Insects live in virtually every freshwater habitat and can be extremely abundant, composing a large part of the animal biomass in lakes and rivers. Aquatic insects have important roles in food webs, acting as decomposers and consumers of aquatic plants. Many are in turn consumed by crayfish, fish, and other predators. But aquatic insects can also be important parts of food webs on land when they emerge from the water and fly over land to find mates. We’ve been studying how neighboring habitats are connected to and affect one another. When midges, mayflies, mosquitoes, caddisflies and many other aquatic insects emerge from freshwater they can become food for predators like spiders, lizards, birds and bats and many studies have shown that these predators are more abundant along the edges of streams and lakes because of the rich food source provided by aquatic insects. In addition to providing food to predators, emerging insects can also have a fertilizer effect on plant communities next to lakes and streams because they act as fertilizer when they die and decompose.
Some aquatic insects emerge all at once in large swarms, resulting in a “pulse” of insects moving from the water to shoreline food webs. We’ve been studying the consequences of this resource pulse using a group of lakes in northeast Iceland. In Iceland, the typical productivity of the land is low (lava fields and heathlands) and dotting this landscape there are lakes, some with many midges and some with few midges, a type of small aquatic insect. We measure midges coming out of lakes, how many end up on land, and how this affects plant growth and the abundance of arthropod predators (e.g., spiders, harvestmen, beetles), detritivores (e.g., springtails [photograph above], mites) and herbivores (e.g., caterpillars, plant hoppers, aphids). The members of this arthropod food web can eat live midges (predators), dead midges (detritivores) or midge-fertilized plants (herbivores).
One way we track midge resources as they move through terrestrial food webs is by measuring carbon isotopes of spiders and insects on land. Carbon from aquatic plants has a different isotopic value than carbon from terrestrial plants. Because midges develop and grow in the water, they incorporate an aquatic carbon signature and we can track their carbon as it is integrated into the terrestrial food web.
In an experiment simulating midge deposition at high-midge lakes (in the photo above you can see David on the right near our midge addition plots at Helluvadstjorn in Iceland), recently published in Oecologia by David Hoekman and others from our Iceland group, we added midge carcasses to a low-midge heathland site. We found that the strongest responders to the dead midge additions were the ubiquitous springtails or Collembola, little insect-like creatures that have a habit of jumping around by flicking a little structure under their abdomens. These tiny (<2mm) arthropods are known to be decomposers of dead and decaying organic matter, likely feeding on the bacteria and fungi growing there. If enough midges are added (for example, if you add midges for two consecutive years), then even larger things like spiders start to show evidence of midge carbon in their tissues. How does it get there? Well, spiders are active predators that eat only live prey, so we surmise that they got the midge carbon in their tissues by feeding on the springtails. We even found some evidence that midge nitrogen was picked up by some of the plants in the area where midges were added. Once the midges are gone, however, the midge carbon quickly disappears from the system. Within one year of the midge additions there is little evidence of midge carbon in the arthropods any more. As the arthropods die and their offspring take their place, they new generation develops in an environment that is now midge-free.
This research is helping us to understand how aquatic and terrestrial systems are connected. Aquatic insects are a major driver of these connections, but other prominent examples include salmon returning to their natal streams and sea birds nesting on land. We’re also interested in how food webs respond to resource pulses, specifically what components of the food web respond quickly to available resources and how ephemeral resources are stored, resulting in long-term effects.