The long-term effects of invasive signal crayfish (Pacifastacus leniusculus) on instream macroinvertebrate communities

Non-native species represent a significant threat to indigenous biodiversity and ecosystem functioning worldwide. It is widely acknowledged that invasive crayfish species may be instrumental in modifying benthic invertebrate community structure, but there is limited knowledge regarding the temporal and spatial extent of these effects within lotic ecosystems. This study investigates the long term changes to benthic macroinvertebrate community composition following the invasion of signal crayfish, Pacifastacus leniusculus, into English rivers. Data from long-term monitoring sites on 7 rivers invaded by crayfish and 7 rivers where signal crayfish were absent throughout the record (control sites) were used to examine how invertebrate community composition and populations of individual taxa changed as a result of invasion. Following the detection of non-native crayfish, significant shifts in invertebrate community composition were observed at invaded sites compared to control sites. This pattern was strongest during autumn months but was also evident during spring surveys. The observed shifts in community composition following invasion were associated with reductions in the occurrence of ubiquitous Hirudinea species (Glossiphonia complanata and Erpobdella octoculata), Gastropoda (Radix spp.), Ephemeroptera (Caenis spp.), and Trichoptera (Hydropsyche spp.); although variations in specific taxa affected were evident between regions and seasons. Changes in community structure were persistent over time with no evidence of recovery, suggesting that crayfish invasions represent significant perturbations leading to permanent changes in benthic communities. The results provide fundamental knowledge regarding non-native crayfish invasions of lotic ecosystems required for the development of future management strategies

Diel patterns of suspended sediment flux and the zoogeomorphic agency of invasive crayfish

The role of biotic forcing in fluvial geomorphology is understudied. This paper investigates the suggestion that the activities of signal crayfish (Pacifastacus leniusculus) can increase suspended sediment fluxes in rivers. Previous field work, supported by mesocosm experiments, suggests that crayfish nocturnalism can cause night time increases in turbidity, but field data are limited to a 16-hour period from a single site where suspended sediment time-series are not available. Here, field data collected over a 28-day period on the Brampton Branch of the River Nene, UK, are used to quantify the impact of diel fluctuations in suspended sediment concentration on sediment fluxes. Field observations and laboratory experiments are used to evaluate the likelihood that crayfish, which are abundant in this river, are responsible for the diel patterns. Turbidity and water stage were measured at 2-minute intervals at a single site. Water was sampled for suspended sediment concentration on a diel cycle and during storm events. A relation between suspended sediment concentration and turbidity, along with a local discharge time-series, permitted calculation of sediment flux and sediment loads. Aquarium experiments with one or two crayfish were used to directly observe the relative impacts of crayfish activity and conspecific interactions on sediment suspension. Over the 28-day period, turbidity and suspended sediment exhibited a strong diel pattern, characterised by night-time increases in the frequency and magnitude of spikes in the turbidity data and by increases in ambient suspended sediment concentrations. Small diel fluctuations in stage were also measured, but the rises in stage were out of phase with turbidity and there was no correlation between stage and turbidity at any frequency. In the absence of a credible hydraulic explanation for the increases in night-time suspended sediment concentration, several lines of evidence, including results from the aquarium experiments, strongly suggest that crayfish are the most likely cause. We estimate that crayfish activity contributed at least 20% of the suspended sediment load over the 28-day period (which included two moderate floods) and this proportion was 47% during the days when baseflow conditions prevailed. This work extends and strengthens the argument that crayfish are important zoogeomorphic agents with potential implications for managing fine sediment pressures. It also highlights the need to better understand the link between crayfish activity, sediment suspension and downstream dispersal, particularly the catchment-scale distribution and seasonality of such impacts