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Letters
MONARCHTRANSFER: A REALCONCERN? Lincoln P. Brower and colleagues ("On the dangers of interpopulational transfer of monarch butterflies," BioScience 45: 540-544) criticized current research using interpopulational transfers of monarch butterflies between the eastern and western populations. They described three potential problems that could occur if this research continues: disease risk due to human-induced transfer, changes in the geof the two netic composition populations, and a lack of methodology for gathering enough data to statistically test a null hypothesis, therefore resulting in an unnecessary manipulation of a natural population. Although it is always wise to question any scientific endeavor that could potentially put a population of organisms at risk, I believe that the first two problems are overly pessimistic. Brower et al. suggest that different strains of the protozoan Ophryocystis elektroscirrha will be passed on with the adult butterflies transferred between populations. A lack of resistance to a new strain could put the target population at risk. The example of a bacterial pathogen infecting populations of Drosophila simulans is cited to demonstrate the possibility of rapid
Letters to BioScience should be addressed to Editor, BioScience, 1444 Eye St., NW, Suite 200, Washington, DC 20005. The editorial staff reserves the right to edit letters for length or clarity without notifying the authors. Letters are published as space becomes available.
spread. However, monarchs have a high population turnover rate due to multiple annual generations and high fecundity, and are thus r-strategists. Natural selection acts quickly on rstrategists, and in a relatively short time resistance would be conferred to monarch populations newly exposed to different strains of a pathogen. This would also be the case with D. simulans. The analogy of influenza impacting human populations is not valid because Homo sapiens is a K-strategist. That is, humans are long lived, take many years to become sexually mature, and do not have multiple annual generations. Natural selection acts slowly on Kstrategists due to low population turnover. In all of humankind's efforts to eliminate insect species with poisons and pathogens, not one has been totally eradicated. This is a salute to the unfailing effectiveness of natural selection. It is likely that, as Brower et al. state, interpopulational transfer studies will cause a "decrease in any existing differentiation between the populations," but will this eliminate the uniqueness of each population? Probably not. For instance, Brower et al. suggest that each population may be adapted to use the indigenous milkweed flora available and to cope with specific microhabitats. If an interpopulational transferee is placed in a region where its adaptations are poorly suited to the biotic and abiotic conditions present, natural selection would act against those offby the alien spring produced individual. Thus, directional selection will eventually eliminate those genes present in submarginal individuals. Brower et al. also express concern for the disruption of migrational timdue to ing in the monarch genetic interinterpopulational change. Once again, natural selec-
tion would eliminate those individuals not in synch with the season. Those that do migrate at the proper time would thrive, and submarginal individuals would be selected against. The example of the tuatara being a set of populations that faced an adverse situation due to interpopulational exchange is once again an example of a K-strategist. Submarginal individuals produced by the exchange would not be eliminated quickly due to the long life span of this animal and genetic disparities would persist. Also, tuatara populations are small, which reduces the opportunity for selection against submarginal individuals. Natural selection would be rapid in monarch butterfly populations, which are large and replete with genetic material. The polymorphic peppered moth (Biston betularia), in which the dark form was selected for in areas of soot-covered trees, is a classic example of rapid natural selection in a large population of highly fecund insects (Kettlewell 1961). Although the concerns of Brower et al. may not be as devastating as they claim, there is truth in their statement regarding the statistical validity of the proposed study. A massive effort would be needed to recapture enough tagged individuals to gather the necessary data. However, introducing members of one genetically distinct population into the range of another is a technique that would probably not be detrimental to the butterflies in the long run. J. B. KEIPER Department of Biological Sciences Kent State University Kent, OH 44242-0001
Referencecited Kettlewell HRD. 1961. The phenomenon of industrial melanism in Lepidoptera.Annual Review of Entomology 6: 245-262.
