S De

La Peña-Lastra

C Gómez-Rodríguez

A Pérez-Alberti

Franck Torre

X L Otero
email: xl.otero@usc.es

Effects of a yellow legged gull (Larus michahellis) colony on soils and cliff vegetation in the Atlantic Islands of Galicia National Park (NW Spain)

Keywords:

come   L'archive ouverte pluridisciplinaire

Introduction

Seabirds are one of the most numerous bird groups in the world, including over 1100 million individuals and comprising around 3.5% of all birds [START_REF] Del Hoyo | Handbook of the Birds of the World[END_REF][START_REF] Otero | Seabird colonies as new global drivers in the nitrogen and phosphorus cycles[END_REF]. During the breeding season, most species concentrate in large colonies in coastal areas, strongly impacting the vegetation and leading to the transformation of plant communities [START_REF] Sobey | The relationship between herring gulls and the vegetation of their breeding colonies[END_REF][START_REF] Hahn | Estimating the contribution of carnivorous waterbirds to nutrient loading in freshwater habitats[END_REF][START_REF] Hiradate | Effects of soil erosion and seabird activities on chemical properties of surface soils on an oceanic island in Ogasawara Islands[END_REF][START_REF] Zwolicki | Importance of marine-derived Nutrients supplied by planktivorous seabirds to High Arctic Tundra Plant Communities[END_REF].

The effect that seabird colonies have on vegetation and flora is complex and multidirectional and as such is sometimes difficult to identify. Thus, low bird densities lead to increased biodiversity, plant biomass and plant height, as well as to enhanced seed dispersal, owing to "ornitheutrophication" of the soil [START_REF] Anderson | Nutrient fluxes from water to land: seabirds affect plant nutrient status on Gulf of California islands[END_REF][START_REF] Sanchez-Piñero | Bottom-up dynamics of allochthonous input: direct and indirect effects of seabirds on islands[END_REF][START_REF] Otero | Seabird colonies as new global drivers in the nitrogen and phosphorus cycles[END_REF]. Nevertheless, seabird colonies often reach high densities, and the pressure they exert on the substrate and vegetation can dramatically transform characteristic plant formations in coastal environments. Uprooting of plants (for nest building), trampling, soil compaction and eutrophication greatly decrease the biodiversity of vegetation and lead to the introduction of allochthonous ruderal and nitrophilous species [START_REF] Kamijo | Effects of short-tailed shearwater on vegetation in Great Dog, Little Dog and Little Green Islands, Tasmania[END_REF][START_REF] Hiradate | Effects of soil erosion and seabird activities on chemical properties of surface soils on an oceanic island in Ogasawara Islands[END_REF][START_REF] Ishida | Effects of the common cormorant, Phalacrocorax carbo, on evergreen forests in two nest sites at Lake Biwa, Japan[END_REF][START_REF] Baumberger | Plant community changes as ecological indicator of seabird colonies' impacts on Mediterranean Islands[END_REF].

For the aforementioned reasons, newly established colonies can sometimes conflict with conservation programs that target certain species or communities. In the Atlantic Islands of Galicia National Park (AINP), a dramatic change in plant communities was observed in coastal cliffs where a yellow-legged gull colony was established, and a new plant community associated with the gull colony was described (e.g. Calendulo algarbiensis-Parietarietum judaicae; [START_REF] Guitián | A paisaxe vexetal das illas Cíes. Xunta de Galicia[END_REF]. Nevertheless, most plant formations present in colonies within the AINP remain uncharacterized [START_REF] Arcea | Influencia da colonia de gaivota común sobre a biodiversidade das illas Cíes: efecto sobre outras aves de interese, sobre a vexetación e sobre os solos de acantilados do Parque Natural das Illas Cíes[END_REF]. In addition to the formal description of plant communities (i.e. identification of the species in each plant assemblage), gradient analysis of vegetation can be used to investigate how plant assemblages respond to environmental factors, such as gull-driven eutrophication. This type of analysis should be based on appropriate metrics of biotic dissimilarity that enable discrimination of two opposed patterns: replacement of some species by others (i.e. turnover) and overall loss of species [START_REF] Baselga | Partitioning the turnover and nestedness components of beta diversity[END_REF].

The AINP includes one of the best representations of cliff vegetation in the NW Iberian Peninsula; in addition, some of the representative species are rare or endemic [START_REF] Guitián | A paisaxe vexetal das illas Cíes. Xunta de Galicia[END_REF]. The plant species are subjected to high levels of environmental stress (e.g. high salinity, strong winds, shallow soils), as well as to pressure from one of the world's largest yellow-legged gull colonies (Larus michahellis) (Barros 2015; [START_REF] Otero | Parque Nacional Marítimo Terrestre de las Islas Atlánticas de Galicia[END_REF]. However, many of the factors influencing the distribution and dynamics of coastal plant communities are unknown. Long-term monitoring is used by different researchers and recommended for studies aiming to determine changes in habitats and species [START_REF] Tilman | Experimentos Ecológicos: Fortalezas y problemas conceptuales[END_REF][START_REF] Stohlgren | Planning Long-Term Vegetation Studies at Landscape Scales[END_REF].

The "ornitheutrophication" [START_REF] Otero | Seabird colonies as new global drivers in the nitrogen and phosphorus cycles[END_REF] of marine ecosystems by seabirds is an environmentally important factor that substantially alters floristic diversity and plant community structure in colonies. The impact of faecal deposition by seabirds and other vertebrates (e.g. the Arctic fox) on the vegetation of subarctic ecosystems has been widely studied [START_REF] Zhu | Geochemical evidence for rapid enlargement of a gentoo penguin colony on Barton Peninsula in the maritime Antarctic[END_REF][START_REF] Croll | Introduced predators transform subarctic islands from grassland to tundra[END_REF][START_REF] Ellis | Nutrient transfer from sea to land: the case of gulls and cormorants in the Gulf of Maine[END_REF][START_REF] Zmudczyńska-Skarbek | An assessment of seabird influence on Arctic coastal benthic communities[END_REF][START_REF] Gharajehdaghipour | Arctic foxes as ecosystem engineers: increased soil nutrients lead to increased plant productivity on fox dens[END_REF][START_REF] Fafard | Nutrient deposition on Arctic fox dens creates atypical tundra plant assemblages at the edge of the Arctic[END_REF], while fewer studies have been conducted in temperate-humid areas [START_REF] Sobey | The relationship between herring gulls and the vegetation of their breeding colonies[END_REF][START_REF] Breuning-Madsen | Influence of perennial colonies of piscivorous birds on soil nutrient contents in a temperate humid climate[END_REF][START_REF] Baumberger | Plant community changes as ecological indicator of seabird colonies' impacts on Mediterranean Islands[END_REF][START_REF] Vidal | Seabirds drive plant species turnover on small Mediterranean islands at the expense of native taxa[END_REF]. In the present study, longterm monitoring of the effect of yellow-legged gull colonies on soils and cliff vegetation was conducted, and the relative importance of the following factors was analyzed by distance-based RDA analysis (dbRDA) of dissimilarity indices discriminating turnover and nestedness components of the variation in plant assemblage: (1) soil properties (pH, electrical conductivity), (2) macronutrient content (N-NO 3 -, N-NH 4

