Consequences of consumer origin and omnivory on stability in an experimental food-web

Background/Question/Methods

There is currently great concern about the stability of both natural and human-managed ecosystems. The introduction of non-native consumers may affect the stability of recipient systems, because non-native consumers usually exhibit higher consumption rates. These stronger interactions may lead to increased temporal variability in lower trophic levels. The introduction of omnivory in particular has been shown to affect stability drastically. While some suggest omnivory can be stabilizing, others show that omnivory can create chaos in simple food-web models.

In this study, we performed a factorial mesocosm experiment to test the interactive effects of consumer origin (native or non-native) and consumer type (predator or omnivore) on stability in a tri-trophic food-web. We used four species of crayfish to act as consumers: two species native to the Great Lakes (Orconectes propinquus and O. virilis), and two non-native species (O. limosus and O. rusticus). Each mesocosm contained one consumer, which acted as either an omnivore (feeding on snails and algae) or a predator (feeding on snails). Half of the crayfish were converted to predators through the surgical removal of setae to prevent algal consumption. Snail and algal biomass was measured every second day for 2-months to determine stability, here measured as temporal variability. 

Results/Conclusions

We observed differential responses across the four species in the experiment. No significant difference was observed between omnivore and predator treatments in O. limosus. However, we did observe a difference between treatments in O. propinquus, O. rusticus and O. virilis. Higher snail predation by O. virilis predators likely generated a trophic cascade by releasing algae from herbivory, resulting in higher algal biomass. The non-native O. rusticus demonstrated the highest snail predation rate – driving snails to extinction in as little as three days. The absence of snails translated into higher algal biomass across time.

Contrary to expectations, our results indicate omnivory did not reduce variation in algal biomass. Mesocosms containing predaceous crayfish showed smaller fluctuations in algal biomass across all four species. Algal abundance varied less in mesocosms containing non-native crayfish compared to those containing native crayfish. We posit this result is mediated by higher snail predation rates of non-native crayfish.

While omnivory can be stabilizing in food-webs, it is dependent on weak consumer-resource interactions. Here we show omnivory can have deleterious effects given strong consumer-resource interactions between crayfish and snails.