Population Bias (aka "The Tulsa Phenomenon")

Earlier this summer I wrote a blog post about the ranges of moths in Oklahoma. I finished that post by mentioning the "Tulsa range phenomenon." This relates to my recent post on observation biases.

When I was flipping through my field guide looking specifically at the range maps I noticed a trend - there were a lot of maps that showed the species occurring in the Tulsa area, but not in the rest of the state. Is there something special about Tulsa that results in a huge biodiversity that is not seen elsewhere in the state? I don't think so. True, Tulsa is in the wetter eastern part of the state where there is a higher species diversity among plants and that results in more species of insects, but the number of species in other eastern parts of Oklahoma should be comparable to Tulsa.


Here is a single example of a range map that displays the "Tulsa phenomenon." This is Lespedeza Webworm (Pococera scortealis), which is shown as only occurring in the northeast part of Oklahoma (Tulsa). However, we have spotted this species at two of our moth nights in Norman.

I believe what we're seeing with this range maps is simply a bias of where observations are being made. Not only is Tulsa the second most populous city in the state, but there have also been a few dedicated individuals making regular observations of Lepidoptera in the Tulsa metro area. In fact, the state keeper of Lepidoptera records lives in the Tulsa area and has been making observations there for many years. So when we see a bulls-eye of species biodiversity in the Tulsa area I think what we're seeing is a reflection of the number of days spent observing.

I was first exposed to this concept of observation bias when I was studying meteorology at the University of Oklahoma. We were looking at a United States map that showed the track of every known tornado observation and you could see there were more observations near populated areas. Why? Historically the reason is because there were people there to see them. Nowadays it is rare for a single tornado to go unnoticed in the United States because of the storm chaser/spotter community and the lead time provided for severe weather by weather models and forecasters. Tornado vortex signatures are detected by radar and there are hundreds of storm chasers roaming the country, scouting out every storm with the potential to rotate. Therefore, the data set is becoming much less biased towards population centers.

Do you think we could ever come to a day when moths, or at least certain moths, could be so well observed? Probably not, but one can dream...

Posted on October 17, 2019 09:08 PM by zdufran zdufran

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