Effects of rapid prey evolution on predator-prey cycles

We study the qualitative properties of population cycles in a predator-prey system where genetic variability allows contemporary rapid evolution of the prey. Previous numerical studies have found that prey evolution in response to changing predation risk can have major quantitative and qualitative effects on predator-prey cycles, including: (i) large increases in cycle period, (ii) changes in phase relations (so that predator and prey are cycling exactly out of phase, rather than the classical quarter-period phase lag), and (iii) "cryptic" cycles in which total prey density remains nearly constant while predator density and prey traits cycle. Here we focus on a chemostat model motivated by our experimental system [Fussmann et al. 2000,Yoshida et al. 2003] with algae (prey) and rotifers (predators), in which the prey exhibit rapid evolution in their level of defense against predation. We show that the effects of rapid prey evolution are robust and general, and furthermore that they occur in a specific but biologically relevant region of parameter space: when traits that greatly reduce predation risk are relatively cheap (in terms of reductions in other fitness components), when there is coexistence between the two prey types and the predator, and when the interaction between predators and undefended prey alone would produce cycles. Because defense has been shown to be inexpensive, even cost-free, in a number of systems [Andersson and Levin 1999, Gagneux et al. 2006,Yoshida et al. 2004], our discoveries may well be reproduced in other model systems, and in nature. Finally, some of our key results are extended to a general model in which functional forms for the predation rate and prey birth rate are not specified.Comment: 35 pages, 8 figure

Modelling nematode life cycles using dynamic energy budgets.

1. To understand the life cycle of an organism, it is important to understand the underlying physiological mechanisms of their life histories. We here use the theory of dynamic energy budgets (DEB) to investigate the close relationships between growth, reproduction and respiration in nematodes. 2. Using a set of simplified equations based on DEB theory, we are able to accurately describe life-cycle data from the literature for the free-living bacterivorous nematodes Caenorhabditis elegans, C. briggsae and Acrobeloides nanus, under different temperature or food regimes. 3. Nematodes apparently differ from other animals, as the initial growth is slower than expected. We explain this phenomenon by food limitation in the larvae, which is supported by more detailed physiological studies. 4. Food density and temperature are shown to have predictable effects on the growth curves (temperature affects only growth rate, whereas food density also affects ultimate size), although the reproduction patterns reveal some deviations from model predictions. 5. The presented model integrates the different aspects of the life cycle into a single framework, and can be applied as such to interpret the effects of various stressor