Environmental Limitations Physiological Response to Stress and Tolerance

Environmental characteristics such as temperature, light, water, and nutrient availability widely vary in natural environments. The variation in environment is accompanied by modifications in organism growth and survivorship. The response to variation of a certain environmental factor is described by idealized physiological response curve that characterizes organism reaction in the absence of other environmental limitations and competition from other species. Organism growth and survivorship are maximal at optimum value of the environmental factor or resource, and decrease below (suboptimal) and above (supraoptimal) values (Figure 1a). Commonly, the physiological optimum is not sharply expressed, but organism performance can be close to a maximum value over a relatively broad range of environmental factors or resources. Therefore, an arbitrary optimum range, within which the organism's performance is close to the maximum value, is often defined (Figure 1a).

Organism growth rates or productivity are generally used as performance indices because they are relatively easy to measure. However, environmental factors commonly alter a series of relevant organism characteristics, and the survival in specific environments does not necessarily rely only on growth and productivity. For instance, in plants, temperature influences growth, maturation, fruit ripening, and seed germination. While a given plant may grow appreciably in a given temperature environment, it will not be able to survive if no viable seeds are formed because of too low or high temperatures or due to limited length of growing season. The concept of physiological response function has also been transferred to population biology, where survivorship or population density can be used as appropriate measures ofpopulation performance.

Broad concepts of ecology - stress and tolerance - rely on physiological response curves. Stress is a reduction of organism performance due to sub- or supraoptimal environmental conditions. Organisms experience stress when the specific environmental factor deviates from the optimum range, whereas the severity of stress increases with increasing the deviation from the optimum range. Tolerance characterizes organism's response to stress and is the organism's ability to grow and survive in conditions of limited or excess of the environmental resource. The 'law of tolerance' formulated in 1913 by Victor E. Shelford states that tolerance to both deficiency and excess determines the total resource or environmental factor range, that is, tolerance range, in which the organism can survive (Figure 1a).

When the concept of tolerance is transferred to population biology, it is important to recognize that survivorship of individuals and population density decreases with increasing environmental stress. In population biology, tolerance range is the environmental range over which population density or survivorship of a given species is above zero.

Figure 1 Physiological response of an organism to an environmental factor is generally described by a bell-shaped symmetric curve with an optimum (a). Traditionally, the response function was a relative performance indicator of representative individuals of given species, but the concept of physiological optimum has also been extended to biological populations. As the physiological optimum can be relatively broad, an arbitrary optimum range within which the organism performance deviates little from the physiological optimum is often defined. Organisms experience stress below and above the optimum range. Organism's ability to survive at the resource availability below the optimum range is determined by the tolerance to resource deficiency, while the ability to survive at supraoptimal resource availabilities depends on the tolerance to resource excess. A given species is not able to survive below and above the resource levels determined by the lower and upper tolerance limits of resource availability. The shape of the relative performance versus resource availability curve is often asymmetric, implying differential tolerance of limiting and excess resource availabilities (b). An example of negatively skewed response curve (dotted line) is plant tolerance of low and high temperature extremes, while positively skewed response function (dashed line) is observed for plant tolerance of nutrient deficiency and excess (see also Figure 2).

Figure 1 Physiological response of an organism to an environmental factor is generally described by a bell-shaped symmetric curve with an optimum (a). Traditionally, the response function was a relative performance indicator of representative individuals of given species, but the concept of physiological optimum has also been extended to biological populations. As the physiological optimum can be relatively broad, an arbitrary optimum range within which the organism performance deviates little from the physiological optimum is often defined. Organisms experience stress below and above the optimum range. Organism's ability to survive at the resource availability below the optimum range is determined by the tolerance to resource deficiency, while the ability to survive at supraoptimal resource availabilities depends on the tolerance to resource excess. A given species is not able to survive below and above the resource levels determined by the lower and upper tolerance limits of resource availability. The shape of the relative performance versus resource availability curve is often asymmetric, implying differential tolerance of limiting and excess resource availabilities (b). An example of negatively skewed response curve (dotted line) is plant tolerance of low and high temperature extremes, while positively skewed response function (dashed line) is observed for plant tolerance of nutrient deficiency and excess (see also Figure 2).

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