Ecological Effects of Environmental Stressors
Summary and Keywords
Regimes of environmental stress are exceedingly complex. Particular stressors exist within continua of intensity of environmental factors. Those factors interact with each other, and their detrimental effects on organisms are manifest only at relatively high or low strengths of exposure—in fact, many of them are beneficial at intermediate levels of intensity. Although a diversity of environmental factors is manifest at any time and place, only one or a few of them tend to be dominant as stressors. It is useful to distinguish between stressors that occur as severe events (disturbances) and those that are chronic in their exposure, and to aggregate the kinds of stressors into categories (while noting some degree of overlap among them).
Climatic stressors are associated with extremes of temperature, solar radiation, wind, moisture, and combinations of these factors. They act as stressors if their condition is either insufficient or excessive, in comparison with the needs and comfort zones of organisms or ecosystem processes. Chemical stressors involve environments in which the availability of certain substances is too low to satisfy biological needs, or high enough to cause toxicity or another physiological detriment to organisms or to higher-level attributes of ecosystems. Wildfire is a disturbance that involves the combustion of much of the biomass of an ecosystem, affecting organisms by heat, physical damage, and toxic substances. Physical stress is a disturbance in which an exposure to kinetic energy is intense enough to damage organisms and ecosystems (such as a volcanic blast, seismic sea wave, ice scouring, or anthropogenic explosion or trampling).
Biological stressors are associated with interactions occurring among organisms. They may be directly caused by such trophic interactions as herbivory, predation, and parasitism. They may also indirectly affect the intensity of physical or chemical stressors, as when competition affects the availability of nutrients, moisture, or space.
Extreme environments are characterized by severe regimes of stressors, which result in relatively impoverished ecosystem development. This may be a consequence of either natural or anthropogenic stressors. If a regime of environmental stress intensifies, the resulting responses include a degradation of the structure and function of affected ecosystems and of ecological integrity more generally. In contrast, a relaxation of environmental stress allows some degree of ecosystem recovery.
Ecology may be simply defined as the study of the relationships of organisms and their environment. Within that context, the study of environmental influences (or factors) that affect organisms and ecosystems is a core subject matter.
Effects of environmental influences may be exerted at any level of ecology: on individual organisms, populations, communities, ecoscapes (landscapes or seascapes), and, ultimately, the biosphere in its entirety. There are also influences on the functions of ecosystems, such as productivity and nutrient cycling. At any particular time or place, the relative importance of those influences can be beneficial, damaging, or inconsequential. The study of environmental factors is an important aspect of ecology because it helps ecologists to better understand the influences that affect, and perhaps even control, the structural and functional attributes of ecosystems and their components. This understanding is of theoretical interest and is also important to the application of ecological knowledge to the resolution of environmental problems.
Arguably, the field of ecology had its genesis when scientific examinations began to be made on the effects of environmental factors on organisms and other elements of ecosystems. The preceding field of study, known as natural history, also considered questions about the influences of environmental factors, but these questions were not studied by using a scientific methodology. The latter involves asking a question (establishing a hypothesis) about the structure or function of the natural world, and then conducting experiments or making careful measurements either in field or laboratory studies to establish whether the hypothesis is viable.
For example, an experiment relevant to environmental factors may involve growing a number of replicate populations of a plant species under conditions in which each is exposed to a different intensity of a particular factor of interest while other influences are kept as constant as possible. The factor that is varied experimentally may be nitrogen in a form that plants can take up from soil and use as a vital nutrient, such as the ion nitrate. One experimental treatment may involve no addition of nitrate to the soil (this is a “control” treatment), while other treatments have the nutrient added in some known amount. A typical result would be a steady increase of plant growth with greater nitrate availability. However, that positive response would eventually level off at high values of the nutrient, and beyond that exposure the nitrate may eventually prove to be toxic. This is a typical response to varying concentrations of many chemicals. They may serve as vital nutrients over a certain range of availability but may be toxic at higher exposures, even to the point of causing death.
Ecologists also use nonexperimental approaches to understand the influence of environmental factors, especially when they are working “in the field,” such as in a natural or managed habitat (the latter may include areas used for agriculture or forestry purposes). A study of that sort may involve careful measurements of the local intensity of a particular factor (such as the concentration of nitrate in soil) as well as the growth rate of plants in those same places. If a study finds a predictable variation of the intensity of a factor, the situation represents an environmental gradient. Field data could then be analyzed to determine mathematical relationships between the intensity of the factor and the response of plants. In this case, the mathematics may involve a correlation analysis that determines the strength of the relationship; if the nitrate concentration is having a strongly beneficial influence on plant productivity, the two factors would be highly correlated. It must be understood, however, that a strong correlation does not necessarily “prove” that the factor is having a presumed ecological influence; in fact, the relationship could be spurious, and a different environmental factor may be controlling the response. This is the reason why correlations observed in field studies of environmental gradients should be followed up by experimental manipulations of apparently key factors, in order to determine whether they are really the controlling influence on an ecological response being examined.
