CONSERVATIONAL STATUS AND DEMOGRAPHIC CHARACTERISTICS OF PATELLA FERRUGINEA GMELIN, 1791 (MOLLUSCA, GASTROPODA) ON THE ALBORÁN ISLAND (WESTERN MEDITERRANEAN)
Mariano Paracuellos1, Juan Carlos Nevado1, Diego Moreno2, Adela Giménez1 & Juan José Alesina1
1 Dpto. de Flora y Fauna, Consejería de Medio Ambiente, Junta de Andalucía, Centro Residencial Oliveros, E-04071, Almería, Spain.
2 C/ Araña, apto. Las Dunas, 2, E-04150, Cabo de Gata, Almería, Spain
Animal Biodiversity and Conservation, 26(2). En prensa. Edición digital: Almediam, julio de 2003
Conservational status and demographic characteristics of Patella ferruginea Gmelin, 1791 (Mollusca, Gastropoda) on the Alborán Island (Western Mediterranean).- Due to the high risk of the global extinction in which Patella ferruginea Gmelin, 1791 is found, it is considered of great interest to describe and quantify its demographic characteristics in those sites where it still persists, as well as to evaluate the reasons which have led this limpet to be one of the most threatened marine species in the Mediterranean Sea. Over the study period (2000-2002), systematic census were made on the perimeter of the Alborán Island (Alborán Sea, westernmost area of the Mediterranean Sea) with the object to quantify the abundance of the species in the locality, as well as their external biometry and spatial distribution. As a result, the presence of a probable reproductive population of P. ferruginea was actually found on the island. The negative effect provoked by the continuous presence of man was proved, prejudicing the population in those zones which were more accessible for their harvesting. For this reason, it is necessary to regulate the use of the natural resources of the island to favour the conservation and spontaneous recolonisation of the zone by P. ferruginea.
Key words: Patella ferruginea, Alborán Island, population, conservation, human influence, Western Mediterranean.
Estado de conservación y características demográficas de Patella ferruginea Gmelin, 1791 (Mollusca, Gastropoda) en la Isla de Alborán (Mediterráneo Occidental).- Dado el peligro de extinción global en el que actualmente se encuentra Patella ferruginea Gmelin, 1791, se considera de gran interés describir y cuantificar las características poblacionales de la especie en aquellas localidades donde aún persiste, así como evaluar las razones que han llevado a situar a esta lapa como una de las especies marinas más amenazadas de extinción del Mediterráneo. Durante el período de estudio (2000-2002) fueron realizados muestreos sistemáticos en el perímetro de la Isla de Alborán (Mar de Alborán, extremo occidental del Mediterráneo) con objeto de cuantificar la abundancia de la especie en la localidad, así como su biometría externa y distribución espacial. Como resultado, se ha constatado la presencia en la isla de una población de P. ferruginea probablemente reproductora. Se ha comprobado el efecto negativo provocado por la continua presencia humana en la isla, perjudicando a la población en aquellas zonas más accesibles para la recolección de ejemplares. Es por ello que se considera necesario regular el uso de los recursos naturales en la isla con objeto de favorecer la conservación y recolonización espontánea de la localidad por P. ferruginea.
Palabras clave: Patella ferruginea, Isla de Alborán, población, conservación, influencia humana, Mediterráneo Occidental.
