Table of Contents
Living organisms are constantly influenced by environmental conditions such as temperature, water availability, light, oxygen, and the presence of salts or toxins. In this chapter, the focus is on how strongly and in what range organisms can tolerate such factors. Two key concepts describe this: tolerance range (or tolerance curve) and ecological potency.
Environmental Factor and Performance
For any single abiotic environmental factor (for example temperature, pH, or salt concentration), the performance of an organism (growth, survival, reproduction, movement, etc.) depends on how close the factor lies to the organism’s “preferred” conditions.
If you plot performance (on the vertical axis) against the value of one environmental factor (on the horizontal axis), you typically get a curve with a characteristic bell-like shape. This is the tolerance curve for that factor.
Important terms on such a curve:
- Minimum:
The lowest value of the environmental factor at which survival is just still possible (below this value, the organism dies). - Maximum:
The highest value at which survival is still possible (above this value, the organism dies). - Tolerance range (tolerance breadth):
The interval between minimum and maximum. It describes the full range of values of that factor in which the organism can survive at all. - Optimum:
The value of the environmental factor at which the organism’s performance is best (for example maximum growth or reproduction rate). - Pessimum range:
Zones close to the minimum and maximum where survival is still possible, but performance (especially reproduction) is very poor. Populations cannot usually grow or remain stable in the long term under pessimum conditions. - Preferendum:
The range around the optimum where performance is high and individuals typically prefer to stay if they can move (for example, fish choosing a particular water temperature).
A species can have different tolerance ranges for different factors. For instance, a plant may tolerate a wide range of temperatures but only a narrow range of soil salinity.
Law of Limiting Factors
Even if most environmental conditions are near the optimum, a single factor outside a suitable range can limit the organism’s performance. This idea is often summarized by the law of limiting factors:
- The environmental factor that is furthest away from its optimum (and closest to or beyond the tolerance limits) limits survival, growth, or reproduction.
- Improving other factors (which are already within the preferendum) will not fully compensate for this one limiting factor.
Examples:
- A plant may have ideal light and temperature but still grow poorly because soil nitrogen is too low.
- A fish species may have enough food but cannot survive in a warm pond because the oxygen content of the water becomes too low at high temperature.
The limiting factor can change in space and time. For the same species, water might be limiting in summer and temperature limiting in winter.
Specialist and Generalist: Eury- and Steno- Types
Species differ in how broad or narrow their tolerance ranges are for particular factors. This is often expressed with the prefixes:
- “Eury-” (Greek eurys = wide):
Describes species with a broad tolerance range for a particular factor. - “Steno-” (Greek stenos = narrow):
Describes species with a narrow tolerance range for a particular factor.
These prefixes are combined with the name of the factor, for example:
- Eurythermic:
Species with a broad tolerance to temperature.
Example: Many species of carp can live in relatively cold and relatively warm waters. - Stenothermic:
Species with a narrow temperature tolerance.
Example: Certain cold-water fish need consistently low temperatures and die quickly in warm water. - Euryhaline:
Species that tolerate a wide range of salinities (freshwater to brackish to marine).
Example: Some shore crabs can live in both brackish estuaries and full seawater. - Stenohaline:
Species that tolerate only a narrow range of salinity.
Example: Many marine invertebrates can survive only in near-constant sea salt concentrations. - Eurytopic:
Species that can live under a wide variety of environmental conditions overall (generalists). - Stenotopic:
Species that are restricted to a narrow “niche” of conditions (specialists).
Generalists (eurytopic species) often cope better with environmental changes and can colonize many habitats. Specialists (stenotopic species) may be very efficient and competitive in their narrow range of conditions but are more vulnerable to change.
Ecological Potency: Tolerance Under Natural Conditions
The tolerance range is determined under idealized conditions (for example in the laboratory) where only one environmental factor is varied while all others are kept optimal and no competitors or predators are present.
In nature, however, many factors act simultaneously, and there are interactions with other organisms (competition, predation, symbiosis). The effective “usable” tolerance range under these realistic conditions is called the species’ ecological potency.
Ecological potency describes:
- How well a species can assert itself (survive, reproduce, and maintain populations) under natural conditions,
- where both abiotic factors (temperature, light, salinity, pH, etc.) and biotic factors (competition, predation, disease, mutualists) are present.
Two aspects are important:
- Fundamental vs. realized possibilities
- The fundamental range of conditions in which survival would be possible (based on physiology alone) may be quite broad.
- The realized range that the species actually occupies in nature can be narrower because of competition, predation, or lack of dispersal.
Ecological potency refers to this realized, “in practice” ability to live and reproduce under natural conditions. - Ecological potency is specific to each factor and context
A species may have high ecological potency with respect to temperature (tolerates and succeeds in many temperature regimes) but low potency with respect to salinity or pH.
Species with high ecological potency can maintain stable populations under a wide variety of natural conditions. Such species are often successful colonizers and tend to occur over large geographic ranges.
Species with low ecological potency require rather specific, stable environmental conditions and often have restricted distributions. Many rare or endangered species fall into this category because human-caused environmental changes easily push them outside their viable conditions.
Ecological Valence and Ecological Indicator Species
Closely related to ecological potency is the idea of ecological valence: how strongly the presence of a species is tied to certain environmental conditions.
- Species with narrow ecological valence (stenotopic, low ecological potency) often occur only where particular conditions prevail (for example only in very acidic bogs, or only in very clean water).
- Because their presence or absence reflects specific environmental factors, these species can act as indicator species. For instance:
- Certain mayfly larvae live only in oxygen-rich, unpolluted streams. Their presence indicates very good water quality.
- Specific lichen species only grow where air pollution (e.g. sulfur dioxide) is very low, indicating clean air.
In contrast, eurytopic species with broad ecological valence and high ecological potency are less informative as indicators, because they can tolerate many different conditions.
Ecological Potency and Species Distribution
Differences in tolerance range and ecological potency help explain:
- Geographical distribution
Why some species occur worldwide (cosmopolitan species) while others are restricted to certain regions (endemic species). - Vertical distribution
For example, in a mountain, as temperature and oxygen availability change with altitude, only species with sufficient ecological potency for these gradients can occur across many altitude zones. - Habitat preferences and zonation
In lakes, streams, oceans, and soils, the distribution of species along gradients of light, depth, salinity, or nutrients reflects their tolerance and ecological potency.
Environmental change (such as climate change, pollution, or habitat alteration) shifts the ranges of environmental factors. Species with:
- Broad tolerance and high ecological potency can often adapt their distribution or behavior, making them more resilient.
- Narrow tolerance and low ecological potency can quickly be pushed beyond their viable conditions and may decline or go extinct.
Summary of Key Concepts
- The tolerance range for an environmental factor extends from minimum to maximum values where survival is possible, with an optimum and pessimum regions.
- The law of limiting factors states that the factor furthest from its optimum limits performance.
- Eury- vs. steno- species differ in the breadth of their tolerance: generalists vs. specialists.
- Ecological potency describes how well a species can survive and reproduce under realistic, natural conditions where multiple factors and interactions with other organisms act together.
- Species with narrow ecological valence and low ecological potency often serve as ecological indicator species, reflecting particular environmental conditions.