Which of the following stressors would be categorized as a cataclysmic event?

Key Concepts

1. Describe the environmental stressors, their causes, and how ecosystems respond to changes in their intensity.
2. Explain the differences between contamination and pollution.
3. Provide examples of natural stressors and explain how knowledge of them can help us understand anthropogenic stressors.
4. Outline the differences among toxicology, environmental toxicology, and ecotoxicology.
5. Explain the differences between voluntary and involuntary risks.
6. Identify how a risk assessment is done of a predicted exposure to a toxic chemical.

• competition, predation, disease, and other interactions among organisms
• constraints related to climate or to inadequate or excessive nutrients, moisture, or space
• disturbances such as wildfire and windstorms

Kinds of Stressors

• Physical stress is a disturbance in which there is an intense exposure to kinetic energy, which causes damage to habitats and ecosystems. Examples include such disruptive events as a hurricane or tornado, a seismic sea wave (tsunami), the blast of a volcanic eruption, an explosion, or trampling by heavy machinery or hikers.
• Wildfire is another disturbance, which involves the uncontrolled combustion of the biomass of an ecosystem. A wildfire can be ignited by people, or naturally by lightning. A severe fire consumes much of the biomass of an ecosystem, but even a less-severe wildfire may kill many organisms by scorching and poisoning by toxic gases.
• Chemical pollution occurs when one or more substances occur in a concentration high enough to elicit physiological responses in organisms, potentially causing toxicity and ecological change. Chemical stressors include pesticides, gases such as ozone and sulphur dioxide, and toxic elements such as arsenic and mercury. Pollution may also be caused by excessive nutrients, which can distort productivity and other ecological functions. Note that the mere presence of a potentially toxic agent does not necessarily cause pollution. (The distinction between contamination and pollution is examined later in this chapter.)
• Thermal pollution is caused by the release of heat (thermal energy) into the environment, which results in ecological stress because species vary in their tolerance of temperature extremes. Thermal stress may occur at natural springs and submarine vents where geologically heated water is emitted. It is also associated with discharges of hot water from power plants.
• Radiation stress is caused by excessive exposure to ionizing energy. The radiation may be emitted by nuclear waste or explosions, or it can be diagnostic X-rays or solar ultraviolet energy.
• Climatic stress is associated with insufficient or excessive regimes of temperature, moisture, solar radiation, wind, or combinations of these.
• Biological stressors are associated with interactions occurring among organisms, such as competition, herbivory, predation, parasitism, and disease. For example, individuals of the same or different species may compete for essential resources that are limited in supply. Herbivory, predation, parasitism, and disease are trophic interactions, in which one species exploits another. Exploitation can be anthropogenic, as when humans harvest wild animals or trees, or it can be natural, perhaps associated with defoliating insects or disease-causing pathogens.
• Biological pollution occurs when people release organisms beyond their natural range. This might involve the introduction of alien species that invade and alter natural habitats, or it may be the release of pathogens into the environment through discharges of raw sewage.

Ecological Responses

• mortality increases, especially of the most vulnerable species
• species richness decreases
• the stocks of nutrients and biomass become depleted
• the rate of community respiration exceeds that of production, so the net production becomes negative.
• sensitive species are replaced by more-tolerant ones
• top predators and large-bodied species may be lost from the ecosystem
• previously self-maintaining ecosystems may require active management to sustain their desirable attributes, for example, to maintain declining populations of rare or economically valuable species that have become threatened

Pollution Can Be Natural

Anthropogenic Pollution

• accidental or deliberate emissions of chemicals into the environment, such as sulphur dioxide, metals, pesticides, and petroleum
• releases of substances that react in the environment to synthesize chemicals of greater toxicity – this is known as secondary pollution (as occurs when ozone is created by photochemical reactions in the atmosphere)
• emissions of chemicals that degrade stratospheric ozone, such as chlorofluorocarbons
• releases of waste industrial heat, as when a power plant discharges hot water into a river or lake
• discharges of nutrient-laden sewage or fertilizer into waterbodies
• emissions of greenhouse gases that threaten global climate
• releases of alien species that cause damage when they invade managed or natural habitats, or are pathogens of people, crops, or native species

• A community-replacing disturbance is extensive in scale and results in a catastrophic destruction of one or more original communities. Natural examples are caused by wildfire, windstorm, avalanche, and glaciation, while anthropogenic ones include clear-cutting and ploughing. These large-scale disturbances may be followed by a successional recovery that eventually regenerates a community similar to what was destroyed. Younger communities in the successional sequence (or sere) are relatively dynamic in their structural and functional properties. They are typically dominated by species that are abundant only during the initial stages of recovery, when competition is not so intense. Community changes in later stages are somewhat less dynamic, until a late-stage community is re-established.
• A microdisturbance involves a local disruption that only affects a small area within an otherwise intact community. Anthropogenic microdisturbances include the selective harvesting of individual large trees or particular animals, while leaving the community otherwise intact. Ecological changes are relatively rapid within a habitat patch that has been affected by a recent microdisturbance, but at the stand level the community is stable. So-called patch- or gap-phase successional dynamics occur in all natural forests but are particularly important during the later stages of succession. This is especially the case in older-growth forest, where individual trees might die from disease, insect attack, or a lightning strike, creating a gap in an otherwise intact canopy.

