population Flashcards
STRENGTH OF
ASSOCIATION
A greater magnitude effect is associated with
exposure to the stressor
* Do the stressor and effect coincide in their distribution?
* Is the effect large relative to the cause?
* Is the prevalence of the effect in exposed populations large to unexposed populations?
CONSISTENCY
A relationship is observed repeatedly in different
persons, places, circumstances, systems, and time
* Need for replication; many environmental impairment studies
the cause is not replicated
SPECIFICITY
The effect is diagnostic of a stressor. The more
specific the effect, the more likely it is to have a single
consistent cause.
* Could the effect be due to a different cause?
* Can alternative hypothesis be eliminated?
* Both effects and causes should be defined as specifically as
possible
* Specificity of the cause is more persuasive; high specificity of
each cause and effect is most persuasive
* x causes only y, and y is only caused by x
TEMPORALITY
The stressor must precede the outcome it is
assumed to affect.
* Episodic effects have episodic causes; may include
natural episodes in the life history of the organism
BIOLOGICAL GRADIENT
The outcome increases monotonically with increasing
dose of exposure
* Monotonic dose response curve?
* Gradient may not occur if interactions between stressors
occur
* Varying levels of SS, TOC, temperature may alter bioavailability
or metabolism
PLAUSIBILITY
Given what is known about biology, physics, and
chemistry of the potential cause, the receiving
environment, and the affect organisms, is it plausible
that the effect resulted from the cause?
* Is there a plausible mechanism?
* Are there plausible routes of exposure?
* Do the stressor and receptor co-occur in space and time?
COHERENCE
A causal conclusion should not fundamentally
contradict present knowledge of natural history and biology. Is the hypothesized relationship between the cause and effect consistent with all available
evidence?
* Does this theory contradict current knowledge?
* Do remedial actions lead to altered frequency or severity of the
effect?
EXPERIMENTAL EVIDENCE
Causation is more likely if evidence is based on
randomized experiments. Do toxicity tests
demonstrate the potential cause can induce the
observed effect?
* May be taken from literature; case-specific studies are more relevant
ANALOGY
Similar stressors cause similar responses. Is the
hypothesized relationship between cause and effect
similar to any well established cases?
* Many or few studies, but clear
* Few, but unclear
* None found
* Found no relationship
DIFFICULTIES WITH ATTRIBUTING
CAUSALITY TO ENVIRONMENTAL
FACTORS
- Difficult to define exposure precisely, or to
measure accurately - Long and variable latency periods for effects to
be seen - Relative rarity of most environmentally related
diseases - Non-specificity of most potential health
outcomes - The possibility that multiple factors may interact
Community
An assemblage of populations living in a prescribed area
or physical habitat: it is an organized unit to the extent that it has characteristics additional to its individual and population components … [it is] the living part of the
ecosystem.
Ecosystem
The biota (community) and abiotic environment are combined into an organized system.
Species interact with each other and loosely interact with their physical environment. Biotic and abiotic components act
together to direct energy flow and cycle materials.
Structure
-The range and variety of species; taxonomic or diversity oriented assessment
- Essentially the imposition of human values on the ecosystem
Function
-Productivity, photosynthesis, nutrient cycling, community
respiration, consumption of biomass, processing detritus etc.
- Ecosystem can’t recognize species
FUNCTIONAL REDUNDANCY
- In a sustainable functioning ecosystem, a decrease in
biodiversity can be tolerated; as long as it isn’t a keystone
species - Multiple species are able to perform each critical function
