Dynamics of sympatric Paratylenchus nanus and Paratrichodorus minor populations in soil under pasture

Populations of both Paratrichodorus minor and Paratylenchus nanus in a grazed pasture in Waikato, New Zealand were assessed in single 2.5 cm diam. core samples over six successive seasons. Neither seasonal periodicity nor a consistent vertical distribution pattern was observed in the P.minor population. P.nanus had distinct spring and summer population maxima and was more abundant at 0-10 than 10-20 cm depth. P.minor but not P.nanus abundance was significantly correlated with rainfall and an activity index combining rainfall and soil temperature for the week before sampling. Significant correlations among P.minor, P.nanus and concentrations of some soil nutrients seemed to reflect soil nutrient interactions with other biotic factors such as plant growth. There was no evidence for competition between these two nematodes. Host root resource, spatial aggregation and differences in morphology that indicate niche differences are discussed as possible reasons for the observed lack of competition.