Influence of reproductive state and temperature on the functional response of the marbled crayfish, Procambarus virginalis

Predictive information on the trophic impacts of an invasive consumer can be gained by experimentally measuring its functional response- the rate of prey consumption in relation to available prey density. However, a common gap in such information is how functional response varies with the consumer's reproductive state and thermal environment. Here, we tested the effect of reproductive state and temperature on the functional response of the marbled crayfish (Procambarus virginalis), a parthenogenetic species that is popular in the pet trade and considered to be an invasion threat to freshwater habitats globally, including the North American Great Lakes. We applied two thermal treatments: 18 degrees C and 26 degrees C, representing current and projected mean summer surface water temperatures, respectively, for nearshore areas of the lower Great Lakes. We found that gravid individuals readily fed on benthic invertebrate prey (chironomid larvae), but their rate of consumption was substantially reduced compared with non-gravid individuals at both temperatures. When tested at 26 degrees C, the functional response of gravid individuals did not change, whereas non gravid individuals exhibited an increase in attack rate and a reduction in maximum feeding rate. A generalized linear model of bootstrapped attack rates and handling times showed that these parameters were affected by temperature and reproductive state. In addition, warming altered reproductive cycles by increasing the frequency and duration of gravid states. Our findings suggest that invasive species risk assessments would be better informed by quantifying variation in invader per capita effects across reproductive cycles and temperatures.