The impact of Ponto Caspian invaders on the parasite communities of multiple hosts within West and Central European river systems

Abstract

Biological invasion has become one of the great threats to ecosystems worldwide. Frequently, invasion is caused by anthropogenic activities either intentionally due to an introduction of free-living organisms for agricultural or recreational purposes as well as unintentionally for instance if species are transported unperceived in cargo containers from one ecosystems to another. However, only a small amount of these species will be able to establish itself after entering a new habitat. As soon as a species is successful they might become invasive and cause negative effects to local biocoenosis that might lead to loss of biodiversity or eventually initiate a meltdown of the entire system. Biological invasion is a global problem, which is exacerbated inter alia by globalization, (wrong) environmental management and climate change. In that context Europe have become one of the hotspot for invasion where various Ponto-Caspian species that are considered very successful invaders established many self-sustaining communities. Ponto-Caspian (PC) species such as the zebra mussel, killer shrimp (Dikerogammarus villosus), round-, bighead-, and monkey goby usually adapt easily to environmental conditions and frequently become dominant species in newly invaded habitats. Additionally various PC species seem to favor the distribution of pathogens and parasites. However, these relatively new discovery are scarcely studied, which represents a general problem in the research of invasion biology since the effects of invasive species on parasites are frequently neglected. The ignorance towards parasites is surprising because they are considered as important response variable for ecosystems health. Generally, parasites contribute to 40 % of the biomass of an ecosystem, govern host behavior, and potentially alter the structure, dynamics and function of food webs.Therefore, the effects of invasion by the example of PC invasive species on local parasite communities were at the focus of the present doctoral thesis. Hence, their impact on parasite prevalence and infestation intensities within various hosts of habitats that were free of or invaded by PC species were compared and evaluated. Additionally, possible interactions of co-introduced PC parasites with local parasites and the consequences for local species were investigated. Thus, within a period of four years three local and one PC fish hosts as well as two different amphipod species were sampled in various rivers (in Germany, the Netherlands, Switzerland) and parasitological examined. The results were compared with each other and subsequently placed into context. The results of various substudies showed that PC species affect the parasite communities of recently invaded habitats on many layers. In this context, the acanthocephalan Pomphorhynchus laevis and the nematode Raphidascaris acus have been identified as key species since an increase in their prevalence in local hosts seem to be related to PC invasion. Furthermore, the interaction between P. laevis and other PC species might have caused the replacement of the local acanthocephalan P. tereticollis in invaded habitats and eventually directly encouraged interactions between P. laevis and other local parasites. The results of the present doctoral thesis have shown that effects of PC invasive species on parasites are multilayered and therefore suggest that not only PC species but invasive species in general might affect local parasite communities. Hence, parasites have to be integrated into proposed or existing conservation efforts, since invasive species might cause an increase of local parasites and/or a loss of parasite diversity, both with grave consequences for local environments.