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 Index	Water Quality	Degree of Organic Pollution
Less than or equal to 3.75	Excellent	Organic pollution unlikely
3.76 to 5.0	Good	Some organic pollution
5.1 to 6.5	Fair	Substantial pollution likely
6.6 to10.0	Poor	Severe organic pollution likely

Table from Stroud Water Research Center http://www.stroudcenter.org



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.



References
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 <http://www.stroudcenter.org>

No Thanks, I'm Full

Nobody likes a bad meal. You can put in the time, energy, and dedication, yet when all is said and done, what you get is disappointment. I don't know if wild animals experience similar events, but I certainly wonder. For instance, could predators pursue and succeed in catching prey, only for prey to taste bad or have some defense that leaves the predator out of luck? The short answer is yes, but the long answer involves the detailed history of predator-prey interactions: the co-evolution of species' physiology to be in a spy versus spy battle of biological weaponry (think camouflage, claws, armor, or a toxic taste). One defensive tactic, known by all owners of adventurous pet dogs, is the skunk's potent stink spray.

As you can imagine, a pungent stench only really works against something with the ability to smell it. The skunks (Family Mephitidae, "of noxious smell") have two common species in North America--striped (Mephitis mephitis) and spotted (Spilogale putorius). Two additional species exists in the southwest near Mexico, but they are rarer (hooded and hognosed skunk). While skunks have fellow mammal predators such as foxes, coyotes, and mountain lions, they are mainly a food source of last resort for these species. Unfortunately for the skunk, it has a more voracious predator, the great-horned owl (Bubo virginianus). It was probably a great-horned owl that left this striped skunk dead on the road.

Great-horned owls, reliant on their sight and not their nose, can have skunks as the main prey-item in their diet. In some cases, owls will have eaten so many skunks that they and their nests give off the skunk musk. The skunk in the picture above appeared dead on the road after one night. It is hard to see in the picture, but on one side of the skunk, there were two sets of three puncture holes, and on the other side, there were two sets of one puncture hole each. It was clear talons had pierced it. Since skunks are mainly nocturnal, it was most likely a great-horned owl that struck it. Other candidates include red-tailed hawks.  While skunks can become pests in urbanized areas because of available food (trash) and shelter (crawl spaces), they are great consumers of rodents, insects, lizards, frogs, salamanders, fungi, roots, and nuts in their natural milieu.

As seen by its small eyes, skunks have poor vision and instead rely upon

strong senses of smell and hearing.

  

In the superfamily of Musteloidea, skunks are close relatives of weasels, badgers, otters, and mink.

Thus, it is no surprise that they spend a lot of time in burrows that they have dug.

I'm not sure what exactly happened between the skunk and its owl predator--maybe the owl got spooked by something else and abandoned its kill--but I prefer to think it was an epicurean owl, disappointed by its foul smelling catch.