Conservation planning usually starts from records of where a species has been seen. A study in Biological Conservation by Markus Franzén and Victor Johansson shows how abundance data can add an important dimension: identifying the relatively small proportion of occupied sites where butterfly counts are concentrated. The findings highlight why occupied sites should not automatically be treated as equally important for conservation.
Applying inequality measures to butterfly data
The researchers applied Lorenz curves and Gini coefficients to three grassland butterflies: the marsh fritillary (Euphydryas aurinia), Apollo (Parnassius apollo) and Large blue (Phengaris arion). The study covered a 60 km² landscape on Gotland, Sweden, divided into one-hectare grid cells.
The analysis included 795 species-by-grid records. Only occupied cells surveyed on at least three different dates were included, and local abundance was represented by the highest daily adult count recorded in each cell during the focal year.
A minority of occupied sites accounted for half of the peak counts
Across the three species, approximately one-fifth to one-quarter of occupied grid cells accounted for half of the summed peak counts. Spatial concentration was greatest in the large blue, intermediate in the marsh fritillary and lowest in the Apollo. Gini coefficients and the other main concentration measures gave the same species ranking.
Habitat associations differed among species. Marsh fritillary peak counts were positively associated with ground moisture and grassland proportion, while Apollo peak counts were positively associated with grassland proportion. Large blue peak counts showed the opposite association with grassland proportion. This negative association should not be interpreted as grassland avoidance: the authors emphasise that broad grassland cover does not necessarily capture the fine-scale conditions required by the large blue.
Implications for conservation
The findings support combining occurrence records with abundance surveys and species-specific habitat assessments. Identifying high-density sites can help conservation practitioners recognise concentrations that presence–absence records alone cannot describe.
The study measured spatial concentration, rather than demonstrating which sites maintain populations over the long term. It used one focal year per species—2017 for the marsh fritillary, 2019 for the Apollo and 2025 for the large blue. Further monitoring is needed to determine whether the same sites remain hotspots across years.
Read the research
Franzén, M. & Johansson, V. (2026). Most individuals, few sites: spatial concentration of threatened butterfly populations challenges occurrence-based conservation. Biological Conservation, 321, 111953.