The study of the influence of environmental factors has a long history in ecology. An important early worker was Justus von Liebig (1803–1873), who discovered that nitrogen is an essential nutrient for plants. He proposed “the law of the minimum,” which suggests that the growth of a plant is not limited by the total amount of necessary resources, but rather by whichever resource is available in the least supply compared with the biological demand for it. This idea is now more commonly referred to as the “principle of limiting factors,” and along comparable lines of Liebig’s original proposition it suggests that ecological productivity is limited by whichever necessary environmental factor is available in the least supply compared to the demand for it. For instance, in a typical oligotrophic (unproductive) freshwater lake, phosphate is the limiting factor for the productivity of phytoplankton. As such, adding phosphorus fertilizer to a lake will increase the productivity of the ecosystem, whereas adding inorganic carbon or nitrogen will not have that effect unless the controlling demand for phosphorus first has been met. This is the reason why managing the rate of phosphorus addition to freshwater lakes is the key to controlling the damaging effects of eutrophication (excessive productivity) of those waterbodies.
In summary, biological and ecological attributes are influenced by a wide range of environmental factors. However, the intent of this article is to examine environmental stressors, or influences that limit biological or ecosystem development or that cause changes that are regarded as being degrading or damaging.
Environmental factors exist as complex regimes of intensities of exposure, which vary on a continuous scale as well as over space and time and also interact with each other. Within the continua of intensity of environmental factors, the range of stressors is marked by a manifestation of detrimental effects on organisms or ecosystem processes at relatively high or low strengths of exposure—in fact, many factors are beneficial at intermediate levels. Although a great diversity of environmental factors are manifest at any time and place, only one or a few tend to be dominant as stressors.
As such, environmental stressors are influences that limit the performance of organisms or of their characteristics at aggregate levels, such as those of a population or community. In the sense meant here, “performance” of an individual is related to such attributes as its productivity and reproductive fitness relative to its genetic potential. Almost always, the realized performance of an individual is less than would be possible under optimal environmental conditions. Depending on the intensity of a stressor regime, the growth and fitness of organisms may be diminished or even be made nonviable. Because individuals may exhibit genetically based differences in their tolerance of stressors, this aspect of the environmental regime has an influence on the evolutionary dynamics of populations and species.
Environmental stressors also affect ecosystems at the larger scales of populations, communities, and ecoscapes. In these cases, “performance” may be indicated either by functional or structural attributes of the system. Indicators of function would include the aggregate rates of productivity or nutrient cycling performed by many individuals compared with what is possible under optimal environmental conditions. Functional indicators are typically expressed as a rate, such as tonnes of carbon fixed per square kilometer (km2) of forest in a year (tC/ha.y). Structural indicators are amounts per unit area (or volume) at a particular time, such as biomass (tC/ha), a population (individuals/km2), or species richness (species/km2). Again, depending on the intensity of a stressor regime, the values of functions or structural attributes may be diminished or even be made nonviable compared with what occurs under optimal environmental conditions.
In any circumstance—meaning at a particular time and place—there is a complex regime of interacting environmental stressors. For certain stressors, referred to as “disturbances,” the exposure is intense but short lived. For other stressors, there is a chronic exposure that does not rapidly change over time.
The study of environmental stressors began with research into environmental factors, such as the pioneering work of Justus von Liebig noted in the Introduction section. Essentially, a stressor is an environmental factor that is available in too limited an amount to satisfy the biological demand, or that occurs in such a great exposure that toxicity or other kinds of damage are caused. Among the earliest researchers of environmental stressors were plant physiologists who were interested in regimes of environmental conditions (such as nutrient and water availability) that would optimize the productivity of crops. Later on, ecologists and toxicologists studied the poisonous effects of industrial chemicals released into the environment, including gases such as sulphur dioxide and ozone as well as metals and pesticides. In the 1960s, other ecologists began to examine how environmental stressors have system-level effects, such as on the structural and functional attributes of ecosystems. Examples of structural attributes that were being examined in earlier research on environmental stressors included the composition and species richness of communities, spatial distributions of the standing crops of biomass and nutrients, and trophic structure, while functional attributes included nutrient cycling, productivity, and behavioral and trophic interactions among species. Pioneers in these system-level studies include Gary W. Barrett (b. 1940), Fakhri A. Bazzaz (1933–2008), J. Philip Grime (b. 1935), Eugene P. Odum (1913–2002), and David Schindler (b. 1940).
To varying degrees, organisms and higher-level ecological attributes (such as populations and communities) have a degree of tolerance to exposure to stressors. The suite of tolerances is an important aspect of the niche of a species within its ecological community. However, when an exposure becomes greater than the limit of tolerance, a response will occur. This conceptual relationship is sometimes called the SER model (i.e., Stress–Exposure–Response). It describes how and to what degree biological and ecological changes will occur when the capacity of a system to endure stressors is exceeded.
Stressors that affect organisms and ecosystems are associated with various kinds of environmental factors, such as:
• regimes of temperature, moisture, or nutrients, or of space, that are insufficient to allow for growth rates that are as high as are genetically possible, as well as difficulties associated with too intense an exposure to those same factors
• constraints associated with biological interactions such as competition, disease, and predation
• short-lived events of disturbance (episodes of mass destruction), as may be caused by a windstorm or by a clear-cut of a stand of forest
The various kinds of environmental stressors are examined in more detail in the Kinds of Environmental Stressors section.
