did they conduct the experiments?]
[What did they find?]
by James Harding
study shows that frogs are likely to be far more sensitive
to pesticides in the real world than traditional laboratory
tests used to establish regulatory standards would indicate.
and Mills reach this conclusion for two reasons. First, the standard
amphibian tests are far too short. Extend their time and low
contamination levels of carbaryl that appeared to cause no deaths
wind up causing significant mortality. [More...]
important, however, is what Relyea and Mills discovered when
they made the experiments resemble the real world by adding a predator
to the mix. They set the experiment up in a way that the tadpoles'
natural predator was present and the tadpoles could detect it, but
that the predator, a type of salamander, could not actually reach
the tadpoles. Not surprisingly, previous work has shown that the
presence of a predator induces stress in prey, even in amphibians.
The combination of stress and pesticide made the pesticide 2-4
times more lethal than the pesticide alone. [More...]
this experiment documents a hitherto completely unexplored variable
affecting the lethality of a pesticide, i.e., the interaction with
predator-induced stress. By itself, that is highly significant and
should be repeated in appropriate experimental designs for a wide
array of organisms.
predation is only one of the many natural factors that stress organisms
living in the real world, including people. Virtually all
toxicity testing for regulatory purposes ignores these realities.
It is highly likely, therefore, that current regulatory science
even in its simplest form (does a pesticide kill) has dramatically
underestimated the impacts of many pesticides.
important dimension to this study is that the levels of carbaryl
used in the experiments was only a small percentage of carbaryl
concentrations measured in ponds in the real-world after application
on crops. Because carbaryl can be persistent under some settings
for years, and because its use is so widespread, there is every
reason to believe that amphibians in many places are encountering
carbaryl levels within the concentration range found in these experiments
to be deadly.
did they do? Relyea and Mills performed a series of experiments
examining the effect of carbaryl,
a commonly used pesticide, and predator-induced stress on survival
of tadpoles of a common frog in North America, the gray treefrog,
Hyla versicolor. These experiments were carried out using
contamination levels of carbaryl that were low compared to the doses
normally required to kill tadpoles.
their experiments they varied the dose of carbaryl and also
whether or not the predator, a caged larval Spotted Salamander
Ambystoma maculatum,was present in the tadpoles' container.
Photo by James Harding
According to Reylea and Mills, "caged predators emit chemical
cues that induce antipredator responses in their prey without allowing
the predators to kill the target animals." By placing the caged
predator in the vicinity of the tadpoles, Reylea and Mills increased
the stress level of the tadpoles.
dose levels they employed were a small percentage of the concentration
of carbaryl normally required to kill tadpoles.
did they find? The first revelation was that simply by extending
the time of exposure of the tadpoles to relatively low levels
of carbaryl, they were able to cause significant tadpole mortality.
testing of carbaryl had lasted 2 to 4 days at much higher levels
of exposure. Relyea and Mills showed that by extending the experiments
out only a few days longer, carbaryl at much lower concentrations
caused tadpole mortality.
from Relyea and Mills 2001
amphibian toxicity testing of carbaryl had been too short to reveal
this effect. "Very low concentrations of carbaryl can still
cause widespread amphibian death, it just takes a few more days
to observe the effect."
and Mills found that the effect of stressing the tadpoles by adding
the predator to their tanks dramatically increased mortality. "Predator
cues made the pesticide 4 times more lethal."
from Relyea and Mills 2001
graph on the left shows the impact of combining predator stress
with carbaryl on tadpole survival. All tadpoles were exposed
to a low level of carbaryl and died at a higher rate than normal
(compare to "water" in graph above). Tadpole survival
drops even lower when predator-induced stress interacts with
and Mills also observed other impacts of carbaryl and predatoress
presence of the caged predator "did not affect tadpole activity
when carbaryl was absent but significantly reducted activity when
carbaryl was present."
did not affect growth in the absence of carbaryl but reduced tadpole
growth by 50% in the presence of carbaryl.