity.
A group of researchers led by experts from Utah State University (United States) has analyzed hundreds of studies and databases on the factors that determine the presence of species in various terrestrial habitats and, among the most important elements, they have confirmed that "Precipitation patterns" (cyclical regime of rain, snowfall...) are much more important than previously believed in the presence and maintenance of mammal populations. Without rain or in situations of serious alteration of the rain cycle, the maintenance of these groups of animals is not possible, indicate the authors of this research whose results have been published (September 2023) in the journal Ecology Letters.
With access to a massive set of global-scale climate data and a novel strategy, a team from the Quinney College of Natural Resources' Department of Watershed Sciences and the Quinney Ecology Center identified several factors to help explain the variability of the number of species in various terrestrial areas of the planet. Thus, they quantified the relationship between feeding and the interaction with climate as elements of great importance in most species.
"Historically, studies looking at the distribution of species along Earth's latitudinal gradient have overlooked the role of trophic ecology: how what animals eat affects where they are," said Trisha Atwood, Ph.D. co-author of the study and researcher at the Department of Watershed Sciences and the Ecology Center, in a note released by Utah State University. "This new work shows that predators, omnivores and herbivores are not randomly scattered around the world. There are patterns in where we find these groups of animals."
Certain places have an unexpected abundance of carnivorous predators: parts of Africa, Europe and Greenland. Herbivores are common in colder areas and omnivores tend to be more dominant in warm places. Two key factors emerged as crucial in shaping these patterns: precipitation and plant growth.
Precipitation patterns over time play an important role in determining where different groups of mammals thrive, Atwood says. Geographic areas where rainfall varies seasonally, without being too extreme, had the highest levels of mammal diversity.
"You have to keep in mind that we are not talking about the total amount of rain," said Jaron Adkins, lead author of the research. "If you imagine ecosystems around the world on a precipitation and seasonal scale, certain places in Utah and the Amazon rainforest are at the extreme with low variability: they have constant levels of precipitation throughout the year. Other regions, such as the South of California, have really high variability, receiving around 75 percent of the annual precipitation between December and March," says this researcher in a note released by his university.
The optimal point for predators and herbivores is located in an intermediate zone between the two extremes, says this expert, referring to the results of his study. Places like Madagascar, where precipitation patterns were evenly split between a wet and a dry season (six months each), had the ideal ecological cocktail to promote conditions for these two groups. The diversity of omnivores tends to thrive in places with very stable climates.
The second major factor related to mammal diversity that the work uncovered was a measure of the amount of plant growth in an area, measured as "gross primary productivity."
"It makes intuitive sense that animals that eat plants would benefit from plant growth," Adkins said.
But this measure actually affected carnivores more, according to the research. The strong relationship between predators and plant growth highlights the importance of plant abundance in the structural integrity of the entire food chain.
"It was surprising that this factor was more important for predators than for omnivores and herbivores," Atwood said. "The reason for this remains a mystery."
Although evolutionary processes are ultimately responsible for stimulating differences between species, climatic conditions can affect related factors (rates of evolutionary change, extinction and animal dispersal) that influence species and trait-based richness, according to the research.
Animal diversity is rapidly declining in many ecosystems around the world due to habitat loss and climate change. This has negative consequences for ecosystems. Forecasting how climate change will alter animal systems in the future is extremely important, Atwood said, and this research is a first step toward better managing future conditions for animals around the world.
"Animal diversity can act as an alarm system for the stability of ecosystems," Atwood said. "Identifying the ecological mechanisms that help drive richness patterns provides information for better management and prediction of how diversity might change in future climates."
In addition to Adkins and Atwood, the research included seven authors currently or formerly associated with the Department of Watershed Sciences and the Center for Ecology: Edd Hammill, Umarfarooq Abdulwahab, John Draper, Marshall Wolf, Catherine McClure, Adrián González Ortiz, and Emily Chavez.
