Biocenotic influence of the great cormorant (Phalacrocorax carbo L.) in the Azov-Black sea region of Ukraine

The great cormorant (Phalacrocorax carbo L.) is a bird species that, due to the peculiarities of its biology, can significantly affect the natural environment. First of all, due to the fact that the species is an obligate ichthyophage and feeds exclusively on fish (causing, in some places, quite significant damage to both the fishery and the natural fish resources of the seas), a large amount of potassium, nitrogen and phosphorus compounds accumulates in its feces. Because of this, the caustic excrements of cormorants have a detrimental effect on herbaceous and woody vegetation in the colonies of this species, near them, as well as in the resting places of the birds. Considering the fact that the nesting colonies of the great cormorant can be quite numerous, reaching tens of thousands of nests (such as the settlement on the Obytichna spit in the Berdyansk district of the Zaporizhzhia region), the impact is observed to be very significant. At the same time, tree vegetation suffers from mechanical damage during the construction of nests, and trees die after 2-3 years. Another type of influence of cormorants is their interaction with other bird species in nesting areas. The high number of cormorants leads to a reduction in the number of some other bird species, the impoverishment of nesting bird complexes (herons, terns); in the conditions of a shortage of nesting sites on the islands, only (Larus cachinnans can nest together with the great cormorant (with its high number). We observed a similar situation in most nesting settlements of the species.


Introduction
The scientific literature has accumulated a lot of information about the impact on the environment (including its physico-chemical and biological regimes) of various organisms [1].The environment-forming activity of birds is most significantly and visibly manifested in the places of their mass gatherings.Large colonies play an important role in soil formation, bringing a large number of organic substances with the products of life (mainly guano).Special ornithogenic soils are formed in places of bird gatherings, in which the concentrations of phosphorus and nitrogen can exceed the corresponding background indicators by two times, and zinc and potassium -by one and a half times.Under the influence of atmospheric precipitation, leaching of easily soluble compounds of phosphorus, nitrates, sulfates and chlorides occurs from the litter.
The participation of birds in the transport of biogenic substances from aquatic to terrestrial ecosystems is indisputable.Japanese scientists analyzed the long-term consequences of the nesting of the great cormorant (Phalacrocorax carbo) and its participation in the nitrogen cycle [2].These birds feed on fish in fresh water bodies, and nests are built on trees in the 1254 (2023) 012013 IOP Publishing doi:10.1088/1755-1315/1254/1/012013 2 forest.In nesting places, trees die, and in the territories of former cormorant settlements, the increased content of nitrogen in litter, soil and plants remains for a long time.Similar pictures are observed in many places along the coasts of different seas.Birds' excrements entering the water sometimes affect the dynamics of biogenic substances so significantly that some foreign colleagues began to use a special term -"guanotrophication".
Another important type of impact of the great cormorant is its relationship with other species of birds, especially, the wetland complex birds, which depends, first of all, on the characteristics of the species that nest in mixed settlements with the great cormorant, as well as the specifics of the nesting sites -the type of nest biotope, shortage of suitable nesting area, etc.The impact can be both direct and indirect, and can be both negative and, in some cases, positive.The material presented below is based on the results of own expedition trips and analysis of literary sources [3,4].

Material and methods
Researches of the ecological impact of the great cormorant were conducted in the Azov-Black Sea region of Ukraine in 22 nesting colonies of this species in the period from 2012 to 2021 (figure 1).Most of the expedition work was carried out using the methods of fixed vehicle, boat and pedestrian census.Surveys were made with Etherna binoculars (10x) and Nikon ACULON A211 10x50 and VIXEN Geoma telescope (20-60x80).Mapping of bird gathering places and spatial characteristics of routes were made using a GARMIN GPSMAP 78s navigator.Biotopes and birds were photographed with Canon EOS 450D and Nikon D700 cameras.The photographs were exported to the FastStone Image Viewer programme, which together with the camera software in the Exif metadata mode made it possible to control the geolocation data of the photographs taken, the date and conditions of shooting.Linear dimensions between objects and object heights were measured using a Nicon Forestry 550 laser altimeter.Statistical processing of the obtained data was carried out in Microsoft Excel 2010 and Statistica Release 8 (Basic Statistic module) programmes.
