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A major current challenge related to invasions is the absence of a comprehensive theoretical basis for preventing the introduction of alien species and controlling their populations. By using a macroecological framework, we aim to examine patterns of symbiotic species diversity across native and invasive hosts to understand the mechanisms underlying the host–parasite system stability. The symbiotic communities of seven gammarid species, two native and five invasive, were analyzed at 16 fresh and brackish water sites along the Baltic coast of Poland. We investigated the influence of four factors—locality, habitat, host species, and host origin—on diversity patterns and assessed their relative contribution to diversity variation. Our results indicate that all factors are crucial in determining the composition and abundance of symbiotic communities in gammarids. Among these factors, locality and host species apparently have a greater influence on symbiotic communities than habitat conditions and host origin. Comparative analysis of diversity indexes of symbiotic organisms from native and invasive hosts showed that the richer communities of native gammarids were paradoxically less diverse. We assume that these rich and uneven symbiotic communities keep the host–parasite system of native gammarids in equilibrium. Highly dominant symbiotic species with a high load per host individual may stabilize the growth of the host population. Symbiotic communities of the invasive host exhibited lower species richness but displayed a relatively even distribution of species with moderate loads per individual, resulting in a scarcity of heavily infected gammarids within the population. Thus, the survival rate of invasive gammarids is expected to be relatively high, facilitating their population growth and further spread. We conclude that complex communities of 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. [ABSTRACT FROM AUTHOR]