Britannica identifies the invasive species as follows: “Invasive species, also called introduced species, alien species, or exotic species, any nonnative species that significantly modifies or disrupts the ecosystems it colonizes.” The way a species arrives in a new environment can be done in numerous ways; either by natural migration or by activities of other species. Humans do it directly through pet trade or global commerce or indirectly by unintentional “hitchhiking” invaders in ships, planes, trucks, shipping containers or packing materials. Another reason for such a phenomenon is that a species can no longer adapt to the environmental conditions of their habitat. Therefore the invasive species may have more opportunities in an alternative future climate than they have at present.
Recently in Greece there have been many headlines about a species that swarmed the Korinthian Gulf- the tropical scyphozoan Rhopilema nomadica, a kind of Jellyfish. This particular species is listed as one of the “100 worst invading species” in the Alien Invasive Species Inventories for Europe (DAISIE 2009).
There are several reasons that an invasive species can flourish. But before blaming it for destroying our weekend by the beach, let’s get to know it better. Rhopilema nomadica known in english as Nomad Jellyfish, is native in East Africa and Red Sea but its known introduced range is Egypt, Israel, Lebanon, Turkey and Greece. Its natural predators are fish, like tuna, mackerel and marine turtles; apparently,overfishing these natural predators helps the species in question thrive. Spawning occurs between June and August and the sexually reproducing swimming scyphomedusae appears when the water temperature exceeds 24º C. It entered the Mediterranean through the Suez Canal in the late 70s and it first appeared in Greece in Lakonikos Gulf in 2006 (Siokou-Frangou et al. 2006). Nonetheless, several other factors could help or hinder the rise of Nomad jellyfishes. Temperature is directly correlated with the population of this particular species and wind traffic at sea affects them. They are offshore and with the help of currents produced by the wind gather up at beaches.
Cost is another important factor that should be considered regarding alien species. When the costs have become apparent, they can be vast; one study in the United States in 2001 estimated that costs associated with alien species amount to some US$136 billion per year and Canada in 2004, estimated that a preliminary review of alien species pegged them conservatively at $13,3 millions to $34,5 millions, annually, for 16 invasive species alone (Joanne Laucious, 2017). These costs must usually be covered by someone other than those who sponsored or promoted the introduction of the species, usually the general public. Decision-makers need to invest more in assessing the potential impacts before allowing potential introductions as well as incorporate more biosecurity measures once the species has been introduced (Jeff McNeely, 2001).
Yet, the nomad jellyfish is a minor incidence of invasion. The most memorable examples of invasive species are the kudzu plant from Japan, the cane toad from America, the asian tiger mosquito, zebra mussels from southern Russia, rats etc. People have always been on the move, carrying other species with them. Australian aborigines brought in the dingo, Polynesians sailed with pigs, and the Asians who first peopled the Americas brought dogs with them (Jeff McNeely, 2001). In the very end,the worst invasive species known to human, is human.
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