+

, and bioavailable P), (3) gull influence (absent, low, intermediate and high density) in different subcolonies in the AINP and in a control site (without gulls)

Materials and methods

The Atlantic Islands Natural Park of Galicia (AINP) is located in the NW Iberian Peninsula (Fig. 1). Seven subcolonies of yellow-legged gull located on cliffs of the main archipelagos (Cíes and Ons) and also a control site with no gulls, located on a cliff in Cabo Home, were selected for study. The cliffs were selected on the basis of the distribution of the main plant communities described in previous studies [START_REF] Arcea | Influencia da colonia de gaivota común sobre a biodiversidade das illas Cíes: efecto sobre outras aves de interese, sobre a vexetación e sobre os solos de acantilados do Parque Natural das Illas Cíes[END_REF] and gull density (Fig. 2, Table 1,S1). Gull density in the AINP has undergone major variations in recent years: the population in the Cíes Islands decreased from 22,220 breeding pairs in 1991 to 3520 breeding pairs in 2015, while the population in the Ons Islands fell from 3747 breeding pairs in 1991 to 2242 in 2015 [START_REF] Munilla | Estudio de la población y la ecología trófica de la gaviota patiamarilla, Larus cachinnans Pallas, en Galicia[END_REF][START_REF] Barros | Censo da poboación reprodutora de corvo mariño cristado (Phalacrocorax aristotelis), gaivota patiamarela (Larus michahellis), gaivota escura (Larus fuscus) e gaivotón (Larus marinus) no Parque Nacional Marítimo-Terrestre das Illas Atlánticas de Galicia: resultados de 2015[END_REF]; Supplementary Material Table S1).

Sampling, floristic inventories and soil analysis

Soil sampling was performed at three different times a year in the periods 2011-2014 and 2016-2017: (1) February-March, before the gulls returned to the subcolonies; (2) August-September, at the end of the breeding season, i.e. when the impact of the gulls on the soil was maximal; and (3) December-January, after strong precipitation and in the absence of gulls.

One to three permanent sampling plots (7 × 5 m), depending on the environmental conditions of each cliff, were established in each subcolony (total, 14 plots). Targeted sampling was performed in each plot by randomly throwing a 50 × 50 cm metal square into the air 6 to 10 times and sampling within the area delimited by the square on the ground. The (0-15 cm) layer of soil was removed within three such squares and composite surface soil sample was obtained for each plot by mixing the three soil subsamples. The plant species present within each square were also inventoried. Cover of each was determined by the Braun-Blanquet method (1979): cover categories: 1: <10%, 2: 10-25%, 3:25-50%, 4: 50-75% and 5: >75%.

Soil analysis was performed on the fine soil fraction (<2 mm), after careful removal of the coarse plant debris (roots and stems): granulometry was determined by the Robinson pipette method; pH was determined in a soil: water ratio of 1:2.5 [START_REF] Buurman | Manual for Soil and Water Analysis[END_REF]; electrical conductivity (EC) was measured in a 1:5 soil/water suspension; and total organic carbon (TOC) was analyzed in a Leco CNS1000 autoanalyzer. Exchange cations (Ca 2+ , Mg 2+ , Na + , K + , and Al 3+ ) were extracted using a 1 M NH 4 Cl solution [START_REF] Bertsch | Methods of soil análisis. Part 3. Chemical Methods[END_REF], except for Al, which was extracted from 5 g of soil with a 1 M KCl solution. The concentrations of the exchange cations were determined by atomic absorption spectrophotometry (Perkin Elmer model 1100B). Exchangeable ammonium (N-NH 4

+

) and nitrate (N-NO 3 -) were extracted from 5 g of fresh soil in 50 ml of a 2 M KCl solution [START_REF] Mulvaney | Nitrogen-Inorganic forms[END_REF]. Bioavailable phosphorus (P-ba) was extracted by the Mehlich 3 method [START_REF] Mehlich | Mehlich 3 soil test extractant: a modification of the Mehlich 2 extractant[END_REF]. Briefly, the P-ba fraction was extracted from 2 g of air-dried soil in 20 ml of Mehlich-III solution (2 M CH 3 COOH, 0.25 M NH 4 NO 3 , 0.015 M NH 4 F, 0.013 M HNO 3 and 0.001 M EDTA), by first shaking the mixture for 5 min and then filtering it through a n • 42 Whatman filter.

The extract was then stored at 3 ± 1 • C until analysis [START_REF] Mehlich | Mehlich 3 soil test extractant: a modification of the Mehlich 2 extractant[END_REF]. The P-bio was determined colorimetrically by the molybdenum blue method, at a wavelength of 800-nm [START_REF] Bowman | A rapid method to determine total phosphorus in soils[END_REF].

