History and complexity in tick-host dynamics: discrepancies between ‘real’ and ‘visible’ tick populations

BACKGROUND: Numerical responses of ticks to changes in densities of their hosts can be complex and apparently unpredictable. Manipulations even of deterministic models can produce counter-intuitive results, including tick populations that either rise or fall under increasing host densities, depending on initial conditions. METHODS: In this paper I use an established simulation model to demonstrate a wide range of numerical responses to different scenarios of host changes, and to examine the basic mechanisms that drive them. RESULTS: The rate and direction of change of host densities affects the extent to which questing tick numbers reflect those of their hosts. Numerical responses differ profoundly between dynamic tick-host systems and those allowed to reach equilibrium. CONCLUSIONS: The key to understanding tick-host dynamics is to understand the difference between ‘real’ and ‘visible’ tick populations. An appreciation of the implications of this difference – and of the conditions that influence it - will benefit the effective interpretation of field data.