Crayfish ecosystem engineering effects on riverbed disturbance and topography are mediated by size and behavior

Abstract

Animals that burrow, forage, or produce physical structures can have substantial impacts on transport processes related to erosion. However, the influence of behavior and body size on regulating the magnitude of engineering effects by animals is not well understood. For example, crayfish are common and abundant freshwater organisms that disturb sediments in ways that influence gravel transport, fine sediment suspension, and bank stability. Animals such as crayfish also display complex territorial and aggressive behaviors, often related to body size, which might influence their ability to influence sediment transport dynamics. We conducted an experiment with spiny-cheek crayfish (Faxonius limosus) to investigate the influence of behavior and body size on substrate disturbance. The experiment included 4 enclosure (0.31-m long x 0.21-m wide x 0.17-m deep) treatments that differed in the body size of individual crayfish and the amount of biomass: 1) 2 small young-of-the-year (YOY) crayfish of 15-mm carapace length (CL) (abbreviated SS), 2) 2 larger, 1 + y-old crayfish of 25-mm CL (abbreviated LL), 3) 1 crayfish of 15- and one crayfish of 25-mm CL (abbreviated SL), and 4) a control with no crayfish. We monitored construction of pits within the gravel bed and the proportion of streambed over which crayfish exhumed subsurface gravels. We also used videography to quantify aggressive encounters between crayfish individuals. We found that body size strongly influenced the amount and type of disturbance, with large crayfish creating a significantly greater number of pit structures than small crayfish. Additionally, surface gravels were moved over 11.4, 9.3, 1.3, and 0.003% of the bed surface area in LL, SL, SS, and control treatments, respectively. On average, 77% of interactions between crayfish individuals were aggressive regardless of size, which may explain why the amount of change in bed topography in the LL treatments did not always exceed that in the SL treatments. Successfully incorporating animal behavior into sediment transport models may require consideration of both behavior and body size.