The Mosquito

Mosquito, <i>Ochlerotatus (Aedes) nigripes</i>

 

A commonly repeated story is that the mosquito arrived on Svalbard in 1918 with water barrels with phosphate miners from Tromsø. While this mosquito is indeed found on mainland Norway this story is unlikely to be true. This species was recorded by Holmgren during a scientific expedition to Spitsbergen in 1868 and was experienced in large numbers at Brucebyen by Summerhayes and Elton during their expedition in 1921. Indeed, it would be more surprising if the mosquito was not here prior to 1918 since it is a large flying insect already present on Greenland and mainland Norway. The distances involved are not great for such an insect and given correct wind directions could easily be blown here. See the story of the Diamond Backed Moth.

Where do they come from?

<em>Plutella xylostella</em>, Diamond back mothKittiwakes on nest, Ossiansarsfjellet.

The invertebrate fauna of Svalbard is relatively young. There is no evidence that invertebrates survived the last glacial maximum in situ. The islands were covered by ice and where there was exposed rock the conditions were probably too extreme for invertebrates such as insects and mites to survive. Even amongst animals with such as amazingly tolerance of extreme conditions as tardigrades there is no evidence that they persisted on Svalbard throughout the ice age. It is therefore likely that all these animals (and plants) have arrived after the retreat of the ice, that is during the last 10,000 years. So where did they come from? For the plants there is growing evidence that source populations were either Siberia or Greenland. (See svalbardflora.net).

However, little work has been done on the invertebrates. By comparing species assemblages, that is what species are present in an area it is possible to draw some conclusions. Many seem to have spread eastwards, surviving the last ice age in refugia in Beringia (see article) before spreading eastwards after the ice began to melt. These conclusions are based on the fact that many species of one type of invertebrate are found North America, a subset of these in Greenland and a subset of these in Svalbard.  Species lists for Russia, including Siberia, are difficult to compare with Svalbard due to the problem of synonyms, that is one species given two or more names. Until better knowledge is available and synonyms clarified it is often difficult to compare directly European Arctic and Siberian invertebrate species lists.

So an obvious question is how did this extensive fauna colonise Svalbard? How does an animal less than one millimetre long travel the 700 km to Svalbard? There are several possible routes by which these species could make the journey. One likely route is to fly or be blown by the wind. Since most invertebrates are small infrequent immigration is difficult to observe directly. However on an island with low growing vegetation and only one common species of resident moth the arrival of new species of butterfly are readily spotted. A good example is the diamond back moth, Plutella xylostella (Picture Albert Vliegenthart).

The species overwinters in southern Europe but each spring begins a migration north following the start of spring. Since this insect is an important crop pest its progress north is monitored by various government agricultural agencies providing an accurate record of the moths migration. On a periodic basis the moth is seen in large numbers on Svalbard. The last such recorded event being in July 2000. The moth was seen in large numbers on the 20th July but within a week the population was dead due to the low temperatures in lack of suitable food plants. Back-tracking using the meteorological records shows that the migrating moths arrived at the north coast of Scandinavia just as the strong south easterly winds set in (picture Flexitrac). Using these winds the moth took approximately 48 hours to travel from the mainland to Svalbard. Recently the presence of mites and Collembola usually associated with the soil has been shown on seabirds. It appears these animals may live for considerable time under the feathers of the birds and so be transported large distances. Another possible route is via the ocean currents. Many species have been shown to be able to survive for many days or weeks on the sea surface. It is not unusual for terrestrial invertebrates to be swept down rivers and into the sea. Invertebrates entering the sea in mainland Norway could potentially be swept by ocean currents to Svalbard. Other studies have shown that many species common on Svalbard can survive four years at below -20oC frozen in ice opening the possibilities that species could travel on driftwood moving from the Siberian rivers with the arctic drift ice.

Wind tracks for three days prior to June 20th, 2000. Note the air moving up from Northern Norway (red line). http://tarantula.nilu.no/trajectories/index.cfm

Now there is a new vector. Man. Insect pests travel throughout the world with man and Svalbard is no exception. For example a small beetle, the mercant grain beetle, is found in dwellings in Longyearbyen. There is also a story that the mosquito Aedes nigripes was imported to Kapp Thorsden with phosphate miners in 1918. However, this story seems to be disappointingly incorrect. As has been reported in Svalbardposten other species do sporadically arrive courtesy of man, for example ladybirds. Nonetheless, some species do seem to have arrived with man. In manure augmented soils around Barentsburg, 26% of the invertebrate species are considered to have originated in southern European Russia and have been imported to Svalbard with the soils intended for use in the greenhouse (click here for paper). Other species have also arrived thanks to man, hitch-hiking with us but most of these are domestic pests, for example cockroaches, flour beetles and flour moths. Very few arriving with man have established in the natural environment. The only good example is the tapeworm parasite of the sibling vole. Occasionally other invertebrates are seen in Butikken, insects associated with the transport of fresh vegetables such as ladybirds. However, very few, if any, stand much of chance of establishing here. The conditions are far too extreme.

Some extra info:

Tardigrade species cluster analysis

Tardigrade species assemblages.

In 1998 Phil Pugh and Sandra MacInnes, both from the British Antarctic Survey in Cambridge, UK, published an analysis of tardigrade species assemblages around the Arctic. Tardigrades are small arthropods (often around 0.5mm in length) best known for their remarkable ability to desiccate and in this anhydrobiotic state survive many years Tardigrades have even been taken up on the Space Shuttle and have recently been shown to survive 10 days exposed to the vacuum of space). Cluster analysis of the species, that is, a statistical procedure grouping different areas based on whether or not their species are similar, indicates two main regions with very different species, the Arctic (northern hemisphere) and the Antarctic. Within the Northern Hemisphere region the Alps and Northern European species groups are clearly different from those in North American and Russia (Novaya Zemlya and Taimyr Peninsula) but Svalbard, Iceland and Greenland all form sub-groups of the North American fauna. The conclusion is that the Tardigrade fauna of Svalbard is composed of species that survived the last ice age in North America and which were spread eastwards with the prevailing winds after the retreat of the ice.

Island Hopping / Land bridges.

Left hand figure: Species dispersing northwards island hop. At each island the species can adapt to new conditions that are only slightly different from the previous habitat. Right hand figure: Species dispersing northwards have no regions on which to gradually adapt to the altered photoperiod and temperature regimes ? lack of ability to adapt. Instead any species arriving at the northernmost region must be immediately able to tolerate the conditions there; that is be pre-adapted.

Animals "hop" from small island to small island enabling adaptation to the environment as the move northwards.Island hopping. Animals have to make a large leap from the southerly source population to the northern regions where the environment may be very different.

Island hopping and land-bridges may be how the Collembola (springtails) recolonised the Arctic, spreading out from refugia. An analysis of springtail communities suggests that observed differences in species community composition are likey to be the result of dispersal routes rather than local environmental conditions.