A Tale of Two Nutty Butterflies – The Buckeyes

Lemon Bay Preserve in southern Venice, FL, is an interesting place for many reasons, but it has some remarkable butterflies illustrating the role of habitat differences in speciation. For example during a nature walk there yesterday we saw a mangrove buckeye butterfly in a tidal area sitting on some saltwort (see photo). The name is a bit strange since the buckeye tree does not occur here; the tree is named for its large brown nuts which somewhat resemble the eye of a deer/buck. Indeed this butterfly does have 6 distinctive eye-spots, likely useful in deterring predatory birds. Buckeye butterflies often sit in open areas with their wings spread, allowing an observer to admire their unusual coloration. What really excites me about buckeyes at Lemon Bay Preserve is that there are actually two different but closely related species there, the mangrove and common buckeyes. The other photo shows a common buckeye that I found there on another day. If you look carefully you will notice some subtle but distinctive differences between these two sibling species. The common has lighter or even white borders to the large eye-spot on the fore-wing; in addition the two eye-spots on its hind-wings are much more different in size than in the mangrove buckeye. While these differences are clear, they are not so striking that it is that easy to recognize them in the field.

So what is going on here? How can two closely related species co-exist in the same area without competing? The common buckeye is by far the most widespread in N. America (larvae feed on gerardia, toadflax & plantain) whereas the mangrove buckeye, as its name indicates, is primarily limited to the sub-tropical tidal coastline, and its caterpillar feeds mainly on black mangroves. This situation illustrates one mechanism by which new species originate- habitat specialization within one species splits off a separate species. Whereas speciation normally requires considerable geographic separation, the degree of isolation here is very small and due to juxtaposition of two distinctively different habitats (saline mangroves and adjacent uplands). Sometimes the argument is made that evolution is hard to observe, but here in our backyard we can actually see the end results among two “nutty” buckeye butterflies!

Bill Dunson, Englewood, FL & Galax, VA
wdunson@comcast.net