A threshold model for heron productivity

We demonstrate the potential of conditionally Gaussian state-space models in integrated

population modeling, when certain model parameters may be functions of previous

observations. The approach is applied to a heron census, and the data are best

described by a model with three population-size thresholds which determine the population

productivity. The model provides an explanation of how the population rebounds

rapidly after major falls in size, which are characteristic of the data. By contrast, a

simple logarithmic regression of productivity on population size was not significant.

The results are of ecological interest, and suggest hypotheses for further investigation.