Looking at the Little Guys

Insect behavior is a curious thing. Those that are terrestrial are easily observed by the average person such as ants on the ground, bees buzzing around flowers, crickets chirping at night or the pesky house fly sitting on the window. However, there are many insects whose behaviors go unnoticed. One such group of insects is a group that lives in water also referred to as aquatic macroinvertebrates.

Aquatic macroinvertebrates exhibit a behavior called drift. A scientist by the name of Mueller (no known relation but I am looking into it) described a phenomenon where some, not all, species of macroinvertebrates release themselves from the substrate and float downstream in the current until they choose to grab ahold again. The macroinvertebrates continue this process of moving farther and farther downstream until they emerge and the adult fly upstream to lay their eggs. Once the eggs hatch the process of drifting repeats itself. Without going into further detail of when, why, types and other patterns, simply put there is still limited knowledge about this behavior.

An increasing number of studies have been done to look at various factors influencing insect drift. One particular study looked at the impact of suspended solids (sediment) on macroinvertebrates drift behavior in an Indiana Creek (USA). Sediment is considered a pollutant, which comes from activities such as mining, farming, and logging. Previous studies have shown that when there has been an observed increase in sediment in water there has been an observed decrease in macroinvertebrate numbers because sediment decreases the amount of light that comes through the water column as well as smothers habitat. This particular study wanted to consider the impact of sediment runoff of a rock quarry. In this study, controlled amounts of sediment (collected from the bottom of the quarry’s settling ponds) were released into the stream from a modified garbage can dispenser. Holes in the bottom of the can allowed water to flow through the garbage can and a known amount of sediment was placed inside therefore they could calculate the total suspended solids released.

Collecting drifting macroinvertebrates is done by using an appropriately named drift net. These nets were placed in the water downstream of the garbage can for 15 minutes during the time the sediment was being added. After 15 minutes, nets were removed from the water and the insects and debris captured were preserved in 70% ethanol. In the laboratory, macroinvertebrates were identified and counted. Water samples were also taken to be checked in the laboratory for the total suspended solids in order to compare the numbers to what was being released. The researchers were responsible and stopped the experiment on a particular day when they noticed sediment starting to collect on rocks in the study site. Lastly, a substrate sample was also taken, on non-experiment days, with a Surber sampler to determine what the natural benthic macroinvertebrate community structure was.

The results of this study were not particularly surprising. There was a linear relationship between the number of macroinvertebrates drifting and the amount of sediment put into the stream. They found that midge, blackfly, caddisfly, and mayfly larvae were the most common drifting species. Riffle beetles were observed in the substrate but did drift and were therefore not as effected by the sediment entering the water. The researchers cannot be sure, but they believe this drifting behavior could be classified as active-behavioral drift. This means that the macroinvertebrates chose to move away from something happening in their environment as opposed to being carried away by a high flow after a heavy rain. Based on their observations, short term re-population of the study area was successful but high levels of suspended solids could be detrimental in the long term.

So even though an impact like dumping sediment into a stream is a pretty drastic situation, it can have further implications on an ecosystem than just making some insects change habitat. Fish rely on macroinvertebrates as a food source. No macroinvertebrates will mean no fish. Therefore, it is critical that more studies like this one occur to find our what human activities have what level of effect on macroinvertebrate behaviors. The same applies for the dumping of chemicals or excess water into streams. What are the part per million or part per billion tolerances of macroinvertebrates? How tightly can they hang onto the substrate when there is an increased flow? There is also limited knowledge of the time scales of which these changes in macroinvertebrate behaviors occur. Therefore, as more an more industry waste is created it is important to remember the little guys even if we don't see them every day.

To see the original paper visit: https://journals.iupui.edu/index.php/ias/article/viewFile/8310/8461

To read more about the original research on drift, locate the paper:
Müller, K. 1954. Investigations on the organic drift in North Swedish streams. Rep. Inst. Freshw. Res. Drottning. 35:133-148.