Tage der Biodiversität 2027

The Una River is known for its crystal clear karst spring water and spectacular travertine formations in the form of waterfalls, rapids, and a multitude of islands. These islands differ in size, height, and distance to the riverbank: a setting that reminds us of island biogeography. We thus asked whether species richness and composition depend on these island features, with the additional complexity that islands along the continuum of a flowing river may have rather peculiar spatial relationships.

We characterized island structure and investigated biodiversity and community composition of vegetation (with a focus on flowering plants) and ground-dwelling invertebrates on 50 islands along about 60 km. Plants were identified in repeated 5 minute-search intervals in the field, while invertebrates were sampled using pitfall traps, with three traps per island exposed for seven days and later identified to species level under the microscope.

For both groups, richness was higher in the lower, wider section of the Una than in the upstream Una canyon. Islands in the upper canyon also tended to a more similar community composition. The most species-rich islands supported >40 plant species, up to 14 ant species, and a range of Isopoda, Aranea, Carabidae, Staphylinidae, and other invertebrates. Vegetation richness increased significantly with island height and size, while it tended to decline with increasing distance from the riverbank. Invertebrates showed a similar pattern for height and size but the opposite trend to distance to the riverbank. The island size effect for both groups and the distance effect for vegetation are consistent with  island biogeography theory. The effect of height may be due to availability of refuges in case of floods or, alternatively, to increased microhabitat heterogeneity. Also, islands offering both tree and ground refuges supported notably higher richness than those with only tree refuges or none at all.

A highlight is the finding of the hypogean ant Strumigenys tenuipilis. This species is a new record for Bosnia and Herzegovina. Unlike related species, it tends to occur in preserved natural habitats rather than urban areas, further underscoring the need to keep the Una protected.