My mom and myself are immune to poison ivy. I’d help her weed it from the gardens. And I’d hatchet it at the base of trees. Fortunately our property had lots of Virginia creeper, which I’d leave alone. Eventually I’d virtually eliminated the poison ivy, as its niche was filled by Virgina creeper.
This might be worth exploring a little further.
There are two main variations of poison ivy in North America. I believe you’re in Arkansas, right? Looks like t. radicans is the more common variety there, and is more likely to be a trailing or climbing vine. T. rydbergii is Canada + most of the US (minus the southeast), and is mostly ground cover, or even grows to large shrub size.
Nettles are fundamental for the life cycle of about a third of the butterfly species in Europe. (Quick cite, superficial but shows my statement to be at least true-ish. For the record: it is 100% true, but my real cite is a book in German)
They are not only herbal medicine, but also great for eating: soup, salad , pesto(after a quick blanching) and more than you would think possible.
And one of the nitrogen richest fertilizers you can make liquid compost-style.
What, me? 
What trouble was I in?
Needless to say (or maybe it needs to be said) this is a dangerous myth.
Farmer Weird may have been one of the 15% or so who don’t react to urushiol. But if you keep getting exposed, your immune system may eventually decide it’s bad stuff and you’ll wind up with a bad case.
I agree but I’ve been reluctant to repeat the experiment on myself.
How did indigenous people use PI to treat warts? I wonder 
The OP applies human-centered reductionism to something of nearly infinite complexity.
In the global ecosystem, there is no concept of “usefulness”. There is symbiosis, there is competition, there is constant striving to not be eaten and to reproduce but only humans think in weirdly constricted terms like ‘usefulness’. Sure lots of organisms are a royal pain in the ass like the deer flies making my livestock’s lives hell right now, or devastating to human agriculture like potato blight – but they don’t have a point, or a reason for existence, or virtues … they simply ARE. Interacting at every moment with every living and nonliving thing they encounter.
Goats are, as any experienced goat keeper knows, incredibly picky eaters. They do like poison ivy however. As well as many cherished garden plants.
Again, there is no “benefit” to any habitat from any organism. That’s not how habitats work.
At least in the case of Poison Ivy, that’s not a defense mechanism. The oil is just for maintaining moisture in the leaf. Humans happen to be prone to an allergic reaction, that’s all.
As a defense mechanism, it wouldn’t be much good. It takes a day or more for the reaction to be noticeable, by which time few animals are going to remember what plants they were near. And the plant is nondescript, not covered in vivid markings like a hornet, so nothing is going to notice it.
To be clear what I was looking for was reliable information on whether hemlock has any particular attributes that have been identified as useful either as a host plant or a source of food or shelter for wildlife.
Well, it is an invasive weed from Europe. Many of these invasives lack beneficial relationships within the ecosystems they invade. It is extremely poisonous to all mammals, but I’ve seen the flowers full of bees and butterflies.
Ahh. Now I know. Thanks.
Right. The stuff is eaten by most herbivores. That allergic reaction is counter productive to poison ivy, because if it didn’t have that feature, humans would actively propagate it. It’s attractive in the spring and summer, and has gorgeous fall foliage. It’s easy to grow in a wide range of the US.
I’m going to agree with @Ulfreida that “beneficial” isn’t really a good way to describe plants with respect to ecosystems. But poison ivy provides forage for many herbivores, nutritious berries for birds and some mammals, and stabilizes the soil in some places.
I was wondering if it was beneficial in that regard. I guess it’s rare for a plant to offer nothing but I do have at least one flowering plant from a commercial nursery that never gets any action from insects. I regret planting it.
I read somewhere that people indigenous to north america did not have skin reaction to poison ivy and poison oak. A long time ago I read this and it might have been a myth. But no other animal has this reaction …
Also, make great cloth like coarser linen.
Yep. Happened to me. I was in the scouts and thought I was immune- even picked a sprig to show as “ID ten local trees or plants”. Later, however, as a volunteer ranger I had to clear some out- I did wear gloves and long sleeved shirt- I wasnt stupid. But there was a small gap between the two at times, and also some hit my neck.
I was in misery for a couple days.
You apply some -just to the wart- and it can clear the wart. Maybe.
I read a study once where the team concluded that if we eliminated the two Human biting mosquitoes from the earth- it would not hurt the ecosystem. Except perhaps for the increase in human population.
You just wait and see, I’ll have poison ivy rash by next week. My life just goes that way.
Read it. I get it.
(Oh, We have been dealing with tomatoes like idiots recently, and due another big deal tomorrow, after the rain yesterday. They make me itch. So this is in the line of full disclosure)
Probably a bit niche. The Cramps.
That would Lux Interior. A one of a kind. Sadly missed.
That would be a huge negative consequence, though.
I would be surprised if the families of swallows who nest in my barn all summer would not suffer, since that’s a major food source for them.
There are tens of thousands of negative consequences due to some humans saying, let’s fix the ecosystem to benefit us more.