Cows graze along a rural mountain road in Tepetiltic, Nayarit in Mexico. (Image credit: Getty Images)

The diversity of life has plummeted over the past 30 years in more than a dozen tropical forest reserves in Mexico, a new study shows.

Even these highly protected areas are seeing the array of plant and animal life follow a now global trend in which a few groups thrive and proliferate in human-altered landscapes where most groups decline.

“We had an idea that this was happening globally. The surprising thing is the omnipresence of this phenomenon of winners and losers in tropical areas,” said senior study author Rodolfo Dirzo, a professor of biology in the Stanford School of Humanities and Sciences, and of Earth system science in the Stanford Doerr School of Sustainability. “This is critical because tropical rainforests are the most important stores of biological richness on the planet.”

The research is based on surveys of more than 60 tropical ecologists who, like Dirzo, have spent decades studying reserves in central Mexico. The 14 studied reserves, which are part of a biodiversity hotspot that spans across Mesoamerica, have each been designated under a UNESCO program aimed at establishing a scientific basis for improving human livelihoods and safeguarding ecosystems.

In and around many of the protected areas, the authors found that new roads continued to go up and trees came down between 1990 and 2020 as people cleared forest for timber or cattle grazing. The abundance of long-lived, shade-tolerant tree species – the kind that make old-growth forests some of our planet’s largest carbon sinks – declined on average across all reserves by more than 25%.

“What is left is just small pieces of forest, and then a lot of disturbances around them,” said lead study author Daniel Auliz-Ortiz, who worked on the research as part of his doctoral thesis at Universidad Nacional Autónoma de México in Morelia, Mexico.

According to the study, published Jan. 22 in Proceedings of the National Academy of Sciences, the fragmented and depleted habitats that remain do not sustain populations of primates, many butterflies, raptors, reptiles, amphibians, and apex predators like jaguars in the numbers they once did. In their place, small, rapidly reproducing animals like rats and mice and weedy, light-loving vines have flourished.

A global challenge

The changes unfolding in central Mexico’s tropical forest reserves illustrate a reality of protected areas around the world: Deforestation is rarely eliminated entirely, with major negative consequences for ecosystems and humans.

Small rodents carry many infectious diseases that can spread and prove dangerous to people. As prolific seedeaters, they can also change where and how various plants become established in an ecosystem, added Dirzo, who is a senior fellow at the Stanford Woods Institute for the Environment and the Bing Professor in Environmental Science. “We are positioning them to be the winners in these ecosystems,” he said.

Also among the winners are fast-growing, pioneering plants that need abundant light and can take advantage of disturbed or clear-cut landscapes, but which store much less carbon than the shade-tolerant tree species they are replacing. This replacement “can significantly limit global carbon storage,” the authors write.

“We had an idea that this was happening globally. The surprising thing is the omnipresence of this phenomenon of winners and losers in tropical areas.”

—Rodolfo Dirzo

Professor of Earth System Science in the Stanford Doerr School of Sustainability and Professor of Biology in the Stanford School of Humanities and Sciences

Expanding opportunities

To prevent further deforestation, the researchers call for expanding opportunities for people to earn a living around protected areas by means other than cattle ranching. They found that forest loss rates were lower and human welfare was higher in regions where economies revolve around alternatives such as eco-friendly tourism, public programs that pay local communities for the environmental services provided by intact forests, or cultivation of multiple crops like coffee and cacao under the shade of a forest canopy.

In the absence of these opportunities, the researchers found local communities often turn to intensive cattle ranching or monoculture farming – and are forced to cultivate more and more land to remain profitable.

“Protection is not a panacea,” said Dirzo. “We need to make sure that the areas we have set aside are not just reserves on paper, but that they’re effectively safeguarding biodiversity within landscapes where the livelihoods of local peoples are also meaningfully supported.”

Lead study author Daniel Martín Auliz-Ortiz is affiliated with Instituto de Investigaciones en Ecosistemas y Sustenabilidad, Universidad Nacional Autónoma de México (UNAM). Additional co-authors are affiliated with UNAM, Universidad de Ciencias y Artes de Chiapas, Secretaría de Medio Ambiente e Historia Natural, Universidad Michoacana de San Nicolás de Hidalgo, Universidad Autónoma del Estado del Morelos, Faktorgruen, El Colegio de la Frontera Sur, Universidad Autónoma Metropolitana Unidad Lerma, Mexico Federal Electricity Commission’s Pre-Planning Center of the Gulf, Universidad Veracruzana, Instituto de Ecología, A.C., Instituto tecnológico de Ciudad Victoria, Comisión Nacional de Áreas Naturales Protegidas in Autlán de Navarro, Universidad de Guadalajara, Regenerando Nuestro Entorno Asociación Civil, Centro de Intervenciones Asistidas con Equinos y Formación para el Bienestar y Sustenabilidad, Universidad Autónoma de Ciudad Juárez, Universidad Autónoma de Tamaulipas, Secretaría de Medio Ambiente e Historia natural, KiekariTerra A.C., Comisión de Parqes y Biodiversidad de Tamaulipas, Instituto Tecnológico de Conkal, Bishop’s University, and Universidad Autónoma de Campeche.

This research was supported by Posgrado en Ciencias Biológicas of UNAM and CONACyT. The authors used ChatGPT 3.5 to support language review and writing.

Media Contacts

Rodolfo Dirzo, Stanford Doerr School of Sustainability and Stanford School of Humanities and Sciences:

Josie Garthwaite, Stanford Doerr School of Sustainability: (650) 497-0947,