Competitive displacement of the native species Bursaphelenchus mucronatus by an alien species Bursaphelenchus xylophilus (Nematoda: Aphelenchida: Aphelenchoididae): a case of successful invasion

The presence of alien invasive species has serious negative impact on endemic biodiversity, especially on native species that occupy the same niche in the ecosystem. To study the influence of the alien invasive species Bursaphelenchus xylophilus on its native sister species B. mucronatus, the two nematode species were mix-cultured in a fungal mat and mix-inoculated into a susceptible host. By comparing the propagation parameters of both species under competitive and noncompetitive conditions it was shown that the propagation level of B. xylophilus was clearly higher than that of B. mucronatus under laboratory culture. Furthermore, the propagation capacity of B. xylophilus under competitive conditions was much higher than that under noncompetitive conditions, both in laboratory culture and with host inoculation. Bursaphelenchus xylophilus also excluded B. mucronatus when the two species were cultured as a mixture for a longer time. The relative abundance ratios of the two species in natural pinewoods were also determined by random sampling of dying pine trees from regions with different invasion histories. It was noted that with an increase in invasion years the distribution frequency of B. xylophilus increased while that of B. mucronatus decreased. Experimental tests verified our hypothesis that because of its high fecundity and strong competitive ability, the invasive species B. xylophilus out-competed the native species B. mucronatus and displaced it in natural ecosystems. The successful invasion of B. xylophilus is attributed to competitive displacement, which may be one of the ecological invasive mechanisms.