Yes, you heard that right. Poison ivy is not poison. I recently posted a YouTubevideo in which I cuddle up next to a poison ivy plant (they are ubiquitous in Oklahoma) to address this topic.
Plants—all plants, all the time—are engaged in a silent struggle for existence. You cannot see it, especially in a peaceful forest, but you can imagine it. Leaves contain thousands of different chemicals that are mildly or very toxic to herbivores. This makes perfect sense, because any plant that did not defend its leaves against herbivores—any leaf that said, “Come and eat me!”—would quickly disappear down the gullets of extinction. Each kind of plant has its own cocktail of poisons.
But when a plant defends its leaves, it is an investment decision. Plants that produce too much defensive chemical, and thus waste their precious resources, risk extinction just as surely as a plant that produces too little. A leaf has to pay for itself by photosynthesis, but also by not having excessive maintenance costs, including defense.
None of this, however, explains poison ivy. In poison ivy and related species within the genus Toxicodendron, a set of chemicals collectively called urushiol harm humans, but not most other mammals. According to this scholarly article (which can be downloaded here), “Deer, goats, mice, and other mammals readily eat poison ivy foliage without apparent discomfort.” I have seen squirrels eating the berries. Clearly, urushol is not a metabolic poison.
The human immune system reacts to urushiol as if it is a pathogen. That is, urushiol is an allergen, not a poison. Poison ivy itch is a massive allergy. Some people are more allergic to urushiol than others, just as is the case with other allergens such as pollen (hay fever), gluten, etc. But why does poison ivy affect only humans? It seems incredible to me that humans might have exerted an evolutionary pressure on poison ivy sufficient to select for the urushiol response.
As far as I can tell, no one has explained why poison ivy plants produce urushiol. It must have some function unrelated to defense. There will always be mysteries and unanswered questions in science!
I am working on a book, tentatively entitled Silent Struggle: The Hidden World of Plants. Watch for it!