ReviewOn-farm diversity offsets environmental pressures in tropical agro-ecosystems: A synthetic review for cassava-based systems

•A crop for millions of smallholders, cassava allegedly contributes to soil exhaustion and biodiversity loss.•Intercropping can augment a bundle of ecosystem services, including soil- and water-related benefits.•Diversified cropping systems can deliver benefits to agro-ecosystems at multiple levels.•Better understanding of ecological processes can guide the design of intercropping systems.

Ecosystem integrity is at risk across the tropics. In the quest to meet global dietary and market demands, tropical agro-ecosystems face unrelenting agricultural intensification and expansion. Agro-biodiversity can improve ecosystem stability and functioning, but its promotion in smallholder-based systems faces numerous practical hurdles. In the tropics, cassava (Manihot esculenta Crantz) is cultivated on over 25 million hectares and features as the third most important source of calories. Cassava crops are often maintained by resource-poor farmers who operate on marginal lands, at the fringes of sensitive, biodiverse habitats. As traditional intercropping schemes are gradually abandoned, monoculture cassava systems face stagnating yields, resource-use inefficiencies and agro-ecosystem degradation. A global literature search identified 189 cassava intercropping studies, covering 330 separate instances of intercropping systems. We employed a vote-counting approach and simple comparative measure across a subset of 95 studies to document the extent to which intercropping sustains a bundle of ecosystem services. Across geographies and biophysical conditions, a broad range of intercrops provided largely positive effects on five key ecosystem services: pest suppression, disease control, land equivalency ratio (LER), and soil and water-related services. Ecosystem services were augmented through the addition of a diverse range of companion crops. Results indicated 25 positive impacts vs. 3 negative impacts with the addition of maize, 5 vs. 1 with gramineous crops, 23 vs. 3 with four species of grain legumes, and 9 vs. 0 with trees. Appropriate intercropping systems can help to strike a balance between farm-level productivity, crop resilience, and environmental health. Our work highlights an urgent need for interdisciplinary research and systems-level approaches to identify intensification scenarios in which crop productivity, provision of ecosystem services, biodiversity conservation, and human well-being are all balanced.