The counter-intuitive interconnectedness of species

Posted on June 5th, 2010 in Climage Change,ecology,Environment,Evolution,Medicine,Nature,Science by Robert Miller

Red Knot Shorebird

Perhaps we need another century or two to understand the species of the world and their inter-dependencies before we make judgments about who should go and who should stay: say goodbye to one and you may have to do the same for a seemingly diverse group of animals for reasons that are highly counter-intuitive. It’s foolish of course to even suggest that we are in a position to make decisions about species survival, because we aren’t knowingly making those judgments, even though events, such as species extinction, are very likely occurring on a regular basis as a result of human interventionism. But, species extinctions are taking place without our knowledge of the cause or even, in most cases, an understanding of the species involved. We keep track of big animals, like lions, tigers, elephants and other large mammals and, though  the future for them is not looking particularly bright, we are completely ignorant about animals a step or two below on the evolutionary ladder–like the now extinct, Gastric-Brooding Frog. Who said goodbye to that species? But, here’s one to ponder for the short-term: are you kidding me?–shorebirds and horseshoe crabs? This survival dynamic may play itself out over the next few years.

The interconnectedness of nature almost dictates that you don’t lose single species, that in in losing one, some other species or fauna will also be put into harm’s way:  the loss of one species may precipitate the loss of one or more others, largely because we are unaware of the biological forces that unite them. I don’t know who else we lost or which other species might have been changed when the Gastric-Brooding Frog disappeared, but it didn’t disappear without impacting other species. Of that we can be certain. But, what connection for example does the continued vitality of the horseshoe crab, Limulus polyphemus, an ancient marine arthropod, have with survival of the Red Knot bird, a migratory shore bird that makes an annual stopover in the region in which the horseshoe crab breeds? The Limulus is virtually unchanged since it first appeared in the Paleozoic, 570-248 million years ago. Though most people have barely heard of the Limulus, anyone who studies vision is well versed with this species, as its compound eye was first used by H.K. Hartline to reveal fundamental mechanisms of visual physiology, for which he went on to win a Nobel Prize in 1967 for his pioneering work. Horseshoe crabs are abundant on the shoreline of Woods Hole Massachusetts, where Hartline did much of his early work. One of Hartline’s students, Robert Barlow, went on to show that the male Limulus uses its eyes to search aggressively for females and looks for the outlines of the carapace as a visual cue for finding a suitable female, at a time when the animals come into the shoreline for laying and fertilizing their eggs, an activity that usually takes place at night. But, who would ever have thought that the seasonal breeding of this ancient marine species, which takes place big time in Delaware Bay on the East coast, would have a dramatic impact on the survival of the Red Knot bird, a migratory shorebird that flies 20,000 miles each year, from South America to the Arctic, where it breeds, and then flies back again. Surely the biologists got this one wrong!

The Red Knot arrives in Delaware Bay just at the time the Limulus has come near the shore for breeding and egg laying. Eggs are laid by the female in the sand and then fertilized externally by the male or males that surround her. It is the nourishment derived from feeding on the newly released Limulus eggs that provides a critically needed source of food for the Red Knot to regain its stamina and prepare for resuming its long journey North.  Once the Red Knots arrive at the Delaware shore, they only have about two weeks to get sufficient nourishment, rebuild their wing muscles and store fat for the flight ahead to their Arctic breeding grounds, where they lay their own eggs and raise their young over the short summer of the region. If  insufficient Limulus eggs are available, the Red Knot does not seem to have a plan B and may be ill-equipped to finish the long journey to the Arctic. In some regions where Red Knots used to breed in the Arctic, they have not been seen in recent years and insufficient Limulus egg nourishment has been regarded as the main deficiency in their failed migratory outcome. In preparation for the long flight from South America (Tierra del Fuego, in Chile/Argentina) the bird’s digestive system shuts down, such that the intermediate stop, to feed on Limulus eggs, provides the bird with a very digestible meal, rich in proteins–apparently the ideal food for building up muscle and fat for an animal with a reduced capacity digestive system. Despite the aggressive feeding of the Red Knot on Limulus eggs, the horseshoe crab population in the region was stable into the 1990s, when fishing with Limulus bait became popular.

