Your browser does not support JavaScript!

Post-graduate theses

Current Record: 629 of 791

Back to Results Previous page
Next page
Add to Basket
[Add to Basket]
Identifier 00035998
Title Βιογεωγραφία, συστηματική και οικολογία των χερσαίων μαλακίων στο αρχιπέλαγος της Σκύρου
Alternative Title Biogeography, systematics and ecology of the land snails of the archipelago of Skyros
Author Τριάντης, Κώστας
Abstract The present M.Sc thesis studies the biogeography, ecology and systematics of the land snails in the archipelagos of Skyros. There are many questions that this study is willing to answer. The most important of them is discovering the degree in which the land molluscs can be used as indicators of evolution in a specific region or else, in what degree can their distributions reveal the evolution of the archipelago and its historical relationships with the nearby areas. On the other hand, an equally important question that is touched in the present study, is whether general patterns observed in nature can be described by mathematical types which will also incorporate the basic ecological functions and at the same time quantify the temperamental characteristics of the studied area and the studied taxonomic group. The archipelago of Skyros is situated in the western part of central Aegean Sea. The island of Skyros is the largest island of the archipelago and also the largest of the Sporades island group, with a total area of 207,9 Κm2. It’s distance from Evvoia is only 19 miles and is surrounded by 32 small islands and islets. In order to collect the material, three different sampling periods were scheduled and carried out: 9 to 12 July of 2001, 20 to 24 January and 10 to 14 May of 2002. Based on the extend of the ground, the geomorphology and the type of vegetation, the island of Skyros was divided in 24 sampling stations whereas the fauna of 11 islets of the island group was also studied. On those islets, there weren’t any determined sampling stations, as the sampling covered them thoroughly. In Skyros archipelago, the total number of land mollusk species that have been reported is 27. Those references are quite circumstantial and some of them are also wrong. With the completion of the present study, I believe that Skyros archipelago contains-bears –carries 45 species, 43 of which are recent. Two of them are new species for the science. The results and the conclusions of the present study can be summarized as follows: The number of land mollusk species that are present in Skyros Island is especially high comparatively to the number in other Aegean islands. Unfortunately, due to the inefficient data on the malacofauna of Evvoia and north Sporades, their comparison with Skyros data is not possible. From all the species that were discovered as subfossils in the island of Skyros, only Pomatias elegans and Helix figulina are not recent. The low percentage of land mollusk endemism in the archipelagos of Skyros contradicts with the long geological isolation of the island, as well as with the presence of the endemic vertebrate. The small participation of the anthropophilic species in the composition of the malacofauna in the archipelagos of Skyros is due to the fact that although humans have a long presence in the island, it was never especially vigorous. The faunas of the small islets of the Skyros archipelagos are mostly composed of steady and accessory species, whereas the accidental species have a very small contribution. Because of that, the faunas of those islets present an important stability and their composition does not depend greatly on accidence. Also, they are not considered –for the land mollusks- as small and isolated land areas, but as parts of greater-larger regions. The processes that have shaped the malacofauna of the archipelagos of Skyros are: · The extinction of species from the islets and the island of Skyros, throughout the fragmentation of the single land mass that formed them of and the raise of the level of the sea. · The colonization of the archipelago by easy dispersed species, which replaced species already present on the archipelago. Η εποίκιση του αρχιπελάγους από ευρύοικα και ευκολοδιασπείρομενα είδη που εν μέρει αντικατέστησαν τα ήδη υπάρχοντα στο αρχιπέλαγος είδη. · The limitation and extinction of the psycrophilous species, which have been settled in the area during the glacial periods. As far as the similarity of the faunas of the islands in the archipelagos of Skyros is concerned, the similarity indexes that have been used show that the islands are grouped according to their size (large-medium and small). The size of the Koulouri Island, based on both indexes, reflects the disjunctive size between the large-medium and the small islands in the archipelago. The above general pattern is verified by the study of the islands of the archipelagos together with the study of Skyros island sampling stations. The faunas of the sampling stations in Skyros do not follow a specific grouping pattern, thus showing that the malacofauna of the island is quite uniform and regions with distinctive characteristics do not exist. The only exception is the existence of the endemic species Helicella n.sp. at the Kohilas mountain, but this one alone can not distinct this region from the others. The only parameter that leads –in a certain degree-to grouping between the sampling stations in Skyros Island, is the human influence. In this way, there is distinction between the highly affected from human systems and those that are practically unaffected at al. The faunas of the small islands of the group are quite distinct from those of the sampling stations in Skyros and the other bigger islands and therefore the groupings sort out these small islands. The relationship between the species number and the area can be derived from the description of the steady characteristic of the ecosystems (increase in the species number follows increase in the area) through an exponential rule. The value of z parameter reveals all the dominant processes that shape the fauna of a region. Those processes depend on the scale of the studied area. The use of geographically close islands in order to study the relationship between the species number and the area without taking into account their basic characteristics that concern the studied taxon, could lead to a decrease of the biological importance of the parameters of this relationship. As far as the land molluscs are concerned, the Skyros island group is not an isolated system, mostly due to the fact that the majority of the species that exist-spread-prevail there are easily dispersed. The z value classifies the archipelagos of Skyros, as far as the land molluscs are concerned, in the category of those regions that constitute parts of the same continental area. The fact that the archipelagos of Skyros behaves as part of a continental area and not as an island group, can explain not only the quite rich fauna of it, but also the big capacity of it that is expressed through the value of the c parameter. The especially high correlation between the species number and the area of the limestone at the archipelagos islands, confirms the correlation between the presence of the land molluscs and the limestone. On the small islands of the group there is not any occurrence of the small island effect. Those islands are quite stable and behave as parts of a larger area. The land molluscs of the islands of the archipelagos of Skyros, could be considered as local populations of a metapopulation. The choros model quantifies the interaction between the area and the habitats, thus expressing their total impact on the species number of an area. It also offers the mathematical base for the combination and the correlation of the mechanisms that determine the species number of an area, something that the area per se and the habitat hypothesis suggest. The choros model describes more effectively than the Arrhenius equation the species number in the case of the archipelagos of Skyros. At the same time, the choros model can be considered as stepping stone at the attempt for understanding the small island effect. Using the choros model and the process of path analysis, one can clearly see that the environmental heterogeneity should be part of the equations that try to describe and predict the species number in a region. I presume that the absolute number of habitats is the more effective term for expressing the environmental heterogeneity in an equation like this, without excluding of course the coming up with-finding of a more suitable one. The number of habitats is strongly correlated with the increase of the area, since in many occasions the area is a restraining factor for the presence of a specific habitat. The total influence of area on the species number depends mostly on the indirect influence of it through the habitats. The direct influence of it in the archipelagos of Skyros is relatively small, mostly due to the big number of specialists species and species with a big dispersion ability. The clear distinction of the ecological terms habitat and biotope is necessary in prospective-future studies. The habitat should be defined on the basis of the ecology of the studied taxon in such a way that the comparison between different data sets would be possible. When this is happening, the choros model presents significantly important correlation between the species number and the choros of the region. The choros model can in the future offer important assistance in the ecosystem conservation since it can also express the environmental heterogeneity of a region.
Language Greek
Issue date 2002-11-22
Date available 2002-10-11
Collection   School/Department--School of Sciences and Engineering--Department of Biology--Post-graduate theses
  Type of Work--Post-graduate theses
Permanent Link https://elocus.lib.uoc.gr//dlib/6/f/0/metadata-dlib-2002triantis.tkl Bookmark and Share
Views 333

Digital Documents
No preview available

Download document
View document
Views : 54