Environmental stressors typically affect different species in a variety of ways. When a stressor regime changes markedly, some species within a community will suffer damage while other species may realize an ecological benefit. Consider, for example, a wildfire that affects a mature stand of jack pine (Pinus banksiana) in an area of boreal forest of North America. Many individual trees are killed by the heat of scorching or by combustion, which results in acute habitat changes that are detrimental to local populations of most plant and animal species of the pre-fire mature forest. Nevertheless, the aftermath conditions of the disturbance provide other species with suitable early-successional habitat, such as the perennial herbaceous plant called fireweed (Epilobium angustifolium). These species increase greatly in abundance after a wildfire and are prominent in the postdisturbance recovery of the ecosystem. Moreover, although most of the mature pine trees are killed by the wildfire, there may be a benefit to their larger-scale metapopulation (or regional population). This occurs because a severe wildfire combusts much of the organic layer of an affected mature forest, which helps to prepare a mineral seedbed of the kind needed for regeneration of jack pine. This can allow a cohort (even-aged population) of seedlings to establish and then to grow, eventually producing another mature stand. Therefore, over the longer term, occasional disturbances by wildfire may be necessary for jack pine and its stands to survive and grow on the boreal landscape. In fact, wildfire as an environmental stressor must have played a role in the evolutionary history that resulted in jack pine being relatively tolerant, at the population level, of this kind of natural disturbance.
The intensity of environmental stressors may be exerted in various ways. For instance, chronic stressors have a relatively continuous presence, and so they exercise their pressure over an extended period of time. The characteristics of soil (such as its mineralogy and contents of water and nutrients), for example, have a relatively uninterrupted influence on the productivity of plants within an ecological community, even though soil factors do vary over space and time. This is also the case for many factors related to climate, such as temperature and moisture availability. Chronic stressors, such as those that occur in the desert or tundra, have a large influence on ecosystems, and if their intensity is severe they may restrict development to a simple condition and low level of biodiversity.
Other stressors have an acute rather than a chronic influence. This effect, referred to as a disturbance, is a short-lived event of destruction that affects dominant organisms in a community or over a larger area such as a landscape. Examples of disturbances include wildfires, a hurricane or tornado, flooding, and irruptive biological agents such as a disease epidemic or an infestation of defoliating insects. A disturbance is followed by succession, which is a period of community-level recovery.
Some disturbances have a large-scale influence in that they affect entire communities and even an ecoscape. These are referred to as stand-replacing disturbances. For instance, in most years, millions of hectares of boreal and montane forest are disturbed by extensive wildfires in remote regions of North America. Irruptive insects may also cause stand-replacing disturbances, such as the spruce budworm (Choristoneura fumiferana), which occasionally kills fir and spruce trees in eastern forests, and the mountain pine beetle (Dendroctonus ponderosae), which causes landscape-scale mortality in pine forests in western regions of North America.
In contrast, other disturbances may be quite small scale in terms of the area affected. The death of a large tree within an intact forest, for instance, will create only local damage. The newly available spatial gap is occupied by smaller trees that grow vigorously in their attempt to seize this temporary, resource-rich opportunity within an otherwise closed forest canopy. This kind of smaller-scale disturbance regime, which is typical of old-growth forests, results in that habitat developing its complex physical and biological structure.
Kinds of Environmental Stressors
Stressors of natural origin have always been part of the environmental context of organisms and ecosystems (Figure 1). Increasingly, however, anthropogenic stressors (those associated with human activities) are exerting an important influence. In fact, anthropogenic stressors are now more pronounced than natural influences over vast tracts of “working” landscapes and seascapes that are being used to serve human economies, such as in agricultural and urbanized areas.
Within this context, however, there are many kinds of environmental stressors. For the purposes of studying their ecological effects, it is useful to examine their complexity within a number of categories (Freedman, 1995, 2010). The main groupings of environmental stressors are climatic stressors, chemical stressors, wildfire, physical stressors, and biological stressors.
Climatic stressors are associated with extremes of moisture, solar radiation, temperature, or wind, and combinations of these and related factors. These are stressors if their condition is either insufficient or excessive in comparison with the comfort zones of organisms or the needs of ecosystem processes. As such, climatic stress can involve too-much or too-little moisture, or too-cold or too-hot temperatures. Climatic stressors may exert themselves year-round, as occurs on glaciers and in the driest desert. Alternatively, climatic stressors may exert their effect in a seasonally predictable manner, as is the case with winter stresses that affect plants and nonmigratory animals in ecosystems located at high latitudes (such as the Arctic) and high altitudes (alpine habitats). Climatic stressors also may involve uncommon events that cause great damage, such as a disturbance by a hurricane, an ice storm, or an extreme precipitation event. Stressors caused by humans include anthropogenic climate change, which may range from regional desertification to a global influence associated with rapidly increasing concentrations of greenhouse gases in the atmosphere.
Chemical stressors involve situations where the availability of certain substances is intense enough to cause toxicity or another kind of physiological detriment to organisms. If enough organisms are affected by toxicity, then there may be larger-scale ecological damage. Any chemical is potentially toxic if the exposure is intense enough, but substances that are most often associated with pollution are gases (such as ozone and sulphur dioxide), metals (such as lead and mercury), various kinds of pesticides. In general, organisms are tolerant of an exposure to relatively small concentrations of potentially toxic substances, and some chemicals may even accumulate in their tissues without causing discernible harm. In this sense, the mere presence of a potentially toxic chemical is referred to as contamination. Pollution, on the other hand, requires that the substance cause a damaging biological or ecological response. Note that even nutrients can be chemical stressors if their supply is excessive, because that circumstance amplifies productivity and distorts other ecological functions, causing, for example, the eutrophication of aquatic ecosystems. Moreover, at even higher concentrations, nutrients may be toxic. There are many sources of anthropogenic releases of chemicals into the environment, including large industrial emissions from power plants and metal smelters as well as smaller-scale emissions such as those that occur with the use of personal motor vehicles or the discarding of medication into the sewage system. Natural sources of pollution also exist, such as sulphur dioxide released by volcanoes, fine carbonaceous particulates associated with forest fires, metal-rich minerals that occur close to the surface and render soil conditions toxic to most plants, and certain bio-toxins such as those produced by phytoplankton, including domoic acid and saxitoxin.
