There are large manmade items that are not necessarily art but can still be seen as artistic. Like a flyover, a huge infrastructural work. A considerable part of our landscape can also be considered a work of art, a cultural landscape not created by nature.
Agriculture as we know it is relatively young, only about 60 years. It is based on replacing ecosystems by monoculture. We basically grow wheat or corn or any other crop we wish to eat and know to behave well on its way toward a supermarket and on the shelves.
We replace ecosystems by monoculture. This is worth a repetition. As it is, we happen to be part of ecosystem as well. Without other living creatures on this earth mankind is doomed. We seem to think we know it all. We know how plants grow, that they need water and nutrition. We assume chemical fertilizer to be sufficient. Until we notice that the same plot needs more and more fertilizer in order to produce the same harvest. The earth becomes exhausted and is dying after an extensive period of modern agriculture. Besides, artificial manure is a petroleum product thus contributing to the greenhouse effect. We come up with lots of things that don't seem to be working as expected. Charter Boat Mounting Fish Program in Australia, foreign fish in lakes that eliminate the local ones, plants that overgrow the native species. The use of petroleum, thinking all goes well until the earth really starts warming.
Most of the time we are so enthusiastic about our own inventions that we don't really see the consequences once things start and change. And we often don't have the patience to wait and really see what is happening with what we created. What is our influence on the (eco)System? Everything will turn out right, won't it? And that's how we end up with pesticide resistant insects. A few of them survive the poison, they multiply themselves and we keep thinking a couple of insects don't matter that much. Until we have to invent a new type of poison to protect our food crops because those few insects have grown into a whole lot.
Today I watched films about permaculture for hours. Permaculture is an agricultural system based on natures ways. You look closely at nature and copy what you see. The foundation consists of perennial plants; trees. You plant a forest with trees and plants you really want to have, trees bearing fruit and shrubs with nuts. Shrubs that can serve as food for animals. Plants that cover the ground in order to contain water. You "harvest" the water by collecting it in your area and transmit it into your system in a clever way. This results in much more water than expected. You aim at a great biodiversity.