In general, such a phenomenon as the influence of the great cormorant on natural and anthropogenic complexes, in particular, the negative impact on vegetation, other species of colonial wetland birds, pond fish farms, technical structures, etc., is quite well known and repeatedly described in the scientific literature.Also, many different publications and studies are devoted to issues of human influence on the number and distribution of the great cormorant, methods of regulating its number.Therefore, the purpose of this research is only to provide data available in Ukraine on the impact of the cormorant on vegetation, soil and ornithocompexes.

Results
The biocenotic significance of great cormorant colonies and their negative impact on vegetation is mentioned in the works of many authors.The forms and scales of this phenomenon depend on the number of cormorants, the method of nesting (tree, shrub or ground colonies), the species diversity of accompanying species, the duration of the impact (intraseasonal and perennial aspects) and other factors.

Impact on vegetation
3.1.1.Terrestrial colonies.One of the most comprehensive researches in Ukraine on the study of the biocenotic influence of ground colonies of the great cormorant was conducted in 1997-2003 on the Lebedyni islands.They made it possible to conclude that this species has a decisive influence on the composition, structure and distribution of vegetation on the islands.Cormorants are most destructive to vegetation during the period when they are building their nests.Nests are completed and renewed during the entire reproductive period, but their building is especially active during the laying and hatching of eggs.Birds collect nesting material within a radius of up to 2 km from the colony.Some birds find the material in the immediate vicinity of the nest, others collect it on neighboring islands and some "steal" it from the nests of other cormorants.The cormorant uses Artemisia sp., as well as last year's shoots of Atriplex tatarica, Pontic catrana, rhizomes and stems of Phragmites australis, pulling them out of the soil to build their nests.The influence of the cormorant on vegetation is not limited to nest-building activities, it experiences a depressing effect as a result of trampling by birds, compaction of the soil, as well as "burning" with excrement, since raw bird feces contain up to 16.3% of nitrogen, 15.4% of phosphorus, 8.5 -potassium, 24.0 -calcium and 7.4% manganese.At the locations of the colonies, there is a change in vegetation cover, while in the first year, as a result of strong nitrification of the substrate, species that prefer nitrogen-rich soil prevail.In the following years, even in the absence of birds, communities dominated by spring or winter annuals are observed in the colony for a long time, the vegetation cover is transformed, including the impoverishment of the coenotic and floristic structure of the island complexes.The restoration of communities with the participation of perennials, including Artemisia sp., Crambe pontica and Leymus sabulosus, does not occur in the first 10 years, and during this period, areas with disturbed plant cover in the conditions of the Lebedyni islands are most often washed away as a result of a decrease in the turf of the substrate, an increase in the influence of wind erosion, decrease in wave resistance of accumulative shores.A similar situation is observed for the islands of Tendra and Yahorlyk bays, where the ground nesting sites of the cormorant very quickly become devoid of any vegetation cover, which is connected both with the direct destruction of the vegetation used for building nests, and with accumulation in the soil out-of-boundary concentrations of phosphorus and nitrogen that enter there from bird feces.At the same time, even after the cessation of the existence of the colony, the restoration of vegetation occurs after at least 2 years.Another negative consequence of the destruction of vegetation at the place of cormorant colonies, if they are located near a water cut, is the acceleration of the erosion process of the coastal edge of the islands and, as a result, a gradual decrease in the area of the islands, including those suitable for nesting.
On Kryva spit, the great cormorant usually occupies nesting sites at the very beginning of the growing season of the main types of plants of the Kryva spit, such as Leymus sabulosus, Melilotus sp., and Crambe pontica.As a result of the impact of cormorant feces, all vegetation in the territory of the colony dies, while in neighboring areas it can reach up to 1 m in height.

Arboreal colonies.
