Symbiotic species diversity can explain invasion success and stability of the host-parasite system: the case of native and invasive gammarids (Crustacea, Amphipoda)

The major current challenge related to alien species is the absence of a comprehensive theoretical basis to explain the resistance from bioinvasions. One of the factors conferring resistance to invasion is the native species diversity of a recipient ecosystem. We aim to examine patterns of species diversity to understand the mechanisms underlying host-parasite relationships in alien species. The dataset represents 20 host-locality samples, including over 15,067,600 symbiotic organisms counted from 612 host individuals belonging to 7 gammarid species, two native and five invasive, sampled at 16 localities in the Baltic region of Poland. We asked whether diversity patterns depended on locality, habitat and host species, and which factors are deterministic in the variation of diversity. Our results suggest that both locality and host phylogeny are critical factors in determining the composition and abundance of symbiotic communities in gammarids. Out of these two factors, locality appears to be more determinant of the symbiotic community of gammarids than host species, whereas habitat condition was a stronger determinant of the symbiotic communities than geographic distance. The rich and uneven symbiotic communities of native gammarids keep the host-parasite system in equilibrium. Sharply dominated symbiotic species with a high load per host individual stabilize the host population. Simplified symbiotic communities of the invasive hosts showed a relatively even distribution of species with moderate loads per individual, with heavily infected gammarids being rare in the population. The survival rate of invasive gammarids is expected to be relatively high, facilitating their population growth and further spread. Complex communities of the native gammarids constructed from many symbiotic species may prevent host populations from undergoing explosive growth, while such mechanisms may be completely or partially reduced in invasive hosts.