Metapopulation dynamics of a persisting predator–prey system in the laboratory: time series analysis

The scarcity of experimental evidence for the persistence of predator-prey systems at the metapopulation level inspired us to developa simple predator-prey experiment that could be used for testingseveral theoretical predictions concerning persistence and its causes.The experimental system used consisted of one or several islands withsmall bean plants, the phytophagous mite Tetranychus urticaeand the predatory mite Phytoseiulus persimilis. In the firstexperiment, one large system was used consisting of 90 small bean plants,prey and predators. The system persisted for only 120 days. Second, asystem was used consisting of eight islands with ten plants each wherethe islands were connected by bridges. Two replicate experiments showedpersistence for at least 393 days. The difference between the first andthe second experiments suggests that the longer persistence is causedby a limited migration between the eight islands. Despite efforts to startboth replicates of the second experiment with similar initial conditions,the dynamics of both replicates varied substantially. In one replicatethe prey and predator numbers showed a trend through time, whereas thenumbers fluctuated around a fixed value in the other replicate. A timeseries analysis of the data of the prey and predators showed the presenceof periodicity with a lag of 8.5 weeks in one replicate, whereas such cyclicbehaviour was not found in the other replicate. The differences betweenthe two replicates suggest that it is difficult to perform experimentswhere one replicate is perturbed and the other serves as an undisturbedcontrol. We suggest using a longer time series, where a system is disturbedonly during the second half of the experiment. The data from the firstand second halves can subsequently be used to estimate the effect of theperturbation. The advantages and disadvantages of this method are discussed.