MANETs are highly vulnerable to attacks due to their characteristics such as the lack of infrastructure and wireless communication. Considerable improvements have been made towards providing ad hoc network security and existent solutions apply cryptography, intrusion detection systems or reputation systems. However, these conventional defense lines are inefficient to put all attacks and intrusions off. In this paper, we suggest the cooperation among preventive, reactive and tolerant defense mechanisms as an approach to reach the survivability of essential services. We highlight survivability requirements for MANETs, and we quantify the survivability of AODV and AOMDV protocols in the presence of blackhole and selective forwarding attacks.