Density‐Dependent Effects of Invasive Red Swamp Crayfish Procambarus clarkii on Experimental Populations of the Amargosa Pupfish

Abstract

AbstractThe adverse effects of invasive species are expected to be proportional to their relative densities. However, interference competition among conspecifics when invasive species are at high densities could limit such effects. These alternative hypotheses inspired us to test the density‐specific effects of red swamp crayfish Procambarus clarkii on experimental populations of the Amargosa Pupfish Cyprinodon nevadensis amargosae. The presence of crayfish had a significant influence on the survival of adult pupfish. In allopatry, adult pupfish survival was nearly 100%. However, the presence of crayfish resulted in significantly lower survival rates for adult pupfish, 84% and 69% survival for the low‐ and high‐density crayfish treatments, respectively. Crayfish also significantly affected the production of juvenile pupfish, with high juvenile production in the allopatric (705 juveniles/mesocosm) and low‐density crayfish treatments (705 juveniles/mesocosm). Juvenile production in both of these groups was significantly higher than that in the groups sympatric with high‐density crayfish (271 juveniles/mesocosm). The same pattern was observed when we evaluated the number of juvenile pupfish that were produced per adult female. We tested the hypothesis that interference competition among the crayfish would decrease predator efficacy by comparing the high‐density crayfish treatment with a fourth high‐density crayfish treatment wherein the crayfish were tethered. There was no significant difference in the rate of adult survivorship of the Amargosa Pupfish in the tethered and untethered high‐density treatment groups (70% and 69%, respectively). However, tethering resulted in a significant increase in juvenile production in the high‐density crayfish treatments (493 and 271 juveniles per mesocosm were produced in the tethered and untethered groups, respectively). The same patterns were observed when we examined the number of juvenile pupfish that was produced per adult female. These findings are inconsistent with the interference competition hypothesis, while supporting the hypothesis of density‐dependent effects of crayfish on Amargosa Pupfish populations. These findings suggest that reducing crayfish density will be a beneficial management tool for reducing the effects of crayfish on benthic fish populations.