Interactions among endoparasitic root-feeding nematodes; consequences for nematodes and host plant

Plants are influenced by above- and belowground herbivores and their interactions. Root feeders, among which are nematodes, generally occur in multi-species communities. The aim of this study was, to determine whether interspecific interactions among endoparasitic root-feeding nematodes ( Heterodera arenaria , Meloidogyne maritima and Pratylenchus penetrans ) would influence nematode abundance and dynamics and subsequently biomass of the shared host plant Ammophila arenaria .In a field experiment, different combinations of the endoparasitic nematodes and other soil organisms were added to A. arenaria . Subsequently, the feedback of the established soil communities was tested in bioassays. Soil from non-buried plants inoculated with a mixture of organisms from the root zone of A. arenaria reduced the biomass of newly planted seedlings. In contrast, a combination of the three endoparasitic nematodes did not affect plant biomass differently from the control. When in the field experiment M. maritima had been added to plants alone, juveniles and males of this species occurred in the new root layer earlier in the year and in higher densities than when H. arenaria and P. penetrans had been added as well. Addition of the other two species apparently forced M. maritima to develop under suboptimal conditions. Addition of the other endoparasite species did not affect H. arenaria and P. penetrans . Greenhouse experiments aimed at studying interactions between different pairs of the endoparasite species did not explain observations from the field experiment.Interestingly, in the field experiment M. maritima reduced plant biomass more when added alone than when added together with the other two endoparasites, whereas addition of root zone soil reduced plant biomass most. Heterodera arenaria and P. penetrans did not affect plant biomass. These results support the view that effects of species identity and diversity may be intermingled and that species traits rather than diversity determines the effect. The ectoparasitic nematode Tylenchorhynchus ventralis spontaneously colonised the field experiment in high densities, but the numbers declined when the experiment proceeded. In a greenhouse experiment, numbers of T. ventralis were reduced by adding an unnaturally high density of endoparasites. Thus, the observed decline in spontaneously established ectoparasite numbers was unlikely to be caused by the endoparasites, but by other (micro-) organisms that spontaneously colonised the soil. Despite their limiting effect, the endoparasites could not prevent T. ventralis from reducing plant biomass.In conclusion, it seems likely that in the field, abundance and dynamics of M. maritima are determined by interspecific competition with H. arenaria and P. penetrans . The latter two endoparasitic nematodes are less influenced by the presence of the other endoparasites than is M. maritima . Interestingly, M. maritima , when added alone, reduced plant biomass, whereas a combination of the three endoparasites did not. Addition of root zone soil of A. arenaria reduced plant biomass most, so that the negative soil feedback is likely to be caused by the whole soil community rather than by endoparasitic nematodes alone