Biotic and abiotic controls of burrowing by signal crayfish (Pacifastacus leniusculus) and the implications for sediment recruitment to rivers

Abstract

The signal crayfish (Pacifastacus leniusculus) is a globally invasive species, and since its introduction to Great Britain has developed a behaviour of burrowing into riverbanks, which has not been documented or observed in its native range. These have the potential to recruit sediment into river systems both directly, through burrow construction, and indirectly, by promoting accelerated erosion and the occurrence of mass failure events. This thesis investigates, for the first time, the drivers of crayfish burrowing, and the effects of crayfish burrows on fluvial geomorphology. The research was conducted using two field investigations that considered the spatial extent and temporal dynamics of crayfish burrows and associated sediment dynamics, and three laboratory studies that considered the physical and biotic mechanisms behind the associations and processes observed in the field. Crayfish burrows were distributed throughout Great Britain across all sampled habitats, and were, on average, 205 mm deep (range 20 – 870 mm), and excavated 1.15 kg of sediment per burrow directly into river systems. In rivers where burrows were present, an average burrow density of 0.39 burrows m1 of riverbank (maximum = 1.13 burrows m-1) was observed, directly contributing 0.93 t km-1 of fine sediment (maximum = 4.14 t km-1) to the channel. However, accelerated erosion caused by the presence of burrows recruited 29.5 times more sediment than burrowing alone, with burrowing and the associated acceleration of retreat and collapse recruiting an additional 25.4 t km-1 a-1 of sediment at one field site; an estimated 29.8% of total bank sediment yield. This was supported by physical modelling, which showed that the spatial distribution of burrows was important for determining retreat rates and mechanisms. Numerical modelling successfully predicted the presence, density, and geomorphic impacts of crayfish burrows. Burrow presence was attributed to associations between crayfish population densities and sediment grain size distributions, and burrow density was dependent on river flow velocity. This modelling was supported by mesocosm and flume experiments with live crayfish. Further, mesocosm experiments undertaken in the UK and the USA showed that all signal crayfish have the capacity to burrow, suggesting that burrowing and associated geomorphic change may occur in any invasive signal crayfish population. This thesis has shown that signal crayfish burrows can have substantial geomorphic impact, and is the first research to quantify the geomorphic role of signal crayfish burrows, and more broadly to quantify the relative importance of biotic and abiotic forcing of fluvial sediments. Future research should better consider animals as geomorphic agents, both directly, but also indirectly, by considering the facilitative role that zoogeomorphology may have on wider system processes.