Reinforcing the Significance of Crop Diversity for Biodiversity Conservation in the Face of Agricultural Expansion

Agriculture has altered the Earth's surface since it first emerged in different regions worldwide. However, its impact on ecosystems and biological communities varies significantly across geographical areas. These variations are due not only to climatic and ecological differences but also to differences in human occupation and land-use history. Overall, tropical ecosystems have proven to be more resilient to traditional agricultural disturbance, showing a greater capacity for recovery after abandonment than subtropical and temperate ecosystems (Chazdon et al. 2020). Yet, the ability of agricultural areas to recover after the abandonment of agricultural activity depends largely on the depth of the transformation, the intensity of use and the duration of the activity (Krause et al. 2016). Ecosystems in temperate and subtropical regions have been profoundly transformed by agriculture since the Neolithic revolution, so that original biological communities have been largely replaced by others adapted to open landscapes and the cyclic disturbance regimes associated with agriculture, in which pioneer and generalist species thus tend to dominate (Ellis et al. 2021). In contrast, the highly rich species pools of tropical ecosystems have persisted in the face of traditional, low-intensity and often itinerant farming, which has allowed much higher niche diversification and degree of ecological specialization (Oakley and Bicknell 2022).

Probably due to such divergence, studies assessing the impact of agriculture on biodiversity have flourished in either type of ecosystem, while comprehensive accounts that globally evaluate and compare the response of biological communities to agricultural transformation are very scarce. Fan et al. (2024) in their work ‘Impact of crop type on biodiversity globally’, recently published in Global Change Biology, is one of those few. Shunxiang Fan and colleagues use large global datasets to assess which major crop types are linked to varying levels of species richness and abundance in birds, arachnids and insects. They compare annual and perennial crops across tropical and non- tropical regions, including subtropical and temperate areas. It can, therefore, be considered a landmark study, whose main result and message is that modern agriculture is causing stronger biodiversity loss in annual than in perennial crops, in monocultural than in mixed or mosaic (including natural vegetation patches) systems and in tropical regions. In the typical vein of good and fertile scientific work, Fan and colleagues leave the ground open for reflection, new questions and further testing of ecological hypotheses.

One of these considerations has to do with the tested and acknowledged recognition of heterogeneity within agricultural landscapes as a critical factor for biodiversity conservation. As agricultural intensification progresses, the resultant simplification of agro-ecosystems constrains the survival of dependent biological communities (Benton et al. 2003). Consequently, complex agricultural mosaics, crop rotations and mixed cropping systems can be instrumental in sustaining richer ecological communities, as pointed out by Fan and colleagues. In response to evidence, agricultural policy developments have begun to recognize the environmental benefits of diversified cropping systems by incorporating measures to encourage diversification (e.g., Maggio and Sitko 2021). However, not all diversification strategies provide equivalent ecological benefits, and trade-offs may arise (Beillouin et al. 2021). From the findings of the Fan et al. study that certain crops support comparatively low levels of biodiversity, it can be inferred that mixed systems consisting solely of these crops may not provide the necessary resources to maintain diverse and functional communities. This observation suggests that some diversification strategies may be inadequate, encouraging further research into effective methods.

The observed variation in biodiversity responses to different crop types is influenced by several factors, including the management practices inherently associated with them, as Fan et al. acknowledge in their paper. To be consistent, we should consider whether the ecological benefits of crop diversification stem from the diversification itself, or from the varying levels of intensity associated with these crops. Fan et al. find that perennial crops are able to support higher levels of biodiversity. However, this conclusion leads to further inquiry: is this capacity inherently linked to the crops' life cycles, or does it result from less intensive management practices typical of such agricultural systems? As stated in the study, by integrating perennial crops, farming systems not only enhance biodiversity, but also provide greater stability by reducing disturbance and input demand. However, the rise of intensive and super-intensive perennial cropping systems would change this statement (Guerrero-Casado et al. 2021). This transformation of permanent crops needs to be carefully monitored, as a misinterpretation of their role in conservation-oriented management could have important implications for biodiversity.

Nevertheless, the study identifies that mixed cropping systems, combining both annual and perennial crops, have a reduced impact on wildlife communities. This may support the increasingly widespread view of agroforestry as a promising strategy for mitigating the impacts of agriculture, suggesting a viable alternative for the future of sustainable agriculture. However, understanding how the expansion of agroforestry may influence the composition of biological communities on agricultural land (e.g., the replacement of certain communities by others) may turn crucial, particularly if we consider the biological singularity of open habitat communities adapted to low-intensity agricultural management in temperate, Mediterranean and subtropical zones (Sainz Ollero 2013). If the uniqueness of these communities is not taken into account, their replacement by a subset of forest or forest-ecotone communities from neighbouring woody ecosystems could have consequences, not only on their conservation, but on the function of the agrarian systems thus established (Tscharntke et al. 2016).

Ultimately, the study by Fan and colleagues exposes the complexity of prescribing a global strategy to minimize biodiversity loss, where solutions untailored to regional climatic, land-use and historical conditions may be inadequate and one size does not fit all. The necessity for tailored approaches to sustainable agricultural management is evident. Given that agricultural systems are predominantly managed for the provision of goods, processes and impacts notably differ between regions, taxa and systems. The paper's findings on biodiversity responses to crop types across such diversity of conditions underline the importance of adopting case-specific conservation strategies and continuing research into the effects that different forms of intensified land use can have on ecosystems.

Irene Guerrero: conceptualization, writing – original draft, writing – review and editing. Manuel B. Morales: conceptualization, writing – original draft, writing – review and editing.

The authors declare no conflicts of interest.