Wildfire is a disturbance in which much of the biomass of an ecosystem is scorched or combusted (Figure 2). Organisms may be damaged or killed by exposure to intense heat and toxic substances, such as smoke and carbon monoxide. Most wildfires occurring in remote areas are naturally ignited by lightning, but wildfires nearer to civilization are usually lit by people. Wildfires are heterogeneous in their effect, affecting tracts ranging in area from hundreds of square kilometers to less than a hectare, and with the burn ranging from a light scorch that many trees can survive to an intensity that consumes most of the organic matter of the ecosystem. Once a wildfire event is over, there is an extended period of successional recovery that may regenerate an ecosystem similar to that present before the disturbance, or a different one if there has been a longer-term alteration of core environmental conditions.
Physical stress is a kind of disturbance in which there is a short-term exposure to kinetic energy that is intense enough to damage organisms and ecosystems. Examples of severe physical disturbances include a volcanic eruption or a seismic sea wave (tsunami), while anthropogenic disturbances include trampling by heavy machinery or the blast of an explosion.
Biological stressors are related to various kinds of interactions that occur among organisms. These may be associated with trophic interactions that involve one species feeding on another, as in herbivory, predation, and parasitism. In addition, biological stressors may occur indirectly through effects on the intensity of physical or chemical factors, as when competition among plants results in a limited availability of moisture, nutrients, or space, or when animals vie for access to territory or mates. Competition may be among individuals of the same species (intraspecific competition) or between different species (interspecific). Biological stressors may be natural in origin, as when trees are damaged by a native insect or disease. They also may be anthropogenic; for example, when trees or wild animals are harvested as economic bioresources. Anthropogenic influences on biological stressors also can be indirect, such as when an invasive alien species or pathogen is introduced to a naive ecosystem. This could be through the accidental or deliberate release of exotic plants, animals, or pathogens that cause damage to natural ecosystems or to agricultural or urban habitats. In these cases of ecological damage, the exotic species are referred to as invasive aliens or “biological pollution.” The release of pathogens by dumping of raw sewage into rivers or lakes or onto agricultural land is another kind of biological pollution.
Virtually all the universe consists of lifeless environments. The only known exceptions occur in the relatively miniscule biosphere of Earth, the bounds of which are defined by the presence of organisms. However, even on our habitable planet most of the physical space is devoid of life. The zones of life extend from places somewhat deep in the crust to tracts in the upper atmosphere, as well as diverse occurrences in between. Within that spatial context, the outer limits of the biotic envelope, where life and ecosystems are barely viable, are referred to as extreme environments. In those places, stressor regimes are only marginally able to support a small number of specifically tolerant species and low rates of ecosystem functions. The highest and lowest of the extreme environments where life is known to occur include the following:
• The stratosphere, or upper atmosphere, which is a highly stressful environment because of its intense cold, severe exposure to solar ultraviolet radiation, and extremes of other climatic factors. Nevertheless, viable microorganisms have been recovered from atmospheric samples collected as high as 41 kilometers (km), where the temperature is about -20°C and exposure to solar ultraviolet radiation is essentially unabated (Wainwright et al., 2003).
• Endolithic (“within-rock”) bacteria survive as deep as 3 km within fissured geological formations in the crust, where environmental conditions are warm and at high pressure (Golubic et al., 1981). Many of the bacteria are chemosynthetic autotrophs that survive by oxidizing sulphide minerals, while other bacteria are heterotrophs that feed on the sparse amounts of organic matter that are present.
Extreme environments also occur in more-intermediate places of the biosphere where stressors are intense because of geological or other circumstances. Examples include the following:
• Thiobacillus bacteria occur in severely acidic environments with pH less than 3, where these acidophilic (“acid-loving”) autotrophs survive by oxidizing sulphides and reduced compounds of iron and other metals. At the other end of the pH scale are alkaliphilic (“alkali-loving”) microbes that can tolerate environments with a pH greater than 9.
• Thermophilic (“heat-loving”) bacteria may survive at temperatures marginally exceeding 100°C, which in nature are associated with geothermal heat, such as eruptions of searing water at geysers on land or at deep-sea vents, or in fissures within the crust. Autotrophic thermophiles are chemosynthetic in their nutrition, because they manage to survive by oxidizing sulphide compounds, and especially at deep-sea vents the community may contain various heterotrophic filter feeders.
• Extremely dry habitats, such as the most-arid deserts, may support xerotolerant (“dry-tolerant”) microbial crusts, such as that formed by Microcoleus cyanobacteria in salt deserts (Campbell, 1979).
• High-altitude habitats above the alpine tundra are environments with extremely stressful climatic and rocky conditions, but there nevertheless may be a meager productivity of lichens and bryophytes, a sparse abundance of invertebrates, and slow rates of microbial activity.