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Lincoln P. Brower et al. reply: occurred naturally between populaOn the basis of generalizations about tions. This would muddle our ability insects' high reproductive capacities to reconstruct the evolutionary hisand short generation times ("r" as tory of the migration in North opposed to "K" life history strate- America as well as how monarchs gies), Keiper rejects our arguments colonized the Pacific Ocean islands (Brower et al. 1995) against intro- and Australia during the nineteenth ducing members of the eastern mi- century. In addition to altering the gratory population of the monarch differently evolved genetic structures butterfly into the western popula- of the recipient populations, transtion and vice versa. Through anal- fers could also hinder our ability to ogy with other insects that have rap- analyze the coevolutionary interactions between host and parasite linidly evolved pesticide resistance, Keiper maintains that natural selec- eages among the different populations. tion would assure that recipient monarch populations would rapidly Monarchs are already subjected evolve resistance to introduced for- to novel selection pressures due to eign strains of pathogens or proto- degradation of their overwintering zoan parasites, such as Ophryocystis ecosystems and the massive contamielektroscirra. nation of their summer breeding aractivities Keiper's argument ignores the fact eas by agroindustrial We that the that most parasites and pathogens submit (Brower 1995). are coevolving entities with genera- basic premise of conservation of the tion times as short as or shorter than endangered migratory phenomenon their hosts (e.g., May and Anderson of the monarch is subverted by ex1983). Consequently, a much longer perimental procedures that potentime would likely be needed for the tially perturb the phenomenon we monarchs to evolve resistance, dur- are trying to preserve without proing which time recurrent epidemics viding significant new knowledge. could lower the monarchs' overwin- At what point will the addition of yet tering population sizes to the point another straw break the camel's at which interactions with known or back? At the very least, those who unknown ecological factors might are doing or proposing transfers for become devastating. For example, in experimental purposes, as well as Mexico orioles and grosbeaks kill those who are inadvertently causing millions of monarchs each year, and transfers by commercial shipments, the percentage killed by the birds is should perform appropriate risk asinversely proportional to the colony sessments and monitor the recipient size (Brower and Calvert 1985, populations for potential impacts. Calvert et al. 1979). Predation dur- We doubt that this can or will be ing an extreme population low might done. well reduce monarch populations to Heeding our conservative advice levels at which the numbers of survi- will cause no harm. But if transfers vors are insufficient to recolonize continue and our concerns are corthe summer breeding range, followed rect, then, at the least, our capacity to understand the evolution of one by collapse of the migration-overwintering phenomenon. Recall that of the greatest biological spectacles the passenger pigeon used to darken on the planet would be diminished; North American skies with its mi- at the worst, the whole migrationgrations, and although hunting dras- overwintering phenomenon of the tically reduced its numbers, its final monarch butterfly would be at risk. demise was due to a population crash We reject Keiper's optimism that the of unknown cause. transfers "would probably not be detrimental...in the long run" and natural that Keiper's argument selection would rapidly eliminate reiterate our conclusion that artifideleterious alleles in recipient popu- cial transfers of monarch butterflies lations is correct, but it ignores the between the eastern and western fact that selectively neutral alleles populations should not be made. would also be introduced by the LINCOLN P. BROWER transfers and corrupt the very data Department of Zoology that geneticists use to estimate the University of Florida Gainesville, FL 32611 magnitude of past gene flow that
LINDA S. FINK
Department of Biology Sweet Briar College Sweet Briar, VA 24595 ANDREW VAN ZANDT BROWER
Department of Entomology American Museum of Natural History New York, NY 10024-5192 KINGSTON LEONG
Department of Biological Sciences California Polytechnic University San Luis Obispo, CA 93407 KAREN OBERHAUSER SONIA ALTIZER
Department of Ecology, Evolution, and Behavior University of Minnesota St. Paul, MN 55108 ORLEY TAYLOR DANEL VICKERMAN
Department of Entomology University of Kansas Lawrence, KS 66045 WILLIAM H. CALVERT
503 East Mary Street Austin, TX 78704
TONYA VAN HOOK ALFONSO ALONSO-MEJIA
Department of Zoology University of Florida Gainesville, FL 32611 STEPHEN B. MALCOLM
Department of Biological Sciences WesternMichigan University Kalamazoo, MI 49008 DENIS F. OWEN
School of Biological and Molecular Sciences Oxford Brookes University Oxford, England OX3 OBP MYRON P. ZALUCKI
Department of Entomology The University of Queensland Brisbane, Australia Qld 4072 References cited BrowerLP. 1995. Understandingand misunderstandingthe migrationof the monarch butterfly (Nymphalidae) in North America:1857-1995. Journalof the Lepidopterists' Society 49: 304-385. BrowerLP, CalvertWH. 1985. Foragingdynamics of bird predators on overwintering monarchbutterfliesin Mexico. Evolution 39: 852-868. Brower LP, et al. 1995. On the dangers of interpopulational transfers of monarch butterflies. BioScience45: 540-544. Calvert WH, Hedrick LE, Brower LP. 1979.
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Mortality of the monarch butterfly (Danaus plexippus L.): avian predation at five overwintering sites in Mexico. Science 204: 847-851. May RM, Anderson RM. 1983. Host-parain site coevolution. Pages 186-206 Futuyma DJ, Slatkin M, eds. Coevolution. Sunderland (MA):Sinauer and Associates.
