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Home    Η εδαφική αραχνοπανίδα της Κρήτης (οικογένεια GNARHOSIDAE) : συστηματική, οικολογία και βιογεωγραφία  

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Identifier 37863
Title Η εδαφική αραχνοπανίδα της Κρήτης (οικογένεια GNARHOSIDAE) : συστηματική, οικολογία και βιογεωγραφία
Alternative Title Ground spiders of Crete (Araneae, Gnaphosidae) : Taxonomy, Ecology & Biogeography
Author Χατζάκη, Μαρία
Thesis advisor Μυλωνάς, Μωυσής
Abstract Spiders are an important element of the terrestrial arthropod fauna and present a high diversity in time and space. Therefore they constitute a very useful tool for any kind of zoological research. Their use in more applied studies, such as agroecology, use of their silk, biochemistry of their venom, makes them even more attractive for current issues of research. Although Greece is one of the richest areas in Europe as far as biodiversity is concerned, arachnological research is lacking. The first arachnological survey in Greece was that of Brull? (1832). More complete contributions are those of Roewer (1928), Bristowe (1935) and Hadjissarantos (1940), while contemporary research has been mainly based on temporary visits of several scientists. Despite their importance, these surveys leave data on the taxonomy and distribution of Greek spiders in a very poor state of knowledge. The current catalogue of the Greek spiders, based on the existing literature, reveals 810 species. Gnaphosidae constitute about 2000 species worldwide and they are the most abundant and one of the most diverse of all spider families on the Mediterranean basin. They show a clear preference to sunny, open grounds in arid environments. Less species are present in the woods and some are synanthropic. All Gnaphosidae are active hunters on the ground. However, this family is not well represented in the spider literature, partly because of the nocturnal activity of many species and because of their complex taxonomic characters. Several papers have dealt with Gnaphosidae in the adjacent areas, such as Italy, Israel, Lebanon and North Africa, revealing the importance of the family in the soil fauna. However, species catalogues and data on their distributions in the whole area of the Mediterranean is far from thorough. The same counts for Greece: until now 34 species were recorded from Crete and 32 from Attiki. Many of them have never been revised after their first description. The main reason for this is the difficulty to collect these animals by hand, their nocturnal activity, the lack of specialists in this family and the focus of most researchers on cave-dwelling spiders of Greece. The aim of the present study is to bring this poorly known fauna to a better state of knowledge, in order to include it in the major topics of ecology, biogeography and conservation of the south Aegean area. More precisely, this study focused on: 1. The identification of most of the species found on Crete and adjacent islands to a level that would permit safe conclusions for the rest of the analysis. 2. The study of ecological variation of the species in space and time along the island of Crete. 3. The biogeographical analysis of the species distributions along the main axes of Crete and among the islands which were included in this study, namely Crete, Antikythira, Karpathos, Kos and the satellite islands of Crete, Dia, Gavdos and Gavdopoula. Comparative material was also used from one locality in Peloponnisos, Mt. Mainalo, 1600m. For this study material from 76 sites was used. In all of them, pitfall traps were set, using ethylene glycole as a killing preservative. At each site, 15-20 traps were set and changed in two-month intervals. Material derived partly from the samplings realized for the purposes of the present study, but also from a series of projects since 1990, organized by the Natural History Museum of Crete. Comparative material loaned from several European museums was also analyzed. No other island was included in the study, as there is no current revision on the taxonomy and the distribution of their species, so data on them was considered as incomplete. Most of the taxonomical part of the study was carried out in the Zoological Institute of the University of Innsbruck, Austria under the supervision of Doz. K. Thaler. The rest of the study was carried out at the Natural History Museum of the University of Crete under the supervision of Prof. M. Mylonas. All quantitative data were transformed into numbers of individuals per 100 trapdays in order to standardize them. Apart from the seasonal variation, all other quantitative analyses include material collected from the period of high activity of this family, i.e. from mid spring to mid autumn. Statistical analysis was based on the measurement of similarity indices, cluster analysis and Correspondence Analysis (CA). In total, 22 genera and 68 species were identified, out of which, 9 are new to science. On Crete, 14.060 mature individuals were counted, belonging to 21 genera and 56 species. The total number of species found on the rest of islands is 23 on Gavdos, 9 on Gavdopoula 14 on Dia, 13 on Antikythira, 16 on Karpathos and 9 on Kos. These numbers are considered to underestimate the diversity of the last two islands, because of the