On-Farm Agrobiodiversity Index

The mathematics is shared, but the object and interpretation are agricultural. A generic ecological index counts wild species in an assemblage; the on-farm agrobiodiversity index measures cultivated and reared diversity — crop species, the landraces and named varieties within them, and livestock breeds that farmers deliberately maintain. This shifts the key decisions: units are defined at the crop, variety or breed level (varietal diversity within a species is often the focus), abundance is area planted or animals kept, and the diversity has direct meaning for resilience, nutrition, livelihoods and genetic conservation. Jarvis and colleagues framed on-farm crop-variety richness and evenness specifically to monitor agricultural diversity and its erosion, which is a distinct, substantive application.

Because two farms can grow the same number of varieties yet be very differently diverse in practice. If one farm devotes nearly all its land to a single variety and keeps the others on tiny plots, while another spreads its land evenly, both have the same richness but very different resilience and conservation value. Evenness measures how balanced the area or abundance is across units, and combined indices like Shannon and Simpson reward both many units and balanced shares. Reporting richness, a diversity index and evenness together gives the full picture; richness alone can make a near-monoculture look as diverse as a genuinely mixed farm.

Unit definition is the most error-prone step. Diversity exists at several levels — species, variety and breed — and indices computed at different levels are not comparable, so the level must be fixed and stated. Variety identification is especially tricky on farms: the same landrace may carry different local names in different villages, and different landraces may share a name, which inflates or deflates richness. Good practice verifies names against germplasm references or morphological characterisation, records abundance consistently (area or count), and corrects for differences in farm size and sampling effort before comparing indices across farms or communities.