Top-down effects of predators in food webs are generally strong in freshwater communities, but not all predators are the same.  Species turnover in space and time may lead to stronger or weaker top-down effects depending on species traits.  Omnivorous freshwater crayfish are known to have strong impacts on grazers and algae in benthic food webs, but it is unknown whether different species have different per capita effects.   Using experimental mesocosms I examined the trophic effects of two species of crayfish (Procambarus fallax and P. alleni) that exhibit species turnover along hydroperiod gradients in Florida wetlands.  The two species have different life history traits, growth rates and maximum sizes.  I examined effects of each species on snails and algae in experimental mesocosms to determine whether they are expected to have similar individual effects on equivalent food webs.

Using 18 mesocosms (500 L each) I established experimental communities with snails and algae. Four crayfish were added to 12 mesocosms (6 mesocosms/species) and 6 mesocosms were used as crayfish-free controls.  Over 8 weeks we counted eggs and then emptied the mesocosms and quantified total snails and algal biomass (AFDM on rope vegetation) in each mesocosm. 

Results/Conclusions

Egg counts declined through time in all tanks, but crayfish tanks had lower egg production after the first date (pre-crayfish) of the study.  Total egg production was reduced similarly by the two species of crayfish, but most of the reduction in egg counts was by one species of snail.  Final snail density was reduced 98% in crayfish treatments relative to the control, but there was no difference between the two species.  Algal biomass (AFDM) was on average reduced by 50-60% and again there was no difference between the two species.   While the context of this experimental study (i.e., no top predators, moderate nutrients) may have an effect on the results, the trends observed in this study coincide with results from other studies of crayfish trophic impacts.