Patella ferruginea Gmelin, 1791 is an endemic species of the Western Mediterranean Sea which, although widespread distributed in this basin during the Pleistocene, in prehistorical and historical times has entered obvious regression owing to the human predation and it is now considered in high risk of extinction (Imperatori, 1968; Templado, 2001). Given this situation, the species has been catalogued in the Annex II of the Convention Relative to the Conservation of the Wildlife and Natural Environment in Europe (Bern Convention, European Council, 19 September 1979; Royal Decree of 13 May 1986; BOE, 235, 1 October 1986) as a strictly protected species, in the Annex II of the Barcelona Convention (Monaco, 24 November 1996) as “endangered or threatened”, in the Annex IV of the Directive 97/62/CE of the Council (Habitat Directive, of 27 October; DOCE, L 305, 8 November 1997) as with a communitarian interest that requires the strict protection, as well as in the National Catalogue of Threatened Species (Order of 9 June 1999; BOE, 148, 22 June 1999) as “in risk of extinction”. Among the causes that are usually described as probable factors of the decline of the species are that the limpet lives on the rocky substrates usually above sea level (upper mesolittoral), easily detectable by man, and that reach considerable size as adult, making it very appreciable for human consumption and for decoration. Other causes that must have influenced its actual scarcity and loss could have been the progressive deterioration of the coastal strip where the species inhabit, the low fecundity of the individuals and their scarce dispersal ability (Laborel-Deguen & Laborel, 1991a, 1991b; Templado, 2001). Therefore, it is considered of great interest to describe and quantify its population structure in those areas where it still persists, as well as to evaluate the reasons that have situated this limpet as one of the marine species with the highest risk of extinction in the Mediterranean Sea (Porcheddu & Milella, 1991; Templado, 1998). However, although various authors have analysed diverse aspects of the distribution and biology of P. ferruginea (Curini-Galletti, 1979; Grandfils, 1982; Biagi & Poli, 1986; Boudouresque & Laborel-Deguen, 1986; Laborel-Deguen & Laborel, 1990, 1991a, 1991c; Porcheddu & Milella, 1991; Moreno, 1992; Laborel-Deguen et al., 1993; Cretella et al., 1994; Aparici-Seguer et al., 1995), to date there are few studies that have quantified the repercussions of the human factor as the cause of its spatial distribution and regression of the populations (nevertheless see, Laborel-Deguen & Laborel, 1991b).
The presence of P. ferruginea on the Alborán Island is described in different studies (i. e., García Raso & Salas, 1984; Yus & Cabo, 1986; Rubio, 2001). However, although some of them point out the species is very abundant in the area (Salas & Luque, 1986), Templado (2001) observed a very low number of individuals (28 in 1996 and 13 in 1998) during the last census (“Fauna IV” and “Alborán 98” respectively), underlining the possibility that the population was, in those years, not viable as reproductive, owing to the low number of detected specimens and that they were distanced among each other, which made fecundation difficult.
In this study, the current diverse aspects related to P. ferruginea on the Alborán Island are described, paying special attention to its demographic characteristics, external biometry, spatial distribution and repercussion of the human factor on its presence and development.
Materials and methods
The Alborán Island (Almería; 35º56’N, 3º02’W) is placed in the centre of the Alborán Sea (westernmost area of the Mediterranean Sea), 46.5 miles from the Iberian coast and 30 from the Maghreb coast, being a small and remote promontory of 7.1 ha in size, 605 m in length, 265 m wide, with a maximum height of 14 m and a perimeter of about 2,000 m of shoreline (Fig. 1).
The lithological nature of the area is essentially volcanic, mainly made up of an andesite tufa substrate (Hernández-Pacheco & Ibarrola, 1970). Its surface is flat, with a contour of vertical cliffs or steep slopes of 8-12 m in height along most of its perimeter (except for two small beaches/landing places). At the base of the border of the island, where the action of waves occurs, there is a horizontal platform which is uncovered at low tide, forming a surrounding stretch of 15-20 m in average width. Apart from the main island, there are some small inlets close to its shores, for example the Nube inlet or the inlet near to the Cuevas Viejas (Fig. 1). Therefore, with the exception to the existent beaches, the rest of the contour is favourable for the colonisation of P. ferruginea.
Mankind has been present on the island throughout history, but mainly since the construction of a lighthouse and associated buildings in the XIX century (Archduke Luis Salvador, 1898; Rubio, 2001). Since 1997 a permanent military garrison composed of a normal complement of 12 members exists. In relation to the man uses in the locality, an Integral Reserve within a Natural Reserve has been declared since 1997 on the surroundings of the island, in the interior limits of which inhabit P. ferruginea. Therefore and since its protection, it is expressly forbidden to collect or fish the flora and fauna in this area (Order of 31 July 1997; BOE, 204, 26 August 1997; Guirado et al., 1999). At present, the global protection of the whole space in and around the Alborán Island as a “Paraje Natural” and the official approval of the Development Plan of the Natural Resources of the Alborán Island are underway, both measures which would help to guarantee the future conservation of the species and its habitat on the island (Agreement of 20 December 1998, of the Council of the Government of the Junta de Andalucía; Pinilla, 2001; Dirección General de Planificación, 2002).
