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Butterfly research spells hope for endangered species
STANFORD -- It is against the law to do bodily harm to any creature listed in the Fish and Wildlife Service's "red book" of endangered species. Plucking a feather from a bald eagle constitutes harassment. Putting a bullet through its heart could lead to imprisonment under the Endangered Species Act.
But what if loggers in need of jobs cut down an endangered animal's forest home? What if ranchers want to graze their cattle in the meadows that harbor a rare insect?
That question -- whether the Endangered Species Act should simply protect rare species from bodily harm or whether their habitats must be protected as well -- is before the Supreme Court and Congress. The court must decide whether to uphold or overrule a lower-court ruling that says, in effect, that it would be OK to fell the tree a bald eagle nests in, as long as you don't lay hands on the endangered bird itself. Congress is considering legislative changes to the act as well that would weaken protection of endangered species' habitats when human economic interests are at stake.
Among those who are particularly exercised about these turn of events is Carol Boggs, a conservation biologist with 20 years' experience studying butterflies in the field and in the lab. Boggs is also director of the Center for Conservation Biology at Stanford, a research center founded by another butterfly specialist, biologist Paul Ehrlich. They are among several hundred scientists worldwide who study butterflies, in part, to learn how insects and other animals interact with their habitats.
Studies of butterflies demonstrate that a population can become extinct if its habitat is disrupted, and offer clues about how habitat protection can prevent many species from becoming endangered. Boggs' work, in particular, shows why it is necessary to protect as much of the natural complexity of a habitat as possible, to aid the survival of a creature like the butterfly. She has learned how simple solutions -- such as putting cows to graze somewhere else while a mountain meadow is blooming -- can lead to peaceful coexistence between economic and environmental interests.
Like white rats, many butterfly species are well studied enough that each scientist's research builds on a solid base of knowledge collected by other researchers. Some of that research has been compiled for more than a century by butterfly collectors who have provided valuable hints about the natural history of the insects.
For scientists, says Boggs, butterflies are relatively easy to study in the lab and easy to see in the field. "Also, they keep bankers' hours. They usually don't fly before 10 a.m. or after 4 p.m., and they don't fly in the rain."
Because butterflies generally need particular host plants for caterpillars to feed on, they're correlated with the plant communities of an area.
"They also often need a particular kind of microclimate -- very localized weather conditions," says Boggs. "Since they have some very specific habitat requirements in order to be able to fly and survive, they're pretty sensitive indicators of the health of a particular area."
Boggs contends that what people do to a habitat can have an impact on the ability of butterflies to reproduce. This finding has relevance all the way through the ecological system, because plant reproduction is dependent on pollinators like butterflies, bees and flies. Pollinators, in turn, need nectar and other food from plants to reproduce. Therefore, if a habitat that supports many species is suffering damage from natural or human causes, a drop in the butterfly population could be the first indicator of damage.
Boggs' research on butterflies west of the Appalachians has focused primarily on the relationship between nectar availability and fecundity, or the number of eggs the insects can produce.
The availability of a good food source for butterfly larvae, or caterpillars, long has been recognized as the critical factor in predicting whether a habitat is being stressed enough to harm a population of rare butterflies. If the larvae have enough of their preferred food (usually leaves of certain host plants where butterflies lay their eggs), it has been assumed that when they pupate and emerge as butterflies they will have had enough nutrition to lay many eggs and manufacture an abundant next generation.
Boggs' research has shown that some butterfly species also need to get nectar from plants that flower when the insects are in the flight stage in order to have enough food to produce many eggs. In other words, some species of butterflies must have a habitat with a broad range of plants. A single conservation planner could inadvertently doom an entire species of butterflies by designating a reserve for them that does not have the proper number and variety of plant varieties.
In several years of summer fieldwork at the independent Rocky Mountain Biological Laboratory, Boggs tested this hypothesis with a laboratory study of the Mormon fritillary butterfly (Speyeria mormonia), which lives in the Colorado mountains. She showed that a reduction in egg production is directly proportional to nectar intake: If intake is cut in half, so is egg production.
The adult Mormon fritillaries were divided into two female groups, one of which was fed "ad lib" -- they could eat as much fake nectar as they pleased. ("Fake nectar," or honey water, is easier for researchers to obtain than real nectar, which comes from flowers.) Individuals in the second group of female fritillaries were fed only one-half or one-third of what the "ad-lib butterflies" had consumed.
Boggs' researchers used a syringe to uncurl the proboscis, or tongue, that butterflies feed with. They would then squeeze out droplets of fake nectar and record how much the butterflies had eaten. Each of the "lucky" ad-lib butterflies was then compared with a butterfly from the "restricted diet" group.
"It turns out that if they are on a restricted diet, fecundity is cut by the same proportion as you cut the feeding," Boggs said.
Study results also showed that the butterflies on the restricted diet actively resorbed oocytes, the immature eggs that are in their ovaries. Because they no longer had oocytes ready to make into eggs, extra food would do the formerly underfed butterflies no good.
In short, Boggs determined that restricted-diet butterflies are, in short, reducing their potential fecundity. Instead of laying mature eggs, the adult survives by using up the rich nutrients in the immature eggs.
This oocyte resorption is the result of nutrient allocation priorities, in which stored nutrients are used first to survive, and then to manufacture the next generation. Nectar, and the plants that make nectar, are thus extremely important in maintaining butterfly population numbers.
