Between 1997 and 2005, a study was made of the Chrysididae, Eumeninae, and Spheciformes wasps in the Arribes del Duero Natural Park (Provinces of Salamanca and Zamora, western Spain), a highly heterogeneous Mediterranean landscape. We collected, respectively, 127, 57, and 230 species of these groups, constituting ≈50% of the species known for the Iberian Peninsula. The inventory was fairly complete according to the final slope of the species accumulation curves. From a biogeographic point of view, the predominant elements of the Arribes del Duero fauna are Mediterranean in the broad sense, together with a high percentage of species of Euro-Atlantic distribution. The proportion of endemic species obtained is similar to those known for the whole of the Iberian Peninsula. The species endemic to the northern subplateau and to the southwestern quadrant predominate. The Arribes del Duero territory is the northern limit of the distribution of some Iberian-Maghrebine species, although it is also the southern limit of species widely distributed throughout central and northern Europe. The Atlantic influence in the territory has facilitated the persistence of some species, with an Atlantic or sub-Atlantic distribution, related in particular to riparian forests. This space constitutes a large eco-corridor that joins the north of the Peninsula to the south, linking communities corresponding to the Eurosiberian and Mediterranean biogeographic regions and to territories encompassed within the Temperate and Mediterranean macrobioclimates. Thus, because of its geographic situation and extensive latitudinal range, together with the fact that it has a good representation of European biodiversity, the Arribes del Duero Park is proposed as a priority area for insect diversity conservation in the Mediterranean region.
Considering the large diversity of insects and the current rate of species loss (Dunn 2005, Samways 2006), the identification of areas with great entomological diversity and with an abundance of endemic and/or rare species (Andriamampianina et al. 2000, Lobo and Martín-Piera 2002, Gutiérrez et al. 2004) is a highly necessary task. Within this context, the Mediterranean basin is considered to be one of the hotspots of biodiversity (Mittermeier et al. 2005). This is partly because of the maintenance of traditional soil uses; highly variable biogeographic patterns and agro-forestry practices can be differentiated within its complex and heterogeneous landscapes (in both space and time). From the botanical point of view, two main centers of biodiversity can be distinguished: a western one, which would include the Iberian Peninsula and Morocco, and an eastern one, which would encompass some parts of Turkey and Greece (Médail and Quézel 1999). Regarding entomological diversity, two centers of endemism coinciding with those areas have been defined (Lumaret and Lobo 1996). Because of its location in Europe, the Iberian Peninsula (included in the western center of biodiversity and endemism) is the richest area, and the number of species of insects inventoried has been estimated at 46,900 (≈31% of those known in the Mediterranean basin (Ramos et al. 2001). Part of its richness is because of the fact that the Iberian Peninsula includes the limit between Eurosiberian and Mediterranean biogeographic regions. Hence, the natural areas placed around this boundary are particularly important within the European biota. This is the case of the Arribes del Duero Natural Park (herein Arribes del Duero for short), which hosts 44% (12 of 27) of the amphibian and 46.3% (19 of 41) of the reptile species present on the Iberian Peninsula (Pleguezuelos et al. 2002); 51.2% (124 of 242) of the species of breeding birds (Martí and del Moral 2003); and 58.6% (54 of 93) of the terrestrial mammals (Palomo et al. 2007). However, knowledge of the insect fauna of the Arribes del Duero is scarce and limited. The butterflies have been reasonably well sampled, but the available information is not reliable at fine scale (Romo and García-Barros 2005). There are just five publications addressing insect communities (González et al. 1998,2002; Gayubo et al. 2000, Baselga and Novoa 2003; De la Nuez et al. 2003). One aim of this paper is to determine to what extent the figures shown by the groups known thus far are also found in solitary wasps.