In some places, pollution can be intense enough to represent an extreme environment. A natural example occurs at the Smoking Hills, a remote coastal place in the western Northwest Territories of Canada (Figure 3). At various places along the seacoast, deposits of bituminous oil shale have spontaneously ignited and have burned for centuries. The resulting “natural pollution” by sulphur dioxide has affected the nearby tundra by the direct toxicity of that gas to plants, as well as by severe habitat degradation by the acidification of soil and water. In fact, the worst affected habitats at the Smoking Hills are so severely stressed by these factors that no plants can survive. Within limits, this natural ecological damage is comparable to that caused by smelters that have released large amounts of SO2, such as those near Sudbury, Ontario (Freedman, 2010).
For that matter, many kinds of habitats that have been created by people are extreme environments. This includes urban tracts of asphalt and concrete on which few organisms can survive, and those habitats mostly in cracks and other interstices. Interestingly, many of these extreme anthropogenic habitats can be naturalized to a degree, by mitigating the severe environmental conditions in ways that would allow some vegetation to establish and grow. That goal is pursued through a naturalization approach to horticulture and also by the ecotechnology of “green roofs,” in which vegetation is grown on rooftops instead of covering them with uninhabitable asphalt or concrete.
Tolerance, Resilience, and Stability
Organisms, populations, communities, and ecoscapes can function in a “healthy” manner within ranges of intensity of any particular environmental factors. However, when the limits of tolerance (or resistance) are exceeded, those influences become stressors and they cause biological or ecological damage to occur.
Tolerance of an increasing intensity of environmental stress can take various forms, some of which are illustrated in Figure 4. Model A shows little biological or ecological tolerance, so that only a small increase in the intensity of a stressor results in a large response. Model B shows a direct proportionality between the stressor and the response. Model C shows some tolerance, but once it is exceeded a small further intensification of the stressor causes a large response to occur, which eventually levels off at a new level of tolerance, which itself can be overcome by a further increase in stress. Model D shows considerable tolerance, but it, too, can be overcome if the stress becomes intense enough.
Resilience refers to the speed and degree by which an organism, population, community, or ecoscape manages to recover to its original condition following a disturbance or after some other stressor lessens in intensity. For instance, boreal stands of trembling aspen (Populus tremuloides) have a relatively high degree of resilience both to wildfire and clear-cutting. Their extensive network of subsurface rhizomes tends to survive the disturbance and then prolifically resprouts soon afterward. This vegetative regeneration results in an enormous population of aspen sprouts that, during the course of succession, self-thins (a kind of intra-individual competition) and eventually recovers as a mature stand that is broadly comparable to the original forest. In contrast, the widespread collapse of populations of cod (Gadus morhua) in the northwestern Atlantic, which was caused by commercial overfishing, has been followed by minimal recovery even though harvesting of that species has been greatly reduced by a moratorium enacted in 1992. In this case, the original cod-dominated ecosystem may have become fundamentally changed by the overharvesting, and that persistent damage has somehow prevented a recovery of the fish stock.
Stability refers to the degree of constancy of a population, community, or ecoscape over time. Although change is a pervasive quality of all ecosystems, old-growth ecosystems are relatively stable, such as aged forests and abyssal communities of the oceans. Stable ecosystems tend to have relatively constant environmental conditions, which result in biological and ecological stability. In addition, a high degree of resistance to environmental changes that do occur also contributes to stability, as does a strong degree of resilience after a perturbation has relaxed.
Ecological Responses to Changes in Environmental Stress
An ecosystem that has been affected by a disturbance typically suffers mortality of some individuals, along with changes to its structural elements (e.g., species composition and the spatial distribution of biomass) and functional properties (e.g., productivity and nutrient cycling). Following a disturbance, the ecosystem recovers through the process of succession, which restores a mature ecosystem if it proceeds for a long enough time.
Chronic stressors influence ecosystems in a more continuous manner, as is the case with many chemical and climatic factors. If there is an increase in the intensity of one or more chronic stressors, organisms may suffer a decrease in productivity and show acute effects such as tissue damage and even death. At the community level, species that are vulnerable to an increased intensity of stress will be reduced in abundance or eliminated. When that happens, part of their functional role in the community (i.e., their niche) may be assumed by species that are more tolerant of the changed stressor regime, which already may be present or may appear as new invaders. An increased exposure to chronic stressors also may result in evolutionary changes within populations, assuming that individuals vary in their tolerance and that those differences are genetically based. Under such a condition, natural selection may eventually result in increased tolerance at the population level.
Longer-term ecological changes occur as a result of a prolonged intensification of chronic stressors. Ecological damage of this sort has been documented around a number of older smelters that, because of lax environmental legislation of the time, were allowed to cause excessive pollution. This has been observed in cases where a new metal smelter was constructed in a forested landscape and proceeded to emit large amounts of pollutants, such as sulphur dioxide and metals. Well-known examples include several smelters near Sudbury, Ontario, and others in the Copper Basin, Tennessee (Freedman, 2010; EPA, 2005). In these cases, the toxic stressors damaged tree-sized plants that had dominated the original forest, eventually causing them to give way to shrub-sized and then herbaceous vegetation. Where the pollution stress was severe enough, the landscape entirely lost its plant cover. In addition, the evolution of local ecotypes of several plant species was documented, specifically in regard to populations that are tolerant of high concentrations of metals present in soil (e.g., Cox and Hutchinson, 1980).
