The remarkable resilience of a tropical rainforest

Tropical rainforests are home to almost two-thirds of all vertebrate species and three-quarters of all tree species: they are the most species-rich terrestrial ecosystem on Earth. However, over half of these diverse rainforests have already been cleared, and their area continues to decline drastically, primarily for agricultural purposes. Is there a chance of regeneration, and can not only trees but also the unique diversity of thousands of animal species return to cleared areas? The Reassembly research group, funded by the German Research Foundation (DFG) (www.reassembly.de), investigated this question. The findings have now been published in the journal Nature.

The answer is surprisingly clear-cut and encouragingly positive: trees regrow rapidly on agricultural land as soon as land use ceases. A diverse range of animal species also re-establish themselves. Biodiversity recovered to over 90% of its original level within 30 years. During this period, as many as three-quarters of the animal and plant species typical of primary forest returned. This applies at least to the study area in Chocó, north-western Ecuador, a region where a few patches of pristine primary forest and larger secondary forests still remain. These form the reservoir for the many returning animal and plant species. To this end, researchers from over 30 universities and institutions have, for the first time, conducted a detailed study of the natural regeneration of 16 different groups of organisms (animal, plant and bacterial species) along a regeneration gradient. A total of 62 sites were compared, all of which have been protected for several decades by the conservation organisation Jocotoco and integrated into a large nature reserve: actively used pastures and cocoa plantations, secondary forests of varying ages that were formerly used as pastures and plantations, and untouched primary forests.

Lead author Timo Metz carried out and summarised the extensive analyses as part of his PhD at Darmstadt Technical University. He emphasises: “Rainforests, as complex ecosystems and species-rich communities, demonstrate remarkable resilience and the ability to return to their original state. This stability has often been modelled theoretically, but until now could not be demonstrated on the basis of such extensive empirical data.” Senior author Nico Blüthgen is Professor of Ecology at TU Darmstadt and spokesperson for the Reassembly research group. He initiated the studies together with Dr Martin Schaefer, director of the Ecuadorian conservation organisation “Jocotoco”. Nico Blüthgen emphasises: “The many rapidly returning animal species are not only beneficiaries of forest regeneration, but are also its key agents: bats, monkeys and other mammals, as well as birds, bring tree seeds back to the cleared areas; dung beetles bury the seeds in the soil; and hundreds of other animal species ensure pollination.” Martin Schaefer adds: “Our findings that 75% of species composition and 90% of species diversity return under their own steam within a single human generation show just how effectively we can protect nature. By purchasing and protecting land, we can preserve the diversity of life and the foundations of our societies – soil, water, and the pollination of the plants that form the basis of our food supply.”

A total of 41 colleagues, mainly from Germany and Ecuador, were able to contribute their expertise to a study of over 8,500 species that is unique to date. Karla Neira and Mark-Oliver Rödel from the Naturkunde Museum Berlin, investigated the frog species and their prey. Frogs are important links in tropical food webs, and due to their narrow habitat requirements particularly suited to track forest regeneration. To date, various studies in Central and South America have clearly documented that the original diversity and biomass of trees require over 100 years to fully regenerate. For most animal species, however, it was previously largely unknown whether and how quickly they could recover. The new study revealed clear differences between species groups: whilst some mobile animal groups regenerated within just a few years, communities of invertebrates in the leaf litter or bacteria in the soil take much longer than tree species. A comparison of pastures and cocoa plantations revealed shorter regeneration times for the latter. Because the plantation trees are left in situ, shade-providing trees and foliage are already present in the early stages, rather than the highly competitive pasture grasses.

Efforts to restore ecosystems are underway all over the world. The study impressively demonstrates that natural regeneration is worthwhile and can therefore help to motivate such conservation projects. Nico Blüthgen, however, stresses how important the protection of intact ecosystems also is: “The rate of deforestation of tropical forests is currently much higher than the measures taken to protect them – almost 4–6 million hectares are lost worldwide every year. These annual losses are thus almost as high as the total area covered by all long-term restoration measures combined.” In addition to restoration, which should be massively expanded, the deforestation of virgin forests still untouched by humans must also stop, as promised in international agreements for this decade but not yet implemented. “Furthermore, rapid natural regeneration only works as long as there are still sufficient intact forests in the landscape to act as seed sources,” adds Blüthgen. There is therefore little time left for the urgently needed turnaround in the climate and biodiversity crisis.