Natural capital metrics as predictors of farm-scale richness of birds and plants

There is increasing pressure for improved traceability through agricultural supply chains and for farmers to report on their natural resource use, environmental performance, and biodiversity management. Metrics or indices of natural capital are often used in sustainability reporting, but to be effective, they need to represent the condition and extent of natural capital assets present on a farm, and their capacity to support biodiversity at the farm scale – the scale at which many management decisions are made. We propose seven metrics to represent natural capital at the farm scale: (i) Ecological Condition; (ii) Woody Vegetation Cover; (iii) Aggregation (of woody vegetation); (iv) Proximity (of production areas to woody vegetation); (v) Soil Condition; (vi) Riparian Buffer Condition; and (vii) Forage Condition. To assess whether these metrics can represent biodiversity values, we tested the relationship between the metrics and empirical data on plants and birds collected from 1155 sites located across 50 farms (total farm area = 135,890 ha) in south-eastern Australia. Farm-scale species richness of woodland-dependent birds was best explained by the proximity of production areas to woody vegetation and the overall cover of woody vegetation on the farm. Ecological Condition was the strongest predictor of farm-scale richness for all plant species, native plant species, native ground cover and perennial ground cover species. While we demonstrated strong relationships between some natural capital metrics and the species richness of native plants and birds, we recommend using a suite of metrics to represent a range of biodiversity components. The natural capital metrics presented here could potentially be used to report on natural capital value and condition and biodiversity value at the farm-scale in sustainability reporting and natural capital accounting. • We introduce a suite of natural capital metrics that collectively, represent natural capital attributes on farms. • Some metrics can potentially be used as surrogates for specific components of biodiversity at farm-scale. • Ecological Condition can represent native plant richness, especially for ground layer species. • Measures of farm-scale tree cover can represent woodland bird richness. • Multiple metrics are needed to represent multiple biodiversity components.