Insect carcasses link aquatic and terrestrial ecosystems: Lessons from the subarctic

Background/Question/Methods

Aquatic insects are ubiquitous and abundant components of terrestrial communities that occur at the water-land interface.  We hypothesized that, despite their small size, as aquatic insects move over land and die, they can at times represent a relatively large contribution to ecosystem C and N fluxes, can affect food web interactions on land and alter the composition of plant and animal communities.  To test this general hypothesis, we have been studying the annual emergences of chironomid (non-biting) midges from the subarctic lake Myvatn in northeastern Iceland to understand the ecosystem and food web effects on near-shore terrestrial ecosystems.  Long-term collections and modeling were used to estimate the amount of midges that deposit themselves on land each year.  In addition, we used manipulative experiments where we added midges to heathland communities that historically experience few aquatic insect inputs.  We measured decomposition rates, microbial community responses and plant communities.  Finally we measured terrestrial arthropod responses as a function of midge inputs.

Results/Conclusions

Over a three-year period, average midge deposition to land reached a peak at about 20 m from shore and contributed as much as 10 to 15 kg N ha^-1 to terrestrial communities.  This amount of N was associated with wholesale changes in plant communities near shore, which had higher N concentrations, and became more dominated by grasses.  Experimental addition of midges produced the same general results, with increasing dominance of grasses and incorporation of midge-derived N into plant tissues.  In addition, soil decomposition rates increased when midges were added to soils.  Insect communities also shifted in composition in response to midges, reflecting an increase in decomposers and predatory taxa.  In short, because most aquatic insect emergence at the water-land boundary goes unconsumed and enters the terrestrial ecosystem as carcasses, soil and plant communities are altered by this input such that terrestrial community patterns cannot be fully understood without taking into account the effects of insect carcasses.  Although the patterns at Myvatn were extreme, the same processes were observed at smaller scale in most other lakes and streams, making the responses we observe likely widespread in other biomes.