The largest colonies of great cormorants located on trees are located on the coast of the Sea of Azov -the Kazantyp Peninsula (Mysove Forestry), Obytichna spit, the Dnipro reservoirs -Kakhovka (Velyki Kuchuhury), Kremenchuk, Kaniv, Kyiv reservoirs, as well as in a number of other places.Everywhere, the negative impact of the nesting of this species, both on herbaceous and tree-shrub vegetation, is noted, which in the case of large colonies acquires a significant scale.The most thorough studies of the influence of cormorant colonies on tree and shrub vegetation were conducted by S. M. Podorozhnyi in June-July 2008 on the islands of Velyki and Mali Kuchuhury, and the previous results were confirmed by our expedition trips in 2012-2017.
They showed that the tree vegetation on the islands is represented by two types of plantings.The first is the most common and is represented by trees located along the coastline of large and small islands that are part of the reserve.Salix alba is the dominant species here.Populus alba and P. nigra were much less common in these stands.The height of the stands of these species is 18-20 m, and the average age is 50-60 years.Morus nigra, Acer negundo, Fraxinus excelsior can be found singly in these plantings.The vast majority of these are low-growing trees of coppice origin.The shrub layer of coastal tree plantings is represented by Sambucus nigra and Amorpha fruticose.
Coastal plantings suffer the most from the settlements of the great cormorant, as they are chosen by the birds for placing their nests in the first place.Tall Salix alba and Populus sp.trees with a good view and an uncomplicated approach to the nest are chosen as nesting biotopes.Nests, as a rule, are arranged in the upper and middle parts of the crown of trees at the base or middle part of branches of the second order.The negative impact on the growth and development of trees is primarily associated with the impact of bird feces with a high concentration of nitrogen, phosphorus and potassium on the vegetative organs of plants during the entire nesting period.This leads to the death of leaves and young shoots as a result of burns.In the first year of the appearance of nests on trees, the leaves of the upper and partly the middle part of the crown "burn".As a result, the bark of the skeletal branches in the upper part of the crown peels off and the tree begins to dry out.
The next year, the upper dry parts of the trees, in most cases, are used by the cormorant as a roost, and the nests are moved to the middle part of the crown, where the leaves are still partially preserved.By the end of the growing season, the leaves and one-year shoots of the middle and lower part of the crown "burn", the bark of the perennial shoots peels off, and the tree dries up completely.
In the future, dead trees are used by cormorants as roosts, and nests are moved to nearby living trees.In the absence of trees, cormorants begin to use shrubs (mainly Sambucus nigra) with a height of 2.5-4.0 m as nesting biotopes.Much less often, tree forms of trees are used for placing nests -Acer negundo, Morus nigra, etc. with a similar height.Amorpha fruticose and coppice forms of trees die in the first year after placing nests on them.Sambucus nigra is more stable and, depending on the intensity of the impact, can vegetate for several seasons, but in the future it also dies.
Monodominant thickets of Phragmites australis later form on the site of dead shrubs, which, as a rule, are not used by cormorants as nesting biotopes.
Thus, we can state the fact that the nesting of the great cormorant on the islands of Velyki and Mali Kuchuhury leads to the degradation of coastal tree plantings and their replacement by unproductive monodominant thickets of Phragmites australis with low biological diversity.In addition, the degradation of coastal tree plantings is extremely negative, as it can lead to the intensification of coastal erosion processes, which endangers the existence of the islands themselves.Dead trees cannot perform the functions of shore protection and shore fortification.To a much lesser extent, it is performed by shrubs and, even more so, reedy coenoses, which is the last stage of succession.
The second type of tree plantations is found in the interior of large islands with a more elevated topography.They are limited to inter-arena depressions and hollows with close groundwater, forming so-called "groves" or "kolkas".The structure of tree and shrub plantings here depends on the degree of moisture in the central part.Kolka plantings are used by the great cormorant as a nesting biotope only when there is a shortage of them in the composition of coastal tree plantings.Cormorants choose tall Salix sp. and Populus sp.trees in the central part for nesting.In dry depressions, Betula sp. and Populus sp. are used to build nests.
