A method for the reintroduction of entire benthic invertebrate communities in formerly degraded streams

Abstract

Reintroduction is a commonly used tool in wildlife conservation. Although freshwater ecosystems are among the most threatened worldwide, reintroductions of entire benthic invertebrate communities have never been reported before. Here, for the first time, we developed a comprehensive method for the reintroduction of entire communities in formerly degraded streams that describes the selection of donor and recipient streams, the transfer of specimens, mortality rates, and a monitoring scheme. The stream selection procedure of the new method is based on widely available environmental variables and richness metrics that are regularly monitored by water authorities to fulfill the requirements of water policy frameworks (e.g. EU WFD). The sampling and transfer of specimens is also based on common and simply applicable methods (i.e. kick sampling). For each recipient stream, three donor streams were selected with a complementary set of reference species missing at the recipient stream. The sampling at each donor stream was done six times a year to cover different seasons and life stages. We applied our method to one stream (10 km2) and one river (1608 km2) in central Germany. In total, we transferred 325,197 and 401,233 specimens from three different donor streams, including 45 (28,682 specimens) and 50 (47,716 specimens) missing reference taxa into each recipient stream and river, respectively. Average mortality rates varied between 0.97% for all specimens and 1.72% for reference specimens. We recommend a post-reintroduction monitoring that includes new methods, i.e. environmental DNA and high throughput sequencing. Our proposed reintroduction method is based on commonly used variables, metrics and sampling techniques. Accordingly, this method requires only minor modifications to become applicable in other countries. The simultaneous reintroduction of a large number of reference taxa will increase the likelihood to reestablish near-natural benthic invertebrate communities in formerly degraded streams. This may increase the public and political acceptance of costly river restoration projects that so far, do not often reach a good ecological status even after decades, due to a large-scale depletion of potential source populations. Our method will help in the reintroduction of reference species in restored streams where natural recolonization is not possible.