by Olga MOCHALOVA, Cand. Sc. (Biol.), Leading Research Assistant of the Laboratory of Botany, RAS FEB Institute of Biological Problems of the North (Magadan); Mariya KHOREVA, Cand. Sc. (Biol.), Academic Secretary of the same institute

As you know, on the small islands of the North Okhotia seabirds form enormous colonies, changing the landscape and vegetation in the nesting areas. Depending on the intensity of ornithological load, scientists have determined different correlations between productivity of these communities and their specific diversity. Normally,

degradation of flora is registered there.

THE INTERACTION

Today, the main focus of scientific studies are cereal tussock areas formed by perennial long-rhisome rough blue-joint C.langsdorfi auctt. We published a detailed article on this subject back in 2009 in the Siberian Environmental Magazine.

The influence of colonial birds on the local vegetation in the multiyear or centuries-old nesting areas has been repeatedly emphasized in relevant research works, but there are only few specialized articles published in Russia. It is only on the islands of the White and Barents seas that their interaction aroused intent interest. One of the practical outcomes (I. Breslina, Plants and Waterfowl of the Sea Islands of the Kola Subarctic Area, L., 1987, 200 p.) is that such places are characterized by special vegetation that differs from the zonal flora, typical of this or that of this ecotopes (i.e. conditions of abiotic environment) by species composition, distribution, and productivity.

The concentration of birds is a primary factor affecting the structure and functioning of the island biogeosystem as a whole. The short-term nesting period af-

Islands of North Okhotia. In brackets--area (km²), total number of marine colonial birds, number of species of vascular plants.

fecting the environment for centuries changes a surface layer of the lithogenic basis, results in the formation of different soils and vegetation, abnormal chemical composition of surface and coastal waters. Most of the abovementioned factors are still almost unknown.

SCIENTIFIC APPROACHES

Initially, our flora studies in the North Okhotia revealed a clear tendency to degradation of vegetation in the habitats of large colonies of sea birds. For example, after comparing specific diversity of vascular plants on different islands of the Taui Bay and the Yam Archipelago, it was established: the considerably fewer species, than in could be assumed, are growing on the Talan, Shelikan and Matykil Islands, taking into account their area.

In its turn, the specific composition of the plants on the Commander Islands (especially on the two smallest ones--Ariy Kamen and Toporkov) is monotonous and scanty too. As for the Shelikan Island, the situation there is both unique and critical: due to the growth of the population of the slaty-backed gull (Larus schistisagus Stejneger), the specific diversity of vascular plants is decreasing.

At present, there are no commonly recognized research methods for studies of interaction between sea birds and vegetation; that is why we applied various floristic and geobotanical methods "adapted" to the conditions of nestling colonies. As for different types of nesting sites and neighboring seaside slopes, we made separate floristic lists. The vegetation was described on the basis of standard methods with specification of a projecting cover, composition and life status of the species at sampling areas and profiles. Finally, the same studies on the sites densely populated with birds were carried out at 1x1 m² areas arranged transactionally from the center to the periphery of the colony (the use of standard 10x10 m² areas turned out to be impossible due to a high tessellation and small area of ornithogenic communities locating by narrow strips or patches along the edges and cracks of rocks). Productivity of vegetation was assessed by way of struts from 25x25 and 50x50 cm² areas.

BIRDS' INFLUENCE ON THE ISLAND VEGETATION

Birds' influence on the vegetation diversity varies depending on the region. In regions with a warmer climate, certain species of plants under the influence of gull colonies are replaced by new, as a rule, weed-infested annual plants. This is typical of small islands of the Mediterranean Sea, the Orlov Island in the Black Sea, etc. According to other works, large rookeries are considered a factor of enriching the flora, for example, on the Furulheim Island in the Sea of Japan.

Besides, many experts point out a role of marine colonial birds in the initial formation of the vegetation on the islands of the Kola Subarctic Area. This is attributed to a tectonic elevation of dry land in the White Sea and emergence of new very small islands colonized by birds bringing plant primordia together with nestling material.

Diagram of ratio "ornithogenic load-productivity of phytocenoses-specific wide range on the islands of North Okhotia: l-productivity, ll-specific wide range; grading according to effect rate; 1-moderate, 2-strong, 3-destructive.

Speaking of the North Okhotia islands, they have a shelf origin--they had separated from the continent in the late Pleistocene*-early Holocene, meaning that formation of the local flora commenced not "from a clear desk", as on the oceanic islands or small islands of the White Sea, it has an earlier, "pre-island" period. Most likely the loss of specific diversity there was a result of their separation from the continent. Later on, colonial birds actively contributed to this process, while the "return" process of the plants to these islands turned out to be difficult. Thus, we found adventive species only at nestling places of slaty-backed gulls and polyphagous insects visiting waste disposal areas (Umara and Shelikan islands). Other colonial birds (planktonphagous and ichthyophagous**) do not bring any new species there.

