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 niche, 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 to carry more individuals and species, even without the need for
an increase in nutrients and ecosystem recovery.