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 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 www.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 continental North America 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. However, much of this is still speculation. 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 (© 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 (Coulson et al. 2002).
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 (pictures of hovering birds) 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 (Coulson et al. 2002). Other studies have shown that many species common in 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
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. Conclusive answers based on genetic analysis of diverse populations is work in progress at UNIS. Some species have 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. However, studies in Barentsburg (Coulson et al 2013a) and Pyramiden (Coulson et al 2015) in the soils that were imported from Ukraine for either the greenhouse or to ‘green’ the settlements have revealed species not seen elsewhere in Svalbard and which most likely came with these soils, for example the only lumbricid earthworms (Coulson et al. 2013a).
In addition to invertebrates, a species of vole was imported along with forage for the horses working in the Grumant coal mine (Henttonen et al 2001). This vole brought with it the tapeworm parasite, Echinococcus multilocularis. 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 (see box – box on island hopping, lack of time to adapt etc).
Box. Tardigrade species assemblages. In 1998 Phil Pugh and Sandra MacInnes, both from the British Antarctic Survey in Cambridge, UK, published an analysis of tardigrate 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. Species did not travel north from Europe or west from Siberia.
Few invertebrate species have been known to have established recently, but see the case of imported soils to Barentsburg and Pyramiden (Coulson et al. 2013a, Coulson et al 2013b, Coulson et al. 2015). Connectivity with mainland sources is relatively high and those that can currently successfully establish have probably done so. But the case of the diamond back moth shows that insects are still arriving and in an era of climate change it is highly likely that in the future species arriving here will find conditions suitable and establish. This is different to the situation found on the sub-Antarctic islands where the isolation has hindered colonisation. Here there has been considerable recent influx of new species, including that of the Diamond backed moth, many following with man.
Given that invertebrates are arriving by air, via transport by man and presumably with the ocean currents, why are none thought to have colonised recently? What prevents establishment? Lack of suitable food plants plays a role. However, another major factor is that of cold tolerance. Invertebrates on Svalbard must be able to tolerate ten months frozen in the soil and, in certain environments where snow cover is thin, survive temperatures approaching -40ºC. To enable tolerance of the sub-zero temperatures invertebrates have evolved a series of physiological strategies. In mainland Europe the majority of invertebrates overwinter by avoiding freezing, the so called ‘freeze avoiding’ species. These species produce compounds that reduce the freezing temperature of the body in the same way antifreeze in a car radiator prevents the coolant from freezing. While some species can resist freezing until below -60°C, in High Arctic regions where temperatures are constantly below freezing for extended periods a different strategy is often employed: ‘freeze tolerance’. These species actively initiate and control the freezing of body water and pass the winter fully frozen. Once frozen they are in a form of suspended animation and so wait until spring arrives. Studies on such organisms have more than just an academic interest. In Canada and northern USA there are several species cold-blooded vertebrate, for example the wood frog, which are also freeze tolerant. http://http-server.carleton.ca/~kbstorey/. Research in to such freeze tolerance has drawn considerable interest from the medical research organisations and companies researching into cryo-preservation of human organs, this sometimes includes whole human preservation, for example: http://www.alcor.org/. Yet other invertebrates have an intermediate strategy. Danish researchers have recently shown that some species desiccate during the winter, loosing up to almost 70% of their body water. In such a state they cannot freeze as there is insufficient water remaining. While this strategy, Freeze Desiccation, enables tolerance of low temperatures it also requires the ability to survive extremely high cell salt concentrations as a result of the desiccation. Given survival of the winter, the invertebrates must complete their lifecycle during the short summer period. Here species either race to complete their lifecycle during the summer or grow slowly over several summers. The two endemic aphids both possess extremely unusual lifecycles for an aphid. Both overwinter as an egg, hatch, mature and lay eggs before the onset of winter which is a common pattern seen in many other species of aphid. What is unusual is that the lifecycle is pre-programmed and does not appear to respond to environmental cues as is usual with other aphids, for example those on roses. Adults of the Svalbard species all die before or during the winter, the aphid overwintering as an egg. While the aphid has a one year lifecycle, the soil mites, animals less than 0.5 millimetres long, can take five years to develop from egg to adult. Elongation of the lifecycle in arctic regions is not uncommon, for example a moth found in Greenland (Gynaephora groenlandica) can take between 7 to 14 years to mature depending on how warm the summer is.
The ‘chronosequence’ and community assembly
The chronosequence is an approach to investigating community assembly when species colonisation new space.
A chronosequence is created in front of a retreating glacier; new ground appearing from under the glacier as it retreats. In effect, walking away from the snout of the glacier is a journey backwards in time as you cross a progressively older and older land surface. By use of, for example historic photographs (including arerial mapping), it may be possible to date the land surfaces and hence the ‘age’, or the period of time available for the flora and fauna to colonise. By surveying the invertebrate fauna and the flora it is possible to observe the sequence in which species collonise this new terrain and how abundances vary.