A smelter is a point source of emissions, meaning that the pollutants are released from a discrete place such as a smokestack. As a result, the intensity of pollution decreases in a more-or-less exponential pattern with increasing distance from the smelter (Figure 5). Consequently, the ecological responses radiate outward from the source of pollution and eventually become manifest as a persistent spatial gradient of community change. Eventually, at some distance away from the point source, where environmental stressors do not exert an effect markedly different from the regional background, the reference ecological conditions are again met.
Other kinds of spatial changes in stressors also result in corresponding patterns of ecological responses. For instance, a linear source of pollutants (such as a highway) will establish a perpendicular gradient that corresponds to the track of the source. Emissions of lead from vehicle tailpipes are distributed along roads in this way, as is the gradient of deicing salt (Freedman, 2010).
If there is an immediate change in the spatial intensity of a stressor regime, the result is a step cline and the boundary is referred to as an ecotone. Rapid changes of this sort exist between a lake and its surrounding terrestrial upland, in which case water saturation is a stressor that affects nontolerant upland plants. Similarly, places where industrial waste (such as mine spoils) has been dumped on land have a toxic ecotone with the surrounding terrain.
As environmental stress intensifies over time, there are predictable changes in the structure and function of ecosystems. Commonly observed ecological changes are summarized in Table 1.
Table 1. Effects of intensified stress. This table shows general trends that are observed in ecosystems that have been subjected to an intensified regime of environmental stress associated with pollution, climate, disturbance, or other factors. Based on information in Odum, 1985; Schindler, 1990; Freedman, 1995, 2010; and Freedman et al., 2014.
• There is a general decrease in net primary production
• The ratio of production:respiration (P:R) becomes unbalanced, generally with P:R becoming <1 so that accumulated biomass decreases
• The net export of biomass increases, such as by erosion
• In aquatic systems, exogenous sources of fixed energy (external to the ecosystem) become relatively more important than endogenous ones (internal)
• The rate of nutrient turnover increases—this is recycling of organically bound forms to inorganic ones that are available for uptake by autotrophs or for leaching out of the system
• Nutrient losses from the ecosystem increase—the system becomes “leaky” of its accumulated nutrient capital
• If the stress is due to biophilic chemicals that bioconcentrate in organisms and biomagnify in species higher in the food web (as occurs with chlorinated hydrocarbons and methylmercury), there may be a cumulative increase in residues of those substances
• The proportion of ruderals and r-strategists (which are better adapted to recently disturbed habitats) increases, while that of competitors and K-strategists (more-stable habitats) decreases
• The size of dominant organisms decreases
• The life spans of organisms or their parts (such as leaf longevity) decrease
• Food chains become shorter because of reduced energy availability to higher trophic levels or greater sensitivity of predators to an intensified stressor regime—a consequence is that top predators may be lost from the ecosystem
• Species richness and diversity become diminished and dominance increases; at the community level there is decreased redundancy of functional attributes, but if the original diversity was low because of dominance by competitors, the reverse may occur
• There is a general biotic impoverishment by the extirpation of sensitive species and increased dominance by relatively few tolerant ones
• The prominence of alien species may increase
• If there are genetically based differences in tolerance within a population, then natural selection may result in evolutionary change and the development of ecotypes that are more tolerant of the stressor
General System-Level Trends
• The ecosystem becomes more open, with inputs and outputs becoming more prominent as internal storage and cycling are reduced
• Successional trends may reverse, with a reversion to earlier stages
• The efficiency of resource use declines
• There is a decrease in positive symbioses such as mutualisms, and an increase in negative ones such as disease and parasitism
• Functional properties (such as community-level metabolism) may be more resistant to intensified stress than are species composition and other structural properties
If an intensified regime of stressors becomes stabilized at that level, then the longer-term ecological change will reflect the kinds of responses noted in this section. Broadly speaking, compared with the original communities, the affected ecosystem will be simpler in its structure and functioning, will sustain less biodiversity, will be dominated by relatively small species and few predators, and will have lower rates of productivity and slower decomposition and nutrient cycling. In the worst cases of an extremely intensified stressor regime, no biota can survive.
Changes such as these may be caused by a natural change in environmental stressors, such as a wildfire ignited by lightning, a flow of lava, or a persistent accumulation of snow or another glacial phenomenon. However, the intensification of environmental stressors may have an anthropogenic causation, such as bioresource harvesting or industrial pollution. In either case, the ecological responses may be interpreted as representing some degree of degradation of the quality and natural condition of the ecosystem (or of ecological integrity; see the Ecological Integrity section) and of its ability to provide natural resources for use by the human economy.
Judgements on the quality of ecological change—whether it is “good” or “bad”—are an important subject area in applied ecology. In general, changes are more likely to be considered as damaging if they are caused by an intensified stressor regime that has resulted from human influence, such as by an industrial activity. If the stressors are natural in origin, then the ecological responses may not be viewed as damaging but rather as representing a value-neutral “change.” These considerations are relevant to the intersection of ecology with environmental planning and impact assessment and are a key aspect of the relevance of the applied knowledge of ecologists.
Of course, the intensity of environmental stress may also relax (or decrease) over time. When this happens, the ecological responses are in many respects the reverse of those observed when stress intensifies. In essence, these changes represent a process of ecological recovery after the relaxation of stress. Depending on circumstances, however, a succession may not recover the original community; resilience is the degree to which that happens.
