City Creek Nature Notes – Salt Lake City

June 13, 2017

June 6th

Sufficiency and Necessity

5:00 p.m. It is the fourth day of near record heat in the nineties. On the heat of the afternoon, only a few birds are heard: Warbling vireo, Black-headed grosbeak, Song sparrow and American robin. There is one each of this season’s common butterflies: Weidemeyer’s Admiral butterfly, Western tiger swallowtail, Mourning cloak butterfly, and Cabbage white butterfly. Because of the heat, there is no one on the first mile of road, although the parking lot is three-quarters full.

Returning down canyon, an American robin alights in the grass of a picnic site, cocks its head to one side, and then jabs down with its bill to pull out a fat earthworm. It comes and goes in a moment.

* * * *

In Thoreau’s “Journal” on June 6th, 1852, he notes the season’s first dragonflies. On June 6th, 1853, he records pink corydalis and side-saddle flowers, He sees blue-eyed grass, a flower, in meadows, lambkill, and huckleberries. He describes a slippery elm and hears bluebirds singing. He notes that under the summer sun, the ground is drying out and notes which spring flowers have peaked due to lack of water. On June 6th, 1854, he smells locust trees. He sees a viola by a river and examines the shapes of leaves of various trees. He again reports dragonflies and Sphinx moths. On June 6th, 1855, records seeing blue-eyed grass blooming. On June 6th, 1856, he admires that bird’s nests are well constructed. On June 6th, 1857, he notes leaves and grass dominate, i.e. not flowering plants. He notes dwarf dandelions are common. On June 6th, 1860, he notes that undergrowth foliage is becoming dense. At night, he observes bats in the sky and water bugs on a stream.

* * * *

In a 1995 article, Montgomerie of Queen’s University and Weatherhead at Carleton University conducted a series of controlled experiments concerning the most common of human experiences – seeing a robin pull of worm from underneath the ground. How do robins so expertly perform this task? It seems as if they can somehow see beneath the ground to locate their prey. I like their study because it nicely illustrates how proof of causation can be shown in laboratory controlled experiments and how designing such experiment is a difficult art. Such causation proofs are done using bi-directional “if and only if” implication, and such logic problems are the bane of graduate school candidates who sit for standardized entrance exams.

Montgomerie and Weatherhead suspected that robins use either sight, smell, vibrations, sound or some combination of the four to locate underground worms. In their experiments, Montgomerie and Weatherhead measured the success rate of robins finding mealworms in four artificial environments: First, live scent, sound, and vibration emitting mealworms were buried in the same cage as smell emitting but soundless and vibration-less dead worms. Second, robins were placed on a layer of soil that contained a buried cardboard sheet that covered soil containing live mealworms. Thus, the robins could not rely on visual cues to find their prey. Third, they buried mealworms in a vertical soil containing wall of a cage. Thus, the robins could not feel the vibrations of the buried worms. Fourth, they placed robins on soil containing buried live mealworms but used a white noise generator to mask any noises that the worms might make. The robins could find worms in the absence of sight, smell and vibration cues. They had less success when finding buried worms when sounds were masked with white noise (id).

Proving causation by bi-directional “if and only if” implication involves showing that cause implies effect using propositional logic, that is “If a (C)ause exits, then the (E)ffect occurs” or C -> E in symbolic logic. In bi-directional “if and only if”, the (E)ffect must also imply the (C)ause, or E -> C. The C -> E part is usually described using the language that “C is a sufficient cause for E”, and the E -> C part is described using the language “C is a necessary cause for E”. The mathematics of propositional logic tells us that C -> E is the same as not(C and not E). Conversely, E -> C is the same as not(E and not C). Thus, E -> C, sufficient causes and necessary causes can be expressed as an overlapping butterfly Venn diagrams. Draw two overlapping circles and label the left circle “C” for causes and the right “E” for the effect. Sufficient causes are in the left circle, and necessary causes are subsets of the effect and appear within the right circle. Necessary and sufficient causes appear in the overlapping area. All of this confusing propositional logic is normally reduced to two simple tests for the propose of analyzing causation and taking graduate school entrance examinations:

• Sufficient cause test: Any candidate Cause that is present when the Effect is absent is eliminated as a candidate sufficient cause. This is equivalent to not(C and not E) in proposition logic symbolism.

• Necessary cause test: When an Effect is present when a candidate Cause is absent is eliminated as a candidate necessary cause. This is equivalent to not(E and not C) in proposition logic symbolism.

• Necessary and sufficient cause test: Is the intersection of the remaining results after application of previous two tests.

Finding a necessary and sufficient cause is accomplished by building a table of causes and the resulting effect. Eliminate causes that fail either the sufficient or necessary causation test and take the intersection of the two lists. The remaining candidates are sufficient and necessary causes. The proof that those candidates are the cause of the effect is done. This is best illustrated by Pasteur’s experiment that demonstrates that the fermentation of liquids like grape juice and milk was caused by particles in the air and did not arise spontaneously from particles in those liquids. This was Pasteur’s proof of the germ theory of disease.

