That depends on the level at which you are observing them. OK, a prairie may not be a good example. What to me looks like "just a grass" is probably a mixture of dozens of grass species. But there are vast forests with no trees other than birch. That is not to say that there are no other species in there, just no other trees. Similarly, a field of wheat has no other species of grass, but there are species of other things. Soil bacteria, fungi, even other plants, insects, birds, mice... Especially with organic farming, but even chemical treatment is not "perfect" and does not kill everything.

I never disputed that. We definitely do. They are many orders of magnitude more prevalent wherever humans are involved. But to say that they do not exist in nature is a bit of a stretch. There is an oxbow lake near where I live. Completely natural. I watched for the past 20 years how it was gradually losing connection to the main stream. Now it's not much more than a shallow bog, completely overtaken (choked) with water lilies. Its banks have similarly been overtaken by stinging nettle. There, two nice monocultures for you None of them are as big as a typical wheat field but they are monocultures none the less. It's just a matter of scale.