The activity of cormorants directly and indirectly affects the natural complex of the Obytichna spit.Arboreal colonies of the great cormorant are a relatively new phenomenon here, and very detrimental to artificial forest plantings on sandy soils.The high content of nitrogen, phosphorus and other chemical elements in bird feces, and their accumulation during the nesting period (from the end of February to the middle of June) leads to the death of almost the entire grass cover.Instead of natural associations, for the 2nd-5th year, ornithogenic ones are formed, represented by lush thickets of weeds (Chenopodium sp., Melilotus sp.etc.).In some areas, vegetation disappears almost completely.According to our observations, restoration of herbaceous vegetation at the site of the colony occurs in the second or third year when the impact is stopped.
Similar processes take place in other colonial settlements of the species, both in Ukraine and abroad.

Impact on soil cover
Waterbirds, such as the cormorant, are very important intermediate links in some food chains and a factor that facilitates the dislocation of matter between aquatic and terrestrial ecosystems.From one perspective, cormorants exclude biogenic elements (along with fish) from aquatic ecosystems.Although cormorants hunt at a fairly large distance from colonies, within a radius of up to 30 km, they leave feces in a relatively small area below the colony and near the lake shore.Nesting colonies of fish-eating birds affect the habitats and phytocenoses they occupy in different ways.During the breeding season, cormorants transfer large amounts of biomass and chemicals from the aquatic environment to colonies located on land.The consequence of the introduction of allochthonous matter into the terrestrial ecosystem can be significant enrichment of soil and plants with nitrogen (N), phosphorus (P) and potassium (K).Elevated concentrations of N and P within the colony may accumulate in the soil and be transported with groundwater or surface runoff to lakes.The range and rate of changes that occur depend mainly on the species of nesting birds, the density and the age of the colony.In addition, cormorants break branches during nest construction, resulting in defoliation of the soil under trees.In areas with high bird density, feces cover herbaceous vegetation, limiting photosynthesis and ultimately leading to plant extinction.
It should be noted that the initial impact of introduced organic elements in the colony leads to an increase in the biological diversity of plants.An assessment of changes in the amount of nutrients accumulated in soils and groundwater, and changes in the floral composition of the forest phytocenosis, which occurred 5 years after the cormorants left the colony, was carried out.The colony, located on a small (0.6 ha) island in Lake Wielkie, has been inhabited by cormorants since the 1980s.The maximum indicated number was about 200 nesting pairs.Research on the impact of the former nest colony on the ecosystem of the island and the surrounding lake was conducted in 2009-2010.Compared to the control station, significantly increased concentrations of nitrogen, phosphorus and potassium were found in the soil under the colony.The highest concentrations of nitrogen were noted in the surface organic layer of soils under the colony.Organic nitrogen dominated among different nitrogen forms (ammonium nitrogen and nitrates were present in the surface layer in much lower concentrations -approximately 200 mg N-NH 4 kg −1 and 40 mg N-NO 3 kg −1 , respectively).Regarding phosphorus, the highest concentration was found in the deeper soil layers below the colony.Statistically, the concentrations of chemical elements in the soil profiles located in the areas under the colony were significantly different from the concentrations at the control stations (ANOVA, p <0.005).Moreover, an increased concentration of nutrients and other chemical elements in comparison with the control stations was found in the groundwater under the cormorant colony.The underground water under the colony contained a significantly larger amount of dissolved mineral salts -the electrolytic conductivity was almost 5 times higher than at the control station.In addition, nitrogen and phosphorus concentrations in groundwater were significantly higher.Comparing the abandoned colony with the control station (an island not occupied by cormorants), it was found that the floristic composition of the phytocenosis of the forest is very low.All trees were destroyed by birds, and the island is covered with nitrophilous Sambucus nigra.A dense canopy of Sambucus nigra trees created shading, so there are no grasses at all.Only in the transition zone between the land (colony) and the lake ecosystem are some herbaceous plants: Urtica dioica and Phragmites australis.Plant biodiversity noted on the control island is much higher -6 species of trees and shrubs and 17 species of herbaceous plants.
