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Saving energy is the most important goal in a sensor network. But shortsighted optimization for energy can lead to sensor networks that can not fulfill their task. Hence, this goal must be balanced with task related goals. One such task related goal is to transmit messages in a sufficiently reliable way. For instance for a monitoring sensor network this means that the messages that arrive at the sink node allow a good overview of the monitored area, while at the same time no energy resources are wasted. In this paper we define a "good overview" as the "informational value" that arrives at the sink node and adapt the reliability of the link layer such that the overall system efficiency is maximized. The system efficiency is defined as the informational value arriving at the sink put into relation with the energy spent by the network to get it there. Our major result is that there exists a rule that describes how to adapt the reliability of the link layer, which can be evaluated by each node using only locally available information. When the reliability of the link layer is adapted according to this rule, the system efficiency can be increased (sometimes by more than 20%) compared to the best performing nonadaptive link layer.