Because of the difficulties involved in obtaining exhaustive information about insect diversity for conservation strategies, different groups with different biological behaviors must be selected, serving as indicators (Samways 2005, McGeoch 2007). In this paper, we use three taxa of Hymenoptera Aculeata as indicator groups: Chrysididae, Eumeninae, and Spheciformes. These three groups pertain to different functional guilds, thus providing a representation of the ecological diversity of all solitary wasps. The Spheciformes wasps are predators that are very efficient as indicators of changes resulting from cultivation (Duelli and Obrist 1998) and have been proposed for studies addressing entomological diversity within Europe (Gayubo et al. 2005). In the vast majority of cases, the Eumeninae, the group with the greatest diversity within the Vespidae, have a solitary way of life, although some show subsocial habits. They are predators and provision their nests with the larvae of diverse insects to feed their offspring (Carpenter and Cumming 1985). The Chrysidids form a group of wasps that act as parasitoids of other insects, mainly solitary wasps and bees (Kimsey and Bohart 1990). On the Iberian Peninsula, the taxonomy and natural history of these three groups can be considered to be fairly well known. Moreover, they are found in a large number of habitats, although some more specialized species have a restricted distribution, such that they offer guarantees of being good biodiversity indicators (McGeoch 2007).
Materials and Methods
More than 120 km in length from north to south, the Arribes del Duero Natural Park, with a surface area of 106,105 ha, is located on the western side of the provinces of Salamanca and Zamora, in the Regional Community of Castile and Leon (Spain) (40°50′-41°35′ N, 6°00′-6°41′ W; Fig. 1), on the border with Portugal. The large dimensions of this natural space, together with its clear north-south orientation, account for its high biodiversity, and it is one of the three areas with these characteristics on the SCIs and SPAs lists of the Natura 2000 network in Spain and the only one in the northern half of the country (Hidalgo 2002). It is a high-quality protected area with singular ecological and geomorphological traits. The morphological homogeneity of the surrounding plains, whose altitude ranges between 700 and 800 m.a.s.l., is broken up by the gorges of the region. The river Duero and its tributaries run through an extraordinary labyrinth of canyons and gorges (called "arribes") (Sanz et al. 2003), with rocky cliffs that often exceed 400 m. The climate, which is affected by the relief (Calonge-Cano 1990), is characterized by fairly mild annual temperatures (11°C, mean for the whole area), an almost complete absence of frosts during the year, and a rainfall of ≈700 mm/yr—characteristic of a Mediterranean mesoclimate. It has outstanding biological importance both for the northern interior part of the Iberian Peninsula and for the elevated plains of the Regional Community of Castile and Leon (Calonge-Cano 1990).
This territory can be said to be a good example of a mosaic and very heterogeneous landscape, with a marked alternation of habitats and land uses. The north-south orientation, along with the marked altitudinal differences in the Duero valley, with protection from winds and marked insolation, have led to the development of a rich diversity of plants. This has enabled the permanence of highly significant, relict plant communities characterized by an abundance of typically Mediterranean species (Navarro and Valle 1987, Santos et al. 2006), where Quercus rotundifolia forests, oak woods with Q. pyrenaica, and formations of cork oak (Q. suber) are predominant (Oria de Rueda and Díez 2003). Because of their greater rainfall and higher temperatures, in the valleys there are many cultivated species such as olives, almonds, oranges, and lemons, which are unusual at these latitudes (Crespo-Redondo 1968). In contrast, on the plains, with a more extreme climate but easier to work, free-range livestock rearing is the main activity, alternating with cereal crops and private orchards (Calabuig 2008).
Selection of Study Sites and Sampling Protocol.
To study the solitary wasp fauna of the Arribes del Duero, between 1997 and 2005, Malaise traps were used, which are recommended for inventories of flying insects in large areas (Finnamore et al. 2002). To ensure that the samples would cover the broadest variety of habitats and land uses (Southwood and Henderson 2000), the 37 traps were placed at different altitudes on sites representative of the plant formations and characteristic crop types (Fig. 1; Table 1). The traps remained installed for a whole year, and samples were collected fortnightly. On all sampling days, an exhaustive review of their state of preservation was carried out. In zones where initially we observed greater diversity or where it was not possible to install Malaise traps, linear transects were followed, sweeping with entomological nets (over different types of vegetation and substrates) and yellow pan traps (Fig. 1). Finally, all existing records were coalesced, although these data are sparse and do cover most of the territory (see Appendix 1).
Two Malaise traps were set up: one of them completely white (as at all sites) and the other with black walls and a white top.
At this site, the white trap was maintained for 2 consecutive yr, obtaining two annual samples.
Estimation of Total Species Richness.