For instance, various acts of legislation enacted since the early 1970s have prevented the further manufacturing and use of organochlorine insecticides and polychlorinated biphenyls (PCBs) in North America. These regulatory actions have allowed a substantial recovery to occur in the populations of various species of predatory birds that had previously been detrimentally affected by those toxic and hormonally active chemicals, such as the bald eagle (Hlaiaeetus leucocephalus), peregrine falcon (Falco peregrinus), brown pelican (Pelecanus occidentalis), and double-crested cormorant (Phalacrocorax auritus). Similarly, the cessation of commercial hunting of gray whales (Eschrichtius robustus) has allowed their greatly depleted population on the Pacific coast to recover to approximately their preexploitation abundance.
A system-level example involves older metal smelters, which once were prodigious emitters of toxic gases and metals but have since substantially reduced their emissions (or have closed operations). The resulting improvements in environmental conditions, which represent greatly mitigated regimes of toxic stressors, have allowed for a substantial degree of ecological recovery in surrounding ecosystems, as has been well documented near Sudbury (Gunn et al., 1995; Keller et al., 2007).
These and many additional cases show that prudent actions to reduce the intensity of anthropogenic stressors can result in great improvements in ecological conditions. However, there is always some degree of lingering damage, and in some cases the degradation is so severe that it may take centuries for a substantial recovery to occur.
Environmental stressors cause a variety of ecological changes, some of which may be regarded as damage to the natural condition of the ecosystem. Several concepts are being used for the purpose of indicating the quality of environmental changes, including those in ecological conditions. Three commonly used concepts are environmental quality, ecological integrity, and ecosystem health:
• Environmental quality (EQ) is a broad concept that is related to the intensity of stressor regimes, particularly anthropogenic ones such as toxic chemicals and disturbances, as well as their effects on human health, economic values, biodiversity, and ecological conditions.
• Ecological integrity (EI) is related to the degree of naturalness of an ecosystem. EI is a component of environmental quality, but it has a focus on changes that are occurring in ecosystems, rather than effects on people or their economy. In general, a high level of EI represents a low degree of system-level damage caused by anthropogenic stressors to populations and communities (additional interpretations are examined later in this section). Areas of wilderness have a high level of ecological integrity, in the sense of being minimally affected by anthropogenic stressors and being characterized by native species, self-organized communities, and ecoscapes that are characteristic for the natural environmental regimes suitable to the ecoregion.
• Ecosystem health does not differ substantially from ecological integrity, although it typically has more of a focus on the functional attributes of ecosystems.
These concepts all have been used to develop indicators, which are relatively simple measurements that are intended to represent complex aspects of environmental quality or ecological integrity. Environmental indicators should be sensitive to variations in the intensity of stressors, and changes in them may be an early warning of much-greater impending damage. Well-chosen indicators are important components of any program of environmental monitoring.
Any indicator of ecological integrity must be associated with factors that are related to the structure and functioning of ecosystems. Obviously, anthropogenic stressors pose a risk of causing damage to ecological integrity (Figure 6). However, the specific changes are complex because stressors may cause damage to some species, communities, and ecological functions while at the same time enhancing others. Nevertheless, it is reasonable to suggest that higher values for any of the following characteristics would imply greater ecological integrity (note that these components of EI are derived from indicators of ecological responses to intensified stress, as depicted in Table 1):
• resistance and resilience in the ecosystem
• complexity in ecological structure and function, including the biodiversity that is supported
• presence of large species and top carnivores
• controlled nutrient cycling—the system is not “leaking” its accumulated nutrient capital
• low concentrations of anthropogenic chemicals that are biophilic and biomagnifying
• dominance of communities by native species rather than alien ones
• the degree to which the ecosystem is subjected to anthropogenic influences, as opposed to natural environmental factors being the primary controls of structure and function
• management is not needed to maintain attributes that are considered desirable; for example, to maintain the habitat of a species at risk or an endangered kind of community
Ecosystems that are chronically subjected to intense but natural stress, such as desert or tundra, eventually develop stable conditions of structure and function that reflect the limitations imposed by their environmental regime. Typically, those terrestrial ecosystems are simple in structure and function, are low in species richness, and are dominated by long-lived plants that are short in stature. These highly stressed ecosystems also have low rates of functional properties such as productivity, decomposition, and nutrient cycling. Nevertheless, in situations where environmental stressors are natural in origin, there are no implications for ecological integrity.
Consider, for example, the effects of severe climate at high latitude in the Arctic. In that difficult environment, ecosystem development is restricted to tundra, which supports relatively few species of short stature but great longevity, with low productivity and slow nutrient cycling. Nevertheless, that tundra represents the greatest degree of ecosystem development that is possible under the particular regime of naturally occurring environmental conditions. As such, the high-Arctic tundra would not be viewed as having a lower level of EI than a tropical rainforest, even though the relatively benign environmental conditions of the latter sustain much more biodiversity, enormously higher productivity and biomass, and faster decomposition and nutrient cycling (this is not to say that species of tropical rainforest do not experience environmental stress—rather, the environmental regime allows the ecosystem as a whole to sustain relatively high levels of biodiversity and productivity, vigorous nutrient cycling, and other attributes that are much more diminished in ecosystems that occur in more-difficult environmental circumstances).
