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1. Biodiversity offsetting is a tool to balance ecological damage caused by human activity with new benefits created elsewhere. Offsetting is implemented by protecting, restoring or managing sufficiently large areas of habitat. While there are concerns about the true feasibility of offsetting, they are becoming a common policy tool world‐wide. Operationally uncomplicated, quantitative approaches to spatial analysis of offsets are rare and their use is often restricted by the availability of suitable spatial data. 2. We describe a practical method for offsets that builds upon two layers of relatively easily sourced spatial data, a balanced spatial priority ranking and a weighted range size rarity map. Together with (a) spatial information about impact and offset areas, and (b) extra parameters for the effectiveness of avoided loss and the amount of leakage expected, we can evaluate whether the proposed offset exchange represents a credible no net loss or net positive impact with an upward trade. The priority ranking and range size rarity maps can be produced in various ways, most notably using existing conservation planning tools. Here we used the standard outputs of the Zonation spatial prioritization software. 3. We illustrate the method and associated visualization in the context of offsetting of boreal forests in Finland, where forests experience high and increasing pressures from forestry and bioenergy sectors. The example is timely as there is political demand for the uptake of biodiversity offset policies in Finland, and boreal forests are the most common biotope. 4. The methods described here are applicable to biomes around the world. The described tools are made available as r scripts that utilize standard Zonation outputs, thus providing direct linkage to any past or future Zonation applications. As a limitation, the present methods only apply to avoided loss offsets.