Ultraviolet Light and Egg Survival in Amphibians:
An Interview with Andrew Blaustein
From the Environmental Review Newsletter Volume Two Number Ten, October 1995
Andy Blaustein is a professor of zoology at Oregon State
University. He received the Ph.D. in biology in 1978 from the University
of California at Santa Barbara. His research interests are in behavioral,
population and evolutionary ecology and in conservation biology; he has
published approximately eighty peer reviewed research papers on mammals, on
parisites and many papers on amphibians.
ER: Professor Blaustein, are amphibian populations in decline?
AB: Generally with regard to amphibian populations there have not been a lot of long term studies. But I reviewed the long term studies that do exist and of those, some may show natural population fluctuations, other longterm studies show population reductions. There are about as many reductions in longterm studies as there are natural fluctuations. So if you look at the twelve or so good, longterm studies that have been conducted, about five or six of those show declining populations, and the rest do not. But it is difficult to assess whether these remaining populations are stable, declining or increasing.
Now, there are range reduction data that are also difficult to come by, that shows in some areas of the world, the geographic ranges of certain species have become constricted without a concomitant move to another part of a range. It looks like there is some kind of geographical pattern here cropping up, where the western US seems to be much more affected by amphibian declines than for example, the southeastern US; Australia seems to be fairly affected by amphibian declines, whereas some parts of South America are not, and some parts of South America seem to be. Southeast Asia, we do not have much data for but the data we do have does not show too much of a problem. And we do not have a lot of data from Africa.
ER: Are amphibians losing habitat?
AB: Yes. Habitat destruction is the biggest problem for amphibian declines. Populations are moving away from their broadest ranges historically to more restricted ranges in some areas; for example, it has happened in the western US. In Central America the golden toad disappeared in about 1989 and has not reappeared yet. The gastric brooding frog in Australia has not reappeared yet and it has not been seen for about nine or ten years.
ER: How long do you have to wait before you can say the species is extinct?
AB: That is a good question. We do not know much about the natural history of some of these species like the gastric brooding frog; maybe there is some population of them underground, estivating or hibernating, waiting for good weather patterns, maybe they will pop up some day. We do not know how long to wait.
ER: I think the news media blew the results of your PNAS paper out of proportion. Can you explain your findings in simple terms?
AB: The PNAS paper was this: We are out here in Oregon and we see the egg mortality - we are not talking about populations of adults right now - and the eggs dying all over the place and we cannot find anything wrong with the water. We have done chemical analyses, we looked to see whether or not there were predators eating them; of course most of the predators in our area do not cause egg deaths. We could not find any bacterium that was attacking them. So we figured since amphibians lay their eggs right out in the open - exposed to solar radiation - that maybe a possible effect of UV radiation was killing the eggs.
ER: Does previous work support the idea of UV killing amphibian eggs?
AB: Yes, there is previous work. Robert Worrest in the 1970s showed that ultraviolet radiation kills amphibian eggs in laboratory experiments. As a matter of fact, he worked on the exact same amphibian populations because he was at Oregon State University. He published several papers on that.
So we did a two-part study: one part was experimental, one part was an assay. The experimental part, we shielded some amphibian eggs from UV-B radiation and left some eggs open to UV-B radiation. [Ultraviolet-B is that light with a wavelength from 290 to 320 nanometers.ed.] A third regime had a shield that allowed UV-B. This was a shield control.
The shielded eggs of three species did better than the non-shielded eggs. In the PNAS paper it only shows two species doing better under the UV-B blocking shields, but we have another paper coming out with the third species. In the PNAS paper one species was unaffected by shielding - Pacific tree frog - so what that led us to believe is that three species - the Cascades frog, the western toad and the Northwestern salamander - do better if they are not exposed to UV radiation: their eggs develop and hatch with better success. Whereas the Pacific tree frog, they hatch very well no matter what. So we thought maybe there was some kind of resistance to UV in Pacific tree frog eggs. So we measured this enzyme known as photolyase, which repairs damage done by UV. This is the second part of the study, the assay.
The damage done by UV happens at the molecular level; it destroys DNA by producing photoproducts which cause mutations or cell death. So the photolyase is a way by which these animals fix themselves back up. It removes the photoproduct. And it turned out that when we assayed the photolyase enzyme, that the Pacific tree frogs had much more of this than the other amphibian species. So we think therefore there is a correlation between repair ability and whether or not they are going to be hit by UV in a bad way.