The data collected were from samples with different sample effort units or grain size: transects and traps. To gain an approximation of the total species richness of solitary wasps, we used the following nonparametric estimators: abundance-based coverage (ACE), Chao1, Jack1, and Jack2 (first-order and second-order Jackknife estimators), and Bootstrap. These five species richness estimators are precise despite grain variations (Hortal et al. 2006). The necessary calculations were done using the EstimateS 8.0 program (Colwell 2006).
Additionally, an estimation based on the species accumulation curve was carried out, using the number of samples to represent sampling effort (Table 1; Appendix 1) and randomizing the data input 100 times with the EstimateS 8.0 program. The values were fitted to the Clench equation, as recommended by Soberón and Llorente (1993), which has been shown to fit many arthropod groups quite well (Petersen et al. 2003, Jiménez-Valverde and Lobo 2004). This function is expressed as:
Assessment of the Quality of the Inventory.
As reported by Jiménez-Valverde and Hortal (2003), it is impossible to record all the species present in a given area. This is a serious methodological problem when attempting to study insect communities, and of arthropods in general. Because nearly all fauna inventories are incomplete, it is necessary to make estimates of total richness. Those based on species accumulation (see Soberón and Llorente 1993) afford more reliable results in comparative faunistic studies addressing different territories, landscapes, or geographic areas. Additionally, they enable the quality of the faunistic inventories to be evaluated (Jiménez-Valverde and Hortal 2003).
Thus, the inventory completeness was assessed by calculating the observed proportion of species richness (Sobs) out of the total predicted by the nonparametric estimators (Sest). Likewise, as recommended by some authors (especially Hortal and Lobo 2005), the final slope of the species accumulation curve was calculated. For the Clench equation, the slope of the curve in a point is r(n) = a/[1 + (b × n)2]. When n is the total number of simples, the value of the slope is the rate of incorporation of new species into the inventory. Mean values of one indicate a good approximation to a complete inventory, in which all the species present have been discovered.
Bioeographic Study of Taxocenosis.
To asses the importance of the Arribes del Duero from a biogeographic point of view with respect to the Iberian Peninsula and Europe, each of the species represented was given a specific geographic distribution category. Because the two types of regional faunas of the Hymenoptera Aculeata in Europe can be differentiated as Temperate and Mediterranean (Barbier 1999), the distribution categories considered are broad: Holarctic, Palaearctic, Eurasian, Euro-Atlantic, and Mediterranean.
With regard to endemicity, species were considered to be endemic when their distribution area was ≤6 × 105 km2, a surface almost identical to that of the Iberian Peninsula (5.8 × 105 km2) (Lumaret and Lobo 1996). Each of the Iberian species was ascribed to a class of endemism defined by the above authors as a proportion of the total surface area of the West Palaearctic region. The four geographic range- size classes considered have the following surface areas: I = 12 × 103 km2; II = 6 × 104 km2; III = 12 × 104 km2, and IV = 6 × 105 km2.
Finally, the differences in community composition of the samples from the northern part of the park and those obtained in the southern part were examined with an analysis of similarities (ANOSIM), using PRIMER v5 (Clarke and Gorley 2001).
A total of 414 species were collected in the study: 18 genera and 127 species of Chrysididae (61.1% of the 208 species known from the Iberian Peninsula; Mingo 1994, Linsenmaier 1999); 21 genera and 57 species of Eumeninae (42.9% of the 133 species known from Iberian Peninsula; Castro et al. 2008); and 63 genera and 230 species of Spheciformes (49% of the 461 species known from Iberian Peninsula; Pulawski 2008). The 230 species represent 38.3% of the ≈600 species of the Spheciformes fauna of Europe (Bitsch and Leclercq 1993, Bitsch et al. 1997,2007). The 3 families and 11 subfamilies present in continental Europe (sensu Pulawski 2008) are also represented.
The relationships and abundance of species, distributed in the corresponding families, subfamilies, and tribes, are shown in Appendix 1.
Total Species Richness Estimated and Inventory Completeness.
