The clearing of forests, grasslands, and other natural ecosystems to grow more crops is one of the most widespread threats to terrestrial biodiversity and continues to degrade ecosystems across the globe. However, changes in agricultural land use are not unidirectional: Substantial areas of cropland have also been abandoned in recent decades, especially as the agricultural labor supply continues to shrink from rural outmigration, urbanization, and geopolitical conflicts.
The impact of cropland abandonment on biodiversity has been widely debated. In some cases, abandoned croplands can offer opportunities to restore ecosystems, recover biodiversity, and sequester carbon. However, according to previous work , this largely relies on abandoned cropland being mostly undisturbed for multiple decades - a less common scenario as these lands are frequently recultivated due to the increasing demand for agricultural products. Furthermore, variation in the spatial distribution and longevity of abandonment can substantially affect biodiversity outcomes and can be especially disruptive to species with narrow habitat preferences and requirements.
For these reasons, whether or not abandonment is a meaningful conservation opportunity has remained a controversial question among environmentalists. According to a newly-released study led by Princeton researchers, the answer to this question depends not only on the distinct preferences of individual species, but also upon the rate of land recultivation and ongoing habitat loss.
In their study, Dr. Christopher Crawford, Prof. David Wilcove , and their team use high-resolution annual land-cover maps of 11 sites across four continents to assess the biodiversity consequences of cropland abandonment for the years 1987-2017. Annual maps of the vegetation at these sites were then combined with range maps and habitat preferences for over 1,300 species of birds and mammals to calculate changes in habitat area available for each species as a result of cropland abandonment.
"Without detailed abandonment maps for broad areas over long time periods, it has been really difficult to get a comprehensive picture of how abandonment was affecting biodiversity," explains lead author Christopher Crawford , a AAAS Science & Technology Policy Fellow at the United States Department of Agriculture. "By leveraging our cutting-edge abandonment maps and looking at entire communities of mammals and birds, with all of their unique preferences, we've been able to understand abandonment's impacts on biodiversity in a more nuanced and detailed way than ever before."
According to the study, most bird (62.7%) and mammal species (77.7%) gain habitat due to crop abandonment, but even more (74.2% and 86.3%, relatively) would have benefited if recultivation of the land had not occurred. Moreover, despite these gains from abandonment, 32.3% of birds and 27.8% of mammals still experienced net habitat loss when accounting for the ongoing losses of natural ecosystems to create new cropland that often occurs before or alongside abandonment.
"These findings are somewhat surprising but make perfect sense," says co-author He Yin , an assistant professor of Geography at Kent State. "In our previous research, we noted that abandoned fields were frequently recultivated. This study helps us understand the extent to which recultivation may impact conservation efforts when abandonment is used as a strategy."
The authors also explored ecological factors associated with species' responses to cropland abandonment. They found that a species' habitat preference (i.e., habitats that a species can occupy and that are suitable to meet its needs) had a significant effect on responses to abandonment. For example, the ability to occupy forest, grassland, or savanna habitats increased the odds of a species gaining habitat from abandonment, while the ability to occupy arable (i.e., cultivated) land was the strongest predictor that a species would lose habitat following abandonment. While not entirely surprising on their own, the authors note that these findings are encouraging from a conservation perspective: grassland species had the strongest association with gaining habitat gains following abandonment, despite long-standing concerns about how these species would fare following the loss of farmland.
"Ultimately, the answer to the question if abandoned fields benefit biodiversity conservation depends on the population trends of all the species in a given area, and how much habitat is available to them elsewhere," says co-author Volker Radeloff , a professor in the SILVIS lab at the University of Wisconsin-Madison. "Broad-brush statements about the conservation value of abandoned fields are bound to be wrong. What is required are species-specific analyses, like the ones we conducted."
Though the study found overall positive outcomes from cropland abandonment, the authors stress that these benefits are dampened by frequent recultivation and continued cropland expansion. Co-author David Wilcove explains the role policymakers will have to play to unlock abandonment's full conservation potential.
"With millions of people around the world leaving rural areas to move to cities, we might expect the resulting land abandonment to provide great opportunities to restore ecosystems and biodiversity," explains Wilcove, a professor at Princeton's School of Public and International Affairs and the Department of Ecology and Evolutionary Biology. "It can, but the benefits to nature won't be realized if the abandoned cropland is recultivated. An important challenge for policymakers is to provide the right financial or social incentives so that abandoned, marginal croplands can regrow into natural ecosystems. At the same time, policymakers must ensure that the lands that stay in crop production produce enough food to feed a growing population."
The paper, "Biodiversity consequences of cropland abandonment," was co-authored by Christopher L. Crawford (Princeton School of Public and International Affairs, Princeton University), R. Alex Wiebe (Department of Ecology and Evolutionary Biology, Princeton University), He Yin (Department of Geography, Kent State University), Volker C. Radeloff (SILVIS Lab, Department of Forest and Wildlife Ecology, University of Wisconsin - Madison), and David Wilcove (Princeton School of Public and International Affairs and the Department of Ecology and Evolutionary Biology, Princeton University). The paper appeared in Nature Sustainability on October 28th, 2024. This research was supported by the High Meadows Foundation (DSW) and the NASA Land Cover and Land Use Change Program.
