Recent surveys have shown that even protected rainforest reserves are experiencing species loss and ecological decline. This decay was thought to be correlated with human disturbance, but the relationship had remained unclear until now.

As part of a four-decade study in Mexico’s Los Tuxtlas Rainforest Reserve, researchers found that forest fragmentation, the breaking of large forest areas into smaller separated pieces, combined with hunting pressures on medium and large mammals led to an explosion in the number of palm trees, which, in turn, eliminated space for other trees species.

 “What we found is that four decades of forest fragmentation in synergy with defaunation at the landscape-level trigger a cascade of ecological changes ultimately affecting forest biodiversity, structure and functioning, even in areas protected for conservation,” said Rodlfo Dirzo, Bing Professor of Environmental Science and a senior fellow at the Stanford Woods Institute for the Environment who co-authored the study.

Researchers found that deforestation for farming, ranching, and other activities on the edges of Los Tuxtlas creates many open spaces in the top layer of the forest where the tallest trees reach. These gaps increase light availability in the understory, and in turn lead to greater seed production and growth of young palms.

The palms’ long leaves block light from reaching plant species beneath them, and intercept seeds from taller trees that would otherwise rain down on the forest floor and promote the growth of other species. Over the course of the 40-year study, the population of palms increased by 326 percent a year on average, and the understory species diversity decreased by 57 percent.

The study also found that because peripheral deforestation, hunting and habitat loss along the edge of the reserve had decimated populations of large herbivores, palm seeds, seedlings and saplings were less likely to be eaten or trampled by species like tapirs, peccaries and deer, further facilitating the survival and abundance of palms.

“Rippling, dramatic effects can be expected, as the understory tree community represents the species-rich regenerating forest of the future,” Dirzo and the research team wrote in a statement.

This research adds to a growing body of evidence that deforestation and the loss of animal species, even if they occur predominantly outside and around protected rainforest conservation areas, still jeopardize the tropical biodiversity and ecological health of these reserves.

Authors of this study include Miguel Martínez-Ramos, Iván A. Ortiz-Rodríguez, and Daniel Piñero of Universidad Nacional Autónoma de México, Rodolfo Dirzo of Stanford University, and José Sarukhán of Universidad Nacional Autónoma de México and Mexico’s National Commission for Biodiversity. The study was published in the Proceedings of the National Academy of Sciences.