The monitoring was undertaken from 2000 to 2002. A first survey was made in June 2000 to only estimate and map the specimens, continuing with total and supplementary census in October 2001 and in July 2002 to the definitive collection of data. The samplings were always carried out during the day, taking advantage of the optimal environmental conditions (calm day at low tide).
With the aim to quantify the population and distribution of the species on Alborán, systematic census of the individuals were carried out around the entire island. For this, transects which surrounded the island and its inlets were made, surveying the mesolittoral level of the peripheral platform, and the close zones of the supra and infralittoral levels. The transects were always done by 1-3 people, using a boat along the stretches where the accessibility was difficult or impracticable. The following data were taken in situ on each of the observed individuals: (1) location (using cartography), (2) length (widest diameter of the shell), (3) width (narrowest diameter of the shell) and (4) height (elevation of the shell from the base to the apex) (also see Grandfils, 1982; Porcheddu & Milella, 1991; Aparici-Seguer et al., 1995). The shells were measured with a vernier calliper, with a precision to a 0.1 mm. Digital photographs of all individuals located were taken in full length.
Taking into account the historical presence of man on Alborán, its accessibility to the potentially habitable zones for the species was evaluated, with the aim to analyse the pressure provoked by the collection for human consumption on the population and distribution of P. ferruginea on the island. For this, it was predicted that the more accessible intervals of shoreline for man must be more exposed to exploitation, presenting a lesser number of specimens and being more reduced in size (less appreciated) than those less accessible zones, with a hypothetical higher number of specimens and bigger sizes (more appreciated), taking into account the specimen’s length as a standard measure (see as well, Laborel-Deguen & Laborel, 1991b). To quantify the level of accessibility to shoreline, the perimeter was divided into 20 strips of similar extension according to the map. In function of the cartography and estimates in the area, two variables directly associated to the human capability of access to the areas were taken into account. On the one hand, the accessibility by land to each of the defined sectors was evaluated. In relation to this, it was determined how (i) highly accessible strips to the shore zones which could be accessible on foot along its entire length from the interior of the island, (ii) medium accessible strips to the shore zones which could be accessible on foot for only part of its length from the interior of the island, and (iii) no accessible strips to the shore zones which are not accessible on foot from the interior of the island. On the other hand, the level of access by sea to the different sectors was estimated, taking into account the distance by swimming or by boat from the nearest beach/landing place to the midpoint of the established sectors (the farther from the beaches/landing places, the more effort to collect and the less accessibility level by sea).
For the statistic analysis, the Analysis of Variance (ANOVA, applying the Duncan Test post hoc) was used, as well as the Simple and Multiple Regression Analysis (Sokal & Rohlf, 1994). When it was needed and in function of the nature of the data, certain variables were used after its trigonometrical transformation.
80 specimens of P. ferruginea were located on the Alborán Island during the previous sampling in June 2000, as well as 92 and 111 during the later systematic census in October 2001 and July 2002 respectively. The number of individuals of this species found in each stretch of shore is shown in Fig. 1, and one of the specimens measured can be seen in Fig. 2. Their length ranged between 21 and 96 mm, showing a bimodal distribution, with a maximum between 30 and 40 mm and another, of a bigger magnitude, between 70 and 80 (Fig. 3a). The width of the individuals varied between 15 and 80 mm, the bimodal distribution being less apparent, with the greatest number between 60 and 70 (Fig. 3b). The height of the shells varied between 7 and 50 mm, the highest number being between 30 and 40 (Fig. 3c). When the different biometrical measures were compared between them, the adjustments to the lineal relations explained a great percentage of the variance with high coefficients of regression (Fig. 4).
