Temporal niche dynamics of spreading native invertebrates underlie doubling of richness in pristine temperate streams.

While biodiversity loss is undeniably a global phenomenon, an increase in taxonomic richness has recently been reported from some ecosystems and spatial scales. A striking increase in abundance and/or species richness has been documented from temperate rivers over the last 25 years, with many of the expanding species (i.e. winners) being native species. However, the lack of repeatedly collected local environmental data prevents the exploration of their niche dynamics and also makes it difficult to distinguish between possible causes. We fill this gap by using species occurrence data from 65 pristine Czech rivers sampled in 1997-2000 and 2015. The same methods were used for sampling macroinvertebrates and measuring environmental parameters in both periods. We selected 43 winners, defined as taxonomically validated and originally non-rare native macroinvertebrate species whose occupancy increased by at least six sites between the time periods. We searched for consistent patterns of niche dynamics (i.e. stability, expansion and restriction) among species that might contribute most to the overall increase in species richness. Using several biological traits, we also compared the winners with the other 253 taxa collected to look for differences. Analysis of the occurrence data showed that niche stability was by far the predominant pattern of the niche dynamics. This clearly indicates that the winners fill their original niches, with a limited contribution of niche shift or expansion, depending on the species. As no significant differences in either temperature preferences or the other biological traits were found between the winners and the other taxa, there is no unique set of functional traits that explain the success of the winners. The observed mechanism of filling the original niche space by the spreading native species not only explains the increase in local species richness, but also contributes to support the hypothesis of a climate-driven increase in ecosystem energy flow from a new perspective. The increased metabolism of the system may relax interspecific competition allowing it to carry more individuals and species, even without the need for an increase in nutrients and ecosystem recovery.