ER: Have you done any lab work where you could just dose eggs with UV?
AB: We have done some preliminary lab work, it is not published and we definitely have the same response in Cascades frogs and Pacific tree frogs, where the Cascades frogs die under certain UV regimes and the Pacific tree frogs do not. Other people have done that too.
ER: Have you looked for a dose response to UV and mortality of eggs?
AB: We are doing that right now. As a matter of fact that is the very experiment we have all over my lab. We do not have the results yet.
ER: Can you explain the difference between a correlation and a cause and effect with regard to UV and amphibian egg mortality?
AB: We show a cause, UV radiation causing an effect which is egg mortality in nature. And we show a correlation between amphibians that have low photolyase showing the mortality, and an amphibian species with high photolyase showing less mortality. My take home message is, ambient levels of UV kill amphibian eggs in nature, of certain species. That's it. I have no idea how this translates to population; it is possible that if it keeps doing this, the population will be affected. There is a correlation but with only a small sample size and I said that up front in my paper; that this is a preliminary study with photolyase levels that fit the amount of egg mortality we have. We now have an additional species and we have another paper coming out. So three species with relatively low photolyase get damaged by natural UV levels, one species that is high in repair enzyme, does not.
ER: The eggs were damaged by natural UV levels.Where the eggs laid in conditions that would enhance their exposure to light?
AB: No. As a matter of fact, those experiments were done right where they lay their eggs. The eggs are outside, the UV levels are not enhanced.
ER: At about the same depth of water and same conditions?
AB: Exactly. That is why we did a field experiment instead of a lab experiment.
ER: I thought the toad lays its eggs in strings in the debris and winds them around in these debris fields.
AB: That's right. And what we do in our experiments, is we put the strings in the experimental cages.
ER: How does the range of temperatures affect how long the eggs are exposed to UV?
AB: I don't know what the temperature ranges were offhand, they were natural temperatures and they were reported in our PNAS paper. There was one scientist who said that the Pacific tree frogs may have grown faster because they were at warmer temperatures (and therefore may have been subjected to less light) and that is wrong. In actuality, at one site Pacific tree frogs were grown at warmer temperatures than the other species, at another site they were grown at the same temperature as the other species, and at a third site they were grown at colder temperatures than the other species. He failed to mention those other two sites in his letter where they were at the same temperature and colder.
I have a major response with five co-authors coming back to him in September and he is not going to be thrilled with it. The thing that bothers us is that the editors at BioScience did not let me know about his letter and they did not referee it. Three very famous scientists told him to either call me up or not publish that letter, and he did it anyway.
First of all, he did not appear to understand the statistics. The major thing is, he does not seem to know what a randomized block design is. We do not compare species, we compare individuals within treatments at a particular lake. He seems to think that we compared between lakes, that is very important. This is a within lake comparison, not a between lake comparison. My study was used in a new statistics text as an ideal example of a randomized block design. This is all in my response, in a couple of months it will be out. He has written letters to several different journals by the way, only one to my knowledge, was accepted for publication because they let me see and comment on them. BioScience really upset us because they did not let us see the letter and we didn't know it was coming out until it came out. And Larry Licht did not show us the professional courtesy of telling us he was writing a letter.
ER: Do you have any idea why?
AB: Yes I do have a possible reason why: He seems to be working on UV in his own lab and I guess I scooped him. That's the way it goes. The fact of the matter is he gave a paper at a meeting where he got results that actually back us up: there is a UV effect on amphibian eggs. Here it is, Effects of Ultraviolet Radiation on Life History Parameters of Frogs from Ontario, Canada: Karen Grant and Lawrence Licht; I just pulled it out. Let's see, no effect of UV on one species, and detrimental effects on another species. But his is a lab study so I didn't scoop him because there have been four or five other lab studies over the years with the results that there are detrimental effects of UV radiation on eggs and larvae of amphibians in the lab.
UV Repair and Resistance to Solar UV-B in Amphibian Eggs: A link to amphibian declines? 1994 Proceedings of the National Academy of Sciences (USA) 91:1791-1795. A. Blaustein, P.D. Hoffman, D.G. Hokit, J.M. Kiesecker, S.C. Walls, J.B. Hays.
Copyright 1995 Environmental Review