Surtsey 60 ára: Landnám plantna og framvinda

INNGANGUR

2. mynd. Surtsey og staðsetning gróðurreita. Innrauð Sentinel gervitunglamynd frá 6. júlí 2019. Rauðleit svæði eru með þéttum gróðri en dökkleit og ljós eru strjálgróin. – Location of permanent vegetation plots (dots and numbers) on Surtsey, infrared Sentinel image from 6 July 2019. Areas with dense vegetation appear in red color.

Vestmannaeyjar eru ungar eldfjallaeyjar sem hafa myndast við gos undir ísaldarjökli eða í sjó á síðustu 100 þúsund árum¹Ingvar Atli Sigurðsson & Sveinn P. Jakobsson 2009. Jarðsaga Vestmannaeyja. Bls. 14−27 í: Vestmannaeyjar (ritstj. Jón Viðar Sigurðsson). Árbók Ferðafélags Íslands, Reykjavík 2009. Yngst eyjanna er Surtsey (1. mynd) sem varð til í eldgosi sem hófst í nóvember 1963 og stóð fram í júní 1967. Það eru því sextíu ár frá því að saga eyjarinnar hófst og er þessi grein skrifuð í tilefni af því. Síðasta eldgos í Vestmannaeyjum fram til þessa varð hins vegar í Heimaey árið 1973. Þar er miðsvæði eldvirkni í Eyjum og hafa endurtekin gos allt frá ísöld byggt upp eyna. Úteyjarnar hafa myndast eftir ísöld í einu gosi hver líkt og Surtsey. Hrina gosa er talin hafa orðið í Vestmannaeyjum fyrir 5.000–6.000 árum. Í henni mynduðust Stórhöfði og Helgafell á Heimaey, og Bjarnarey og Elliðaey þar skammt norðaustur af.¹Ingvar Atli Sigurðsson & Sveinn P. Jakobsson 2009. Jarðsaga Vestmannaeyja. Bls. 14−27 í: Vestmannaeyjar (ritstj. Jón Viðar Sigurðsson). Árbók Ferðafélags Íslands, Reykjavík 2009. Surtseyjargosið varpaði skýru ljósi á eðli neðansjávargosanna sem mynduðu úteyjar Vestmannaeyja fyrir þúsundum ára og einnig smærri eyjar undan Reykjanesi.²Sigurður Þórarinsson 1965. Neðansjávargos við Ísland. Náttúrufræðingurinn 35. 153–181. Jafnframt hafa rannsóknir í Surtsey eftir að gosi lauk leitt í ljós hve rofgjarnar eyjarnar eru á fyrsta skeiði og hve mikilvæg móbergsmyndun er þeim til viðhalds þegar frá líður.³Sveinn P. Jakobsson & Guðmundur Guðmundsson 2003. Rof Surtseyjar: Mælingar 1967–2002 og framtíðarspá. Náttúrufræðingurinn 71. 138–144. ⁴Birgir Vilhelm Óskarsson, Kristján Jónasson, Guðmundur Valsson & Bel, J.M.C. 2020. Erosion and sedimentation in Surtsey island quantified from new DEMs. Surtsey Research 14. 63−77. https://doi.org/10.33112/surtsey.14.5

Lífríki Vestmannaeyja er ekki síður áhugavert en jarðfræði þeirra. Hafsvæðið umhverfis Eyjar er frjósamt, ríkt af sjávarlífi og fiskimið þar gjöful.⁵Ólafur S. Ástþórsson, Ástþór Gíslason & Steingrímur Jónsson 2007. Climate variability and the Icelandic marine ecosystem. Deep-Sea Research 54. 2456−2477. ⁶Karl Gunnarsson 2007. Marine life. Bls. 39–43 í: Nomination of Surtsey for the UNESCO World Heritage List (ritstj. Snorri Baldursson & Álfheiður Ingadóttir). Náttúrufræðistofnun Íslands, Reykjavík. Mergð sjófugla af mörgum tegundum verpur í eyjunum og hefur mikil áhrif. Langstærstur er stofn lunda (Fraticula arcticus).⁷Jóhann Óli Hilmarsson 2009. Fuglalíf í Vestmannaeyjum. Bls. 28–73 í: Vestmannaeyjar (ritstj. Jón Viðar Sigurðsson). Árbók Ferðafélags Íslands, Reykjavík 2009. Æðplöntur eru hins vegar fremur fábreyttar í flóru eyjanna, um 170 tegundir, og finnast þær allar í Heimaey með fáeinum undantekningum.⁸Náttúrufræðistofnun Íslands, óbirt gögn, gbif.org 2023 . GBIF: Global Biodiversity Information Facility. Free and open access to biodiversity data. Varpsvæði sjófugla utan bjarga einkennist af gróskumiklu en fremur tegundasnauðu graslendi. Gróður er hins vegar fjölbreyttari en gróskuminni þar sem áhrif af sjófugli eru lítil.⁹Sturla Friðriksson & Björn Johnsen 1967. The vascular flora of the outer Westman Islands. Rit Vísindafélags Íslendinga 4(3). 37–67. ¹⁰Sturla Friðriksson, Bjartmar Sveinbjörnsson & Skúli Magnússon 1972. On the vegetation of Heimaey, Iceland. II. Surtsey Research Progress Report VI. 36−53. ¹¹Borgþór Magnússon, Sigurður H. Magnússon, Erling Ólafsson & Bjarni Diðrik Sigurðsson 2014. Plant colonization, succession and ecosystem development on Surtsey with reference to neighbouring islands. Biogeosciences 11 5521–5537. https://doi.org/10.5194/bg-11-5521-2014 Við myndun Surtseyjar skapaðist einstakt tækifæri til fylgjast með landnámi lífvera og framvindu samfélaga á nýrri ey í norðurhöfum, og þótt víðar væri leitað.¹²Thornton, I. 2007. Island colonization. The origin and development of island communities. Ritstj. T. New. Cambridge University Press, Cambridge. 302 bls. ¹³Kristín Svavarsdóttir & Walker, L.R. 2009. The value of Surtsey for ecological research. Surtsey Research 12. 133–148.  ¹⁴del Moral, R. & Borgþór Magnússon 2014. Surtsey and Mount St. Helens: A comparison of early succession rates. Biogeosciences 11. 2099–2111. https://doi.org/10.5194/bg-11-2099-2014

