Ecological communities are often exposed to different kind of disturbances of varied magnitude. The response to disturbances can be affected by, among other factors, the strength of interactions between species within the community. The main purpose of this thesis is the study of the relation between interaction strength (distribution and positioning of strong and weak links) and the response of model communities to disturbances. Additionally, we wish to identify keystone species and keystone links and to recognize species vulnerable to secondary extinctions with respect to the interaction strength concept.

The first part of this study (paper I) deals with species removal from model ecological communities. The number of secondary extinctions following the loss of one species measures the response to this disturbance. The loss of the following species categories triggered, on average, the largest number of secondary extinctions: a) rare resources strongly interacting with many consumers b) abundant intermediate consumer species strongly interacting with many resources and c) abundant weakly interacting resources. Species vulnerable to secondary extinctions were mainly weakly interacting consumers and strongly interacting resources (excluding primary producers). This vulnerability to secondary extinctions was not only dependent on characteristics of the species themselves but also on which species was initially removed. Some species were facing a high probability of extinction after the deletion of a wide range of species categories, while others were vulnerable only to the deletion of few species categories.

The second part (paper II) deals with less severe disturbances; small changes in the intrinsic growth rate of species and small changes in the interaction strength of links. In this way the importance of both species and links was revealed. The response of the community was measured as the effect of these changes on the resilience of the system. Overall, the resilience of the model communities was more sensitive to changes in weakly interacting species or weak links.

The main message of this work is that keystone species and keystone links could be context dependent, both with respect to characteristics of the community and the stability criterion used. Studying the effects of severe disturbances, such as species loss, and using the number of secondary extinctions to measure the community's response, the disturbance of strongly interacting species affected the community most. On the contrary, by applying small perturbations to community parameters and measuring the response as the effect on resilience, the perturbation of weakly interacting species and weak links affected the community most. Thus, both strong and weak agents of the community can be important for stability properties under different regimes of disturbance.