Chapter 1 – The Web of Life
Case Study: Deformity and Decline in Amphibian Populations
Worldwide, the frequency of amphibian deformities was rising, and the global
amphibian populations were declining.
Researchers were worried about amphibians declining for three reasons
The decline appeared to have started recently across wide regions of the
Some of the populations in devline were located in protected or pristine
regions far from the effects of human activities.
Many scientists view amphibians as good “biological indicators” of
environmental conditions for a number of its characteristics:
Permeable skin with no hair/scales/feathers protecting them, and their
eggs also lacked protection.
Spends most of their life in the water and partly on land therefore
exposed to a wide range of threats such as pollution, changes in
temperature, and amount of UV.
Many do not move from their birthplace, therefore a decline in the
population indicates the deterioration of the environment.
Introduction: Humans have such a large influence on the planet on both land and oceans
leading effects such as global climate change, decline in fish stock, “dead zones” in the
ocean etc. But we are also part of the environment and we are beginning to realize that
we need to understand nature and its systems, hence understand ecology.
Connections in Nature
The events in the natural world can be linked or connected to one another, as the
organisms interact with one another/with the environment.
They don’t have to directly interact but by sharing an environment they are indirectly
interacting with each other.
Early observations suggest that parasites cause amphibian deformities
A researcher, Ruth, hypothesized that a parasite (Ribeiroia ondatrae) caused the deformity.
Implanting a glass bead in a developing tadpole by the not yet grown limbs tested the
They reported that the beads caused deformity similar to those found in amphibians
A laboratory experiment tests the role of parasites
At first when the deformities were found in the amphibians, Ruth believed that they
were an isolated local phenomenon.
Although Ruth and others provided indirect evidence of the parasite causing
deformities, another researcher, Johnson, set out to provide a more direct test of the
Johnson and other researchers focused on two of the four ponds where they found
deformed frogs, and thought that the deformities were caused by pollutants (ex
pesticides, and heavy metal) but they were not found in the ponds.
The four ponds they focused on were the only ponds with the frogs and an aquatic snail
which was a host required for the Ribeiroia parasite to complete its life cycle and
produce offsprings. The dissection of the frogs from these four ponds also contained
This again was only indirect evidence; hence an experiment was carried out, where
tadpoles from ponds that was not known to have any deformities were put in
containers with different amounts of parasites.
Johnson found that as the number of parasites increased fewer tadpoles survived, and
the majority of the ones that did survive were deformed. So this supported the
hypothesis and also suggested that the parasites could contribute to amphibian
A field experiment suggests that multiple factors influence deformities
Kiesecker conducted a field experiment in six ponds that all had Ribeiroia and only
three with pesticides like a farm to examine the possible joint effects of parasites and
In each pond six cages were put in that contained tadpoles, three of where parasites
could pass through, and the other three that could not. Tadpoles couldn’t escape.
The results showed the frogs raised without the parasites had no deformities regardless of whether the lake had pesticides or not. Deformity was found in frogs
that were exposed to Ribeiroia. But there was a higher percentage of deformities in
the frogs that were in the lake with pesticides.
Kiesecker hypothesized that pesticides may decrease the ability of frogs to resist
infection by parasites. This hypothesis was tested in a lab exposing all tadpoles to
Ribeiroia and half with pesticides and half without. The results suggest that
pesticide exposure may affect the frequency with which parasites cause deformities.
Connections in nature can lead to unanticipated side effects
Although we know that the parasites cause deformities why has the frequency of the
deformity increased so much compared to the past? increase in pesticides
Pesticides decrease the ability of amphibians to protect themselves from parasite
Other environmental changes such as addition of nutrients to natural/artificial ponds
can lead to increase in parasites, algae formation lead to an increase in snail
population which are hosts to parasites hence increase in deformity.
We live in an ecological world
When people alter one aspect of the environment, we change another aspect even if it
is not intended.
Changes in our environment not only leads to deformity in amphibians but also risks
human health. Ex) spread of may diseases.
No matter how far we are from the natural world in our every day lives, we are part of
an interconnected web of life.
Ecology = the sc