The average density of the specimens encountered was 0.06 individuals/m over the entire rocky shoreline. The distribution of the limpets was not homogeneous along the whole perimeter of the island, given that the greater part of the population was concentrated in its northeastern sector (Fig. 1). Taking into account the different available accesses by land to the delimited strips of littoral, it could be observed that significative differences existed in the density of the limpets according to the degree of access, owing to the fact that the more easily transited areas from the interior of island had a lesser number of specimens/m of shore than the less transited (Fig. 5a), principally due to the existing contrasts between the intervals with high and null accessibility (P = 0.004). Although the located limpets in the more inaccessible areas by land were in many cases bigger in size than those found in more accessible zones, there were no significative differences in the length of their shells according to such variable (Fig. 5b). When relating the distance by sea from the nearest beach/landing place to each of the strips with its average number of limpets/m of shore and with the average size of the encountered individuals, a direct and significative relationship could be observed between the variables. In this sense, those stretches farther from the beaches or landing places usually had a higher density and a bigger size of the specimens than the closest ones (Fig. 6). However, the maximum percentage of variance was explained when relating both variables of accessibility jointly, by land and sea, with the number of limpets/m of shore in each strip: D = 0.07 – 0.04 arcsine (√(x)) + z (r2 = 0.53, F2,17 = 9.63, P = 0.002, N = 20); and with their average size in each strip: S = 65.30 – 1.96 arcsine (√(x)) + 0.03 z (r2 = 0.51, F2,13 = 6.80, P = 0.009, N = 16), where “D” represents the number of limpets/m of shore, “S” the average size of limpets, “x” the proportion of length of accessible strip by land according to the continuous values from 0 (totally inaccessible strips) to 1 (totally accessible strips), and “z” the distance to the nearest beach/landing place.
The obtained results regarding the number of the specimens of the population of P. ferruginea on the Alborán Island surpassed by more than 8 times the number cited by Templado (2001). The data given by this author show a very scarce population of the species during 1996 and 1998, disperse and composed mainly of females (large specimens), aspects which made it probably non-reproductive. However, the characteristics of the population found during the present study (2000-2002) leads one to think that it was potentially fertile. It could be favoured by the relative high number of individuals present, with the greatest numbers found in a reduced sector of shore (more than 90% of the specimens were observed in a sector of about 1,000 m), permitting the external fertilisation of the eggs. According to the obtained measures (some of the biggest observed for the species), nearly all the located individuals were possibly adults (Grandfils, 1982; Laborel-Deguen & Laborel, 1991a; Templado, 2001). In this sense, the distribution of sizes, with a maximum of specimens with a length between 70-80 mm and another between 30-40, could have indicated the mixed presence of adult females (bigger and around the former maximum) along with the reproductive males (smaller and around the latter maximum), also allowing the cross fertilisation (see Templado, 2001). However, in the near future it would be necessary to urgently approach the research on the possibility of recruitment of new specimens of this population of the limpet to evaluate its conservational status on the island over the following years.
The height vs. length ratio of the shell on Alborán Island was high when compared with those given for other populations in the Mediterranean (see data of Grandfils, 1982; Porcheddu & Milella, 1991). The conical profile of the shells in the study locality could be related to the level inhabited by the species with respect to the sea level. In this sense, limpets living in the high part of the mesolittoral normally required a greater hydric reserve and, therefore, higher shells (with a higher internal volume) than those others located in the lower part of the mesolittoral and exposed to a greater hydrodynamism, with flatter shells (Grandfils, 1982; Templado, 2001). Although in this study we didn’t estimate the level where the individuals were found, the proportionally high shells of specimens of Alborán probably indicated a place of the limpets at a certain height with respect to the sea level, the same as on the Chafarinas Islands (Grandfils, 1982).
It could be deduced that the distribution and range sizes of P. ferruginea on the island during the study period was found to be very affected by the human accessibility to the places where it lives. In this way and in spite of being a species still frequent in the locality, the initial hypothesis is verified. According to this, the zones more accessible for the collection of limpets, with easy access on foot from the interior and being near to the usual swimming areas or landing places, were found to be more uninhabited than those other areas more impassable by land and sea, which concentrated the main surviving population composed, in many cases, of older individuals. Consequently, the continued human presence on the island seems to lead to a probable decline of the limpet in some of its zones by harvesting, which could be confirmed by the presence of shell remains of the species in rubbish tips in the island (pers. obs.). Another fact that seems to argue the negative effect provoked by man on the species is related with the possible increase and recovery of its population in the last years, coinciding with the definitive installation of the military garrison and the declaration of the Integral Reserve on the island, as dissuasive factors of the despoliation on the littoral perimeter of Alborán (comparing data of the number of individuals from Templado, 2001, and present study). This confirms the need to regulate the anthropic uses in the island, as it appears in the Development Plan of the Natural Resources of the Alborán Island awaiting approval, with the object to favour the conservation and the future recolonisation of the entire littoral perimeter of the island by the existing population of P. ferruginea.
