Nature has been idealized for a long time, hasn't it? We've seen the near-apotheosis of it in Thomas Cole's Course of the Empire, and artists like John Constable painted landscapes that became instantly popular for their beauty and serenity. Nature, as we discussed, was held in high regard during the Romantic Period (one has only to read Wordsworth, Blake, or Keats to understand that most clearly), but another characterization of nature sprouted around the same time; and this alternate approach was the work of a separate sect of rebellious writers and artists called the Dark Romantics. The Dark Romantics (as their name suggests) approached the same Romantic subjects as their contemporaries—i.e., nature, the noble savage, and the dual concepts of both pastoral and ideological utopian perfection—but approached them in a darker manner. Mary Shelley's Frankenstein perfectly exemplifies the style, for in the novel the "noble savage" concept is flipped into the genre of Gothic horror; it gets pushed to unsettling extremes. Nathaniel Hawthorne, Edgar Allan Poe, and Herman Melville are among the most prolific writers of this movement. Contrary to the beautiful paradise which the Romantics had made of nature, the Dark Romantics took a decidedly negative, but no less reverent view, of the natural world. In Dark Romantic literature, nature is a scary and dangerous place, and mankind, whether bred in tranquil pastures or industrial mills, is a dark being, naturally corrupt, not to be trusted, and often evil to the core. The subjects range from the immediately didactic to the tacitly creepy. Poe's poem "The Raven" subtly disquiets readers with eerie portrayals of nature's cold-heartedness in the form of the mysterious, black raven, and Herman Melville's epic novel Moby-Dick depicts anything but the friendly side of nature. Hawthorne's The Scarlet Letter takes a dark view of humanity's moral state in an otherwise pure, natural setting (the Puritan colonies of the New World, in which the story is set), and Robert Browning's poem "Porphyria's Lover" goes quite a step further in characterizing the heart of man in only the most horrific and disturbing light. Some of this stuff is pretty creepy! (Goya's later art would fit well here, I suppose).
All this to say, whether or not you would call this darker interpretation of nature and the human condition a more "realistic" worldview does not enter into the classification of these literary works as Romantic—they are Dark Romantic. Likewise, art of this time period which produced a negative view of nature was perhaps in opposition to the ideological tenets of Romantic philosophy but was no less Romantic in subject matter. Therefore, at least traditionally, it falls under the label of Romantic artwork. However, I must stress again the reality that art forms blended heavily together during the mid-19th century. Romantic art is sometimes more Realist in execution, and vice-versa. So, I learned Joseph M. W. Turner's work as being a part of Romanticism, even though he painted nature in quite a different light; but be aware that his work draws heavily from the soon-to-be-established Realists, which we will see in the next section.