Types of Responses

From a strictly ecological standpoint, organisms are usually not exposed to one stressor at a time but rather multiple stressors; hence, it is very difficult to decipher which stressor is producing which response. Generally, the resulting effect (response) from multiple stressors follows one of four patterns: potentiation, additive, antagonistic, and synergistic (see Figure 1). Pattern of responses among interacting environmental stressors could also be studied using the toxic units (TU) concept. TU is defined as the concentration of a chemical in a toxic

Synergistic/potentiation

Degree of response

Synergistic/potentiation

Degree of response

Mixture of stressors

Figure 1 Main types of responses from exposure to mixture of stressors.

Mixture of stressors

Figure 1 Main types of responses from exposure to mixture of stressors.

Synergistic

Additive

Antagonistic

Figure 2 Combined effect of quantifiable mixture of stressors expressed as toxic units (TU). Both A and B are independent stressors acting as a composite mixture on an entity. The x-axis represents the quantity of the composite mixture and the y-axis represents the toxic units. Reproduced by permission of Taylor & Francis.

mixture divided by its individual toxic concentration for the endpoint measured (e.g., its individual 48 h lethal concentration 50 (LC50) value.). Mixtures with summed TU values close to 1.0 are considered to be additive in toxicity (Figure 2). Those with summed TU <1.0 are less than additive in toxicity (antagonistic) and those with summed TU> 1.0 are greater than additive in toxicity (synergistic).

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