The values of nonparametric estimates of species richness for the three groups are provided in Table 2. In the mathematical analysis of the species accumulation curves, a good fit for the species richness values (coming from the randomized samples) to the Clench asymptotic model (percentage of explained variance: R2 = 0.9975, Chrysididae; R2 = 0.9929, Eumeninae; and R2 = 0.9955, Spheciformes; P < 0.001; Fig. 2) was obtained. The values of the asymptote, which indicates the number of species predicted for the Arribes del Duero, were 153, 63, and 247, respectively. The fauna inventoried varies from 70 to 89 (Chrysididae), 80 to 96 (Eumeninae), and 87 to 96% (Spheciformes) with respect to the total number of species predicted by the nonparametric estimators for the taxocenosis studied (Table 2). Because the proportion of species found was >70% estimated, and the values obtained for the final slope of the species accumulation curve were in the three cases <1 (0.55, 0.17, and 0.46, respectively), the inventory can be considered fairly complete.
Sest, estimated species no.; Sobs, no. of species observed; Sobs/Sest, percentage of representation.
From the biogeographic point of view, the Arribes del Duero has a predominance of Mediterranean species (in the broad sense, including the western Mediterranean, the Ethiopian-Mediterranean, and the Iberian-Maghrebine species), with ≈46.5 (Chrysididae), 29.8 (Eumeninae), and 36.1% (Spheciformes) of species represented (Fig. 3). However, the presence of elements of Euro-Atlantic distribution should be stressed, which for the Chrysididae and Spheciformes represents almost 20% of the species.
Additionally, the proportions of species obtained with an exclusively Iberian distribution were 13.4 (Chrysididae), 14.1 (Eumeninae), and 7.4% (Spheciformes). These values are similar to those known for the whole of the Iberian Peninsula. Species ascribed to endemism classes II and III predominate; in particular, those endemic to the northern subplateau and to the southwestern quadrant of the Peninsula.
At a local scale, on comparing the distribution of biogeographic elements at the different sites sampled with Malaise traps, greater similarity may be seen between the four localities in the northern part of the park and for a group of the eight localities in the southern part, with differences between the two groups (ANOSIM; R = 0.279, P = 0.048, 495 permutations).
From the extensive inventory of the groups of solitary wasps used as biodiversity indicators, the Arribes del Duero can be said to harbor approximately one half of the species known in the Iberian Peninsula and a very high number of taxa already known in Continental Europe. These data agree with those described for the different groups of vertebrates (see above). Besides the importance of species richness as a means for the evaluation and conservation of a given natural space, rarity also is important (Rodrigues and Gaston 2002, Williams et al. 2008), as may also be inferred from the wasp data of the Arribes del Duero. Specimens of several species were collected that are potentially new to science (see Appendix 1), as has been found with other groups of insects (Novoa and Baselga 2007). Demographic rarities with large local populations were also recorded. Examples include Alysson tricolor Lepeletier and Serville or the Iberian endemic Bembecinus carpetanus (Mercet). For certain taxa, the Arribes del Duero represents the limit of their geographic distribution (see remarks in Appendix 1).
Species that are distributed throughout the warmer zone of the western quadrant of the Mediterranean region and that are particularly abundant in warmer, southern areas of the Iberian Peninsula have their northern limit in the Arribes del Duero, as is the case of certain Iberian-Maghrebine elements. This may be because of its Mediterranean mesoclimate, which makes it particularly favorable for the northwestern sector of the Peninsula. The Arribes del Duero is also at the southern limit of species widely distributed throughout central and northern Europe, a limit also shown by Diptera (Carles-Tolrá 2004). From the distribution of earthworm species in the western Iberian Peninsula, Rodríguez et al. (1997) found that the limits between the Eurosiberian and Mediterranean Regions are not always well defined and that there are overlapping transition zones; in particular, the zone studied here. In turn, Baselga and Jiménez-Valverde (2007) have reported that the Chrysomelidae (Coleoptera) fauna present at the Arribes del Duero strongly resembles that of certain mountainous zones in Galicia, even though there is a predominance of species with a Mediterranean distribution, and also that of some sectors of the Iberian Central System mountain range. The Arribes del Duero contains some Palaearctic species that, in the Iberian Peninsula, are distributed across the northern mountain ranges and the Iberian Central System.