Statistical analysis

In order to determine the impact of the gulls on cliff vegetation in the AINP, a group of 17 plant species were selected for analysis. The selected species had been identified in previous studies as typical of the plant communities on cliffs in the NW Iberian Peninsula or as ruderal (characteristic of disturbed environments) and nitrophilous species (Guitián andGuitián, 1989, 1990;[START_REF] Fernández | Flora y Vegetación de las Islas Cíes (Parque Nacional Marítimo-Terrestre de las Islas Atlánticas de Galicia)[END_REF], Veen et al., 2009). The characteristic and accompanying species in plant communities of cliffs in the NW Iberian Peninsula selected were Angelica pachycarpa, Anthyllis vulneraria iberica, Armeria pubigera pubigera, Crithmum maritimum, Festuca rubra pruinosa, Leucanthemum merinoi, Matricaria maritima, Plantago coronopus, Rumex acetosa biformis, and Silene uniflora, while the ruderal (characteristic of disturbed environments) and nitrophilous species selected were Calendula suffruticosa algarbiensis, Cochlearia danica, Dactylis maritima, Holcus lanatus, Leontodon autumnalis, Parietaria judaica, and Urtica membranacea.

Two-way ANOVA was used to compare the influence of gulls (control, colonies) and season (spring, summer, winter) on soil nutrients, acidity and salinity (i.e. electrical conductivity) in the plots. All statistical tests were performed using SigmaStat software version 3.5.

Differences in plant community composition among plots were assessed following the beta diversity partitioning framework developed by [START_REF] Baselga | Partitioning the turnover and nestedness components of beta diversity[END_REF]. Pairwise turnover and nestedness components of biotic dissimilarity were computed using the 'beta.pair' command in the R package "betapart" [START_REF] Baselga | Betapart: an R package for the study of beta diversity[END_REF]. Community composition was based on presence/absence data of each plant species in each plot (species-by-site matrix) over the entire study period.

To assess the relationship between environmental variables and plant community composition, turnover and nestedness-resultant dissimilarity matrices were used as response variables in independent , N-NO 3 -and bioavailable P, P-ba) were first subjected to Principal Component Analysis (PCA) with varimax rotation. Three PCA components, accounting for 76% of the variance, were retained for downstream db-RDA analysis. A manual forward stepwise selection of predictors based on significant contribution to the model (p < 0.05) was applied using the 'dbrda' command in the R package "vegan" [START_REF] Oksanen | Vegan: Community Ecology Package. R package version 2[END_REF]. Significance was assessed with a permutation test (n = 10,000) with sequential tests for each constraining variable conducted using the 'anova' command in the "vegan" package.

When no significant predictors were selected, dissimilarities among plant assemblages were represented by Non-Metric Multidimensional Scaling (NMDS) implemented using the 'metaMDS' command in the "vegan" package.

In order to determine whether differences in plant community composition are related to current and/or historical gull densities, PERMANOVA [START_REF] Anderson | A new method for non-parametric multivariate analysis of variance[END_REF] was conducted using the turnover dissimilarity matrix, and the nestedness-resultant dissimilarity matrix was used independently as a response variable. The gull densities in 1991 and in 2011 were used as ordered grouping factors. Density levels represented by only one case were excluded from the analysis, resulting in two levels (high and intermediate) for 13 cases in 1991 and two levels (low and intermediate) for 14 cases in 2011. The analysis was conducted using the 'adonis' command in the "vegan" package.

Results

Soil properties and composition

General characterization of soils

Soils in the control site and in gull subcolonies showed limited development (15-25 cm depth). Texture was loamy sand to sandy loam, indicating a clear dominance of the sand fraction, with values over 78%, while silt (11.9 ± 2.8%) and clay (8.3 ± 2.6%) were much less abundant (Table 2, Fig. 3).

Mean organic C content was very high in most sites, exceeding 7% in all cases (Table 2). Regarding the cationic exchange complex, the dominant cation was Ca (mean values: 4.87-21.5 cmol (+) kg -1 ), followed by Mg (3.93-14.5 cmol (+) kg -1 ), Na (2.11-7.81 cmol (+) kg -1 ), and K (0.48-2.23 cmol (+) kg -1 ). Conversely, in almost all samples Al was either below detection limits (DL < 0.01 cmol (+) kg -1 ) or the values were very low (Table 2), which is consistent with pH in water values generally over 5.5 and with the presence of a quartz sand layer fossilizing the granitic substrate. Despite the higher concentrations of Na and Mg in seawater, the dominance of Ca in the soil cation exchangeable complex may be due to the greater affinity of colloids for this cation.

pH and electrical conductivity

The soil pH values were acidic in the control site and subcolonies, with mean values ranging between 5.0 and 7.0 (Table 3, Supplementary Material Table S2); no significant differences were observed between the control zone and the colonies, or between seasons (Table 3). Electrical conductivity ranged from environments with low ionic load to slightly saline environments. The lowest values corresponded to the Percha cliffs (PERSUP: 416 ± 227 µS cm -1 ; PERINF: 574 ± 329 µS cm -1 , Table 3), while the highest mean values were observed in soils from the control site (CH: 1308 ± 1319 µS cm -1 , Table 2) and from the PXUB plot, in Ons Island (1060 ± 562 µS cm -1 , Table 3). EC differed significantly between the control site and colonies (F 1,679 = 54.739, p < 0.001), as well as between seasons (F 1,679 = 86.223, p < 0.001, Table 3, Supplementary Material S2), and the values were generally highest in winter.

Inorganic nitrogen in the soil

The concentrations of N-NO 3, Supplementary Material Table S2).

Bioavailable phosphorus (P-ba)

Mean P-ba concentration was significantly higher (F 1,679 = 42.072, p < 0.001) in gull subcolony soils (162 ± 160 mg kg -1 ) than in the control site (38.3 ± 22 mg kg -1 ) (Table 3). However, no significant seasonal differences were observed (F 2,679 = 0.209, p = 0.811, Table 2). Plot by plot analysis revealed a gradient from sites with the highest to the lowest gull influence, which was consistent with the results obtained for N: CH (control site) <Fig. 2 <MH <Fig. 1 <PERSUP <RZ <PXUB <FP1 <FP2 <PERINF <CENA (Supplementary Material Table S2).