At the same time, the Northern Pacific Area*** (northern part of the Pacific Ocean characterized by enormous reserves of bioresources) stands out as a region of increased concentration of seabirds on the global level. Therefore, their role is evident both in the composition and specific diversity of the island flora. Only one island colonized by birds--Umara Island--is still an example of the maximal diversity on the minimal territory.

PLANT COMPOSITION

Birds change habitat of plants. Besides, the main active factor is an extra supply of mineral elements, first

* Pleistocene--an epoch of the Quarternary period, which commenced 2.58 mln years ago and ended in 11.7 thous. years ago.--Ed.

** Polyphagous insects--plurivorous; planktonphagous--eat plankton; ichthyophagous--eat fish.--Ed.

*** See: A. Ivanov, "Yamskie Islands: a North Pacific Phenomenon", Science in Russia, No. 2, 2007.--Ed.

of all nitrogen and phosphorous, i.e. enrichment and salinification of soils. Some species, for example, ericaceous shrubs typical for the zonal communities of islands and the coastline, react negatively on such impact and disappear promptly from the nestling areas of colonial birds. Other species--nitrophilous and halophilous plants--proliferate and form appropriate communities, which is a result of allogenic successions.

However, the composition of the latter is usually scanty--on the islands under consideration we registered meadows of rough blue-joint (Calamagrostis langsdorffii) with the minimal inclusion of accompanying species. The list of other community-forming species is not that large--sea coast angelica (Angelica gmelinii), wild-rye (Leymus mollis), white-leaf sagebrush (Artemisia leucophylla), nettle (Urtica angustifolia), strawberry cinquefoil (Potentilla fragiformis), cloudberry (Rubus chamaemorus), beach lovage (Ligusticum seoticum), spreading wood fern (Dryopteris expansa), dwarf cornel (Chamaepericlymenum suecicum), etc.

By the way, one and the same species may behave as ornithophilous or ornithophobous, depending on the load level. For example, cloudberry bears fruit on the uplands of the Talan Island (largest colony of sea birds in the Taui Bay), while on its northern slope it forms thickets but bears almost no berries. In 1986, experts working on the Shelikan Island registered an intense expansion of cloudberry around abandoned nests of seagulls; today they are gone. Dwarf cornel on the Talan and Matykil Islands behaves as an ornithophilous plant; in the meantime, it disappeared long ago on the Shelikan Island. As for rough blue-joint, it first grows intensively, forming a specific peaty soil, sometimes directly on the material rock, but then, with the "press" becoming heavier, it "loses its positions", leaving a thick layer of free-flowing wastes, washed out by water and taken away by wind. In other words, productivity of flora communities or gain of biomass for an area unit depends on the intensity of the "press" effect.

For assessments, we used the ratio "ornithogenic load--productivity of phytocoenoses--species diversity" on the islands of the Northern Okhotia. It was established: the effect of the above factor on the vegetation in a specific territory can be split into a number of levels. In case of a medium load, we saw a certain increase of productivity and specific diversity of endemic phytocoenoses (Umara Island); in case of a heavy load we registered generation of unique communities and soils (Yam Islands and Talan Island). Their productiv-

ity increases and specific diversity reduces. Increasing number of birds results in a catastrophic destruction of the vegetation (Shelikan Island).

BALANCE OR CRISIS?

The system "marine colonial birds--vegetation" can reach balance both under moderate and excessive intensity exerted by birds on plants, if the number of the latter remains more or less stable from year to year. If the rookery disappears one day, the system itself will strive for a new balance. The sharp increase of the number leads to imbalance and can cause a crisis, as it is happening now on the Shelikan Island. Besides, reduction of nestling colonies of birds has not been registered there, which clearly proves environmental safety of the adjacent waters.

As for the stability and instability on the islands under consideration, there are two main types of interaction between birds and plants. The first type are relatively stable, long-living, "balanced" ornithogenic complexes. As an example, we can propose ecosystems on the Talan and Matykil islands, where nestling colonies have been likely existing for some thousand years. Extra inflow of mineral elements has the same direct

and relatively stable ecological value, as light, heat and precipitation. Therefore, in the past some plant species disappeared there followed by regeneration and stabilization of local vegetation communities and the flora as a whole.

Most colonies on the continental coastline can be considered as balanced ornithogenic complexes. The nesting density is lower than on the islands, and, as a result, the natural vegetation cover is maintained. At the same time, we registered a great number of ornithophilous species, such as sea coast angelica, white-leaf sagebrush, wild rye, etc.

The second type are unstable complexes with an upset balance in the system "birds-plants" in terms of a sharp increase in the number of birds. We'd like to point out that, according to our data, trees and bushes are disappearing on the Shelikan Island, and herbaceous vegetation is forming there, while the most heavily affected areas are characterized by a total degradation of vegetation and soils. Birds change the environment and make it more comfortable for nesting, which leads to a further expansion of the colony. Probably this process can last until larch and stone birch forests completely disappear.

