A stressor can be any event, influence, or environmental condition that causes stress to an individual or population. Stressors are related to the concept of allostasis in that they place added physiological and psychological demands on its functions. In biology, this is often called a “stressor”. Stressors affect an organism’s homeostatic balance.
An example of a common stressor among humans is being stuck in traffic while late for work. This will elicit the “fight-or-flight response” causing the heart rate to increase and energy levels to decrease, but if a person were given a drug that counteracts these effects they would not be considered stressed.
This is the process through which individuals, populations, or species adapt to potential or actual environmental stressors. Any response that returns an individual or population to its normal condition from a perturbed state is considered an adaptation. If the stressor is more frequent and/or sustained than the ability of an organism to adapt it may result in decompensation, a state of being less adapted than before. This may result in either a temporary or permanent reduction of fitness within the population depending on whether the individual can recover.
Differential reproduction among individuals
This is a process through which changes in environmental conditions or some other stressor results in differential reproduction among individuals which increases their frequency relative to other members
of a population. An example of this is when organisms live in areas with food shortages where they can reproduce much more because other individuals within the population are dying from starvation.
The ability of an individual or species to resist becoming stressed, often called “stress resistance.” Stress resistance is dependent upon both biological factors such as age and immune function and external factors such as social support and coping style.
This is the process through which an organism returns to homeostasis after experiencing a stressor, often called “stress recovery.” This includes post-stress functioning and behavior that continues until the individual returns to its normal condition. An example of this is when someone cuts their hand and it heals. Stress recovery is often divided into three distinct phases: “alarm,” “resistance” and “exhaustion.” These three stages vary among different organisms, different stressors, and the same individual experiencing different stressors.
This process refers to a change in environmental conditions or some other stressor that results in reduced reproduction. This may occur through various mechanisms including reduced fecundity or heightened levels of natural mortality. If an organism cannot adapt rapidly enough to new conditions it may simply die off.
Threats are very important for determining fitness
All organisms are constantly exposed to stressors that threaten their survival, so their ability to cope with these threats is very important for determining fitness. When the environment changes rapidly the phenotype of a species may need to change rapidly to survive, so individuals who are better able to cope with stressful conditions will be more likely to reproduce and pass on this ability. Therefore organisms evolved mechanisms for coping with stress such as the fight-or-flight response which prepares them to either escape or engage a stressor. This increased preparation comes at a cost, however, because it can cause exhaustion which decreases fitness.