UV RADIATIONAND FIELDEXPERIMENTS I was glad to read the response by Andrew R. Blaustein and colleagues ("Field experiments, amphibian morand UV-B, radiation," tality, BioScience 46:386-388) to criticism of their studies of the sensitivity of amphibian embryos to solar ultraviolet-B (UV-B; Licht 1996). The direct and indirect effects of UV-B on living systems are complex (Bothwell et al. 1994, Culotta 1994), and the ecological field studies conducted by Blaustein and his colleagues are essential to understanding them (see Blaustein et al. 1995, Kiesecker and Blaustein 1995). Although my specialty is the measurement of ozone, UV-B, and haze, studies like Blaustein's have stimulated my ongoing investigation of the response of mosquito larvae to sunlight. When wild Culex pipiens larvae are placed in an outdoor tank half covered by UV-B-absorbing film and half by UV-B-transmitting film, most of the larvae wriggle to the UVB-free side of the tank. I am now conducting field studies to determine if female mosquitoes prefer to deposit their eggs in water shielded from UV-B. Consider the implications if UV-B limits the number of nursery sites or even the population of mosquito larvae. Would fewer larvae reduce the population of the larvae's predators while enhancing the survival of algae and bacteria that form the larvae's diet? Conversely, is the mosquito population enhanced when UV-B and visible sunlight are reduced by severe air pollution? If so, because malaria is the most common infectious disease in humans, then the effects of lower-than-normal UV-B may be as significant as the effects of higherthan-normal UV-B. The decline in the obzone layer since the 1980s should have caused UV-B to increase. But the expected increase has been observed only in
Antarctica during the annual ozone minimum popularly known as the ozone hole and at an alpine site (Blumthaler and Ambach 1990). The latter finding has interesting implications for Blaustein's study of anurans in the Oregon Cascade Range. Other than occasional sharp increases in UV-B caused by low ozone accompanying weather systems (Kerr and McElroy 1993, Michaels et al. 1994, Mims et al. 1995), elsewhere there are no clear UV-B trends. Indeed, a ten-year study in the United States found a downward trend in UV-B (Scotto et al. 1988). Because the instruments in this study were located in or near large cities, it is likely that increasing air pollution has caused UV-B to decline (Liu et al. 1991). The sulfate smog that blankets much of the eastern United States every summer can reduce UV-B by more than 20%. Black carbon particulates such as those created during biomass burning are even more efficient at absorbing UV-B. During the 1995 burning season in Brazil, I measured reductions in UV-B of more than 80% (Mims 1995a) and reductions in visible sunlight (500 nm) of 40% (Mims 1995b) in a region harboring various disease-transmitting mosquitoes. These measurements were made hundreds of kilometers south of the most concentrated burning, when satellite images and visual observations from aircraft showed that smoke covered much of Brazil and its neighbors. Does the significant reduction in UV-B and visible light during the burning season in Brazil enhance the mosquito population, especially malaria carriers of the genus Anopheles, whose larvae float fully exposed on the surface of water (Burrows 1968)? Because UVB is both bactericidal and viricidal, do significant reductions in UV-B in regions where UV-B is ordinarily high enhance the population of pathogenic organisms on exposed surfaces and suspended in air and water (Mims 1995b)? Questions like these emphasize the need for field studies that thoroughly investigate the complex effects of UV-B on living organisms. I encourage Blaustein and colleagues to continue their field studies because much remains to be learned and more spe-
cies need to be studied. Also in need of investigation are the effects of turbid water on UV-B (Yan et al. 1996), increased diffuse UV-B caused by anthropogenic haze (Mims 1994), and sharply increased UV-B caused by scattering from cumulus clouds (Mims and Frederick 1994). FORRESTM. MIMS III Sun Photometer Atmospheric Network (SPAN) 433 Twin Oak Rd. Seguin, TX 78155 References cited Blaustein AR, Kiesecker JM, Hokit DG, Walls SC. 1995. Amphibian declines and UV radiation. BioScience 45: 514-515. Blumthaler M, Ambach W. 1990. Indication of increasing solar ultraviolet-B radiation flux in Alpine regions. Science 248: 206208. Bothwell ML, Sherbot DMJ, Pollock CM. 1994. Ecosystem response to solar ultraviolet-B radiation: influence of trophiclevel interactions. Science 265: 97-100. Burrows W. 1968. Textbook of microbiology. Philadelphia (PA): W.B. Saunders. Culotta E. 1994. UV-B effects: bad for insect larvae means good for algae. Science 265: 30. Kerr JB, McElroy CT. 1993. Evidence for large upward trends of ultraviolet-B radiation linked to ozone depletion. Science 262: 1032-1034. Kiesecker JM, Blaustein AR. 1995. Synergism between UV-B radiation and a pathogen magnifies amphibian embryo mortality in nature. Proceedings of the National Academy of Sciences of the United States of America 92: 11049-11052. Licht LE. 1996. Amphibian decline still a puzzle. BioScience 46: 172-173. Liu SC, McKeen SA, Madronich S. 1991. Effect of anthropogenic aerosols on biologically active ultraviolet radiation. Geophysical Research Letters 18: 2265-2268. Michaels PJ, Singer SF, Knappenberger PC. 1994. Analyzing ultraviolet-B radiation: is there a trend? Science 264: 1341-1342. Mims FM. 1994. Beware the glare of black light. New Scientist 144: 71-72. . 1995a. Smoke and rainforests. Science 270: 5243. . 1995b. Aerosol optical depth, ultraviolet-B and total-sky irradiance during SCAR-B. Final Report to National Aeronautics and Space Administration, Goddard Space Flight Center, nr 3-59036Z. Available from:
[email protected] Mims FM, Frederick JE. 1994. Cumulus clouds and UV-B. Nature 371: 291. Mims FM, Ladd JW, Blaha RA. 1995. Increased solar ultraviolet-B associated with record low ozone over Texas. Geophysical Research Letters 22: 227-230. Scotto J, Cotton G, Urbach F, Berger D, Fears T. 1988. Biologically effective ultraviolet radiation: surface measurements in the United States, 1974 to 1985. Science 239: 762-763.
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