Carl Linnaeus, also known as Carl von Linné or Carolus Linnaeus, is often called the Father of Taxonomy. According to the University of California Museum of Paleontology:
His system for naming, ranking, and classifying organisms is still in wide use today (with many changes). His ideas on classification have influenced generations of biologists during and after his own lifetime, even those opposed to the philosophical and theological roots of his work.
John James Audubon is famous for his bird portraits, of course. Says Wikipedia:
Audubon's influence on ornithology and natural history was enormous. Nearly all later ornithological works were inspired by his artistry and his high standards. Charles Darwin quotes Audubon three times in The Origin of Species and also in later works. Despite some errors in his field observations (e.g., he thought an immature bald eagle to be a separate species), his notes were a significant contribution to the understanding of bird anatomy and behavior. Birds of America is still considered one of the greatest examples of book art.Although not primarily known as a naturalist, Albrecht Durer's watercolor of a hare is one of my favorite nature illustrations, and he has many more. Dürer believed that "…art must be based upon science—in particular, upon mathematics, as the most exact, logical, and graphically constructive of the sciences." In his Treatise on Proportion, he commented, "Life in nature makes us recognize the truth of these things, so look at it diligently, follow it, and do not turn away from nature to your own thoughts…. For, verily, art is embedded in nature; whoever can draw her out, has her."
Beatrix Potter -- author of Peter Rabbit and many other children's books -- was not only a writer and an artist; she was an accomplished, if not acknowledged, scientist. As Wikipedia relates:
An uncle attempted to introduce her as a student at the Royal Botanic Gardens at Kew, but she was rejected because she was female. Potter was later one of the first to suggest that lichens were a symbiotic relationship between fungi and algae. As, at the time, the only way to record microscopic images was by painting them, Potter made numerous drawings of lichens and fungi. As the result of her observations, she was widely respected throughout England as an expert mycologist. She also studied spore germination and life cycles of fungi. Potter's set of detailed watercolours of fungi, numbering some 270 completed by 1901, is in the Armitt Library, Ambleside.There's more about Beatrix Potter, scientist, at The Scientist.
In 1897, her paper on the germination of spores was presented to the Linnean Society by her uncle Sir Henry Enfield Roscoe, as women were barred from attending meetings. (In 1997, the Society issued a posthumous official apology to Potter for the way she had been treated.) The Royal Society also refused to publish at least one of her technical papers. She also lectured at the London School of Economics several times.