Predator driven changes in prey size distribution stabilize secondary production in lacustrine food webs

Data from 12 Colorado mountain lakes, some containing invertivorous fish and some without fish, show that fish suppressed biomass of benthic invertebrates and zooplankton and reduced their mean and maximum body size, but did not alter invertebrate production. Resilience of invertebrate production occurred because the smaller body size of invertebrates in lakes with fish raised the community P/B ratio and simultaneously caused an increase in abundance of small invertebrates (<10 mm benthic invertebrates; < 1.5 mm zooplankton), probably by releasing small invertebrates from competition with large invertebrates. Together, these two changes in the invertebrate communities stabilized secondary production even though total invertebrate biomass was reduced. Compensatory stability of herbivore production in response to size selective predation by fish may be a common phenomenon in aquatic food webs. Because it involves both an increase in P/B ratio and an increase in biomass of invertebrates with high P/B ratio, compensation may to some degree explain the commonly observed failure of predation pressure by primary consumers to cause an increase in biomass of primary producers for trophic cascades with three levels, even when invertebrate biomass is reduced by predation. Addition of a fourth trophic level consisting of piscivorous fish would likely have less predictable effects because piscivores produce offspring that are invertivorous predators, thus potentially adding a qualitatively different compensatory mechanism to the trophic cascade.