Natural Disturbance

Anthropogenic Disturbance

Anthropogenic Stressors in Context

• the cycling and transportation of potentially toxic chemicals
• their transformation into other substances (which may be more, or less, poisonous than their precursors)
• the determination of sinks where chemicals may accumulate in especially high concentrations, including within the bodies of organisms

1. Biological Sensitivity

2. Inherent Toxicity

• chemicals vary enormously in relative toxicity
• at a large enough dose, any chemical may be toxic

3. Exposure

4. Indirect Effects

All Chemicals Are Toxic

Interpretation of Damage

• Are measurable changes seen in the populations of affected species? From an ecological perspective, population-level damage is the most important consideration, even while it is acknowledged that the death of an individual organism is regrettable. Populations of all species have a certain degree of resilience and can tolerate some mortality caused by toxic chemicals without suffering an overall decline.
• Are affected species important in maintaining the integrity of their community? Ecological philosophies suggest that all species have intrinsic value. Nevertheless, species do vary greatly in their contribution to the functioning and structure of their community. So-called keystone species have a dominant influence (Chapter 9). Substantial changes in their abundance should be judged as relatively important compared with damage inflicted on more minor species.
• Is the damage of economic importance? This consideration involves damage to resources that are needed by humans and therefore have economic value. In this sense, damage is judged to be relatively important if it is caused to hunted animals such as deer or trout, to trees that can be harvested to manufacture pulp or lumber, or to vital ecological services such as the provision of clean water and air. From a purely utilitarian perspective, damage caused to non-economic values, both species and services, may be viewed as being less important.
• Other considerations, less tangible than those just mentioned, involve appraising damage in aesthetic or ethical terms. These considerations are also important, but they are difficult to interpret in terms of risks or benefits to human welfare. As a result, aesthetic or ethical considerations are rarely reflected in regulatory criteria or in the management of potentially toxic chemicals in the environment.

Environmental Risk Assessment

1. the likelihood of encountering the hazard
2. the likely intensity of the hazard
3. the biological damage that is likely to result from the predicted exposure

• highly energetic (ionizing) radiation associated with ultraviolet-B, X-rays, and gamma radiation
• polycyclic aromatic hydrocarbons (PAHs), such as benzo(a)pyrene
• polychlorinated biphenyls (PCBs) and certain pesticides
• methyl mercury and some other metals
• aflatoxin present in mouldy nuts and grains
• dimethyl nitrosamine present in nitrite-treated foods
• diesel exhaust
• effluent from pulp mills
• tobacco and barbecue smoke

• adrenaline (epinephrine) and noradrenaline (norepinephrine), which are adrenal hormones that stimulate the body to react to a stressful condition by increasing the blood pressure, blood sugar, and heart rate (this is sometimes known as a “flight or fight” response)
• estrogen, a female sex hormone produced by the ovaries, and androgens, male hormones produced by the testes
• insulin, formed by the pancreas to regulate the use and storage of carbohydrates (including blood sugar)
• thyroid hormone, which influences the growth and metabolism of virtually all body cells

• organochlorines, including dioxins (such as TCDD), polychlorinated biphenyls (PCBs), and the insecticides DDT, dieldrin, and lindane
• other kinds of pesticides, including atrazine, permethrin, and trifluralin
• tributyltin, which is used as a marine antifoulant
• alkylphenols used as surfactants, such as nonylphenol
• certain placticizers, such as dibutyl phthalate and butylbenzyl phthalate
• natural hormones and synthetic steroids from contraceptives that are released to the environment in sewage or occur as residues in food, including estradiol, estrone, and testosterone
• phytoestrogens in pulp-mill effluents, including coumestans, isoflavones, and lignans

1. What are the various kinds of environmental stressors? Provide an example of each.
2. Explain the difference between pollution and contamination?
3. How do toxicology, environmental toxicology, and ecotoxicology differ?
4. Use the data in Table 15.4 as the basis of a brief essay about the risks of death. Make sure that you relate the risks to environmental exposures, where appropriate.

1. Identify examples of naturally occurring pollution and disturbance in the region where you live. What considerations determine how society judges the importance of the natural and anthropogenic sources?
2. Compare the ecological effects of a community-replacing disturbance and a gap-phase microdisturbance. How is knowledge of these effects useful for designing ecologically appropriate practices for resource harvesting and management? Use old-growth forest as an example.
3. What do you consider to be the five most important risks to your health? Compare your list with the data in Tables 15.4 and 15.5. What are the similarities and differences? Why do they exist?
4. If all chemicals are potentially toxic, should society allow any exposure to these potential health risks? Discuss this statement and its conceptual fallacy.

1. How can the data presented in Table 15.2 for the herbicide glyphosate be used to assess the probability of people or animals suffering toxicity when this chemical is used in agriculture, forestry, or around the home (these are all common uses)? What additional information would you need to perform a comprehensive risk assessment for this chemical? What about consideration of the indirect effects on wild animals caused by changes in vegetation in treated areas?

Which of the following are considered cataclysmic events?

What are the three categories of stressors quizlet?

Which of the following are examples of stressors?

Which of the following is an example of a stressor quizlet?