Flora and vegetation

The total number of vascular plant species was similar across sites. A total of 46 plant species were inventoried in the Cíes Islands subcolonies, while 49 species were inventoried in the control (Cabo Home) site and Ons Islands. However, analysis of the different subcolonies revealed substantial changes in floristic composition. Fig. 3 shows variations in plant cover for the 17 most frequent vascular plant species in cliffs within the AINP in relation to seagull influence (Table 1). Cover by the most representative taxa of plant communities in the NW Iberian Peninsula generally decreased as seagull influence increased. In the sites with the highest influence, species such as Armeria pubigera, Chrithmum maritimum and Festuca rubra are replaced by meadows dominated by the grass Dactylis maritima and with the presence of species characteristic of ruderal and eutrophic environments, such as Urtica membranacea, Parietaria judaica, Chochlearia danica, Plantago coronopus and Matricaria maritima [START_REF] Gillham | Destruction of indigenous heath vegetation in Victorian sea-bird colonies[END_REF]Veen et al., 2009) or megaphorbic formations of Angelica pachycarpa, as on north-facing cliffs (e.g. Ruzo in the Cíes Islands, or Punta Centulo in Ons Island) [START_REF] Otero | Islas Cíes. Guía del Parque Natural[END_REF].

K + Al 3+ µS cm -1 % -------------Exchangeable Cations (cmol (+) kg -1 )----------- CH 5.
The results obtained for each subcolony in relation to the degree of yellow-legged gull influence are detailed below (see also Supplementary Material Table S3). Note that a subcolony can include more than one plot, depending on the complexity of vegetation.

Site without gull presence

The total number of species in the control site was 29, and higher numbers were recorded in winter (15) than in summer (10). Armeria pubigera was the most abundant species (10-25% cover), followed by Anthyllis vulneraria iberica, Dactylis maritima and Festuca rubra pruinosa, with cover slightly below 10%. Crithmum maritimum was occasionally present. Moreover, Daucus carota gummifer was also present, while no ruderal or nitrophilous species, such as Urtica membranacea, Holcus lanatus or Parietaria judaica, were found (Fig. 3).

Sites with low and intermediate gull influence

The diversity and dominance of plant species were highest in plot Fig. 2 (30), followed by FP1 (27), MH (24), PXUB1 and Fig. 1 (22 each) and, finally, FP2 (13). In general, the numbers of different species were highest in spring and lowest in summer (Fig. 3, Supplementary Material Table S3).

Dactylis maritima, Armeria pubigera, Anthyllis vulneraria iberica and Festuca rubra pruinosa were the dominant species in plots MH, FP1, and PXUB1. However, the individual cover varied between plots. Thus, for example, in plot PXUB1, Armeria pubigera was the most abundant species on these cliffs (mean cover 25-50%), while Festuca rubra pruinosa was the most abundant species in plot MH, with cover varying seasonally between 10 and 25% and 25-50%. In plots Fig. 2, FP2 and Fig. 1, the dominant species were respectively Dactylis maritima and Silene uniflora, Calendula algarbiensis and Parietaria judaica, and Cochlearia danica and Matricaria maritima (Fig. 3, Supplementary Material Table S3).

The ruderal species Urtica membranacea was present in all the plots in some of the inventories (Supplementary Material Table S3). The invasive grass species Panicum repens (GEIB, 2007) also appeared in plot PXUB1.

Sites with high density of gulls

Ruzo (RZ) was the plot with the highest number of species (32), followed by PERINF (26), PERSUP (21) and CENA (15). Similarly to sites with low-intermediate influence, the number of species was highest in spring and lowest in summer (Fig. 3, Supplementary Material Table S3).

Dactylis maritima was present in all plots, with cover reaching 50-100% in PERSUP plot. However, Parietaria judaica was the species with the highest cover in plot RZ, and Holcus lanatus and Angelica pachycarpa were also abundant in RZ and CENA (Fig. 3, Supplementary Material Table S3).

In these plots, Urtica membranacea cover was much higher (i.e. it was one of the most abundant species in the plot, with cover generally > 20%) than in sites with low-intermediate influence, with cover even reaching up to 50% (Fig. 3, Supplementary Material Table S3).

Differences between plots

The environmental variability was summarized in three Principal Components, which accounted for 76% of the variance (Table 4, and Fig. 4). The first PCA component (RC1) was closely correlated with N-NO 3 -and P-ba (explained variance = 35%), while the second and third components were related to EC (i.e. salinity; RC2, explained variance = 22%) and N-NH 4 + (RC3, explained variance = 19%), respectively.

RC1 and RC2 significantly explained the dissimilarity in plant communities derived from the replacement of some species by others (turnover: dbRDA F 2,12 = 4.4, p = 0.008, explained variance = 42.4%), while no predictor was significant according to nestedness-resultant dissimilarity analysis (Fig. 4). In other words, N-NO 3 -, P-ba and electrical conductivity were related to changes in assemblage composition caused by the substitution of some species by others, but not to changes due to species loss. In the turnover analysis, both predictors explained a similar amount of variance, showing that their contribution to structuring plant assemblage composition was similar: RC1 (dbRDA F 1,12 = 4.8, p = 0.010) explained 23.1% of the variance and RC2 (dbRDA F 1,12 = 4.0, p = 0.038) explained 19.4% of the variance. Analysis of the turnover component of beta diversity revealed a historical signature, as

Table 3

Seabird and seasonal influence on soil properties and nutrient concentrations (mean ± standard deviation, median in brackets) and ANOVA results. Two-way ANOVA results for seabird influence, season and site * season interaction. Statistically significant effects were found for seabird influence and season for each of the dependent variables. For electrical conductivity (EC), the interaction between seabird influence and season showed a statistically significant effect, which indicates that the effect of one factor depends on the status of the other. Significant values are shown in bold.

