Several times each summer Doug Taron walks a precisely plotted route through Bluff Springs Fen and counts butterflies. He notes his starting and finishing time and records each sighting along the way. His tally sheet is divided into five columns so he can separate the insects seen under the shade of the old burr oaks in the savanna from those encountered on the gravel hill prairie, in the fen itself, in degraded portions of the preserve, or in areas that are being restored.
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Bluff Springs Fen, an Illinois Nature Preserve, is one of the most beautiful places in northeastern Illinois. It lies on the eastern outskirts of Elgin, just far enough east to be in Cook rather than Kane County, and encompasses 91 acres. It is owned by the Metropolitan Water Reclamation District. It is quite hilly, and the hills are kames. Kames form under glaciers from materials carried by meltwater. They are generally made of gravel, which makes them very attractive to companies that make concrete. Most of this region’s kames have been mined down to ground level.
Precipitation flows easily into the gravel at the top of the kame. It percolates down through the gravel, emerging in seeps and springs on the slope or at the foot of the kame. The gravel is mostly limestone, so the water picks up large amounts of calcium and other minerals in its travels. Where it emerges, specialized plants that thrive in the extremely alkaline water form communities called fens. Where there are specialized plants there are often specialized animals as well.
Our next butterfly was a monarch, the first of many, and then a banded hairstreak fluttering up from a burr oak limb after Taron smacked the limb with his net. “We probably miss most of these,” he told me. “They sit on the limbs, but most of them are up in the crown where we can’t see them. But we can get relative abundance.”
Our route takes us right through the fen past shrubby cinquefoil, a rare and specialized plant now in golden flower. Viceroys are mixed with monarchs out in the wetlands. The two butterflies are almost identical, providing an excellent example of protective mimicry. Monarch caterpillars eat milkweed, a diet that makes both them and the adults taste terrible. A bird that has never seen one before will eat one, but never two. The viceroy’s mimicry protects it from predators. Taron points out the black streak on each hind wing that makes it possible to tell one from the other. Later we see a viceroy female laying eggs on a willow, the preferred food plant of the species.
The fen has been under an active fire regime for several years, but no species has shown any decline. Thus far, all have been either helped or unaffected by the prescribed burning.
