Ecological death is the inability of an organism to
function in an ecological context, leading to death.This term can be used in
many fields of biology to describe any species. In the context of aquatic toxicology, a toxic chemical, or toxicant,
directly affects an aquatic organism but does not kill it; instead it impairs
an organism’s normal ecological functions which then lead to death or lack of
offspring. The toxicant makes the organism unable to function ecologically in
some way, even though it does not suffer obviously from the toxicant.
Ecological death may be caused by sublethal toxicological effects that can be behavioral, physiological,
biochemical,
or histological.
Types of Sublethal Effects Causing Ecological Death
Sublethal effects consist of any effects of an organism
caused by a toxicant
that do not include death. These effects are generally not observed well in a
shorter acute toxicity test. A longer, chronic
toxicity test will allow enough time for these effects to appear in an
organism and for them to lead to ecological death.
Behavioral Effects
Toxicants can affect an organism’s behavior, which with
aquatic organisms, may impact their ability to swim, feed or avoid predators.
The impacted behavior can lead to an organism’s death because it may starve or
get eaten by predators. Toxicants may affect behavior by impacting the sensory
systems which organisms depend on to collect information about their
environment or by impacting an organism’s motivation to properly respond to sensory
cues. If an organism is unable to use sensory cues effectively, they may be
unable to respond to early warning signs of predation risk. Toxicants can also
affect later stages of predation by impacting an organism’s ability to respond
to predators or follow through with escape strategies.
Physiological Effects
Toxicants can affect an organism’s physiology which may
impact its growth, reproduction, and/or development. If an organism does not
grow correctly and is undersize or has growth defects, it will be more likely
to be eaten by predators. If an organism’s reproduction is impaired, it may not
directly die, but it will be unable to pass on its genes to the population. The
organism will no longer be representative in the population’s gene pool.
Biochemical Effects
Toxicants can alter the enzymes or ions present in an
organism. If this alteration does not directly cause death, but impacts the
behavior or physiology of the organism, it can also lead to ecological death.
Histological Effects
Toxicants can alter an organism’s tissues. If this
alteration does not directly cause death, but impacts the behavior or
physiology of the organism, it can also lead to ecological death.
Toxicant Examples Leading to Ecological Death
DDT
An effect caused by DDT is shell thinning in
bird eggs, leading to the death of the chick. Once DDT has been accumulated by
an adult bird, it is metabolized into the form DDE which is both stable and toxic.
Once in the form of DDE, the chemical impacts the metabolism of calcium in
adult female birds’ shell glands, ultimately causing a decrease in eggshell
thickness. At high concentrations of DDT, the eggshells will no longer be able
to support the incubating parents’ weight and will lead to the death of the
unborn chick. This is an example of physiological and biochemical sublethal
effects leading to ecological death of the chick.
Diazinon
An effect caused by diazinon is a
decrease in response to predator cues in Chinook
salmon (Oncorhynchus tshawytscha). Diazinon, an organopesticide, was
exposed to juvenile Chinook salmon for two hours at 1 and 10 μg/L, and these
concentrations were enough to eliminate the behavioral responses of the fish to
predator chemical cues. If the fish cannot recognize that a predator is nearby,
it is likely to be eaten. This is an example of a behavioral sublethal effect
leading to ecological death.
Pentachlorophenol
An effect caused by pentachlorophenol
is a decrease in response to predator attacks in guppies (Poecilia
reticula). Pentachlorophenol was exposed to juvenile guppies at 500 and 700
μg/L, and both concentrations decreased the guppies’ reactions to predators.
The predators did not have to strike as frequently, did not have to pursue as
frequently, or have to pursue the guppies as long as guppies that had not been
exposed to these levels of pentachlorophenol. The guppies that were exposed to
this chemical were more likely to be eaten due to their slower responses. This
is another example of a behavioral sublethal effect that leads to ecological
death.
Copper
An effect caused by copper is impacting
the salmon olfactory
system. The olfactory system is used to gather important information about
one’s environment. In the case of salmon, olfactory cues can gather information
about habitat quality, predators, mates and more. Salmon can detect distinct
copper gradients using their olfactory system, and use this information to
avoid contaminated waters. However, when salmon are affected by copper, the
olfactory system can be impacted in a matter of minutes. If the fish is no
longer able to gather environmental information, it may be at risk for
predation or unable to find resources for survival. This is an example of a
physiological sublethal effect leading to ecological death.
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