Pseudo replication for integrated population models

State-space models are widely used in ecology. However it is well known that in practice it can be difficult to separate process and observation variances. We consider this issue for integrated population modelling. To some extent the mechanism of such models protects against this problem, but it can still arise, and two illustrations are provided, in each of which the measurement variance is estimated as zero. In the context of an extended case study involving data on British Grey herons we consider alternative approaches for dealing with the problem when it occurs. In particular we consider penalised likelihood and introduce a method of pseudo replication. We recommend the use of pseudo replication, which is undertaken via a simple bootstrap procedure. For the case study of the paper it is shown that when it occurs, an estimate of zero measurement variance may not be important for inference relating to the model parameters of primary interest. The wider implications of the work are discussed.