seabirds influence -------------season--------

Discussion

Soil ornitheutrophication

Seabirds establish breeding colonies in areas of land such as cliffs and other coastal formations, in some cases reaching high densities. The continued presence of large numbers of individuals in the same space strong alters the original characteristics of the habitat [START_REF] Sobey | The relationship between herring gulls and the vegetation of their breeding colonies[END_REF][START_REF] Hahn | Estimating the contribution of carnivorous waterbirds to nutrient loading in freshwater habitats[END_REF][START_REF] Zwolicki | Importance of marine-derived Nutrients supplied by planktivorous seabirds to High Arctic Tundra Plant Communities[END_REF]. The eutrophication gradient associated with bird colonies (ornitheutrophication) explains the observed variation in plant communities, with replacement of some species by others depending on the concentration of N-NO 3 -and bioavailable P in soil. Overall, the results showed that the presence of yellow-legged gull colonies in the AINP led to ornitheutrophication, with high concentrations of inorganic N and bioavailable P but not to salinization, which has been observed in Mediterranean colonies [START_REF] García | Seagull influence on soil properties, chenopod shrub distribution, and leaf nutrient status in semi-arid Mediterranean islands[END_REF]. The high levels of precipitation in the AINP (1419 mm; De La Peña-Lastra et al., 2019) favour leaching of salt from the soils [START_REF] Otero | Phosphorus in seagull colonies and the effect on the habitats. The case of NW Spain)[END_REF]. Numerous studies have associated nitrogen enrichment with the loss of plant species diversity in temperate terrestrial ecosystems (e.g. [START_REF] Moore | Conservation of wetlands: do infertile wetlands deserve a higher priority?[END_REF][START_REF] Wheeler | Above-ground crop mass and species richness of the principal types of herbaceous rich-fen vegetation of lowland England and Wales[END_REF][START_REF] Stevens | Impact of nitrogen deposition on the species richness of grasslands[END_REF]; for this reason, many authors refer to the species present in these environments as nitrophilous [START_REF] Gillham | Vegetation of sea and shore-bird colonies on Adabra Atoll[END_REF][START_REF] Hogg | The effect of nesting gulls on the vegetation and soil of islands in the Great Lakes[END_REF]. Our results also show a clear inorganic N enrichment. However, the effect of P on plant diversity has seldom been considered. In a study performed along a P gradient, [START_REF] Wassen | Endangered plants persist under phosphorus limitation[END_REF] showed that many more endangered plant species persist under phosphorus-limitation than under nitrogenlimitation and concluded that phosphorus enrichment is more likely to be the cause of species loss than nitrogen enrichment. Our findings are Fig. 4. Ordination (PCA) of sampling plots according to the environmental variables, as summarised by the Principal Component axes: RC1 and RC2 (a) and RC1 and RC3 (b). The first PCA component (RC1) was highly correlated with N-NO 3 and P-BIO, the second PCA component (RC2) with electric conductivity (i.e. salinity) and the third PCA component (RC3) with N-NH 4 (RC3). Size colour represent gull density levels in 1991: control (black), low (red), intermediate (green) and high (blue). Symbol shape represents gull density levels in 2011: control (square), low (circle), intermediate (triangle). An interactive 3D-scatterplot of PCA scores can be also found in Supplementary Material. Site numbers are: 1) Cabo_Home (CH; control plot), 2) Monte das Herbas (MH), 3) Figueiras-1 (Fig. 1), 4, 5) Figueiras 2 (Fig. 2) 6,7) Ruzo (RZ), 8,9) Faro da Porta 1 (FP1) 10) Faro da Porta 2(FP2), 11)Percha inferior (PERINF), 12)Percha Superior (PERSUP), 13) Punta Centulo (CENA), 14, 15) Punta Xubenco (PXUB). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Fig. 5. Ordination (db-RDA) of sampling sites according to their plant species composition. Two separate ordinations were performed to describe the variation in plant communities due to the turnover (a, b) and nestedness-resultant (c, d) components of dissimilarity. Therefore, distances in the ordination space in (a, b) reflect the substitution of plant species between sites, while distances in the ordination space in (c, d) reflect richness differences in nested subsets between sites. Significant PCA correlates are shown for the turnover component (a, b) while for the nestedness component (c, d) no significant predictor was found in the distance-based RDA analysis, and hence a Non-Metric Multidimensional Scaling is used to represent the ordination of sampling sites. Size and colour represent gull density in 1991 (a, b) and 2011 (c, d). Site numbers are: 1) Cabo_Home (CH; control plot), 2) Monte das Herbas (MH), 3) Figueiras 1 (Fig. 1), 4, 5) Figueiras 2 (Fig. 2) 6,7) Ruzo (RZ), 8,9) Faro da Porta 1 (FP1) 10) Faro da Porta 2(FP2), 11)Percha inferior (PER-INF), 12)Percha Superior (PERSUP), 13) Punta Centulo (CENA), 14, 15) Punta Xubenco (PXUB). consistent with this idea. A major portion of N is leached from subcolony soils during the winter; therefore, in some subcolonies in the AINP, inorganic N content in early spring was observed to be similar to that in the control site [START_REF] Otero | Seasonal variation in inorganic nitrogen content of soils from breeding sites of Yellow-legged Gulls (Larus cachinnans) in the Cíes Islands natural park (NW Iberian peninsula)[END_REF][START_REF] Otero | Nitrógeno (NH 4 + , NO 3 -), Fósforo asimilable y Metales traza (Hg, Cd, Pb, Zn, Ni y Cu) en suelos de la colonia de la gaviota patiamarilla (Larus cachinnans) en el Parque natural de las Islas Cíes[END_REF]. However, the total P content (summer, 815 ± 189 mg kg -1 and winter, 1133 ± 194 mg kg -1 : [START_REF] Otero | Phosphorus in seagull colonies and the effect on the habitats. The case of NW Spain)[END_REF] and bioavailable P in soil remain high throughout the annual cycle as P is adsorbed to Fe and Al oxyhydroxides in acid soils (for further details, see [START_REF] Otero | Phosphorus in seagull colonies and the effect on the habitats. The case of NW Spain)[END_REF]. In this respect, P enrichment in seabird colony soils is an irreversible process, with high concentrations of P persisting thousands of years after colony extinction (see [START_REF] Hawke | Soil indicators of pre-European seabird breeding in New Zealand at sites identified by predator deposits[END_REF][START_REF] Sun | A 3000 year record of penguin population[END_REF][START_REF] Otero | Phosphorus in seagull colonies and the effect on the habitats. The case of NW Spain)[END_REF]. In addition, our results show that P bioavailability did not vary throughout the annual cycle (Tables 3 andS2), while inorganic N decreased significantly in winter (Table S2). Therefore, P should be considered a key geochemical factor in the mechanisms leading to changes in plant communities, and the species associated with colony soils should be termed "eutrophic vegetation" or "eutrophic plants" [START_REF] Nagy | Plant growth and reproduction on a toxic alpine ultramafic soil: adaptation to nutrient limitation[END_REF][START_REF] Rotha | Nitrogen deposition is negatively related to species richness and species composition of vascular plants and bryophytes in Swiss mountain grassland[END_REF] rather than nitrophilous plants, as they are usually referred to in the literature.