high heterogeneity of their landscape that cannot be thoroughly studied with only one site of collection that was used in the present study. The taxonomical study revealed 53 fully identified species on Crete and 3 species identified to genus level. Three more fully identified species are added to this catalogue occurring on the other islands. These species do not present great intraspecific variation, with few exceptions (i.e. Camillina metellus, Drassodes lapidosus, Gnaphosa bithynica, Zelotes labilis, Z. tenuis), so some subspecies or new species may be created in the future. The taxonomical status of very few species is still dubious (i.e. Zelotes cf. ilotarum, Z. labilis, Leptodrassus pupa). However, the present study more than doubled the number of species occurring on Crete and gave a clearer idea of their distribution on south Aegean islands. As far as species numbers are concerned, the genus Zelotes is by far the most numerous, being represented by 13 species on Crete, which constitute 23% of the Gnaphosidae recorded from it. The family presents higher diversity in the Mediterranean countries, when compared with data from the rest of Europe. Some genera present mostly in the East Mediterranean are not represented in Europe at all, but occur on Crete. Therefore Crete is the intermediate link of two almost distinct faunas, resulting from Asia and Europe. Divided in nine chorotypes, Gnaphosidae of Crete belong mainly to endemic (17%), widespread (17%) and east Mediterranean (15%) elements, while Greeksouthaegean and Mediterranean elements are represented by 11% species each. As far as frequency of appearance is concerned, most of the species are rare (44%) and only 20% are common on Crete, being present at more than half of the sites. The most successful species (high frequency and abundance) are P. lentiginosa, N. excerpta, Z. subterraneus, Z. tenuis, Z. caucasius, Z. labilis και D. praeficus. These species have mainly Mediterranean or east Mediterranean distributions, and they present wide ecological preferences, as far as both habitat and altitude are concerned. Most of the Cretan endemic Gnaphosidae are quite common along the island, but not very abundant. This is in contrast to previous studies on Cretan land snails and Coleoptera, in which the endemic species form the main bulk of the total fauna of the island. This difference may result from the lower percentage of Cretan endemic Gnaphosidae, but also from differences in the ways that these faunas were formed, related to the greater dispersal abilities of Gnaphosidae spiders. Compared to them, both Coleoptera and land snails were more isolated and therefore the creation of endemic taxa was favored. As a result of a poorer fauna due to isolation and to several destructive events during the paleohistory of the island, these taxa became the dominant animals on Crete. As far as spatial variation is concerned, very few species show narrow preferences (20%). In order to test for this, the habitats of Crete were divided in six different categories, based on the habitat type (phrygana close to the periphery, inland phrygana, maquis, pine forests, wetlands, subalpine shrublands). The ten most abundant species in all these habitat types were almost identical. The habitat types that showed the most specialized fauna were the wetlands, the coastal phrygana and the subalpine shrublands. These habitats attract species with more hygrophilous, thermophilous and temperature tolerant character respectively, which is indicative of the main ecological factors that influence the distributions of the spider species, at least in the Mediterranean region. As far as numbers of species are concerned, the sites set in phrygana were the richest. This shows that Gnaphosidae are well adapted in the typical habitat of Crete, which is the result of an old co-evolution. A great number of species was also found in sites close to wetlands, among which a great number of rare and endemic species, which must be due to the more favorable climatic conditions prevailing at these areas. As far as altitude is concerned, Crete was divided in six altitudinal zones of 400m each, in which both species composition and abundance were tested. Most species occur in more than three zones, showing a great range of altitudinal distribution. Along the altitudinal gradient the number of species declines almost linearly (tenfold decline in each zone), while at the summits there is a dramatic fall, leaving only 5 species present on them. Quite in the contrary, the total abundance does not change very much until 1200m, then rises until 1950m and finally falls at the summits in about half of the abundance of the first zone. There are two main patterns of community structure: the pattern of the lower zone (0 to 800m), in which many species share the same or equivalent abundances and that of the higher zone (above 800m), in which very few species dominate with remarkably higher abundance compared to the rest of the species. These facts can be explained by ecological reasons, i.e. the relaxation of interspecific competition, as the species become fewer with the raise of altitude, the generalized predation that permits no limitation in food resources and the shortening of the favorable period, which causes higher activity of the animals