Sunday, September 21, 2014

Bugged by Pollution

One would think that the word "wildlife" would involve all living creatures, but in reality, it traditionally refers to vertebrates. Thankfully, in the past few months, I have had the opportunity to branch out of the traditional realm of the wildlife biologist and have been introduced to the overwhelming world of arthropods. The phylum Arthropoda, meaning "jointed leg", has over one million described species, and it is estimated that there are four to nine million species that have yet to be formally described (Ødegaard 2000). Arthropoda includes many types of animals: crabs (crustaceans), spiders (arachnids), and butterflies (insects).

Today, I am going to focus on three insect orders. These insects are useful in understanding the health and condition of the environment around us--specifically water quality. Water quality directly affects our well being in many ways. For recreation, poor water quality may mean that the local summer swimming spot is closed because of harmful bacteria. High water quality may mean the creek nearby can support healthy fish  for whatever fishing habit we may have. Economically, water quality also impacts the amount of effort it takes for water treatment plants to make our water potable. So, how do we measure and understand water quality?

Fishing Creek, Clinton Co. Pennsylvania.

While there are many techniques one can do in the lab, surveys for aquatic invertebrates provide a convenient way to quickly assess water quality in the field. Mayflies (Ephemeroptera "short-lived wings"), stoneflies (Plectoptera "folded wing"), and caddisflies (Trichoptera "hairy wings"), hereafter referred to as EPT, are three insect orders that are indicators of high quality water. This means there are sufficient levels of dissolved oxygen, minimal runoff that carries pollutants and silt, and low levels of organic matter pollutants.

Mayflies, as their scientific name would suggest, only have a brief window of time in which they are mature adults. Some species are adults for a few minutes and others just for a few days. Interestingly, these species molt twice to reach adulthood, transforming from aquatic nymph to an immature-winged subimago, where the subimago must use its wings to break out of the water before it can molt once more to become an adult. The Brown Drake (Ephemera simulans), pictured above, comes from a burrowing group of mayflies that have theirs gills located on their back. Once adults, these mayflies can form impressive swarms during the summertime.

Stoneflies (right), are perhaps the most sensitive indicator species of water quality. Common to streams, members of the Perlidae family (above) are predatory on other invertebrates. Upon reaching adulthood, they will "drum" at select frequencies, communicating with potential mates by sending vibrations through the ground. Caddisflies (left), are noteworthy for making homes for themselves. Families can be distinguished from one another depending on the materials and style, and in streams, members of Limnephilidae usually construct their cases out of heavier materials like sand and pebbles. As adults, caddisflies have silky hairs on their wings.

While the method varies by region and agency, the Biotic Index is a common analysis that uses the numbers of EPT and other aquatic invertebrates to calculate a measure of water quality. This achieved by assigning each family of aquatic insects a pollution tolerance rank (0-10, 10 being high pollution tolerance). Then the tolerance rank is multiplied by the number of individuals from each family. This total tolerance per family is then summed across families and then divided by the number of all individuals, resulting in a value between 0 and 10.

Biotic IndexWater QualityDegree of Organic Pollution
Less than or equal to 3.75ExcellentOrganic pollution unlikely
3.76 to 5.0GoodSome organic pollution
5.1 to 6.5FairSubstantial pollution likely
6.6 to10.0PoorSevere organic pollution likely
Table from Stroud Water Research Center

Next time you are out playing in the water, consider looking under a rock or two and see if you can find any of these prized indicator species.

Marshall, Stephen A. 2006. Insects: Their Natural History and Diversity. Firefly Books, Buffalo, N.Y. 718 pp.
Ødegaard, Frode (2000), "How many species of arthropods? Erwin’s estimate revised." Biological Journal of the Linnean Society 71 (4): 583–597
Stroud Water Research Center <>

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