As scientific thinkers, we need to re-examine our beliefs constantly and question what we “know” to be true. As we learn, our cumulative knowledge increases, often giving us a better perspective. For example, I recently had a wake-up call that forced me to re-evaluate the idea of the windshield effect.
The windshield effect is a phenomenon that people have noticed the world over. In decades past, car travel from town to town often entailed a good bit of windshield cleaning as the glass became laden with squashed bugs that blocked the driver’s view. In those days, gas station attendants often did this for us as they filled the tank, and many travelers kept a supply of cleaners, rags, and scrapers in the car.
When I was young I couldn’t reach the windshield, so I was given headlight duty. The lenses would get so burdened with dead insects that light barely passed through, and the slop needed to be removed frequently in order to see the road. I can still smell those ammonia-soaked rags and the wormy odor of bug goop that permeated my clothes.
For the most part, we no longer clean car parts between stops, which most of us see as a good thing. But environmentalists around the world are horrified by clean windshields and blame pesticides, urbanization, climate change, and habitat loss for the missing bugs. So last year, when an entomologist told me the windshield effect was nonsense, I was taken aback. Then I decided he was simply too young to remember.
A different view
But several months later, when I was interviewing Dr. John Ascher of the National University of Singapore, I asked him about the windshield effect, and his answer stopped me short. He explained that the relative number of bugs on windshields then and now doesn’t mean much because we have no idea what was in the mix. Did it contain a great diversity of critically important insects or did it comprise several species in huge out-of-control numbers like a plague of locusts? In short, are we mistaking insect “blooms” for healthy and diverse ecosystems? Are we confusing biodiversity with sheer volume?
The truth is we will never know. No one back then thought the bugs were of any long-term consequence, so no one saved the windshield scrapings for posterity. We’ll never know if those legions of road bugs were a good thing or a bad thing because we don’t know what species they were or their relative abundance in those ecosystems.
And other variables can enter the picture, too. In the past, busy roads often traversed agricultural land, but now much of our cropland is set back from heavy traffic. Also, cities and towns frequently spray herbicides along roadways, which suppresses insect populations along long stretches of pavement.
In truth, we probably do have fewer insects than we used to and less biodiversity, too. But the windshield effect doesn’t prove that theory because it’s merely anecdotal evidence.
What is anecdotal evidence?
An anecdote is a story, so anecdotal evidence is story evidence. Right now, I’ll tell you a story about swarms. This spring I had three swarms in the trees in my backyard, and all three swarms selected my one and only top-bar hive as their preferred home and nixed my baited Langstroths. True tale. From this observation, I could easily conclude that honey bees are attracted to top-bar hives, or perhaps they are repelled by Langstroth hives. The evidence for such a conclusion is story evidence and it is useless. Countless factors could have swayed the bees’ decision, so the story is merely a red herring.
You scoff and say, “No one would come to that conclusion from just three swarms.” Maybe not, so let’s take another, one that regularly arises on my website. Someone writes, “I had a colony that was strong and doing great when I checked on it four months ago. But last week when I opened the hive to harvest the honey, there was nothing left but wax moths. Wax moths totally destroyed my colony.”
My turn to scoff. You had a strong colony, didn’t check on it for months, and now the hive is full of moths? Sounds normal. Moths love a weak or dying colony and somehow you provided one. But you can’t conclude moths killed the colony because you have nothing but anecdotal evidence — a story about what you observed. Oftentimes, anecdotal evidence illustrates a truth, but a story by itself cannot prove it.
What about that red herring?
A red herring is a distraction that throws someone off course. The phrase comes from a fictional story, written in the 1800s by William Cobbett, about a boy who saves a hare from the hounds by diverting the dogs with a dead fish, which just happened to be a red herring. Red herrings are common literary devices, often used in mystery novels, to divert the readers’ attention from the true villain, but the term is also commonly used to describe logical fallacies.
In the wax moth example, the presence of so many moths and larvae fouling the interior of the hive diverts you from the true cause of the colony’s demise. We follow the deceptive trail which leads us away from the answer.
Both of these stories ignore the many possible variables that could have produced the outcome you saw. A variable is an element, feature, or condition that can easily change. Going back to my swarms, perhaps it wasn’t the architecture of the top-bar hive that attracted the swarms, but maybe it was the amount of sunlight. Maybe it was the odor of the comb or the recently deceased colony that used to live there. Maybe it was the size of the entrance hole, the height off the ground, or the internal volume of the hive that attracted the bees.
Sometimes conditions exist that we don’t account for, simply because we have no idea they are important or because we forget about them. In science, variables that are outside the scope of the experiment, but still affect the outcome, are called extraneous variables or confounding variables.
I knew an entomologist who did extensive controlled testing to learn how much of a certain pesticide caused harm to bumble bees. All the colonies received carefully measured ….