"With that experiment, we learned something about reductions in adult feeding throughout a butterfly's life," Boggs said. "But how resilient are butterflies to stresses of shorter duration?"
If butterflies resorb oocytes when no nectar is available, when does that resorption take place?
Boggs and her co-workers tried to answer that question by stressing a group of butterflies -- by giving them restricted diets for five days. Interestingly enough, some individuals recuperated, suggesting that they had not started resorbing oocytes when the stress period began. For others, even though all were members of the same population, the fecundity completely collapsed; they acted as if they had been "stressed" their entire lives.
"What we're looking at now is the hypothesis that, first of all, there may be genetic variation among individuals in a population," Boggs said, "in which case you might have selection for ability to withstand stress." Those that withstand this type of stress would survive in greater numbers.
"The second hypothesis is that there's a tradeoff, an interaction between the number of eggs that the butterflies lay early, and their ability to withstand stress."
Butterflies that lay many of their eggs early in their life spans use a lot of nutrients early, as well. This means they have fewer reserves to draw upon when they are stressed by a restricted diet.
"If you took somebody, a human being, who is relatively skinny, and starved that person for several days, it would have a greater health impact than if you took somebody who was relatively fat," Boggs said.
The butterflies that reproduce early are equivalent to a skinny person, whereas those that reproduce late are hoarding their resources, or "fat," and as a result are less susceptible to short-term stress.
However, there is an evolutionary advantage to early reproduction. The Mormon fritillary, for example, has an average life span of only five days.
"So the faster a butterfly can get her eggs out, the more eggs she's going to lay before a hailstone hits her over the head, a mouse eats her, a horse steps on her or a butterfly researcher catches her," Boggs said.
This "short-term stress" scenario parallels the kind of natural conditions under which butterflies emerge. Like many insect species, butterfly populations survive partly because of strategic timing. Even though each individual lives for only a few days, some butterflies emerge from the pupal stage almost every day over a period of two months. The lucky individuals spend their flight time during days when there is no rain, no drought, no intense cold, plenty of flowering plants, the right amount of humidity -- in short, when conditions are right for laying a lot of eggs that will mature over the following year.
Boggs' nectar-feeding research suggests that there may be another tradeoff between butterflies that produce eggs as soon as they emerge -- and thus thrive in years with good weather and good food supplies -- and butterflies that produce late eggs, but are still able to produce offspring during periods of stress.
The tradeoff between nectar and oocyte-resorption applies differently to different butterfly and moth species. Some moths, for example, feed only as larvae; as adults, they are essentially "egg-laying machines." Nectar is irrelevant for these moths because they simply do not eat at all as adults.
Other butterflies and moths depend on nectar to mature most of their eggs. However, unlike the Mormon fritillary, these butterflies have some eggs available, regardless of whether the local plants have come into flower. A local Bay Area species, the Bay checkerspot (Euphydryas editha), has made this ready-egg adaptation. Most temperate-zone butterflies resemble the Mormon fritillary or the Bay checkerspot.
Then there are tropical butterflies that emerge with no mature eggs but can feed on different kinds of foods and are not limited to nectar.
What do studies of these butterflies tell about insects in general?
"From a basic biology viewpoint, we see that there may be whole groups of organisms whose environment is often fairly variable," Boggs said. "They have evolved mechanisms to enhance survival or preserve survival, possibly at the cost of reproduction, in order to maintain the ability to reproduce again when the environment improves."
Most species of insects are robust, but not hardy enough to survive permanent damage to their habitats -- particularly if they cannot migrate to a better habitat when their essential food supply is gone. Alien plant species, like the sagebrush that is replacing mountain meadows in Colorado and the iceplant that has invaded coastal parts of California, can crowd out native host plants that butterflies and other insects depend on. Some conservation biologists have predicted large-scale species extinction in the southwestern United States if the mix of plants there is altered by global warming.
Boggs' butterfly studies also suggest how species can be preserved. For example, some ranchers in Colorado are beginning to be concerned about the impact of their grazing practices on native plants and insects. For the meadowlands that host the Mormon fritillary, a simple change would make a big difference. If only a few cows were allowed to graze in meadows during the blooming season, then insects would have enough flowers left for nectar, the flowers would be pollinated, and both the meadow and the butterflies would continue to thrive.
Basic research like these butterfly studies has convinced a panel from the National Academy of Sciences that the Endangered Species Act should be strengthened to slow a rate of extinction that the panel has characterized as "great as any in the fossil record" of life on Earth. In its May 16 report, the panel said specifically that species cannot be saved unless the habitats they depend on are preserved.
That is the way the act is currently interpreted. An Interior Department regulation prohibits landowners from degrading the habitat of endangered species in a way that kills or injures wildlife.
The Endangered Species Act is up for re-certification this year, and it is likely that the Republican Congress will substantially weaken the legislation, with the result that habitat protection would be difficult.
"People are trying to argue that the definition of harm to a species, or to an individual within a species, should be restricted to physically battering, physically killing, physically maiming, physically capturing, physically chasing it," said Boggs.
In addition, the Supreme Court now is considering a decision made last year by a Court of Appeals in a lawsuit brought against Secretary of the Interior Bruce Babbitt. The lower court ruled that the habitat regulation is unconstitutional, and defined the word "harm" to mean "the perpetrator's direct application of force."
Washington insiders say if the Supreme Court upholds this decision, there is virtually no chance that Congress will restore habitat protections.
Jee-Young Shin is a science writing intern at the Stanford News Service.
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