3. mynd. Landnám æðplantna í Surtsey 1965-2021, heild og fjöldi sem fundist hefur á lífi ár hvert. – Surtsey colonizing curve for vascular plants during 1965–2021.

Á líkan hátt og Surtseyjargosið opnaði augu manna fyrir myndun og mótun Vestmannaeyja hefur Surtsey veitt sýn um hvernig lífverur námu land í eyjunum og vistkerfi mótaðist í fyrndinni. Í þessari grein fjöllum við um landnám æðplantna í Surtsey og um framvindu gróðurs og dýralífs. Hverjir voru fyrstu landnemar og hvers vegna? Hvað hefur einkennt landnám seinni ára? Hafa fuglar og jafnvel selir haft áhrif á landnám og framvindu? Hvernig horfir um framtíð Surtseyjar?

LESA ALLA GREIN 

SUMMARY

Surtsey Volcanic Island At Sixty: Vascular Plant Colonization and Ecosystem Development

Surtsey volcanic island rose out of the sea in the fall of 1963, formed in a volcanic eruption that lasted into the summer of 1967. It is the youngest island of the Vestmannaeyjar island archipelago off the south coast of Iceland. From 1964, colonization of life and ecosystem development has been studied on Surtsey. The colonization of vascular plants has been studied in greatest detail and annual records from the first finding in 1965 exist. In 2022 a total of 78 species had been recorded on the island, of those 63 species had living individuals and over 40 species had formed viable populations. Shore species were the first colonizers of the island, their seeds washed by the sea upon the northern shores. They are adapted to sandy, infertile substrates and able to establish under harsh conditions. Most of the shore species were successful and started spreading within the next few years. Following their establishment plant colonization slowed markedly down. In 1985, a dense breeding colony of seagulls (Lesser black-backed gull, Herring gull and Great black-backed gull) started to form on the southern part of the island. It was followed by a sharp increase in plant colonization. The birds dispersed new plant species to the island and improved soil conditions through their nutrient transfer from sea to land. Within the breeding area plant growth was greatly enhanced. This second wave of colonization continued for about two decades when it slowed down. However, colonization by new vascular plant species is still ongoing and the flora of Surtsey has become considerably richer than that of the other but smaller neighbouring islands. In the later years, the number of wind-dispersed species has increased considerably. They are more nutrient demanding and some of them need fungal symbionts for establishing. These conditions and amelioration were brought about by the breeding seabirds (gulls and fulmars) upon the island. Our studies indicate that about 10% of the current vascular plant flora was dispersed by the sea to the island, 70% by birds and 20% by wind. All but few of the plant species of Surtsey are found on the other Vestmannaeyjar islands, the others on the mainland of Iceland.

Since 1990, plant succession has been studied in permanent vegetation plots on the island. The study has revealed a distinct difference between areas affected by the seagulls and those where their impacts are limited. Plant growth and succession has been determined by nutrient supply, mainly nitrogen and phosphorous. Seagulls and fulmars breeding upon the island transport nutrients from the sea to their colonies and greatly enrich the soil. Within their breeding grounds a dense, lush grassland has formed, expanding from year to year. It has become increasingly dominated by the long-lived rhizomatous grass species Festuca richardsonii, Poa pratensis and Leymus arenarius along with the herbs Stellaria media and Tripleurospermum maritimum, outcompeting smaller ruderal species. The vegetation of these areas has now strong affinities with vegetation of bird colonies of the older, neighbouring islands. Besides the greening of Surtsey by the seabirds there are indications that grey seals breeding on the northern, lowland spit have enhanced vegetation growth in their hauling-out area. Vegetation of areas on Surtsey that are little affected by seabirds or seals is still very sparse. However, their vegetation cover and species richness is gradually increasing. Their characteristic species are Honckenya peploides, Leymus arenarius, Silene maritima, Rumex acetosella and Arabidopsis petraea.

The increased breeding of seabirds on the island and transfer of nutrients from sea to land was followed by a jump in ecosystem development on the island. In the expanding and lusher grassland vegetation, invertebrate life became richer and more diverse. In 1996, the first land bird, snow bunting feeding its young on insects, was found breeding on the island. Few years later the insectivorous passerine species, meadow pipit and white wagtail, also started breeding. In 2002, a pair of greylag geese bred on the island, feeding within the grassland area. A raven started breeding on the island in 2006, sitting at the top of the food chain.

The erosion of Surtsey by the ocean continues. In 2021, more than half of the island had disappeared. Within the coming decades and centuries, it will take the shape and size of the neighbouring islands and its biota will become alike. A decline will occur; habitats on Surtsey will be lost along with many species. It is probable that puffin, currently found in limited numbers in the sea cliffs, will invade the expanding grassland of Surtsey and form a large breeding colony. This will have profound effects on succession and ecosystem processes on the island as within their colonies on the other Vestmannaeyjar islands.