To prove that particles in the air causes liquids to ferment and not particles in the liquid, Pasteur constructed flasks with a long-thin neck. Potential causes of fermentation are air in the flask, air in the liquid, and air outside the flask. In his first test, he heated the grape juice (or milk) to destroy any bacteria in the milk and the long-neck excluded outside air. The liquid did not ferment. The result where fermentation is absent shows under the sufficient cause test, that neither particles in the liquid or in the flask’s air cause fermentation. That left the air outside the bottle as an untested candidate sufficient cause. In his second test, he again heated the liquid and air inside the flask to kill all bacteria, but then broke off the long neck that excluded outside air. The liquid fermented. This result where fermentation effect is present shows that under the necessary cause test, air outside the flask contains particles that cause fermentation, but not the sterilized air or liquid in the flask. In his third test, Pasteur heated a tilted the flask so liquid filled the long neck, exposing only the liquid to outside air. Fermentation occurred, and under the necessary cause test, this excluded the air inside the flask as a necessary cause of fermentation. The intersection of the candidate sufficient and necessary causes is the air outside the flask. Conversely, fermentation does not occur spontaneously from particles in the air or liquid inside the sterilized flask.

Applying this type of causation proof to how robins find worms, Montgomerie and Weatherhead thought that there were four candidate causes or means by which robins found worms (the effect): smell, vibration, sound or vision, or some combination of the three. The effect is present when birds found the worms and absent when they did not. In their first experiment involving scent, birds were no better at finding dead worms than chance or than finding live worms, and under the sufficient cause test, this eliminated smell as a sufficient cause. The birds did much better than chance in finding live worms using the remaining auditory, visual or vibration cues, but since smell was also present, no candidate cause was eliminated under the necessary causation test. In their second experiment that removed visual cues but left smell, vibrations and sounds, the birds found live worms better than chance. Under the necessary test, eliminated sight as a necessary cause. In their third test involving vibration, birds were still able to find worms when only vibration was removed, and vibration was eliminated as a necessary cause. In their fourth test, birds were unable to find worms when only sound was removed, and smell, vibration, and sight were eliminated as a sufficient causes. The necessary and sufficient results of their four tests can be reorganized as follows:

• Smell – Effect absent – sufficiency test: Smell-possible, Sight-impossible, Vibration-impossible, and Sound-impossible. Result: Smell was eliminated as a candidate sufficient cause.

• Sound – Effect absent – sufficiency test: Smell-possible, Sight-possible Vibration-possible, and Sound-impossible. Result: Smell, Sight and Vibration are eliminated as a candidate sufficient causes. Sound remained as a candidate sufficient cause.

• Sight – Effect present – necessary test: Smell-possible, Sight-impossible, Vibration-possible, and Sound-possible. Result: Only sight was eliminated as a necessary cause.

• Vibration – Effect present – necessary test: Smell-possible, Sight-possible, Vibration-impossible, and Sound-possible. Result: Only vibration was eliminated as a necessary cause.

• Smell – Effect present – necessary test: Smell-possible, Sight-possible, Vibration-possible, and Sound-possible. Result: This test eliminated no candidate necessary causes.

Looking at the intersection of all of these tests, only sound remains as both a sufficient and necessary cause. Thus, Montgomerie and Weatherhead concluded that, “[T]hese results strongly suggest that the robins located buried mealworms and earthworms by using auditory cues (id. at 149). Their conclusion varied from the only similar prior study that concluded that robins use visual cues to find underground worms.

For most questions involving nature (and economics and sociology), causation cannot be shown in controlled experiments, and investigators must rely only on statistical proof of sufficient causal agents. Both necessity and sufficiency can never be shown due to the multiplicity of causal factors, from ethical restraints that prevent the use of controlled experiments, or excessive cost, and proof of causation is relegated to argumentation from signs (April 27th). For such matters we are forced to rely on humanistic-based judgments when issues involving nature and biology have implications for human society. Much of modern political argumentation is based on politicians intentionally citing only one of several candidate sufficient causes, and then questionably ignoring other obvious, likely causal factors in order to fashion a passably convincing position. The only antidote is an educated citizenry armed with critical thinking skills.

When our canyon robin cocked its head to one side today, it was listening for the buried worm.

* * * *

On June 6th, 2009, three-hundred volunteers pulled invasive weeds like myrtle spurge and toadflax from the Morris Reservoir area overlooking City Creek Canyon (Salt Lake Tribune). On June 6th, 1993, the Salt Lake Tribune overviews hiking access points along the Salt Lake Valley, including at Ensign Peak, City Creek Canyon, and the Tomahawk Drive access to Little Twin Peaks in the Avenues. The Tribune notes reductions in hiking access as luxury subdivisions, such as the Turville-Robinson development below Ensign Peak, develop the foothills. Salt Lake City has regulations requiring foothill developers to include access points in their plans. On June 6th, 1905, boxer Jack O’Keefe trained by running up City Creek Canyon (Salt Lake Herald). On June 6, 1904, wildflowers are in full bloom on the foothills and in the canyons (Salt Lake Herald). City Creek has Stansbury’s phlox, that has a purple flower and yellow balsam root (id).


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