Marion et al [5] found large differences in nutrient content between soil within bird colonies and areas without them at Lake Grand Lieu (western France) (8 times more nitrogen and 42 times more phosphorus at nesting colony sites).In Poland, despite a small proportion of nutrients introduced by cormorants directly into the Dobczyce Reservoir, a high concentration of nutrients was found in the soil and sediments within the cormorant colony [6].In another research, soil from an island occupied by birds was characterized by higher concentrations of NO − 3 , NH + 4 , and total N than soil from an island without colonies [7].Similarly, soil samples in the area of cormorant colonies had many times higher content of all investigated chemical elements compared to control samples.The amount of NO − 3 in the soil of the great cormorant colony in Kąty Rybackie (northern Poland) was 94-216 times higher in the surface layer and 7-12 times higher in the deeper layer than in the control plots.The high concentration of NO − 3 showed that nitrification is a very important and intensive process in soil exposed to bird colonies [8].

Impact on ornithocompexes
The relationship of the great cormorant with other waterbirds depends, first of all, on the characteristics of the species that nest in mixed settlements with the great cormorant, as well as the specifics of the nesting sites -the type of nesting biotope, the lack of suitable nesting areas, etc.The impact can be both direct and indirect, and can be both negative and, in some cases, positive.
The analysis of the available data on the 56 species associated with the great cormorant in 20 mixed settlements (table 1) allows us to state that, depending on the colony, the composition of the avifauna complex can vary greatly, ranging from 3 to 20 species.On the Obytichna spit, the growth of the great cormorant population affects the species composition and the number of other satellite birds, although this is clearly manifested only at high values.The limited nesting of great cormorants on 1-2 islands (until 2002) did not affect other nesting bird species, but after the spread of cormorants on 5 islands (2007-2008), significant changes in bird communities became noticeable.The number of species of the avifauna complexes of the Obytichna spit is inversely proportional to the increase in the number of great cormorants.Until 2002, 8-11 species of waterbirds nested here, and in 2007-2008 -only three species.Since 1993, the cormorant has gradually displaced such species as Egretta alba, Egretta garzetta and Ardea cinerea, as well as Sterna hirundo and Sterna albifrons from the nesting sites.In conditions of scarcity of nesting sites on the islands, only Larus cachinnans can nest together with the great cormorant (with its high number).Experiencing territorial competition, he nevertheless takes advantage of joint nesting by eating clutches, cormorant nestlings and collecting food scraps in his colonies.
After the cormorant changed its nesting strategy from terrestrial to arboreal, predation by gulls became even more active.Research conducted during 2015-2021 showed that the first gulls appear in the colony even before sunrise, at 4 o'clock in the morning.After 1.5-2 hours, massive wave-like raids of gulls begin with a frequency of about once an hour.They show the following tactics: while one part of the birds circles over the nests, scaring the cormorants, the other destroys the nests.At the same time, cormorants are reluctant to defend their nests, in most cases they simply fly away or sit on nearby trees.
The intensive increase in the number of cormorants on the spit provides, in addition to Larus cachinnans, a number of other species, in particular, Pica pica and Corvus cornix, as a food base.Pica pica on the islands steal eggs from terrestrial colonies of cormorants (n = 12), Larus cachinnans (n = 3), Sterna hirundo, Talasseus sandvicensis (n = 32), and in reeds -eggs from nests of herons (Ardea cinerea, Egretta alba, E. garzetta) (n = 5), warblers (Acrocephalus arundinaceus, A. scirpaceus) (n = 3), small nestlings of great cormorants and herons are occasionally dragged (n = 7).Therefore, in the conditions of the spit, Pica pica have developed and maintain new specific biocenotic relationships.During the nesting period, she actively gathers food in the nearest colonies of great cormorants, gulls, terns and herons on sea outcrops along the edge of the water, makes significant flights, up to 500-2000 m from the nest outside the nesting territories (data from route records and visual observations).
