MICROBIOLOGY OF AQUATIC SYSTEMS Prey-Dependent Mortality Rate: A Critical Parameter in Microbial Models Ewan J. A. Minter & Andy Fenton & Jennifer Cooper & David J. S. Montagnes Received: 3 December 2010 / Accepted: 18 February 2011 # Springer Science+Business Media, LLC 2011 Abstract Protozoa are key components of a wide range of ecosystems, but ecological models that incorporate these microbes often suffer from poor parameterisation, specifically of top-level predator loss rates. We (1) suggest that top-level predator mortality is prey-dependent, (2) provide a novel approach to assess this response, and (3) illustrate the ecological relevance of these findings. Ciliates, Paramecium caudatum (prey) and Didinium nasutum (predator), were used to evaluate predator mortality at varying prey levels. To assess mortality, multiple (>100) predators were individually examined (in 2-ml wells), daily (for 3 days), between 0 and 120 preys ml -1 . Data were used to determine non-linear mortality and growth responses over a range of prey abundances. The responses, plus literature data were then used to parameterise a predator–prey model, based on the Rosenzweig–MacArthur structure. The model assessed the impact of variable and three levels of constant (high, average and low) mortality rates on P. caudatum–D. nasutum population dynamics. Our method to determine variable mortality rate revealed a strong concave decline in mortality with increasing prey abundance. The model indicated: (1) high- and low-constant mortality rates yielded dynamics that deviate substantially from those obtained from a variable rate; (2) average mortality rate superficially produced dynamics similar to the variable rate, but there were differences in the period of predator–prey cycles, and the lowest abundance of prey and predators (by ~2 orders of magnitude). The differences between incorporating variable and constant mortality rate indicate that including a variable rate could substantially improve microbial-based ecological models. Introduction Protozoa are key components of a wide range of natural and artificial ecosystems (e.g. food webs in lakes and oceans, sewage processing systems (e.g. [2, 4, 18])). Ecological models, including those that incorporate protists, often suffer from poor parameterisation of the loss of top-level predators (e.g. [9]). Typically such models assume constant, density-independent predator mortality rates. The only attempt to incorporate more complex mortality functions, to our knowledge, come from ocean ecosystem models, most often in the form of nutrient-phytoplankton- zooplankton models [6, 19]. In these works, the authors use intraspecific density-dependent mortality functions for the consumer (i.e. consumer mortality increases with consumer density), often mimicking higher predation by trophic levels not explicitly modelled. Fulton et al. [9] consider the ramifications of this and conclude that imposing, in their case, a quadratic mortality function improved realism of the predicted dynamics in the absence of explicitly modelled top-predators. However, even these models ignore a fundamental aspect of predator mortality, starvation at low prey densities. Here, we provide an alternative form of variable top-level predator mortality and prey-dependent mortality and illustrate how it may be important in terms of microbial, and general, ecology. Equations based, directly or indirectly, on the Rosenzweig– MacArthur predator–prey model are fundamental in microbial and general population ecology. This model comprises a pair of coupled differential equations (Eqs. 1 and 2), describing E. J. A. Minter : A. Fenton : J. Cooper : D. J. S. Montagnes (*) Institute of Integrative Biology, Biosciences Building, University of Liverpool, Liverpool L69 7ZB, UK e-mail: dmontag@liv.ac.uk Microb Ecol DOI 10.1007/s00248-011-9836-5