Effect of gull colonies on plant biodiversity and vegetation structure

Plant communities on cliffs in the NW Iberian Peninsula are arranged in strips that run approximately parallel to the coast according to their tolerance to salinity, wind-related effects and soil development [START_REF] Guitián | A paisaxe vexetal das illas Cíes. Xunta de Galicia[END_REF][START_REF] Fernández | Flora y Vegetación de las Islas Cíes (Parque Nacional Marítimo-Terrestre de las Islas Atlánticas de Galicia)[END_REF]. However, this distribution is greatly altered in yellow-legged gull colonies within the AINP.

The control site corresponds to the Crithmo maritimi-Armerietum pubigerae association, a formation that typically occupies the lower section of cliffs in the NW Iberian Peninsula. This association is characterized by the presence of halochasmophytic herbaceous plants such as Chrithmum maritimum, Armeria pubigera and Anthyllis vulneraria iberica. This association persists on cliffs with a low influence of gulls, such as FP in the Cíes Islands and PXUB in the Ons Islands, although there are some changes in terms of the floristic composition and structure (Figs. 3 and4). These differences manifest as lower levels of cover by species characteristic of this association and a higher variability in floristic inventories. The most relevant floristic aspects are the absence of Daucus carota gummifer and the presence of eutrophic and ruderal species (e.g. Matricaria maritima, Parietaria judaica or Urtica membranacea; [START_REF] Guitián | A paisaxe vexetal das illas Cíes. Xunta de Galicia[END_REF][START_REF] Arcea | Influencia da colonia de gaivota común sobre a biodiversidade das illas Cíes: efecto sobre outras aves de interese, sobre a vexetación e sobre os solos de acantilados do Parque Natural das Illas Cíes[END_REF]. For these reasons, the assemblage could be considered a facies of the Crithmo-Armerietum pubigerae association, although more detailed studies are necessary to confirm this.

The Dauco gummiferi-Festucetum pruinosae association occupies the middle section of the slope, where some incipient soil formation is occurring. This herbaceous plant formation is mainly dominated by Festuca rubra pruinosa, with 75-100% cover, and Daucus carota gummifer, Rumex acetosa biformis and Armeria pubigera, with 10-25% cover [START_REF] Rivas-Martínez | De plantis hispaniae notulae systematicae, chrologicae et ecologicae[END_REF][START_REF] Prieto | Estudio de las comunidades vegetales de los acantilados costeros de la cornisa cantábrica[END_REF]. The most similar vegetation to this formation can be found in cliffs in the AINP such as Monte das Herbas (MH), a site with low to intermediate influence of gulls and corresponding to a recently established gull colony (≈30 years; Tables 1,S1) with low bird density [START_REF] Munilla | Estudio de la población y la ecología trófica de la gaviota patiamarilla, Larus cachinnans Pallas, en Galicia[END_REF][START_REF] Arcea | Influencia da colonia de gaivota común sobre a biodiversidade das illas Cíes: efecto sobre outras aves de interese, sobre a vexetación e sobre os solos de acantilados do Parque Natural das Illas Cíes[END_REF]. In this case, differences relative to the typical association manifest as lower cover by the species that define the association, the absence of Daucus carota gummifer and Jasione gallaecica and the presence of ruderal and eutrophic species (e.g. Parietaria judaica, Urtica membranacea and Cochlearia danica, Fig. 3).

The remaining cliff sites with intermediate to high gull influence showed a marked transformation in terms of floristic composition and structure of the vegetation relative to the typical associations on cliffs in the NW Iberian Peninsula. These differences were closely correlated with P-ba and N-NO 3 -contents in soil and can therefore be attributed to the effect of subcolonies (Fig. 4). However, no pattern or gradient could be established for the effect of gulls on plants and vegetation in the AINP, possibly because the effect is due to the current and also the past presence of gulls on cliffs (see also [START_REF] Young | Plants cause ecosystem nutrient depletion via the interruption of bird-derived spatial subsidies[END_REF][START_REF] Baumberger | Plant community changes as ecological indicator of seabird colonies' impacts on Mediterranean Islands[END_REF][START_REF] Magnússon | Plant colonization, succession and ecosystem development on Surtsey with reference to neighbouring islands[END_REF], along with more local factors such as cliff slope and orientation [START_REF] Guitián | La influencia de las colonias de aves marinas en la vegetación de los acantilados del noroeste ibérico[END_REF]. In this respect, [START_REF] Barcena | Las colonias de aves marinas de la costa occidental de Galicia. Características, censo y evolución de sus poblaciones[END_REF] first referred to changes in plant communities in gull colonies within the AINP. This author observed that Silene uniflora meadows had been substituted by Angelica pachycarpa formations in northern cliffs of Monteagudo Island. [START_REF] Guitián | La influencia de las colonias de aves marinas en la vegetación de los acantilados del noroeste ibérico[END_REF] described new endemic haloornithocoprophilic communities (e.g. Calendulo algarbiensis-Parietarietum judaicae, Cochleario danicae-Matricarietum maritimae and Angelica pachycarpa) and assumed a nitrifying effect by gulls. More recently, [START_REF] Arcea | Influencia da colonia de gaivota común sobre a biodiversidade das illas Cíes: efecto sobre outras aves de interese, sobre a vexetación e sobre os solos de acantilados do Parque Natural das Illas Cíes[END_REF] tried to establish a pattern in the distribution of plant communities, taking into account the presence of seagulls as well as other environmental parameters, such as cliff orientation. However, the study findings did not enable a clear relationship among these factors to be established. The distribution of plant species along this N-NO 3 -and bioavailable P gradient in the soil reveals niche segregation (Fig. 5). Changes in nutrient availability affect interspecies competition, given that those species that can rapidly respond to increases in nutrients have a competitive advantage over species typical of oligotrophic environments [START_REF] Hill | Vascular plant success in a warming Antarctic may be due to efficient nitrogen acquisition[END_REF][START_REF] Tilman | Resources, competition and the dynamics of plant communities[END_REF]. Thus, in this case, species with a high resistance to environmental stressors (salinity, wind etc.) but a low tolerance to nutrient contributions, such as Armeria pubigera, Festuca rubra pruinosa, Anthyllis vulneraria iberica and Crithmum maritimum [START_REF] Jefferies | The effects on the vegetation of the additions of inorganic nutrients to salt marsh soils at Stiffkey[END_REF][START_REF] Kiehl | Nutrient limitation and plant species composition in temperate salt marshes[END_REF][START_REF] Otero | Seabird colonies as new global drivers in the nitrogen and phosphorus cycles[END_REF], are replaced by species that are efficient users of N-NO 3 -and bioavailable P (Angelica pachycarpa, Holcus lanatus, Parietaria judaica, and Urtica membranacea) [START_REF] Zelenskaya | Growth of the nesting colony of slaty-backed gulls (Larus schistisagus) and plant cover degradation on Shelikan Island (Taui inlet, the Sea of Okhotsk)[END_REF][START_REF] Critchley | Association between lowland grassland plant communities and soil properties[END_REF]Castroviejo, 1986Castroviejo, -2012)). These findings are consistent with those of previous studies in which N, P, and K enrichment in soils of Larus argentatus colonies were found to lead to the replacement of native perennial meadows mainly composed of Festuca rubra by a pauci-specific plant community dominated by annual ruderal species such as Atriplex hastata, Holcus lanatus, Poa annua, Rumex acetosa, R. crispus, Stellaria media and Tripleurospermum maritimum [START_REF] Sobey | The relationship between herring gulls and the vegetation of their breeding colonies[END_REF]. Likewise, [START_REF] Hogg | The effect of nesting gulls on the vegetation and soil of islands in the Great Lakes[END_REF] observed the same phenomenon in ring-billed gulls (Larus delawarensis) in the Great Lakes (Canada). More recently, [START_REF] Baumberger | Plant community changes as ecological indicator of seabird colonies' impacts on Mediterranean Islands[END_REF] also observed ruderalization of vegetation in areas with high densities of yellow-legged gull (Larus michahellis) and found that lower densities of nests did not necessarily lead to the restoration of previous plant composition patterns. In the present study, the percentage of ruderal species ranged between 10% and 25% and was particularly high in those colonies with higher densities of yellow-legged gulls, as in Punta Centulo (CENA (in the Island of Ons), Percha (PERSUP, PERINF) and Ruzo (RZ) (in the Cíes Islands). Similar values have been observed in colonies in King George Island (Maritime Antarctic), where 25% of vascular plant species were found to be significantly associated with ornithogenic fertilization [START_REF] Zwolicki | Importance of marine-derived Nutrients supplied by planktivorous seabirds to High Arctic Tundra Plant Communities[END_REF]. Moreover, these authors also observed that increases in nutrients led to a decrease in cover by some species, while eutrophic species such as Prasiola crispa became dominant.

