Developing spatially comparable biodiversity indicators using objective scale-dependent species selection

Multi-species indices (MSIs) are an important tool for monitoring progress towards conservation policy targets from the local to the global scale. The choice of constituent species for habitat-specific indicators often reflects context-specific knowledge, policy needs and data availability. This makes direct comparisons of equivalent indicators across various locations challenging, and potentially reduces their representativeness if subsequently applied to other locations or spatial scales. In recognition of this, there is growing demand to develop standardised approaches to species selection that produce more spatially comparable MSIs. Using forest bird species in Europe, we use an objective, niche-based framework for indicator species selection to derive standardised indices at national, regional and pan-European scales, and explore the implications for species composition on indicator trends when adopting three alternative species-selection strategies: selecting species representative of a given spatial scale (“geographically-targeted”), disaggregating a species set representative of a broad-scale for use at smaller scales (“top-down”) and aggregating species lists representative of smaller scales for use at larger scales (“bottom-up”). We show that although the composition of indicator sets varied according to the species’ selection approach, resultant index trends for a given location were generally comparable. However, “geographically-targeted” indicators tended to be comprised of more specialist species and were more representative of the wider community. Whilst existing biodiversity indices provide critical insights into the state of nature across spatial scales, our study provides the basis for the development of complementary, standardised indicators that are spatially comparable.