We would like to give our gratitude to Juan García and Antonio Rodríguez, the crew of the AMA VII (Junta de Andalucía), without whose ever available collaboration and help the access to the island and the sampling of the limpets wouldn’t have been feasible. The authors also like to thank to José Templado (Museo Nacional de Ciencias Naturales, Madrid) his comments as a referee, to José Carlos García-Gómez (Sevilla University) the contributed information and to Philip Kramer the translation to English.
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CAPTIONS OF THE FIGURES
Fig. 1. The Alborán Island and its geographical location. Dotted lines delimit the distinct stretches of shore taken into account, showing in each of them the numbers of specimens of Patella ferruginea found during July 2002. The accessibility by land to each of the stretches is also shown: dotted, high accessibility; striped, medium accessibility; in white, null accessibility (see Field Methods).
Fig. 1. La Isla de Alborán y su ubicación geográfica. Se delimitan mediante líneas discontinuas los distintos tramos de orilla tenidos en cuenta, mostrando para cada uno de ellos el número de ejemplares de Patella ferruginea encontrado durante julio de 2002. También se señala la accesibilidad por tierra a cada tramo: punteado, alta accesibilidad; rayado, media accesibilidad; en blanco, nula accesibilidad (ver Field Methods).
Fig. 2. Specimen of Patella ferruginea found on the Alborán Island during July 2002.
Fig. 2. Espécimen de Patella ferruginea encontrado en la Isla de Alborán durante julio de 2002.
Fig. 3. Frequency distribution found for (a) the length, (b) the width and (c) the height of the shells of Patella ferruginea in the population of the Alborán Island during October 2001.
Fig. 3. Frecuencia de distribución encontrada para (a) la longitud, (b) la anchura y (c) la altura de la concha de Patella ferruginea en la población de la Isla de Alborán durante octubre de 2001.
Fig. 4. Statistical relationships (Simple Regression Analysis) between (a) height vs. length, (b) width vs. length, and (c) height vs. width of shells of Patella ferruginea on the Alborán Island during October 2001. Average value ± SD is also expressed for each relationship (y/x).
Fig. 4. Relaciones estadísticas (análisis de regresión lineal simple) entre (a) altura vs. longitud, (b) anchura vs. longitud y (c) altura vs. anchura de Patella ferruginea en la Isla de Alborán durante octubre de 2001. También se expresa para cada relación (y/x) su valor medio ± SD.
Fig. 5. Statistical differences (ANOVA) of (a) the density (average number of limpets/m of shore + SD) and (b) the size (average length of limpets + SD) of Patella ferruginea according to the degree of accessibility by land to the different delimited stretches on the Alborán Island during July 2002.
Fig. 5. Diferencias estadísticas (ANOVA) de (a) la densidad (nº medio de lapas/m de orilla + SD) y (b) el tamaño (longitud media de las lapas + SD) de Patella ferruginea según el grado de accesibilidad por tierra a los diferentes tramos delimitados en la Isla de Alborán durante julio de 2002.
Fig. 6. Statistical relationships (Simple Regression Analysis) between (a) the density (number of limpets/m of shore), as well as (b) the size (average length of limpets), of Patella ferruginea in the different stretches delimited on the Alborán Island and the accessibility by sea to the same (distance by sea from the nearest beach/landing place) during July 2002.
Fig. 6. Relaciones estadísticas (análisis de regresión lineal simple) entre (a) la densidad (nº de lapas/m de orilla), así como (b) el tamaño (longitud media de las lapas), de Patella ferruginea en los diferentes tramos delimitados de la Isla de Alborán y la accesibilidad por mar a los mismos (distancia por mar desde la playa/embarcadero más cercana) durante julio de 2002.