Exploitative competition between two seed parasites on the common sedge, Carex nigra

We present results from a two-year experiment studying the competitive interaction between the gall mite Phytoptus caricis and the smut fungus Anthracoidea heterospora . A factorial addition/removal experiment showed that a negative interaction occurs between A. heterospora and P. caricis , presumably due to competitive interactions in the early infection process of their shared resource, developing utricles on the common sedge, C. nigra . Strong effects of interspecific competition for both species were only evident in 1995 while A. heterospora showed some signs of suffering negative effects from competition also in 1994. Results thus suggest that significant effects of competitive interaction may only be apparent when densities of the two competitors are fairly high. The interaction was strongly asymmetric, with A. heterospora being more affected by the presence of P. caricis than vice versa. Estimates of interaction strength for A. heterospora were about twice as large compared to that for P. caricis . The difference in interaction was significant when data were pooled over the two years of the study. The study highlights the need to consider interspecific interactions between different plant parasites when studying the effects of plant pathogens.