COLONIZATION OF THE LAND

Higher Plants

How Are the Plants Studied?

At the beginning of research in Surtsey, each new individual among higher plants was observed and its place of discovery and growth registered on a map of the island. Each new plant was labelled with a wooden stick and given an identification number. The growth of the plant was measured and its flowering and seed formation recorded. This was possible while the number of individual plants was small, but when the plants started to disperse their seeds in Surtsey and the rate of reproduction increased, it became impossible to label each individual and observe its development. Instead, permanent plots were set up in different habitats and locations on the island in order to follow plant succession. In addition, changes in the soil and invertebrate fauna in the plots are observed. Furthermore, each summer the island is combed for new pioneers, and the status of all species growing outside the permanent plots is updated. This has given a clear picture of the colonization of higher plants and the increase in the number of species on Surtsey.

Coastal Species Were the First Pioneers

In the first two decades, the sands and lava of Surtsey were quite barren and soil development was poor. Few species are adapted to such conditions and able to grow and reproduce under them. Shore plants that grow on sandy beaches and in windblown sand are adapted to nutrient-poor soils and can survive in severe conditions. Such plants were the first pioneers to colonize Surtsey and were characteristic of the vegetation during the first decades, along with the mosses and lichens that were found early on the island. This first period of plant history in Surtsey was therefore characterized by species adapted to dispersal by the sea or wind and capable of growing and surviving under extreme conditions.

The first higher plant species found in Surtsey was sea rocket (Cakile arctica) in 1965. It was also found there the following year along with sea lyme grass (Leymus arenarius). In 1967 these species were joined by oyster plant (Mertensia maritima) and sea sandwort (Honkenya peploides). In the late summer of that year the sea rocket flowered and was the first species to reach that stage. However, in these first years no higher plants survived overwinter in Surtsey, as they were either buried in the sand or washed away by the winter surf.

The first higher plant to survive over winter was the sea sandwort. The species overlived in the winter of 1968-69, and it has done very well since then. Only a few years passed before it had flowered and formed seeds. The first seeding taking place in 1971, which his was a turning point in the dispersal of this plant, and consequently it´s spread all over the island as the years passed.

Sea sandwort is now by far the most common higher plant species on Surtsey, growing everywhere that any pumice and sand can be found. The number of plants is most likely several million, and the largest ones have formed hummock-like patches with an area of several square meters. Sea sandwort has a well-developed fibre root system growing deep into the sand and beyond the plant surface above ground. In this way it can utilize nutrients from a large area to grow and develop.

It took sea lyme grass and oyster plant a longer time to reach the flowering and seeding stages. Consequently, these plants began spreading and forming populations later than the sea sandwort. It was in the years 1977 - 1979 that sea lyme grass and oyster plant started seeding and spreading throughout the sands and pumices. Sea lyme grass is a hardier species and is now one of the most common species on the island. It has formed several sand dunes decorated with sea sandwort and oyster plant. In this manner, there has developed on Surtsey a community of shore plants quite similar to the vegetation on sandy shores of Iceland. The annual species sea rocket and smoothish orache (Atriplex longipes) have occasionally been found on the northern ness, but they have not succeeded in establishing stable populations on the island.

During the period 1975-1985, following the colonization by the shore plants, very few new species were added to the young flora of Surtsey, and the succession slowed down (graph).

Enhanced Plant Succession with Increased Gull Breeding on Surtsey

The first birds to nest in Surtsey were the fulmar (Fulmarus glacialis) and the black guillemot (Cepphus grylle) in 1970. In 1974, they were joined by the great black-backed gull (Larus marinus), in 1975 by the kittiwake (Rissa tridactyla) and in 1981 by the herring gull (Larus argentatus). Breeding birds were nevertheless very few to begin with, and their influence on the vegetation development was rather limited. In 1986, a lesser black-backed gull (Larus fuscus) was discovered breeding on Surtsey for the first time. At this point in time, a gull colony was beginning to develop on the lava on the southern part of the island, where 10 lesser black-backed gull and herring gull nests were found. A sharp increase in the breeding population occurred over the next few years, and the dense breeding population that had soon developed grew larger each year. In 1990, the number of breeding pairs had risen to 200. By that time lesser black-backed gulls, herring gulls and great black-backed gulls were found in the colony.

