Ultraviolet Light and Egg Survival in Amphibians:
An Interview with Andrew Blaustein
From the Environmental Review Newsletter Volume Two Number Ten, October 1995
Introduction:
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.
Literature Cited
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