Recovery and Degradation Drive Changes in the Dispersal Capacity of Stream Macroinvertebrate Communities

Abstract

Freshwater ecosystems face significant threats, including pollution, habitat loss, invasive species, and climate change. To address these challenges, management strategies and restoration efforts have been broadly implemented. Across Europe, such efforts have resulted in overall improvements in freshwater biodiversity, but recovery has stalled or failed to occur in many localities, which may be partly caused by the limited dispersal capacity of many species. Here, we used a comprehensive dataset comprising 1327 time series of freshwater macroinvertebrate communities ranging from 1968 to 2021 across 23 European countries to investigate whether dispersal capacity changes with the ecological quality of riverine systems. Sites experiencing improvements in ecological quality exhibited a net gain in species and tended to have macroinvertebrate communities containing species with stronger dispersal capacity (e.g., active aquatic and aerial dispersers, species with frequent propensity to drift, and insects with larger wings). In contrast, sites experiencing degradation of ecological quality exhibited a net loss of species and a reduction in the proportion of strong dispersers. However, this response varied extensively among countries and local sites, with some improving sites exhibiting no parallel gains in macroinvertebrates with higher dispersal capacity. Dispersal capacity of the local species pool can affect the success of freshwater ecosystem restoration projects. Management strategies should focus on enhancing landscape connectivity to create accessible "source" areas and refugia for sensitive taxa, especially as climate change reshapes habitat suitability. Additionally, biodiversity initiatives must incorporate adaptive decision-making approaches that account for the site-specific responses of macroinvertebrate communities to changes in ecological quality.

Keywords: Europe; dispersal capacity metric; ecological quality ratio; freshwater macroinvertebrates; restoration; river; time series; traits.

FIGURE 1

(a) Sampling sites (points) and number of sampling years (color and size of points) across 23 European countries (light gray). (b) Number of time series and mean number of sampling years (color) by country. AT, Austria; BE, Belgium; BG, Bulgaria; CH, Switzerland; CY, Cyprus; CZ, Czechia; DE, Germany; DK, Denmark; EE, Estonia; ES, Spain; FI, Finland; FR, France; GB, United Kingdom; HU, Hungary; IE, Ireland; IT, Italy; LT, Lithuania; LU, Luxembourg; LV, Latvia; NL, Netherlands; NO, Norway; PT, Portugal; SE, Sweden. Map lines delineate study areas and do not necessarily depict accepted national boundaries.

FIGURE 2

Relationship between the dispersal capacity metric of the community (CWM‐DCM) and the ecological quality ratio (EQR). Higher EQR values indicate communities that have become more similar to reference conditions, while lower values indicate degradation. Data points are represented by gray circles and fitted models by solid lines. The colored lines correspond to a model where the smooth term was replicated for each country. The black dashed line corresponds to a model in which a single slope but different intercepts were fitted to all countries and is represented here with the mean intercept. Note that trends for countries with less than three sites with long enough time series were not represented, although they were included when calculating the overall trend. BE, Belgium; BG, Bulgaria; CZ, Czechia; DE, Germany; DK, Denmark; EE, Estonia; ES, Spain; FI, Finland; FR, France; GB, United Kingdom; HU, Hungary; IE, Ireland; IT, Italy; LT, Lithuania; LU, Luxembourg; LV, Latvia; NL, Netherlands; NO, Norway; SE, Sweden.

FIGURE 3

Relationship between changes over time in the dispersal capacity metric of the community (CWM‐DCM slope) and changes in the ecological quality ratio (EQR slope). Green dots represent recovering sites, gray dots represent stable sites, and orange dots represent degrading sites. The black line corresponds to a linear model in which a single slope was fitted to all countries. Recovering sites: Increasing ecological quality (n = 433), stable sites: No significant changes in ecological quality (n = 842), and degrading sites: Decreasing ecological quality (n = 52).

FIGURE 4

Estimates of the dispersal capacity metric (DCM) and dispersal‐related traits of stream macroinvertebrate communities to changes in the ecological quality ratio (EQR). Traits positively correlated to increments in EQR are indicated by green bubbles and negative responses by orange bubbles; non‐significant trait responses are not shown. Slope estimates of CWM slope vs. EQR slope (horizontal axis), conditional R ² (vertical axis), and marginal R ² (bubble size) are shown for each linear mixed model. ALS, adult lifespan; DS, dispersal strategy; FWL, female wing length (mm); LCD, life‐cycle duration (years); LF, lifelong fecundity (number of eggs per female); MBS, maximum body size (cm); RCY, potential number of reproductive cycles per year; WPT, wing pair type.

FIGURE 5

Mean dispersal capacity metric (DCM) of new taxa: Not present initially and then present later, remaining taxa: No change and extirpated taxa: Present and then not present in recovering sites: Increasing ecological quality (n = 433), stable sites: No significant changes in ecological quality (n = 842) and degrading sites: Decreasing ecological quality (n = 52). Bubble size indicates the average proportion of new, remaining and extirpated taxa in each site category. Black crosses indicate the weighted mean DCM value for each site category. See Figure S4 for further details.