Researchers have analysed data from 127 studies to reveal "thresholds" for when logged rainforests lose the ability to sustain themselves.
The results, published in Nature, could widen the scope of which forests are considered 'worth' conserving, but also show how much logging degrades forests beyond the point of no return.
The first-of-its-kind study was led by researchers from the Department of Life Sciences at Imperial College London with collaborators from around the world, including Professor Nigel Stork and Professor Roger Kitching from Griffith University.
Professor Kitching led the Australian Research Council-funded study with Professor Stork as co-principal investigator. Griffith graduate Dr Sarah Maunsell led the field team as a post-doctoral fellow. Former Griffith research students Dr Lois Kinneen and Dr Jane Hardwick tackled projects within the study.
The Griffith team joined dozens of other researchers from around the world all focussed on the same set of experimental plots in Borneo.
Overall, the international team looked at data from 127 plant and animal surveys covering more than 10 years in the same site in Sabah, Malaysia. The Griffith crew, with Malaysian based co-workers Tom Fayle and Kalsum Yusuf, focussed on insects and the seedlings on which they fed.
Both Professors Kitching and Stork brought many years of Borneo experience to the project, having worked in the field there over several decades.
The study site, named the Stability of Altered Forest Ecosystems (SAFE) Project, included a full gradient of landscapes, including unlogged primary forests, selectively logged forests, protected riverside 'buffer' forests, and forests converted into oil palm plantations that represent 99% of forest removal.
While no level of forest degradation through logging was too low to have an impact on the ecosystem, the results showed that forests that had lost less than 29% of 'biomass' (total weight of organic matter) retained relatively high biodiversity and ecological value, and, if left alone, were likely to recover.
Above 68% biomass removal, however, many types of plants and animals formed communities that were effectively 'scrambled' by extinctions and invasive species. In these areas, serious proactive conservation would be needed to maintain biodiversity. Beyond this, the forests rapidly lost the ability to sustain themselves - to act as complete, functioning ecosystems.
"Most of the vast biodiverse forests of south-east Asia are now modified by human activity particularly feeding the insatiable demand for palm oil," Professor Kitching said.
"These forests contain a vast diversity of wildlife of vital importance to the health of the planet. It Is unrealistic to expect these to continue to survive in the small areas of undisturbed forest left in reserves.
"We have shown that, within limits, the flora and fauna can survive, even thrive, in forests that have been harvested for timber. These logged-over forest are worthy of preservation for their biodiversity values.
Professor Roger Kitching
"Of course, the preserved areas of primary (undisturbed) forest are vital reservoirs of natural life but the surrounding modified forests are invaluable adjuncts to tropical conservation efforts and are well worthy of conservation as part of landscape-level efforts to preserve our biological heritage and the many services - including CO2 removal - that they provide to humankind."
Professor Stork said: "Insects are by far the most diverse group of animals on Earth and with the highest proportion being found in tropical rainforests, we cannot rely solely on the small area of pristine forest to conserve rainforest biodiversity.
"This collaborative study with researchers from around the world shows that we need to nurture our logged forests as well."
Lead researcher Professor Robert Ewers, from the Department of Life Sciences at Imperial College London, said: "A study like this has never been done, where so much data across landscapes, species and time have been collected together to determine critical thresholds. We now have a framework for assessing where these change points might be in other ecosystems."
The Malaysian SAFE Project surveys counted the occurrences of 590 plants (including grasses, herbs, and woody trees), 88 mammals (including bats), 161 birds, 9 reptiles, 42 amphibians, 26 fish, and 635 invertebrates (including 263 beetles, 199 moths and butterflies, 130 ants, and 33 spiders), noting the impacts on each of the extent of logging damage.
The team are now constructing a 'Virtual Ecosystem' that can track the birth, growth, reproduction and death of organisms within a changing ecosystem, and are planning to use the data from this study to generate a virtual model of a Bornean rainforest.
This will allow researchers to address ecological questions that cannot be answered through field observations, such as how to optimise the ecological recovery of degraded tropical forests.
The study 'Thresholds for adding degraded tropical forest to the conservation estate' has been published in Nature.
