Biodiversity in agricultural land: better in bioenergy crops

Bioenergy is set to take an expanded role as we decarbonise our economies, as a means of replacing fossil fuels and as a potential pathway to generating ‘negative emissions’ via carbon capture and storage (BECCS). Bioenergy is versatile, using the energy from organic materials—biomass—capable of delivering a range of decarbonisation opportunities including electricity generation, aviation fuels, and heating, and BECCS is unique as the only technology identified capable of removing carbon dioxide from the atmosphere at a large scale. Whilst biomass material can be collected as a by-product of forestry and farm activities, high bioenergy demand requires using land to grow dedicated bioenergy crops, such as Miscanthus grass or poplar trees. Bioenergy is therefore land-use intensive, and with global pressures on land already severe, there is valid concern that an expanded role for bioenergy will come at the loss of important ecosystems for biodiversity; previous research has highlighted the negative impacts that bioenergy crops have on biodiversity when grown in natural ecosystems.

With agricultural land already covering over a third of the terrestrial surface of the world it appears clear that no further conversion of natural ecosystems should take place. Bioenergy crops could be planted on agricultural lands if they are freed-up, as a result of increased farm productivity and dietary shifts away from land-intensive meat and dairy production, whilst the bioenergy crops would still provide an income for land managers.

We completed the first meta-analysis to understand the biodiversity impact of growing non-food bioenergy crops on food-based agricultural land (managed grasslands or arable crops), instead of natural ecosystems. Screening over 4,000 studies for our meta-analysis, we used data from 21 field-based studies which met our strict screening criteria. Our study exclusively looked at non-food or dedicated bioenergy crops, including energy grasses of Miscanthus and switchgrass and short-rotation energy trees of willow and poplar. 

We found that biodiversity increases 75% under land-use change from food-based agricultural land to non-food bioenergy crops, with bird abundance increasing 81% and bird species richness rising 100%. And insects, plants, and soil biodiversity also benefit from this. We found that these bioenergy crops improve farm-scale biodiversity compared to food-based agricultural land-use for three main reasons: reduced management intensity on bioenergy field sites, the provision by these crops of features more similar to natural ecosystems than arable crops, and increasing complexity or heterogeneity in the landscape. 

We caution that our positive results rely upon agricultural land being freed-up to be used for bioenergy crops, which could result from increased productivity of land and dietary shifts away from meat and dairy products. For these benefits to be realized, an increase in the cultivation of bioenergy crops will need agricultural lands to be freed up, otherwise, potentially negative indirect land-use change, such as natural ecosystems being converted to food production, could result. 

Additionally, our positive results are derived from farm-scale studies where field sizes were typically under 10 ha; uncertainty remains concerning the biodiversity impact of very large field sizes of bioenergy crops, such as 100 ha. Two key uncertainties regarding bioenergy expansion are the size of the fields and the landscape-scale at which these crops would be grown at. 

In our paper we also considered the impact that bioenergy crops would have on cultural ecosystem services because, along with their effect on biodiversity, they will likely be important to the acceptability of expanded bioenergy. We screened over 2,000 papers for evidence of the visual aesthetic and recreation impact of non-food bioenergy crops, with just 12 studies providing relevant information highlighting an important knowledge gap in this area. Whilst we found evidence that the visual impact of bioenergy crops is not currently a primary concern of the public, and that these crops can fit in and even enhance the visual attractiveness of a landscape, the evidence is limited, and further work is required to determine public attitudes towards large-scale deployment of these energy crops.

These results of our systematic review and meta-analysis are timely after the November COP26 climate change summit in Glasgow, where policymakers are under pressure to raise the level of mitigation actions. At the same time, biodiversity is in a critical condition globally, with one million species of plant and animal at risk of extinction. The implications of our results for policy makers are that farm-scale biodiversity can be supported as bioenergy crops are expanded in the landscape, providing that agricultural land can be freed-up. However, there are potential risks associated with large field sizes or monocultures of bioenergy crops. Meanwhile, we find a crucial knowledge gap of the cultural ecosystem service impacts of non-food bioenergy crops and further public engagement is required to determine the visual impact and public acceptability of these crops in local communities.

Caspar Donnison is a postdoctoral researcher in bioenergy and climate change mitigation at University of California, Davis. His research explores the social and environmental impacts of bioenergy with carbon capture and storage (BECCS) technology. Follow him on twitter at @caspardonnison. 

Gail Taylor is professor and chair of plant sciences at the University of California, Davis. Follow her on Twitter at @taylorlabsoton.