Species coexistence in a food web with intraguild predation

We studied species coexistence in a protist community consisting of a bacterial resource, five protist consumers, and an intraguild predator, Blepharisma americanum. Spatial heterogeneity was manipulated by constructing spatial arrays of interlinked bottles. In the heterogeneous treatment species were permitted to disperse naturally between bottles whereas in the homogenous treatment half the water was removed from the bottles every second day and redistributed throughout the array. Contrary to expectations, the final number of species able to coexist was lower in the spatially heterogeneous treatment. In the spatially heterogeneous treatment, global extinctions were observed for Paramecium caudatum, P. multimicronucleatum, and P. bursaria, and frequent local extinctions were observed for Colpidium sp. In the spatially homogeneous treatment, Colpidium and P. bursaria were the only species to become globally extinct. Paramecium aurelia and B. americanum, the intraguild predator, persisted under both treatment conditions. There were no differences between treatments in community density, community variability in density or population variability in density. The population dynamics of the intraguild predator, B. americanum, P. bursaria, P. multimicronucleatum, and P. caudatum did not differ between treatments. In contrast, the population dynamics of P. aurelia differed strongly between treatments. In the spatially heterogeneous treatment, P. aurelia was apparently able to escape predation through either direct escape behavior or by being unavailable as a prey. In the spatially heterogeneous treatment the high population growth rate of P. aurelia may have resulted in the competitive exclusion of the other consumer species, whereas in the spatially homogenous treatment, intraguild predation by B. americanum on P. aurelia appeared to reduce competition between P. aurelia and the other consumer species, facilitating coexistence.