This population explosion in the gull colony on Surtsey after 1986 was accompanied by a new wave in the colonization of higher plants and the succession of vegetation. The stagnation that had characterized previous years was broken. A number of new species began to colonize the island, and most of them were discovered within the gull colony. It is likely that many of the species were dispersed to the island by the gulls. In the period 1985-1995, the number of higher plant species on Surtsey grew from 21 to 44. In recent years the increase in number has slowed down. In the summer of 2004, 60 species of higher plants had been found on Surtsey of which 54 had living representatives on the island that year. This shows how the gull invasion has affected plant colonization and survival on Surtsey (graph).

In the gull colony, not only did the species increase in number but the vegetation also became denser due to the fertilizing effects of the birds. In only a few years, the black pumices and lava sands changed into a lush grassland and forb community. By the year 2004, the breeding area on the southern part of the island had expanded to about 10 hectares.

Comparison of Vegetation outside and inside Gull Colony

The succession of vegetation has been observed closely, in permanent plots both outside and inside the gull colony, since 1990. Changes have been slow outside the colony, the most interesting being that species characteristic of gravel plains, especially northern rock cress (Cardaminopsis petraea), sea campion (Silene uniflora) and thrift (Armeria maritima), have been found and are beginning to spread over the island in areas formerly colonized by the sea sandwort, lyme grass and oyster plant. In permanent plots (100 m²) outside the gull colony, the number of plant species recorded in the summer of 2002 ranged from 1-5, and the total plant cover did not reach 30% in any of them. Inside the gull colony there were, on the other hand, up to 10 different species in each plot, and there was a continuous plant cover in some of them.

In sandy areas inside the colony, the most abundant species in the vegetation are the sea sandwort, sea lyme grass, annual meadow grass (Poa annua), common meadow grass (Poa pratensis), common chickweed (Stellaria media), common mouse-ear (Cerastium fontanum) and the sea mayweed (Matricaria maritima), while in the lava the reflexed saltmarsh grass (Puccinellia distans), scurvy grass (Cochlearia officinalis) and procumbent pearlwort (Sagina procumbens) are most prominent. Arctic fescue (Festuca richardsonii) has been found at several sites where it has formed homogenous patches that increase in size from year to year. These patches are similar to grass swards of Arctic fescue, which dominate in many bird colonies of the neighbouring islands.

Northern green orchid (Platenthera hyperborea) and lady’s bedstraw (Galium verum) were found for the first time on Surtsey in the summer of 2003 (see article here, in Icelandic), and both these plants were in lush vegetation in the gull colony. They are examples of species that colonize land where vegetation has been developing for a while. The same is true of a few species of grassland mosses that have begun to colonize the gull colony on Surtsey. This confirms that plant succession on Surtsey is no longer in the primary stage.

Late Invasion by Willows

Dwarf willow (Salix herbacea) was discovered on Surtsey in 1995 and was the first willow species to colonize the island. A few individuals of this species have been found, and in recent years both tea-leaved willow (Salix phylicifolia) and woolly willow (Salix lanata) have been discovered on the island. The willow plants have been found both inside and outside the gull colony. It is likely that the seeds of all these species were dispersed by wind to Surtsey in the early days, but that conditions for their growth and development were not favourable until during the last decade. The improved soil conditions following the gull colonization are probably the main reason for this invasion of willows on Surtsey.

Future Development of Vegetation on Surtsey

The number of higher plants growing on Surtsey has become considerably higher than of most of the neighbouring Vestmannaeyjar (Westman Islands), which are considerably smaller in size. These islands host 2-30 species, and there is a close relationship between island size and species richness. On Heimaey, which is the largest of the Vestmannaeyjar there are about 150 species of higher plants. The island is seven times the area of Surtsey.

The number of species on Surtsey will continue to rise during coming decades. It is unlikely, however, that the number will ever reach 100. In the future, Surtsey will continue to erode and shrink, with a consequent loss of habitats and species. Surtsey will become similar to its small neighbours, Geirfuglasker and Súlnasker, which have less than 10 plant species in their flora.

See a slideshow of higher plants on Surtsey

(authors: Sturla Friðriksson, Borgþór Magnússon - borgthor@ni.is)

- last updated 06-May-2007