Organisms have evolved a variety of defence strategies against environmental threats, like predation or diseases. Defences are expressed on the level of behaviour, morphology, life history and/or physiology. Furthermore, they can be constitutive or inducible. Commonly, not just one defensive trait is expressed at a time, but rather a complex interplay of several traits constitutes the total defence of the organism. Studies investigating the effect of different stressors (e.g., toxins, or temperature) on defensive traits often simply take isolated traits and derive unsatisfactory conclusions about the overall effect on food-web processes from their experiments. This deficiency, however, is not easily solvable. To get a comprehensive picture in multi-stressor studies on how an organism suffers from stressors, it is necessary to investigate effects on the total defence instead of isolated traits. Therefore, the relative contribution of each trait should be identified. For this, elaborative experiments are required. Furthermore, often, not all traits can be assessed in one organism, due to the experimental design (e.g., the organism has to be dead for trait assessment). We developed the defence index framework on the example of predator-prey interactions. A set of simple equations and algorithms that unite all recorded defensive traits of an organism into one parameter. This parameter reflects the total defence of that organism against its respective predator or stressor. It improves and simplifies the comparison of results across experiments and species. Furthermore, the framework allows to qualitatively assess the mal-/ adaptivity of single traits without performing time-consuming experiments. We show the quality of predictions and applicability of this framework with simulations, with an extensive feeding experiment and with literature data. The defence index simplifies studies of adaptive traits to any stressor, and improves our knowledge of this complex research field.