within a shorter period of time. The high abundance of Gnaphosidae at the higher altitudes is also due to the appearance of new taxa above a certain altitude; these species are able to tolerate the harsh conditions of high mountain habitats and thrive in them. As previously suggested, the origin of the high mountain fauna and flora of Crete is related to the course of paleoevents that gave rise to its mountains. Due to the relatively short history of the Cretan mountains (created in Pleistocene, after the full isolation of the island by any other land mass) the species occurring on them derived mainly from the most tolerant lowland species. Gnaphosidae also support this. The period of high activity in Gnaphosidae of Crete is between mid-spring to midautumn. Very few species are found in pitfall traps during the rest of the year. During the favorable period, there is usually one peak of activity, the exact time of which is species dependent. Usually the peak occurs in the end of spring or in the beginning of summer. Earlier studies suggested that Gnaphosidae and/or spiders in total show two peaks of activity, one in early summer and one in autumn, which in reality is the phenological pattern of very few species (P. lentiginosa and Z. subterraneus). This misunderstanding must have been due to the combination of the peak of activities of many species in different moments within the favorable period. In general, phenological patterns of each species are quite stable, with minor differences, which may result from the particular conditions of the different years in which sampling occurred or of the different sites. Species that occur on the high mountains restrict their activity and some change their phenological pattern towards the first months of autumn. As far as niche overlap is concerned, spatial overlap of the species is greater than overlap in time. A possible reason for this is related to the generalized predation of these animals. This feature enables species to feed on different kinds of prey depending only on the size of the latter, and this in turn depends on their developmental stages, which are time specific. Therefore spiders have to differentiate in time in order to get the suitable size of food and to avoid interspecific competition. Instead, active hunting of these spiders forces them to be more widespread in space, therefore a spatial homogenization occurs. In total view, Gnaphosidae is a spider family with xerophilous character and wide ecological tolerances, which accounts for the distributions of its species along the island of Crete, much more than historical reasons relating to the formation of the island or ecological gradients along its main axes. The reasons that lead to this conclusion are the following: the non - dominance of the endemic species, the greater diversity of the central part of the island (which is the most degraded and the newest, as far as history is concerned), the widespread, almost accidental distributions of many species and the clear faunal separation only of the higher altitudes of the Cretan mountains. These features are indicative of an opportunistic pattern of distribution, shown by a fauna that has mainly been formed through dispersal from adjacent regions and from different directions. Gnaphosidae is the first group of animals that present this kind of pattern, all the other taxa that have been studied to a satisfactory level, reflecting a clear picture of the history of the island with their patterns of distribution. Comparing Gnaphosidae of Crete with the rest of islands under investigation, Crete holds its isolated, central position in all types of analyses, leaving the rest of islands divided to clusters that follow the geographical map. This was expected from previous data, but in the case of Gnaphosidae, the faunal differences between east and west islands are less pronounced. Kos is characterized mainly from species of eastern origin, Karpathos from south Aegean species and Antikythira from Mediterranean species. The satellite islands of Crete are mainly composed of species that occur on Crete, without any taxonomical differentiations. The fauna of Gavdos is more similar to that of Crete, compared to the rest of islands, Gavdopoula and Dia, due to the greater size and heterogeneity of its landscape that permits more species to occur on it than on the other two. However, the structure of the communities of Gnaphosidae is different on Gavdos, therefore when a comparison on a site level is realized, then Gavdos becomes more isolated than the rest of islands. This must be due to the greater time that Gavdos was isolated from Crete, its greater distance form it and the fact that its fauna must have been formed completely by dispersal, as the geological findings on the island suggest that it was submerged and then reappeared during late Miocene. This gave rise to a different structure of the communities under the pressure of the new dynamics occurring on this island, but not necessarily to different composition of species altogether.
Language Greek
Issue date 2003-07-10
Date available 2004-01-16
Collection   School/Department--School of Sciences and Engineering--Department of Biology--Doctoral theses
  Type of Work--Doctoral theses
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