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 counterintuitive 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. [ABSTRACT FROM AUTHOR]