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4 O.E. TUREYEN

1994 [6]. Caulerpa taxifolia, because of its rapid spread and high growth rate, it can easily be
the dominant in the habitat and when Mediterranean Sea was invaded by it, the killer alga was a
serious problem for the Posidonia oceanica which is extremely important for marine ecosystem.
It invades the areas of these sea grasses and causes the extinction of them. It releases a kind of
acid which is toxic for many species. It does not have any harm to human health but it alters the
habitats and reduces the biodiversity [7].

At any given moment some 10,000 different species are being transported between
biogeographic regions in ballast tanks alone [8]. If invasive species finds a new environment
welcoming and become established, eradication is nearly impossible [9]. However negative
impacts can be controlled by early detection, prevention of new introductions and effective
management methods [10]. On the other hand, dealing with invasive species requires to
establish priorities such as the most harmful organisms, the most likely pathways and the most
dangerous routes. With prioritization, limited sources and funds can be distributed effectively
and the most effective invasive species management plan can be chosen [11].

Negative effects of invasive species can not be generalized even for a single specie because the
effects of a single invasive species can be different for different biogeographic regions. For one
ecosystem the invasive species can be very harmful by leading native species extinction
however for another ecosystem it can have no harm to native species at all. Therefore ecological
impact of any invader needs to be studied with local approaches. Generalization of negative
effects of any single invader is misleading.

There are some studies focusing on global investigation of invasive species with a global
approach and assume that the invaders have the same negative impacts for any ecoregion where
they are not belong[12-14] Those studies give really important ideas about how dangerous the
invasive species can be, however most of the invaders have different effects on different areas.
For example, Mnemiopsis leidyi was the cause of the anchovy extinction in early 1990s in Black
Sea and it was the reason of huge economic collapse of fishing industry. The main reason for
those situations was that Mnemiopsis leidyi was the predator of anchovy and there was no
predator of the Mnemiopsis leidyi in the environment so their population reached high amounts
while anchovy population decreased dramatically. Obviously Mnemiopsis leidyi is a massive
problem for Black Sea and new introductions of that species needs to be prevented. On the other
hand, when Mnemiopsis leidyi is introduced into a new ecoregion which has Beroe ovata as
native species, there will be considerably less ecological impact on that ecoregion. Beroe ovata
keeps Mnemiopsis leidyi population under control by preying on that species. That example
indicates that investigations of the negative effects of invasive species are not only related with
the characteristics of invaders but also related with the environmental conditions. While
determining a risk index value or ecological impact value, the environmental conditions and
characteristics of species should be considered together. Therefore, the risk index value for
every invasive species will be special and different for each different ecoregions.

In this study the aim is to investigate invasive species in Turkish coasts and to determine an
ecological impact factor for each species by concerning their negative effects on native species
and natural ecosystems in national scale. According to general risk definition, risk is related
with frequency or possibility and the significance of the consequence. Ecological impact factor
of invasive species is used as consequence factor for risk assessment and total ballast water
volume carried by ships from the natural environment of invasive species to Turkish coasts is
used as frequency factor. This risk ranking method developed in this study can be easily used

GiDB|DERGi Sayı 5, 2016