The recent 50th commemoration of man’s first successful jaunt to the moon is a fitting backdrop against which to discuss alien species.
In reality, alien species are not a figment of our imagination but are very real and ominous for native ecosystems worldwide. In fact, the spread and establishment of alien species (known alternatively by a variety of synonyms, including non-indigenous, non-native and exotic species) is nowadays considered to be the second most insidious cause of biodiversity loss worldwide, following on the heels of habitat destruction.
Alien species are, by definition, those that have been introduced, via human activity, to a new geographical area outside their native range. The Maltese islands are no stranger to the impacts of terrestrial non-indigenous species (NIS), with some of the most common and invasive flowering plants on the islands (e.g. Cape Sorrel, hottentot/kaffir fig) being native of the Cape region of South Africa, which is endowed with a similar Mediterranean climate to ours.
Similarly, freshwater habitats of the islands have in recent years been invaded by a multitude of crayfish species, which mainly hail from the southeastern US and Australia.
Given the greater degree of connectivity and the lower degree of geographical barriers to dispersal in the marine domain when compared to the terrestrial one, it follows that marine NIS are currently hogging the limelight. Professional and sports fishermen, anglers, scuba divers, snorkelers and every rank and file of sea buff have witnessed the rapid shift in the species composition of some of our flagship marine communities, with the new species making a greater stand than ever before.
It is a small (generally taken as 10 per cent, as a rule of thumb) subset of the aliens that catch the eye. In fact, most alien species keep a low profile, managing only to establish modest populations in the invaded area and not exerting a major impact on indigenous communities.
Invasive alien species (IAS) are those that merit our attention, given their unremitting potential to spread within and to replace indigenous communities, thus ushering in a welter of undesirable impacts. IAS normally exhibit aggressive behaviour, a high fecundity (ability to reproduce), few (if any) predators and grazers among the indigenous community and a high tolerance to fluctuating environmental conditions.
Besides the mooted ecological impacts, IAS can trigger economic impacts, the scale of which can be of leviathan proportions. The nomadic jellyfish, for instance, native of the Persian Gulf and first recorded from the Mediterranean in the 1970s, regularly shuts down electrical power stations in Israel given its propensity to clog cooling water intake pipes. Beaches are closed off when the infestation is severe, given the stinging nature of this large jellyfish species.
Impacts of marine IAS on the fishing industry as well as on public health are also well documented, with the silver-cheeked toadfish (the fugu puffer fish, already caught a number of times in Maltese waters) voraciously feeding on indigenous fish species of commercial importance and tearing nets, besides being a hazard to public consumption by virtue of its high TTX content.
A species of rabbitfish – the dusky spinefoot – has earned the dubious silver spoon of being the first such species to be given a Maltese monicker – qawsalla (rainbow) – by virtue of its variable body colouration. The herbivorous species of fish, native of the Red Sea, is in direct competition with our own native herbivorous fish species – in particular the salema – and some mathematical models are predicting that the ‘newcomer’ will outclass the native in the years to come.
Unremitting potential to spread within and to replace indigenous communities, thus ushering in a welter of undesirable impacts
Other marine NIS currently running riot in local waters include species of green and red algae, as well as of seagrass, all of which opportunistically invade barren patches of Posidonia seagrass meadows after they have been hit by anchors. The blue cornet fish and reticulated leatherjack are two additional well-established fish NIS in our waters, while invertebrate NIS include the spotted and the ragged sea hare, blue swimmer crab, the oriental mussel and the Red Sea mantis shrimp.
So which human activities are responsible for helping alien species to hopscotch from one marine sector to another?
The blue swimmer crab, native of the Persian Gulf and Indian Ocean, is an edible species that is currently running riot in shallow Maltese waters.Shipping is probably the number one culprit, with the Mediterranean’s status as a global shipping hotspot translating into a global hotspot for marine NIS. The Suez Canal, which was opened in 1869 and enlarged on three occasions (most recently a few years ago), is the world’s largest in terms of shipping volume and does not feature any locks, unlike the Panama and the Kiel canals, thus rendering the installation of any mitigation measures, such as a ‘salinity barrier’, virtually impossible.
Species entering the Mediterranean through the Suez Canal are dubbed ‘Lessepsian species’, in honour of Ferdinand de Lesseps, a French diplomat and engineer who was seminal for the canal’s opening, while tropical Atlantic species entering the Mediterranean through the Straits of Gibraltar are technically not branded as alien species but rather as ‘range-expanding species’, given that no human activity is directly involved in their spread.
Besides the passive entry of marine NIS of Indo-Pacific and Erythrean (Red Sea) origin, the attachment of such species to the hulls of vessels (the so-called ‘fouling’ of vessels) as well as the entry of larval forms within vessel ballast tanks, are two additional introduction pathways for marine NIS which are linked indirectly to the shipping vector.
No wonder the International Maritime Organisation has ushered in the Ballast Water Management Convention. This places onerous responsibilities on shipping companies through, for example, the installation of on-board ballast water treatment systems, as well as on port authorities in a bid to stymie the further influx of such species through ballast water.
The deliberate release of marine NIS, especially those which are popular in tropical aquaria (e.g. different species of damselfish and angelfish) or which are reared in mariculture (e.g. Pacific oyster) is yet another introduction pathway, as is aquaculture (through live bait or as a contaminant on nets).
The scale of the marine NIS phenomenon is truly daunting – over 1,000 such species have been recorded to date from the Mediterranean. The European Commission has formulated a list of 49 terrestrial and freshwater priority IAS, for which no trade throughout the continent is permissible, in a bid to stave off further spread of these invasive species. A similar list for marine IAS is still lacking.
Given the disparate nature of the marine alien invasion phenomenon, citizen science is an essential tool in allowing researchers to keep tabs on the scale of the problem, while simultaneously involving the wider public and popularising the science through a participatory approach. Marine alien species-spotting citizen science campaigns have proliferated around the globe.
Most of these campaigns use technology like websites, social media and smart phone apps. Two such local campaigns are the Spot the Alien Fish and Spot the Alien campaigns (www.aliensmalta.eu), featuring a total of 36 fish and 24 algal and invertebrate alien species, respectively, on user-friendly, easy-to-interpret posters. They are coordinated by the University of Malta’s Department of Geosciences and supported financially by the Environment and Resources Authority and the International Ocean Institute.
ERA last year launched a national strategy for the mitigation and control of IAS, including eight complementary codes of good practice for different stakeholders. The strategy has now been approved and is available online at era.org.mt.