Human mediated species introductions are often irreversible,
represent a world-wide problem and are considered one of the threats to marine
biodiversity. Invasions can have an impact on the native species and
communities, but invasions also affect the demography, dynamics, and genetic
composition of the invading species population. Thus, newly introduced species
provide a unique opportunity to study different ecological and genetic
processes associated with invasions, such as invasion dynamics, build-up of the
spatial and temporal population genetic structure, understanding the roles of
added or removed functions in an ecosystem and the direct impacts of the
invaders.

In my thesis, I studied population demography, dynamics, and the
genetics of a newly introduced mud crab, Rhithropanopeus
harrisii, in the Archipelago Sea in the northern Baltic
Sea. In addition, I studied the impacts of this novel invader to the native
species and tested whether new molecular tools could be used to detect and
monitor this newly introduced species.

The results show that R.
harrisii has established a relatively stable
reproductive population in the invaded area, has spread along the coast, and
can be found in several habitats that range from soft sediments to the
macroalga Fucus vesiculosus having negative impacts on the abundance, richness, and
diversity of its prey species. The abundance of the introduced population in
the Archipelago Sea is lower than in its native range of North America, although
the individuals grow substantially larger in the northern Baltic Sea increasing
the reproductive output. The overall abundance of R.
harrisii appeared to follow the boom and bust
pattern with a rapid initial abundance increase and subsequent decline. The
population dynamics of R. harrisii was mainly driven by changes in the survival of the reproductive
females. In addition, environmental variables, such as temperature, was related
to the recruitment and growth of juveniles, thus indicating that increasing
temperature caused by climate change could potentially increase juvenile
recruitment in the future. There was no significant spatial genetic divergence
within Finland, which probably reflects the short invasion history of the
species in the area. However, the presence of a significant temporal variation
between the cohorts (juvenile vs. adult) indicated a lack of temporal stability
in this species. Finally, a molecular environmental DNA (eDNA) approach can be
used to detect R. harrisii from water samples although the detection rate was fairly low.
Therefore, more studies are needed to optimize the eDNA method and to evaluate
its usability in monitoring.

In conclusion, R. harrisii has established a population in the northern Baltic Sea having a
negative impact on the native species and communities. However, despite the
initial population size increase, the population has declined to the similar size
where it was at the beginning of the six-year monitoring. Although the
population size seems to be stabilized at the monitoring sites, R. harrisii continues
to expand its distribution range, and the rapid initial population increase
together with the negative impacts, is likely occurring at the newly invaded
sites. The results of my thesis provide new information about the population
ecology and genetics of a newly introduced functionally novel species that can
be used for further research and management purposes in the future.