Finally, a stable complex characterized by a rather degraded vegetation and a numerous colony of marine birds can form, as it happened on the islands Three Brother in the Taui Bay (where only two species of vascular plants have survived), on the Ariy Kamen on the Commander Islands (5 species) and on the Iona Island in the central part of the Sea of Okhotsk (2 species). At the same time, you should not forget periodical variations of the number of colonies and even their probable disappearance due to an increase of forage resources,

necessary for birds, epidemics, appearance of carnivores, etc. with a subsequent regeneration of vegetation.

Recently, we have registered signs of instability on the Umara Island. Here the growth of the sea gull population reveals itself in the look of vegetation--in the change of the number and living condition of a number of plants, but has no effect on their specific diversity. Though, probably, we will observe first losses of the most vulnerable vascular plants in the coming decade.

According to the results of studies carried out by scientists L. Abramova and A. Shipunov on the islands of the Kem-Lud Archipelago in the White Sea with an interval of forty years: the birds' role consists in maintaining stability of the flora rather than changing its composition. One of the opportunities to get the ornithogenous system balanced is to adapt plants to rookery-specific terms by way of changing their appearance.

BIOMORPHOLOGICAL ADAPTATIONS

What is meant here is a response of ornithophilous plants allowing them to exist in extreme conditions for most other flora species. The constant additional organic fertilization (in the form of excrements, pellets* of undigested food, feathers fallen out during ecdysis,

*Pellets--pressed undigested remains of food of animal origin, for example, bones, hair, feather, insect chitin, etc.--Ed.

shells, and dead bodies of nestlings) results in an active development of vegetative organs of most plants growing there, in some cases at the expense of generative organs. For example, the starflower (Trientalis arcticd) on the Shelikan Island regularly forms additional leaf whorls in place of a flower (results for 1997, 2002 and 2005). On the Talan and Matykil islands leaves of cloudberry growing along the boundaries of colonies and in places enriched with organic fertilizers are larger (2-3 times) than in tundra.

Other irregularities of vascular plants discovered by scientists were abnormal branching, intensified short formation and proliferation of rhizomes and caudex*. Most pronounced signs of these abnormalities were registered on the Yam Islands with proliferating "cushions" of the roseroot (Rhodiola rosea) often used by fulmars to make nests. Sometimes, proliferation of rhizomes is registered for Arctanthemum arcticum and cinquefoils (Potentilla fragiformis).

However, the most interesting are "cereal tussocks" forming ornithogenic vegetation on islands. In places occupied by densely populated colonies of slatybacked gulls and tufted puffins (Lunda cirrata), rough blue-joint C.Langsdorfi auctt. and sometimes wild rye form large tussocks.

*Caudex outwardly is very similar to a short rhizome. It is a thickened underground or partially underground formation without leaves formed from shoot bases.--Ed.

We still do not know exactly how long it takes to form one tussock: according to preliminary data, it takes more than 10-20 years. In terms of a multiyear and relatively stable load, tussock bogs can survive quite a long time.

For example, on the Aina Islands in the Barents Sea, there was found a unique grassy (Leymus arenarius) bog formed as a result of a multiyear nesting of silver gulls (Larus argentatus) in the same nest. On the Commander Islands, first of all, on the Toporkov Island, bluegrass (Poa tatewakiana) tussock bogs are spread mixed colonies of glaucous-winged gulls (Larus glaucescens) and tufted puffins. "Bluegrass tussocks" (0.5 m high) alternate with areas of normally developing bluegrass. By the way, in the North Okhotomorye, like on the Commander Islands, scientists never found gull nests within a "gramineous" tussock. These communities, like the excessive proliferation of the caudex of roseroot, can be treated as ornithogenic ecobiomorphs (typical adaptive systems of the organism existing in certain conditions) not to be associated with feeding behavior of birds--they are mostly caused by excessive supply of biogenous materials. Gulls directly interact with plants, plucking and pulling them out to make nests or demonstrating aggressive behavior while protecting nesting areas, and tufted puffins, damaging and trampling plants near their holes.

To sum it up, it is worth saying that you can register all types of impact produced by seabirds on plants on the islands of the North Okhotia. Depending on the intensity of impact, it can show up in different ways. First, as a factor of selection of resistant forms and species (forming specific ornithogenic communities and ecobiomorphs). Secondly, as a factor of destruction resulting in a catastrophic degradation of vegetation (excessive load). Thirdly, as an environment-forming factor (in case of relatively stable number of birds within a long-lasting period--hundreds and thousands of years) when the ornithogenic load, specific diversity and productivity of plant communities are in balance.

Illustrations supplied by the authors