Corvus cornix is also an active predator of cormorant eggs and young nestlings.This, possibly unintentional, "cooperation" between Corvus cornix and Larus cachinnans has often been observed: when gulls raid the colony, scaring the cormorants and forcing them to leave their nests, the nests are simultaneously "visited" by crows, which take out eggs and nestlings.The increase in the number of Corvus cornix on the Obytichna spit is directly correlated with the increase in the number of cormorant colonies observed in recent years.To a lesser extent, similar behavior is characteristic of Corvus cornix, for example, on the Velyki and Mali Kuchuhury islands, where it is a companion species for all cormorant colonies known here.
We observe a different situation in Syvash.The competition of species for nesting territories determines the conditions of their existence, which we traced on the islands of Central and Eastern Syvash for the great cormorant and Larus cachinnans, as the most massive colonial species.Research (retrospective analysis and own data) covers the period from the late 1990s to 2021, when the location, number and species composition of colonial bird settlements were characterized by the method of absolute records.
The main perennial nesting sites of this pair of species have been identified.In Central Syvash, this is Kytai island, and in the East -the islands of Chonhar, Koyanly, Arabat spit, Soleprom and Polyhonna spit.The analysis of the dynamics of the number of species showed the relationship of these areas, expressed in the redistribution of colonies, in which the increase in the number in Eastern Syvash led to its decrease in Central and vice versa.Territorial relations between birds nesting on the Kytai island, are determined by the limited capacity of nesting biotopes, and in the pair "great cormorant -Larus cachinnans there is a noticeable inverse correlation (r = −0.54),which confirms the territorial competition of these species.At the same time, in relation to Larus ichthyaetus, which also nests on the Kytai island, such competition is not observed.
The great cormorant forms 5 sub-colonies on the island among the associations of Suaeda salsa and Halocnemum strobilaceum.Larus cachinnans also forms 5 sub-colonies in the areas devoid of vegetation, as well as among Suaeda salsa and Ajuga chia.Larus ichthyaetus forms 2 sub-colonies among the associations of Hordeum murinum and Cerastium holosteoides with interspersed Matricaria perforate.At the same time, 1 subcolony of Larus cachinnans and 2 subcolonies of the cormorant were located in the immediate vicinity of the subcolony of Larus ichthyaetus, among the areas occupied by Hordeum murinum and Cerastium holosteoides.Due to the fact that neither Larus cachinnans nor the cormorant occupies the nesting stations of Larus ichthyaetus, but its number is decreasing, it can be assumed that there is some reduction in the areas occupied by Hordeum murinum and Cerastium holosteoides by the cormorant and Larus cachinnans, which start nesting before Larus ichthyaetus.The conducted studies prove this: these types of plants are subject to intense trampling and the influence of guano, which leads to a reduction in areas.
On the Chonhar islands, with a noticeable increase in the number of great cormorants with a maximum in 2012 (2.500 nests), there was no noticeable decrease in the number of subdominant species, among which Larus cachinnans is traditionally present (r = −0.31).The maximum number of colonial birds on the Koyanly islands is about 17.000 nests, with smaller numbers there is no territorial competition.A regularity was revealed, in which the increase in the number of cormorant species not only does not lead to a decrease in its number, but also significantly correlates with it (r = 0.57).A similar picture is observed on the Polyhonna spit.On the islands of Soleprom, a significant decrease in the number of associated species has been observed over the past 15 years due to joint territorial pressure from Larus cachinnans and the great cormorant.The cormorant occupies large areas, and Larus cachinnans acts as an active predator in relation to other species.Thus, between the great cormorant and Larus cachinnans, with a shortage of suitable nesting areas, the competition is fully manifested in Central Syvash.To a lesser extent, competition exists on the islands of Eastern Syvash, where situations where these species did not compete were also observed.Rarer and more interesting were the facts of the joint influence of great cormorants and Larus cachinnans on the species accompanying them in the colony.