In contrast, exposure of those or any other natural ecosystems to intense anthropogenic stressors, perhaps associated with disturbances or pollution caused by industrial activities, would result in changes that degrade ecological integrity. Such effects may include diminished structural or functional complexity, reduced numbers of large species or top predators, and increased prominence of alien species. Because these kinds of anthropogenic changes represent damage, they degrade ecological integrity and environmental quality.
In Canada, the conservation of ecological integrity has achieved a degree of legal standing because it is a key aspect of the legislated mandate for the management of national parks. According to the Canada National Parks Act (Parks Canada, 2000; section 8.2), the “maintenance or restoration of ecological integrity, through the protection of natural resources and natural processes, shall be the first priority . . . when considering all aspects of the management of parks.” In this context, Parks Canada defines EI as follows: “Ecological integrity means, with respect to a park, a condition that is determined to be characteristic of its natural region and likely to persist, including abiotic components and the composition and abundance of native species and biological communities, rates of change, and supporting processes.” Therefore, a national park would be judged to have a higher level of ecological integrity if it supports native species and functional attributes that are expected in its natural ecoregion, if its populations of plants and animals are likely to survive into the future, and if the patch dynamics of communities on the ecoscape continue to be dependent on natural influences, such as wildfire, windstorms, and biotic interactions.
Monitoring and Research
Threats to the ecological integrity (EI) of protected areas and “working” ecoscapes are particularly severe in regions where the human footprint is most intense—where the human population is largest and economic infrastructure is most developed. This includes cities, agricultural regions, and areas where forestry and other resource-extractive industries are dominant in land use. While all these are essential aspects of the modern human economy, their associated suites of environmental stressors are great threats to natural values and other aspects of environmental quality.
To deal with these problems, a number of integrated actions must be implemented by responsible authorities to mitigate the resulting stressor regimes (Figure 7). These actions include regulating the kinds of activities that people and companies are allowed to undertake, while ensuring that society is provided with suitable advice by ecologists and other specialists, and increasing the environmental literacy of citizens so they will be more likely to support measures that conserve EI and other aspects of environmental quality. Within the realm of ecology, a key aspect is the need for programs of monitoring and research that contribute toward the ecological dimensions of improved sustainability both for the human economy and the natural world.
In general, the intent of monitoring and research programs is to detect or predict stressors that are threats to environmental quality, and to find ways to avoid or repair them to the degree that may be possible. Ecologists have an important role to play in these programs because many of the indicators being examined are ecological in character and are related to EI, such as populations of native species, the extent of certain habitats or communities, productivity and carbon storage, pollutant residues in biota and elsewhere in the environment, and so on.
However, the objectives of monitoring programs can vary enormously in scale and intent. Most are relatively focused, smaller-scale programs that are associated with the need to manage certain protected areas or to ensure that particular industrial facilities are complying with environmental regulations. Other programs are intended to examine changes that may be occurring over large regions or entire countries, and even globally. In either case, the resulting data and knowledge are used to guide decision making in government and industry, to assist the work of environmental nongovernmental organizations (ENGOs), and to provide materials for environmental education.
Environmental monitoring involves repeated measurements of variables that are related either to the abiotic (inorganic) environment or to the structure and functioning of ecosystems. Of course, ecosystems are extremely complex and not everything can be monitored. Therefore, monitoring programs require that limited numbers of representative indicators be sensibly chosen for sampling. In addition, research is integral to monitoring programs because it investigates questions that are relevant to the possible causes and consequences of environmental changes, or to the structure and functioning of ecosystems.
Environmental indicators are relatively simple measurements that are intended to represent complex aspects of environmental quality (EQ) or ecological integrity (EI). Indicators are often sensitive to changes in the intensity of stressors (see Figure 4). Two examples of ecological indicators are chemical residues in organisms and changes in the abundance of species known to be sensitive to certain stressors, such as those affected by acidification or by clear-cutting.
Residues of chemicals in the tissues of top predators are often used as an indicator of the degree of contamination of their ecosystem, especially for persistent substances that biomagnify in carnivores, such as organochlorines (e.g., DDT, PCBs, dioxins, and furans). Various studies of this sort have shown that the residues of these chemicals in predators have declined substantially since their manufacturing and use were curtailed in the 1970s (Figure 8).
Changes in the abundance of species are often monitored, because widespread declines are worrisome and should be dealt with by discovering the causes and implementing appropriate conservation actions. For instance, the population status of the grizzly bear (Ursus arctos) is often considered an indicator of the EI of the various communities and extensive landscapes that they depend on. Other indicator species are spotted owl (Strix occidentalis) in western old-growth forest, salmon in rivers, and orca (Orcinus orca) in coastal marine ecosystems.
Multimetric indicators have also been developed for use in ecological studies, particularly for freshwater ecosystems. They are useful because they provide insight into the effects of environmental stressors on ecological responses, including those relevant to EI. The best-known indicators are Indices of Biotic Integrity (IBI), which were initially developed by James Karr (1981) in studies of fish communities. Subsequent applications involve composite indices that integrate metrics of species richness, the relative abundance of functional groups, trophic structure, and sometimes the incidence of developmental abnormalities associated with pollution. This multimetric approach has since been extended to studies of other groups of organisms, including aquatic invertebrates, birds, and plants, as well as habitat types such as prairie, forest, and wetlands (LaPaix et al., 2009).
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