  It is a challenge to predict the response of a large, complex system to a
perturbation. Recent attempts to predict the behaviour of food webs have
revealed that the effort needed to understand a system grows quickly with its
complexity, because increasingly precise information on the elements of the
system is required. Here, we show that not all elements of the system need to
be measured equally well. This suggests that a more efficient allocation of
effort to understand a complex systems is possible. We develop an iterative
technique for determining an efficient measurement strategy. Finally, in our
assessment of model food webs, we find that it is most important to precisely
measure the mortality and predation rates of long-lived, generalist, top
predators. Prioritizing the study of such species will make it easier to
understand the response of complex food webs to perturbations.
