Long-term trends and drivers of biological invasion in Central European streams

Rates of biological invasion continue to accelerate and threaten the structure and function of ecosystems worldwide. High habitat connectivity, multiple pathways, and inadequate monitoring have rendered aquatic ecosystems vulnerable to species introductions. Past riverine invasion dynamics were largely restricted to large rivers, leaving out smaller rivers that commonly harbour high freshwater biodiversity. Moreover, biodiversity time series have rarely been used to investigate invasions across larger spatial-temporal scales, limiting our understanding of aquatic invasion dynamics. Here, we used 6067 benthic invertebrate samples from streams and small rivers from the EU Water Framework Directive monitoring program collected across Central Europe between 2000 and 2018 to assess temporal changes to benthic invertebrate communities as well as non-native species. We assessed invasion rates according to temperature, precipitation, elevation, latitude, longitude, and stream type. Overall, average daily temperatures significantly increased by 0.02 °C per annum (0.34 °C in total) while annual precipitation significantly decreased by 0.01 mm per annum (-67.8 mm over the study period), paralleled with significant increases in overall species richness (12.3 %) and abundance (14.9 %); water quality was relatively stable. Non-native species richness increased 5-fold and abundance 40-fold, indicating an ongoing community shift from native to non-native species. The observed increase in invasions was stronger in low mountain rivers compared to low mountain streams, with the share of non-native species abundance and richness declining with increasing elevation and latitude but increasing with temperature. We found thermophilic non-native species invasion success was greatest in larger sized streams, at lower latitudes, lower elevations and higher temperatures. These results indicate that widespread environmental characteristics (i.e., temperature) could heighten invasion success and confer refuge effects (i.e., elevation and latitude) in higher sites. High altitude and latitude environments should be prioritised for prevention efforts, while biosecurity and management should be improved in lowland areas subject to greater anthropogenic pressure, where non-native introductions are more likely.

Occurrence of the invasive Spanish slug in gardens: can a citizen science approach help deciphering underlying factors?

BACKGROUND

The Spanish slug (Arion vulgaris, also known as A. lusitanicus) is considered one of the most invasive species in agriculture, horticulture and private gardens all over Europe. Although this slug has been problematic for decades, there is still not much known about its occurrence across private gardens and the underlying meteorological and ecological factors. One reason for this knowledge gap is the limited access of researchers to private gardens. Here we used a citizen science approach to overcome this obstacle and examined whether the occurrence of Arionidae in Austrian gardens was associated with meteorological (air temperature, precipitation, global solar radiation, relative humidity) or ecological factors (plant diversity, earthworm activity). Occurrence of the invasive A. vulgaris versus the similar-looking native A. rufus was compared using a DNA-barcoding approach.

RESULTS

Slugs were collected from 1061 gardens from the dry Pannonian lowland to the wet alpine climate (altitudinal range 742 m). Slug abundance in gardens was best explained and negatively associated with the parameters "sum of the mean air temperature in spring", "number of frost days in the previous winter" and "mean daily global solar radiation on the day of data collection". Precipitation, plant diversity and earthworm activity were also related to slug abundance, but positively. Out of our genetic sampling of collected slugs, 92% belonged to A. vulgaris.

CONCLUSIONS

Our study showed that citizen science (i) is a feasible approach to record species occurrence in restricted areas across a wide geographical range and (ii) could be more widely employed in order to identify underlying environmental factors of species occurrence.

Herbivory of an invasive slug in a model grassland community can be affected by earthworms and mycorrhizal fungi

Invasion of non-native species is among the top threats for the biodiversity and functioning of native and agricultural ecosystems worldwide. We investigated whether the herbivory of the slug Arion vulgaris (formerly Arion lusitanicus; Gastropoda), that is listed among the 100 worst alien species in Europe, is affected by soil organisms commonly present in terrestrial ecosystems (i.e. earthworms-Annelida: Lumbricidae and arbuscular mycorrhizal fungi-AMF, Glomerales). We hypothesized that slug herbivory would be affected by soil organisms via altered plant nutrient availability and plant quality. In a greenhouse experiment, we created a simple plant community consisting of a grass, a forb, and a legume species and inoculated these systems with either two earthworm species and/or four AMF taxa. Slugs were introduced after plants were established. Earthworms significantly reduced total slug herbivory in AMF-inoculated plant communities (P = 0.013). Across plant species, earthworms increased leaf total N and secondary metabolites, AMF decreased leaf thickness. Mycorrhizae induced a shift in slug feeding preference from non-legumes to legumes; the grass was generally avoided by slugs. AMF effects on legume herbivory can partly be explained by the AMF-induced increase in total N and decrease in C/N ratio; earthworm effects are less clear as no worm-induced alterations of legume plant chemistry were observed. The presence of earthworms increased average AMF colonization of plant roots by 140 % (P < 0.001). Total shoot mass was significantly increased by AMF (P < 0.001). These data suggest that the feeding behavior of this invasive slug is altered by a belowground control of plant chemical quality and community structure.