Pollution and predators: a double whammy for tadpoles.
Copper levels in water that are considered safe by regulatory agencies can slow wood frog tadpoles enough to make them easier prey for predators, report researchers in the journal Ecosphere. The slowing combined with the normal quiet triggered when a natural predator approaches can so diminish a tadpole’s movements that it is even more vulnerable to predators that can injure or eat it.
The results of the study highlight how low levels of contaminants can indirectly influence animal fitness in unexpected ways. It also shows the importance of examining toxicity effects in a broader ecological context.
There is considerable concern over the effects of multiple stressors on tadpoles, because worldwide, certain amphibian populations are declining. While experts debate the causes, they generally agree that more than one factor may be at play in their demise, including contaminants, disease and atmospheric changes.
More broadly, frogs, salamanders and other amphibians are important because they act as sentinal creatures that indicate the health of the environment. Their unique absorbent skin and dependence on water at some point in their life cycle means they are highly sensitive to environmental changes.
Copper is a trace metal that enters waterways from road runoff and is also a waste product of hard rock mining. While copper is considered essential for growth and survival, high doses lead to toxic effects in many aquatic organisms.
Previous studies have found that copper can interfere with smell and alter fish behavior by impairing homing ability. Other studies show that tadpoles change their behavior in response to environmental cues, such as chemicals released when a predator eats another tadpole.
This new study looks at what happens to tadpoles when exposure to what is considered safe, low levels of copper in the water is combined with a routine, ecological stress – in this case, the presence of a dragonfly predator.
In an earlier study, the researchers found that copper and predators were associated with limb abnormalities, that smaller frogs were found in sites with high copper levels and that the smaller animals were more likely to have limb abnormalities than larger frogs.
To understand how the combined presence of copper and predators might detrimentally alter tadpole behavior and lead to limb abnormalities, the authors collected tadpoles undergoing hind limb development from multiple sites within the Kenai National Wildlife Refuge. They exposed the tadpoles to a low, nontoxic level of copper (5 micrograms per liter), a predator chemical cue or both. They recorded the number of times the tadpoles moved in two hours.
The tadpoles exposed to copper at this low concentration were half as likely to move compared to tadpoles not exposed to copper at all. Predator cues also reduced tadpole activity, but to a lesser extent than copper. Older tadpoles responded less to predators and were more likely to remain active than younger tadpoles. When combined, copper and predators reduced tadpole activity in an additive way. The change in behavior was particularly strong in less developed tadpoles.
While reduced activity can help avoid predation, the additional slowing from the contaminants can lead to less feeding, which can delay growth and development, increase vulnerablity to predators and decrease survival and reproduction. These results help explain the researchers’ previous study in which they found smaller frogs with more abnormal limbs in copper-laden water. The scientists think that the slower-moving tadpoles are more vulnerable to dragonfly predators, which tend to eat the tadpoles developing hind limbs. In this case, the copper indirectly leads to hind limb abnormalities.
The findings show how low levels of copper, thought to be nontoxic to amphibians, can lead to a change in behavior that is likely to be harmful. However, further studies will need to directly measure the effect of copper on feeding or growth and address whether reduced activity leads to changes in survival or reproduction.
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