7.2 Stress Tolerance Overview
KEY CONCEPTS
By the end of this section, you will be able to do the following:
- Define stress tolerance
- Explain why stress tolerance varies among species
- Evaluate the differences between acclimation and acclimatization, and how they affect stress tolerance
- Apply your understanding of these concepts to the stresses described in future chapters
As previously discussed, many environments can be stressful. Indeed, many environments are inherently variable, constantly challenging the homeostasis of organisms and their cells. For example, temperature fluctuates on both a daily and seasonal basis. Water availability in terrestrial (land) environments also changes over time with alternation between rainy and dry days and seasons. Estuaries (where rivers meet oceans) exhibit daily fluctuations in salinity as the tide comes in and out. Some environments are not particularly variable, but always challenging – for example ocean water near the poles is always cold. Because most environments are stressful in some way, most organisms need some kind of stress tolerance.
Definitions of Stress Tolerance
Just as there are different ways to think about stress, there are different ways to think about stress tolerance. In its simplest form, stress tolerance is the ability to recover normal function following exposure to a stressor. We can think about this in the same context as in Chapter 7.1, in which a stress-tolerant organism is able to recover homeostasis during or after exposure to a stressor via an effective stress response (Figure 7.5A). However, there are other ways an organism can tolerate stressful conditions, especially when we consider that stress can cause damage to cells – a more extreme situation than temporary deviations from homeostasis. If an organism is damaged by exposure to a stressor, that organism could exhibit stress tolerance by being able to repair that damage and eventually restore homeostasis (Figure 7.5B). Alternatively, an organism might be stress-tolerant by pre-emptively decreasing/preventing deviations from homeostasis and damage (Figure 7.5C) or by altering its homeostatic state. For example, mammals and birds that hibernate can survive winter partly because they intentionally drop their body temperature during hibernation (alter their homeostatic state), along with multiple other adjustments that prevent damage from occurring while they are cold.
Variation in Stress Tolerance Among Species
While many organisms exhibit some level of stress tolerance, that stress tolerance varies among species. In general, the extent to which a particular species can tolerate stress depends on the environments they are adapted to. Organisms that are adapted to living in freshwater (e.g., lakes and ponds) don’t experience high concentrations of salt often, and are unlikely to have the appropriate adaptations to tolerate high concentrations of salt. Organisms that live in marine environments (oceans, seas, 3% salt) ar