On the islands of Tendra and Yahorlyk bays, according to the nature of the impact of the great cormorant on the most massive species of the island bird complex, the latter can be divided into two groups: 1 -species on which the great cormorant has a negative impact to varying degrees; 2species on which the nesting of the great cormorant has a positive effect.First of all, it should be pointed out that the great cormorant is a strong territorial competitor and the places occupied by the colonies of this species, as well as the areas adjacent to them, become inaccessible for nesting of most of the traditional for the islands' colonial Charadriiformes species.In addition, great cormorant nesting sites attract many Larus cachinnans to the island, whose predation usually makes nesting on the same island almost impossible for other species of gulls and terns, with the exception of Larus ichthyaetus and Sterna hirundo.Also, the cormorant's ground nesting places on the islands very quickly become devoid of vegetation.Destruction of vegetation makes areas where cormorants' nest unsuitable for nesting ducks.The great cormorant has a definite positive effect on Pelecanus onocrotalus.This species is the center of social attraction for the pelican.Other researchers believe that the nesting colonies of the great cormorant for Pelecanus onocrotalus are a marker of the security of the nesting territory.In any case, the presence of nesting colonies of the great cormorant turns out to be an important factor in the restoration and further preservation of the nesting settlement of Pelecanus onocrotalus in the Black Sea Biosphere Reserve.
Another species on which the great cormorant has a positive effect on the islands of Tendra and Yahorlyk bays is Larus cachinnans.And although, at the first stages of the formation of a nesting settlement on the Kins'ki islands, the great cormorant displaced the gulls from their traditional nesting biotopes in the lower part of the island, where a rather significant death of its nestlings due to rushes was noted, in the future the joint the nesting of these two species turned out to be very beneficial for Larus cachinnans.Colonies of great cormorants are a very rich source of food for gulls, which they obtain here both by kleptoparasitism (passive and active) and by predation on eggs and nestlings.It should also be mentioned that some species can use old abandoned colonies of the great cormorant.Pica pica and Somateria mollissima 1254 (2023) 012013 IOP Publishing doi:10.1088/1755-1315/1254/1/01201311 can nest in the nests of old colonies, and within the abandoned colonies themselves, where the vegetation has not yet had time to recover, sometimes nesting Thalasseus sandvicensis and Sterna hirundo.According to the above-mentioned authors, in the conditions of low-lying islands, often flooded during rains and driving winds, such a strategy can increase the survival of these species.However, the scale of such a positive effect is extremely insignificant and is completely neutralized by the whole set of negative effects of the great cormorant on the valuable components of the island avifauna complexes of the Tendra and Yahorlyk bays.

Conclusion
An analysis of both published and original information concerning the great cormorant in the region allows us to draw the following conclusions: 1.The activity of cormorants affects various components of the natural complexes of the Azov-Black Sea region of Ukraine.Since the great cormorant is an obligate ichthyophage, it exerts a certain influence on the fish productivity of the coastal zones of the Azov and Black seas; fish resources experience a heavy load from the species, especially during the feeding of nestlings.2. Its impact on herbaceous vegetation is manifested both in a change in the species composition of phytocenoses and in the subsequent disappearance of vegetation in certain areas.Tree vegetation is also negatively affected -mechanical damage to trees by birds when collecting branches for nests, their destruction under the weight of nests, drying of trees due to the effect of large doses of nitrogen and phosphorus compounds from birds' excrements.And in the case of a large area and number of cormorant colonies, the drying of trees occurs on large areas, which can reach tens of hectares, which undoubtedly causes damage to forestry.3. The high number of cormorants leads to a reduction in the number of some other species of birds, impoverishment of nesting bird complexes (herons, terns); in the conditions of a shortage of nesting sites on the islands, only Larus cachinnans can nest together with the great cormorant (with its high number).

Table 1 :
[9]ociated species in great cormorant colonies in the Azov-Black Sea region of Ukraine (Nesting colonies,[9]and our data).