"Nature" is a tricky word in Arcadia – what appears natural on the surface is rarely so. Exhibit A is the garden of English manor house Sidley Park, whose style undergoes an extreme makeover as ideas of what "nature" is (or perhaps should be) change over time. Making "nature" subject to popular trends seems kind of, well, unnatural. But it also makes us wonder what other things thought of as "natural" might just happen to be in fashion at the present moment. Arcadia suggests that humans have a bigger role in shaping our understanding of what is and isn't considered natural than we might think.
Bernard's preference for beauty over fact regarding the natural world indicates the danger of looking at the world through a purely artistic lens.
The "natural" is that which is supposedly untouched by human interference. By showing how ideas of the "natural" are actually influenced by fashion, Arcadia questions the possibility of looking at nature objectively.