The shortage of Limulus eggs seems to reflect an overly aggressive harvesting of animals, particularly gravid females used by fisherman as bait for catching eels and conch (marine snails): this has led to a significant decline in the number of Limulus eggs laid on the shoreline, down to perhaps 2/3 of previous estimates and the magnitude of this decline has been implicated in the reduced numbers of Red Knot birds making it to their Arctic breeding grounds. Indeed, it was the alarmingly fast reduction in the Red Knot population, by about 70%, that led to the discovery of their dependence on Limulus eggs in Delaware Bay.  The decline in Limulus breeding and egg-laying seems to be the tipping point that could wipe out the Red Knot and could do so very quickly if a better balance isn’t restored.   There is now a two-year moratorium on using Limulus for fishing bait in the region and researchers are busy trying to find artificial bait substitutes that could be used in place of the real thing. An excellent video about this species interdependency was shown recently on PBS and can be seen here.

How long this interconnectedness between a marine animal that is roughly 350 million years old and a bird, whose evolutionary record goes back 150 million years, is not a matter that can be resolved through the fossil record. At some point, the Red Knot’s migratory flight to Delaware Bay was initiated to be well-timed to the breeding season of the Limulus.  This synchrony could be seriously interrupted further by global climate change which might affect one or the other of these tightly timed mechanisms. Some biologists believe the Red Knot could be extinct within five years. At some point, you reach a bird density wherein birds can’t find one another to mate.

Limulus polyphemus

The fishing industry is not the only survival challenge that Limulus faces. The blood of  Limulus has been used for many years because of its unusual properties. Limulus blood is blue because it uses a copper protein as an oxygen carrier. But, of more importance is the fact that Limulus blood clots whenever it comes into contact with endotoxins. Extracts of Limulus blood have been used for decades to test for bacterial contamination. One quart of Limulus blood is valued at about $15,000. Currently, the FDA insists that all intravenously administered drugs should be exposed to a Limulus blood amebocyte lysate as a test for endotoxins. This is a significant improvement over the prior process of injecting a rabbit with the substance and then waiting to see if the animal gets sick and develops a fever!  The discovery of Limulus amebocyte lysate also took place at Woods Hole, through the observations of scientist Fred Bang. This insight and its technological development has reduced the endotoxin analysis test from days to about 45 minutes. Instead of killing the horseshoe crabs and then bleeding them, the pharmaceutical industry harvests blood from live animals, who are then returned to their native habitat. Thus, some former fisherman, who used them for bait, now collect them for blood letting in a laboratory environment and then release them to the same location. Last year, 300,000 horseshoe crabs were bled and then released; about 13% do not survive this blood-letting procedure, which extracts about 2/3 of their blood.

The counter-intuitive interconnectedness of the Red Knot and the horseshoe crab could only be revealed by extensive field studies that involved capturing, tagging and measuring birds along the pathway of their extensive, almost incomprehensible, migratory flight pattern. These are dedicated scientists who share a passion for this bird and its preservation. Why a bird would exist under the harsh conditions of the Tierra del Fuego, near the Strait of Magellan, then fly to the even harsher climate of the Arctic for breeding and the early rearing of their young, before flying off again on another 10,000 mile trip, is well beyond our capacity to comprehend. Perhaps it got started before tectonic plates rearranged the land masses. The migratory pattern of North America by non-indigenous Homo sapiens was primarily East to West, which is a little easier to understand. In contrast to the rational, the Red Knot flight plan is not one that any of us would recommend to serve as the basis for a committed, routine lifestyle, unless it was one we recommended to our Republican friends. I can imagine Rush Limbaugh feeding on Limulus eggs in search of a new high. Let us hope that the Red Knot survives and the current iteration of the Republican Party goes the way of the Dodo bird as its major flight plan glides it  into extinction. There are many signs that such a glide pattern is already underway. We will undoubtedly hear more about each species in the coming years.


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