The diversity of the Arribes del Duero fauna can be explained thanks to the length and north-south orientation, because significant differences can be attributed to the latitude of the sampling sites. Also important are the Atlantic forest formations, where certain species are isolated. In this sense, the different patches studied of subhumid oak caches of Quercus pyrenaica (characteristic of this part of western Iberia at altitudes of ≈700 m.a.s.l), a well-conserved chestnut grove, with a rich nemoral flora, and, in particular, the zone where the River Tormes meets the Duero, a site known as "Ambasaguas," are of relevance. This is the only zone of the territory where it is possible to find a valley bottom with riparian forests (in this case alder [Alnus glutinosa Linnaeus (L.) Gaertn.] groves, which are characteristic of water courses that are not subject to pronounced drops in levels). Since the 1930s, because of its peculiar geomorphological configuration, the whole of the international stretch of the Duero between Spain and Portugal has been exploited for the production of hydroelectricity. The river is plagued with dams.
Nevertheless, one of the most striking aspects is that this protected area is a conspicuous European "biogeographic crossroad." Detailed analysis of the current geographic distribution of Iberian Peninsula species indicates that four types of distribution area, or chorotypes, occur, which for simplicity will be referred to here as chorotypes 1-4: 1, Southern Mediterranean; 2, Southwestern Iberian-Coastal Mediterranean; 3, Eurosiberian; 4, Mountainous (Fig. 4).
Species that colonize the warmest zones of the southwestern quadrant of th e Iberian Peninsula have their northern limit in the Arribes del Duero and are fairly abundant. Of special interest are the crabronid wasp Miscophus albufeirae Andrade and, among the Chrysididae, Hedychridium carmelitanum Mercet (Table 3). This chorotype also corresponds to the known distribution on the Iberian Peninsula of other insect species, such as Iberian-Maghrebine elements Agapanthia annularis (Olivier) (Coleoptera: Cerambycidae) and Gonioctena aegrota F. (Coleoptera: Chrysomelidae), both present in the zone studied (Baselga and Novoa 2003, Calvo Sánchez 2004).
ADNP, Arribes del Duero Natural Park.
Included are species that are distributed on the Iberian Peninsula throughout the warmer areas of the southwestern quadrant and on the Mediterranean coast, their southern limit (as in the previous case) corresponding to the Arribes del Duero. There are some examples of different groups of insects, such as Phytoecia rufipes (Olivier) (Coleoptera: Cerambycidae), which are species in the Iberian Peninsula that are linked to olive cultivation (Calvo Sánchez 2004, González et al. 2007). Among the different species of solitary wasps represented, of relevance are Odynerus eburneofasciatus Dusmet, Leptochilus medanae (Gribodo), and Tachysphex albocincta (Lucas) (Table 3). Also, among the aculeates, the citation of Arachnotheutes turgidus (Tournier) (Pompilidae) should be added. This is an eastern Mediterranean element represented in the Arribes del Duero (Wahis 2005).
The Atlantic influence in the territory has facilitated the continued presence of some plants with an Atlantic or sub-Atlantic distribution that are strongly linked to aquatic media, such as Littorella uniflora (L.) Ascherson or Cicendia filiformis L. Delarbre (Santos et al. 2006). Also, there are plant associations endemic to this part of the Duero valley that harbor a certain number of species with a northern distribution (Aguiar et al. 2003, Bernardos et al. 2004). The examples of insect species are less numerous but include Megalocoleus tanaceti (Fallén) (Hemiptera: Miridae), a Eurosiberian species, extended throughout Europe and reaching Kazakhstan (Kerzhner and Josifov 1999, Casaseca 2003), and Chaetocnema obesa (Boieldieu) (Coleoptera: Chrysomelidae), a European element (Baselga and Novoa 2003). Among the Spheciformes wasps is Rhopalum coarctatum (Scopoli) (Crabronidae), a Eurasian wasp common in Europe except on the Iberian Peninsula (Gayubo et al. 2006), which has been found at the Arribes del Duero at the locality called Ambasaguas (Table 3).
