Tag Archive | Charles Darwin

What came first—the proboscis or the nectar spur?

The Fakahatchee Strand in South Florida is one of the last places where the elusive and nearly-extinct Florida panther still roams. The panther, which has been extirpated from most of its former range due to hunting and habitat loss still prowls here due mostly to the Strand’s isolation and covering of dense, nearly impenetrable swamp.

Fakahatchee_Strand_Preserve Miguel Vieira Wikimedia Commons CC

Fakahatchee has been called the ‘Amazon of North America’—it is dense, swampy, and difficult to access. Photo by Miguel Vieira.

But Fakahatchee’s swamps shelter another, perhaps more unexpected endangered organism, Dendrophylax lindenii—the ghost orchid. The orchid’s common name refers to the flower that appears to float in midair, since it has no visible leaves—photosynthesis occurs in the tangled roots that anchor the plant to a tree. However, the name could just as easily refer to its rarity—very few plants remain in the wild.

ghost orchid David McAdoo Flickr CC

The ghost orchid, Dendrophylax lindenii. Photo by David McAdoo.

The ghost orchid, I should mention, is breathtakingly beautiful. The flower has delicate arms, which are actually the lower petal that bifurcates into two long tendrils that twist downwards. These are complimented by a long spur exiting the back of the flower that serves as its nectary, where pollinators acquire their reward for pollinating the flower. Indeed, the tantalizing beauty and rarity of the ghost orchid have made it among the world’s most sought-after specimens for unscrupulous orchid collectors—as a result, poachers have decimated wild populations.

But poachers are not the only threat to ghost orchid numbers. Even before collectors caught on to the orchid’s splendor, one of the very plant’s intrinsic characteristics had long ago sent it on an evolutionary trajectory towards rarity. The key to that trajectory is the long, delicate nectar spur, one of the plant’s most striking and mysterious features.

In 1862 Charles Darwin received a package from the horticulturalist James Bateman. In it, he found specimens of Angraecum sesquipedale, another species of orchid from Madagascar, off the eastern coast of Africa. Darwin expressed surprise at the ‘astonishing length’ of the nectar spur found on the flowers; he measured them at one foot long! Darwin predicted that there must be a moth with a proboscis (the equivalent of a moth tongue) long enough to reach the nectar at the bottom of the spur. As the moth extends its proboscis into the flower to feed, Darwin guessed, it would inadvertently pick up pollen on its face and deposit it on subsequent flowers, thereby pollinating the orchids.

Angraecum_sesquipedale_-_Warner,_Williams_-_Select_orch._plants_1,_pl._31_(1862-1865)

Angraecum’s long nectar spurs are clearly visible in this drawing by Walter Hood Fitch.

Several years later Alfred Russel Wallace—Darwin’s contemporary and co-founder of the famous theory of evolution by natural selection—published a report in which he described a hawkmoth, Xanthopan morganii, from eastern Africa with a proboscis almost long enough to reach the bottom of the Angraecum nectar spur. Wallace predicted that such a moth would eventually be found in Madagascar.

Sphinx Moth Fertilizing Angraecum Sesquipedale in the Forests of Madagascar

Drawing by Thomas William Wood in 1867, before the moth had even been discovered.

Sure enough, such a moth was eventually found there—it turned out to be Xanthopan morganii, and members of the population in Madagascar had especially long proboscises. In fact, the hawkmoth’s proboscis is so long that it must back up over one foot from the flower before uncoiling its long proboscis to probe for nectar in the bottom of the Angraecum spur!

While incredibly fascinating for biologists, the problem with all this for the orchid is that there are few things that can pollinate it—or, rather, the flower rewards only those with proboscises long enough to obtain the nectar hidden one foot down at the end of the long spur. The only pollinator known to do this is the hawkmoth Xanthopan morganii and, without being able to receive any prize, no other pollinators visit it. If the hawkmoth disappears or becomes rare, so too does the orchid.

Which brings us back to our ghost orchid of Fakahatchee. An American species of hawkmoth—Cocytius antaeus—is the only moth on the continent with a proboscis long enough to reach the nectar at the bottom of the ghost’s nectar spur, and thus is its only pollinator. Cocytius is rare, and therefore so is the ghost orchid.

Polyrrhiza lindenii OS104W_Debra Carey

Dendrophylax lindenii—the ghost orchid—and its only pollinator, a hawkmoth called Cocytius antaeus. Original artwork by Debra Carey. Click on the image above to visit the website.

Such specialization is not unique to orchids. Posoqueria, which is found in the rainforests of tropical America, has long corollas that store nectar at their base, and which can only be accessed by animals with very long tongues or proboscises—most likely, a moth pollinates these flowers as well. These plants’ long nectar spurs have all been favored by the constant forces of natural selection—as the spur becomes longer over time and successive generations, so too does the moth’s proboscis to keep up. The flower benefits by having just one pollinator that is very efficient at its job—since nothing else visits the flower to seek its reward, very little pollen or nectar are wasted. The moth benefits as the sole beneficiary of the plant’s nectar reward and thus faces no competition from other pollinators.

Posoqueria_Los Amigos

Posoqueria, a member of the coffee family native to the Neotropics. Like our orchids, Posoqueria offers nectar only to those pollinators with proboscises long enough to reach it.

Nor is specialization unique to plants and insects. Indeed, recent studies have shown that specialization is a common trait across groups as varied as fish, birds, and mammals.

And while specialization might be beneficial in terms of improved pollination efficiency, foraging, or other essential survival skills, ultimately the road to specialization is the road species take to extinction. Without versatility, specialized organisms survive only at the whim of finicky pollinators, resources, and changing environmental conditions.

Hopefully, the beautiful ghost orchid of Fakahatchee is not yet at the end of that road.

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