By Dr. David Wartinbee, for the Redoubt Reporter
Photo courtesy of Dr. David Wartinbee. Chironomid pupal skins are seen after a large number of them have emerged from the Kenai River. With so many emerging at one time, predators get satiated or confused with all the individuals.
In my backyard the sun has just popped out from behind a cloud and there are patches of brightly lit grass in the yard. Right above several of these bright spots are swarming aquatic insects. These are the nightclubs where males meet females. The males do their own version of karaoke and attract the females into the swarm with high-pitched notes. Within the swarm is where exchanges of gametes take place, so females can then deposit their fertilized eggs in an appropriate setting.
These insect swarms demonstrate a couple interesting ecological phenomena. First, it shows that many members of the same species have synchronously gone from being an aquatic larva to an aerial adult. Most aquatic insect adults live only a very short period of time. Many live only a week but some are limited to a couple days.
During that short window of opportunity they have to find members of their same species, attract a mate and take care of reproductive imperatives. There is no time for delay since the clock is rapidly ticking away. It is important that the larvae of the species be watching the same play book and that the majority of the population do their emerging at the same time.
Remember that most aquatic insects spend about a year as an aquatic larva or nymph. Many then go through a short pupal period and leave the water to become an aerial adult. With lots to do and very little time to do it, it is critical that the majority of the individuals become adults all at the same time. In order to have everyone emerge at the same time, each species seems to look for a particular environmental cue or series of cues. Light provides a major clue for many species. That light cue may be a change in light, such as the change from light to darkness in the evening, or the other way around, such as when the light breaks the darkness in the morning.
Another frequently used environmental cue is water temperature. Many insects require a number of days at a particular temperature in order to complete their growth before emergence. Then, once growth is completed, they need a temperature cue to time their emergence. Some insects emerge when the water temperatures are starting to rise or perhaps when the water temperatures peak.
These environmental cues or triggers also help the particular species go through emergence, when they will have the greatest margin of safety from potential predators. For example, a larger-sized insect might emerge after a change from light to dark, so the adults emerge in darkness. This provides the new adult with some initial protection from visual-feeding predators like dragonflies.
Most insect species seem to follow a particular emergence pattern. No matter what cues they follow, by synchronously emerging, the members of the population are then likely to find a mate and successfully reproduce. The patterns of emergence throughout the summer months are known as seasonal phenology, while the hourly timing sequences are known as diel emergence patterns.
Another aspect that these swarms demonstrate is there is relative safety in large numbers of individuals. While the swarm is an obvious display to potential predators, predators have difficulty feeding in these situations because of the huge numbers of individuals. The large numbers of individuals and their constant movement is confusing to a would-be predator.
As an example, when you see a swarm of insects or a flock of birds, try to concentrate on following a single individual. Nearby individuals will distract your concentration and then the original individual is gone.
Another reason these swarming activities are fairly successful is the sheer number of insects in the swarm. The potential prey individuals overwhelm the needs of local predators. If a swarm consists of a thousand individuals, the loss of even a dozen is a loss of only 1 percent to 2 percent, and that reduction will probably have little impact on the number in the next generation. Essentially, the predators become satiated and stop their feeding efforts.
As an example, a large brown bear will only consume about eight to 10 salmon before leaving the stream area to sleep off its gluttony. Meanwhile, the rest of the migrating salmon move on unmolested.
In many cases, predators are widely scattered due to territories they vigorously defend. If you have a large dragonfly that buzzes around your yard, it is probably the only one present since the male sets up and maintains a well-defined feeding area. The only other dragonflies allowed into the area are females. Once that territorial dragonfly has fed to its limit, and no other dragonflies are allowed into the area, the remaining swarms are left undisturbed.
The easily spotted insect swarms along rivers and lakes turn out to be fairly safe meeting places for insects as they complete the reproductive phase of their life cycles.
David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Kenai River and Cook Inlet watershed.