The effect of a midge pulse on a terrestrial arthropod food web

Background/Question/Methods

The movement of insects from aquatic into terrestrial habitats can dramatically affect the recipient ecosystem.  However, the short-term effects of this transfer on the terrestrial food web remains understudied, especially surrounding lakes.  Moreover, many insects synchronously emerge, creating a “pulse” of aquatic-derived resources in the terrestrial environment that may elicit temporary changes in predator foraging.  At a lake in northeast Iceland called Mývatn, or “lake of the midges”, massive swarms of midges (Diptera; Chironomidae) descend upon the shores of the lake during brief periods of the summer.  To investigate the effects of the midge pulse on the terrestrial arthropod food web at Lake Mývatn we used manipulative and observational methods to assess short-term changes in food web dynamics.  Hypothesizing that live midges act as alternative prey with positive indirect effects on in situ prey via shared predators, we contained a predator (wolf spiders; Pardosa sp., Lycosidae) and herbivores (Cicadellidae, Delphacidae) together with varying levels of midge density (none, pulse, and press) into mesocosms.  To assess a similar response in the field we placed live sentinel prey (Drosophila melanogaster larvae) into areas of varying midge abundance to measure relative rates of predation through the course of a midge pulse.

Results/Conclusions

Midges had positive effects on herbivore and sentinel prey survival.  The predation rate of herbivores and sentinel prey was significantly reduced in the presence of midges than without.  The number of herbivores remaining in mesocosms after a three-day midge “press” was nearly twice that of herbivores enclosed with spiders alone.  Similarly, twice as many sentinel prey were recovered in spatio-temporal zones of high midge abundance as compared to those of low midge abundance.  This agreed with our prediction that the presence of midges during a pulse would benefit in situ prey, and suggests short-term positive indirect effects of aquatic insects in terrestrial arthropod food webs along lake margins.  Predator switching allows in situ prey a temporary refuge from predation during the height of the midge pulse.  However increases of predators due to the midges subsidy has the ability to reduce herbivore numbers over the long-term, which has been observed in surveys of the terrestrial arthropod community.