Some Eurosiberian and Circumboreal plants spread beyond the refuges of the northern mountains of Iberia to reach habitats typical of this type of landscape. This is the case of Cephalanthera damasonium (Mill.) Druce, which is found in riparian forests on the banks of the Duero (Santos et al. 2006). Some examples of insect species found in the Arribes del Duero are as follows: Heterocordylus tibialis (Hahn) (Hemiptera: Miridae) (Casaseca 2003), Lytta vesicatoria L. (Coleoptera: Meloidae) (García-París et al. 2003), Chrysolina latecincta (Demaison) (Coleoptera: Chrysomelidae) (Baselga and Novoa 2003, Petitpierre 2005), and the butterfly Nymphalis antiopa L. (Aguado 2007). The best examples among the aculeates are the Spheciformes Crossocerus vagabundus (Panzer) and Nitela spinolae Latreille and the cuckoo wasp Cleptes putoni Buysson (Table 3).
Complementing the above, consistent with botanical studies (Santos et al. 2006), in the Arribes del Duero, apart from endemisms, there are species that are of great interest as biogeographic indicators that show important traits and characteristics of the fauna of the territory: these are rarities, among which it is possible to distinguish relict species (which have been stranded as a result of geological or climate phenomena over a long period of time) and species whose populations are considered finicolous (those that show their distribution limit in the area).
Proposal as a Priority Area for Insect Diversity Conservation.
As reported, the Arribes del Duero is a territory where biogeographic assemblages intersect, a "biogeographic crossroad," and as such acquires a high value in the process of setting conservation priorities (Spector 2002). Furthermore, it is a long ecological corridor (for all forms of life) that joins the north of the Iberian Peninsula with the south, linking communities corresponding to the Eurosiberian and Mediterranean biogeographic regions, to the Atlantic-Central European and Western Mediterranean subregions, to the Cantabrian Range and the Iberian Central System, and to territories encompassed within the Temperate and Mediterranean macrobioclimates (Rivas-Martínez et al. 2004). This corridor permits the communication of the different populations and maintains the integrity of the Iberian ecosystems, preventing habitat fragmentation.
The Arribes del Duero is a place that is indeed very important for regional biogeographical processes and therefore for the development of successful reserve networks. Because of its geographic situation, broad latitudinal range, biogeographical significance, and ecological characteristics (a protected area of great entomological richness and with a strong representation of European biodiversity), the Arribes del Duero fulfils all the conditions needed to be a candidate as a priority area for insect diversity conservation in the Mediterranean region and for inclusion in European projects, as it has been within the sphere of the diversity of Europe's landscapes because of the high value of its landscape resources: spectacular geomorphologic landscapes in the gorges (Wascher and Pérez-Soba 2004). Nevertheless, further research aimed at determining the species richness of other groups of insects is necessary, including time series, both at species and community levels.
The authors acknowledge the collaboration of the Park Rangers and several residents in the zone. They also thank Prof. F. Strumia (Università di Pisa) and F. Sanza and L. Castro for their help in the identification of the Chrysididae and Eumeninae, respectively, and M. E. Archer (York St. John College of the University of Leeds) for critical reading of the manuscript. This work was mainly funded through a specific collaborative agreement between the Environmental Agency of the Regional Government of Castile and Leon and the University of Salamanca for the analysis of the diversity of the entomological fauna of the network of Natural Spaces in Castile and Leon, by means of the use of bioindicator wasps, and Project CGL2004-01006/BOS (M.E.C.).
List of species studied, indicating the number of individuals collected. GD (geographical distribution): H = Holarctic; P = Palaearctic; EAS = Eurasian; EAT = Euro-Atlantic; M = Mediterranean; IB = Iberian endemism. CE: class of endemism. Codes of samples studied: 1 = old bibliographic records; 2 = communities analyzed by González et al. (1998) and Gayubo et al. (2000) in an orchard zone; 3 = sporadic sampling with entomological net and yellow pan traps; 4 = sampling performed with yellow pan traps and the locality known as "Ambasaguas" (Villarino, province of Salamanca) between 2000 and 2004; 5 = sample from Ambasaguas obtained by sweeping with entomological net (2000-2004); 6 = sampling with yellow pan traps performed in the orchard zone at Villarino (Salamanca) in 2001; 7 = sum of the 37 samples obtained with Malaise traps, installed over a complete annual cycle in the different habitats and land uses characteristic of the territory. See list of potentially new species for science, remarks, and references at the end of the appendix.