The Web of Life
Evens in the natural world are interconnected
Ecology is the scientific study of interactions between organisms and their environment
Ecologists evaluate competing hypothesis about natural systems with observations,
experiments, and models.
Natural systems are driven by the ways in which organisms interact with one another and with
their physical environment
To understand how natural systems work, we must understand ecology
Ecology: scientific study of how organisms affect and affected by their environment and other
Connection in nature: events in the natural world can be linked or connected to one another
Does not mean all the organisms affect one another that live in a given area
Organisms are connected to feature of their environment such as food, space, and other
resources, species, physical environment
So they can all be connected indirectly
Deformity and Decline in Amphibian Populations: A Case Study
o High incidence of deformities in amphibians
o Declining populations of amphibians worldwide
Amphibians are “biological indicators” of environmental problems.
o Skin is permeable; pollutant molecules can pass through easily.
o Eggs have no protective shell.
o They spend part of life on land and part in water—exposed to pollutants and UV in both
Observation of Pacific tree frogs suggested that a parasite can cause deformities.
Small glass beads implanted in tadpoles to mimic the effect of cysts of Ribeiroia ondatrae, a
trematode flatworm, also produced deformities.
Stephen ruth, found pacific tree frogs, and long toed salamander
o They had extra limbs, missing limbs, and other deformities
o All contained a parasite, known to be Ribeiroia ondatrae (trematode flatworm)
o They implanted small glass beads, to mimic the effect of rebeiroia
o Which produces cysts close to the area where limbs form as a tadpole begins its
metamorphosis into an adult frog
o Beads cause similar deformities
Pieter Johnson, surveyed 35 ponds, found pacific tree frogs deformities in 4 ponds
o No pollutant, pesticide, PCB or heavy metals were found in the pond
o Difference was that it contained aquatic snail o The snail was one of the 2 intermediate hosts req. for Ribeiroia
o They performed controlled experiment
Experimental group being compared to control group
o Collected frog eggs that were from undeformed frogs
o Than assigned them to random treatments
o As the number of parasites increased, few survived, and the ones that did survive, 88%
o Control group, had 88% survive and no deformities
Ribeiroa was clearly important, pesticides were known to contaminate ponds and cause deform
In Kieseckers study, conducted field experiment, 6 ponds all contained Ribeiroia
o All contained Ribeiroia but only some contained pesticides
o 3 were close to farm fields
o Other 3 were not and so did not show detectable levels of pesticides
o 6 cages were placed in each pond, 3 had a mesh which Ribeiroia parasite could pass
o Other 3 cages had a mesh too small for parasites
o Results showed that all contained Ribeiroia caused limb deformities
o No deformities were found in frogs with small mesh that prevented Ribeiroia, regardless
o Deformities were found in large mesh which let in Ribeiroia
o Greater percentage of frog had deformities in ponds that contained pesticides
Pesticides might decrease the ability of frogs to resist infection by parasites
Tadpoles exposed to pesticides had fewer white blood cells, and a higher rate of Ribeiroia cyst
Why has the frequency of amphibian deformities increase?
The addition of nutrients such as when rain or snowmelt washes fertilizers form an agricultural
field, to natural or artificial ponds, can lead to increase in parasite infections and amphibian
Fertilizer inputs can trigger the growth of more algae, and aqua snail eat algae, and they harbour
Skerrat et al. (2007) argued that some declines may be due to pathogens such as a chytrid
fungus that causes a lethal skin disease, and has spread rapidly in recent years.
o But climate change and altered conditions may be favoring growth and transmission of
Hatch and Blaustein (2003) studied the effects of UV light and
nitrate on Pacific tree frog tadpoles.
o At high elevation sites, neither factor alone had any
affect. But together, the two factors reduced tadpole
o At low elevation sites, this effect was not seen.
When people alter one aspect of the environment, directly or
indirectly, we can cause other changes that we do not intend or anticipate We tend to forget that everything we do depends on the natural world hence we live in a
shelter that keeps us worm or cool, and buy clothes in a store