Implications for management and research

Seabird colonies cause dramatic changes in plant communities, with native plants being replaced by new ruderal plant formations that are still poorly characterized. Soil ornitheutrophication is one of the factors explaining the observed changes in yellow-legged gull subcolonies. For approximately the last 15 years, yellow-legged gulls have been establishing new colonies within the AINP in areas considered of high environmental value due to the presence of rare or endemic species such as Corema album, Iberis procumbens and Armeria pungens (e.g. in the dunes of Punta Muxieiro and Rodas). For this reason, future efforts should be aimed at reducing the bioavailability of these two nutrients for a stronger focus on conservation management. In dune systems, the combined addition of N and P has been observed to cause a decrease or even the disappearance of characteristic pioneer dune species and mosses [START_REF] Lammerts | Nutrient deficiency in dune slack pioneer vegetation: a review[END_REF].

For the above reasons, conservation of the aforementioned taxa and of priority habitats 2130 -Fixed coastal dunes with herbaceous vegetation ("grey dunes") and 2120 -Shifting dunes along the shoreline with Ammophila arenaria ("white dunes") within the AINP may be compromised if appropriate management measures are not implemented. The measures adopted should take into account the direct effect of gulls on plants (e.g. uprooting for building nests; trampling) as well as the bioavailability and mobility of N and P in dune system soils. In fact, the waters of the Nenos lagoon, in the Cíes Islands, currently show clear symptoms of eutrophication, which may have been one of the causes of the disappearance of the Zostera marina meadow approximately 15 years ago (for more details, see [START_REF] Otero | Phosphorus in seagull colonies and the effect on the habitats. The case of NW Spain)[END_REF].

Conclusions

Yellow-legged gull colonies are powerful environmental modulators that have greatly altered nutrient biogeochemical cycles in soils and floristic composition of plant communities on cliffs in the AINP.

Colony soils are significantly enriched in nutrients (inorganic N and bioavailable P), and therefore use of the term "eutrophic plant species" is probably more accurate than the commonly used "nitrophilous species".

Yellow-legged gull colonies have irreversibly transformed the cliff vegetation, generating a new environmental system that is exclusive to this National Park and which has not yet been well defined. Species whose presence seems to be favoured by the influence of seabirds include Angelica pachycarpa, Holcus lanatus, Parietaria judaica and Urtica membranacea, while Armeria pubigera, Festuca rubra pruinosa, Anthyllis vulneraria iberica and Crithmum maritimum seem to be receding in areas that are intensely impacted by gulls.

Finally, the colonization of new areas with a high environmental value, such as the Punta Muxieiro dunes, could substantially alter this habitat, classified as priority habitat due to its plant diversity, according to European Directive 92/43/CEE.

Fig. 1 .

 1 Fig. 1. Location of study plots within the Atlantic Islands of Galicia National Park (NW Spain). H: high seabird influence, LI: low-intermediate seabird influence, CS: no influence.

Fig. 2 .

 2 Fig. 2. Characteristic plant species of cliffs in the National Park. Receding species, presumably due to the action of seagulls: (A) Armeria pubigera meadow, (B) Festuca rubra pruinosa, (C) Silene uniflora. Ruderal eutrophic species that are dominant in cliffs with high seagull influence: (D) Dactylis maritima, (E) Angelica pachycarpa, (F) Cochlearia danica, (G) Calendula suffruticosa algarbiensis, (H) Urtica membranacea.

Fig. 3 .

 3 Fig. 3. Seasonal summary of inventories performed in study plots (mean ± SD). Values correspond to plant cover categories according to the phytosociological characterization scale by Braun-Blanquet (1979): 1= <10%; 2 = 10-25%; 3 = 25-50%; 4 = 50-75%; 5 = 75-100%. Characteristic species of plant communities in cliffs in the NW Iberian Peninsula were Angelica pachycarpa (Angel), Anthyllis vulneraria iberica (Anth), Armeria pubigera pubigera (Arme), Crithmum maritimum (Crith), Festuca rubra pruinosa (Festu), Leucanthemum merinoi (Leuca), Matricaria maritima (Matri), Plantago coronopus (Planta), Rumex acetosa biformis (Rumex), 2), and Silene uniflora (Silene). Characteristic ruderal (disturbed environments) and nitrophilic species were Calendula suffruticosa algarbiensis (Calend), Cochlearia danica (Cochl), Dactylis glomerata maritima (Dact), Holcus lanatus (Holcu), Leontodon autumnalis (Leon), Parietaria judaica (Parie), and Urtica membranacea (Urti).

Table 1

 1 Characteristics of plots in the AINP. The influence of seagulls was established on the basis of the census carried out in 1991 (see TableS2).

	Atlantic island of Galicia Natural Park (Spain). Huso 29.
	Plot	Coordinates UTM	Orientation	Plant association	Representative species **	influence of gulls
		(XY)
	Cabo Home (control zone)
	Cabo Home (plot CH)	511270.408	W	Grassland of Armeria pubigera	A. pubigera	Control plot, without
		4680186.325				seagulls
	Cíes islands Monte Herbas (plot MH)	507595.772	N-NE	Dauco gummnifer-Festucetum pruinosae	Festuca pruinosa; Koeleria glauca	Low (<30 pairs ha -1 )
		4674540.514
	Figueiras 1 (plot FIG1)	507,662,000 4675667.000	N-NO	Cochleario-Matricarietum maritimae	Cochlearia danica; Matricaria maritima	Intermediate (30-100 pairs ha -1 )
	Figueiras 2 (plots FIG2a,b)	507663.946	SW	Grassland of Dactylis maritima y Silene	Silene uniflora;
	Faro Porta 1 (plots FP1a,b)	4675668.334 507173.000	S-SW	uniflora Grassland of Armeria pubigera	Dactylis glomerata A. pubigera	High (>100 pairs ha -1 )
		4673353.600
	Faro Porta 2 (plot FP2)	507120.000	S	Calendulo algarbiensis-Parietarietum	Chrithmum maritimun; Calendula
		4673364.000		judaicae	algarbiensis
	Ruzo (plots RZ a,b)	507086.817	S-SW	Grassland of Holcus lanatus	Holcus lanatus; A. pachycarpa
		4673685.139
	Percha Inf. (plot PERINF)	507452.250	N-NW	Cochleario-Matricarietum maritimae	Cochlearia danica; Matricaria
		4676835.310			maritima
	Percha Sup. (plot PERSUP)	507468.880	N-NW	Grassland of Dactylis maritima y Silene	Silene uniflora;
		4676710.230		uniflora	Dactylis glomerata
	Ons Island
	Punta Centulo (plot CENA)	506376.120	N-NE	Community of Angelica pachycarpa	A. pachycarpa	*No data for 1991
		4694105.095
	Punta Xubenco (plots	505646.900	O-NO	Grassland of Armeria pubigera	A. pubigera
	PXUB1 a,b)	4693728.900

distance-based Redundancy Analysis (db-RDA), which allows the use of non-Euclidean dissimilarity indices

[START_REF] Legendre | Distance-based redundancy analysis: testing multispecies responses in multifactorial ecological experiments[END_REF]

. The mean and standard deviations of environmental variables related to natural gradients (i.e. pH and conductivity) and "ornitheutrophication" gradients (i.e. N-NH 4 +

  ± 33 mg kg -1 ) (Tables 3, Supplementary Material TableS2). A gradient was identified in the subcolony soils, ranging from high values in sites with stronger gull influence (high density and long history of breeding), such as the Percha, Ruzo and Punta Centulo cliffs, to lower values in lower density and/or more recently established colonies (see alsoOtero et al., 2015, Table 3and Supplementary Material TableS2). On the other hand, N-NO 3

	3 -and N-NH 4 + were significantly lower (N--: F 1,679 = 38.234, p < 0.001; N-NH 4 NO 3 + : F 1,679 = 12.555, p < 0.001) in the control site soils (N-NO 3 : 16.4 ± 13 and N-NH 4 + : 22.5 ± 18 mg kg -1 ) than in gull subcolony soils (N-NO 3 -: 43.3 ± 37 and N-NH 4

+ : 35.1 -and N-NH 4 + showed significant seasonal variations (N-NO 3 -: F 2,679 = 3.593, p = 0.028; N-NH 4 + : F 2,679 = 4.283, p = 0.014), with the lowest values occurring in winter, intermediate values in spring, and the highest values in summer (Table

Table 2

 2 General characteristics of soils.

	pH	EC	TOC	Ca 2+	Mg 2+	Na +

Acknowledgements

The authors thank the authorities and guards at the Atlantic Islands National Maritime-Terrestrial Park for providing support during fieldwork. This work was financed by a 2016 Leonardo Grant for Researchers and Cultural Creators, the BBVA Foundation, the Autonomous National Parks Organization of the Spanish Ministry for the Environment, Rural and Marine Affairs (Ref. 041/2010) and the Consellería de Educación, Universidade e Formación Profesional-Xunta de Galicia (Axudas á consolidación e estruturación de unidades de investigación competitivas do SUG del Plan Galego IDT, Ambiosol Group, ref. 2018-PG036). Saúl de La Peña was in receipt of a predoctoral fellowship from the FPU Program of the Spanish Ministry of Education and Innovation. Thanks are also due to María José Santiso, for assistance with laboratory work, and to Esther Sierra Abraín, Pablo Montero and Adrián González, for help with sampling.

Authors' Contributions

SPL, XLO, and APA conceived and designed the study and performed the research; FT, CG-R, SPL and XLO carried out the statistical analysis; SPL, CG-R, AG and XLO wrote the paper with the collaboration of FT and APA.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary material

Supplementary data to this article can be found online at https://doi. org/10.1016/j.catena.2020.105115.