| Abstract |
The Aegean (Greece) is an archipelago of extreme botanical interest, ideal for the
investigation of species richness patterns. The aim of this thesis was a contemporary
phytogeographical study of the central and southern part of the Aegean archipelago, through
various applications of the theory of island biogeography. In this frame, the relationship
between islands’ area and their number of vascular plant species, the small island effect, the
degree of endemism (number of endemic species) compared to the islands’ surface, the
different expressions of the species-area relationship (SAR) at the plant family level, are
examined. In addition, a more recent approach, which takes the environmental heterogeneity
into consideration, is integrated in the classical phytogeographical analysis.
This study refers to 197 Aegean islands and concerns 2,313 vascular plant species in
total. Thirty-six of the islands are located in the Kyklades complex, which constitutes the
central part of the Aegean, 92 islands are located in the eastern part of the Aegean (Samos’
complex and part of the insular district of Dodekanessos), and 69 islands belong to the
Southern Aegean island arc, which constitutes an island bridge connecting Peloponissos with
Asia Minor. The islands are of various sizes, from tiny rocky islets (minumum island area:
0,00044 km2) to large inhabited islands, including Crete (8,264.62 km2), which is one of the
largest islands in the Mediterranean sea.
Floristic data from 67 studies, which were published during the last four decades, after
extended botanical explorations on various islands, were gathered in a database and were
elaborated, so that the number of species which occur on each island was obtained.
Unpublished data on the flora of Patmos’ island were kindly provided by Dr. Kirsten Bruhn
Møller and Dr. K.I. Christensen from the Botanical Institute of the University of
Copenhagen, with their permission to use them in the present study. The flora of Milos’
island was studied through the identification of herbarium specimens collected during past
explorations. Additional specimens were collected and identified. Varieties and subspecies,
even if more than one under the same species, were counted as one single species. Plants
which were recorded in floristic inventories as cultivated or introduced but not naturalized,
and as doubtfully present on the islands, due to possibly dubious records or misidentified
specimens, were not counted in the total number of species. The total number of vascular
plant species on islands varied from one, on few tiny islets, to 1,795 on Crete.
The Arrhenius’ dynamic model (1921) was applied in its log-log form, to describe the
SAR: (1) for all 197 islands and for the three phytogeographical regions of the study area,
namely Central, Eastern and Southern Aegean, (2) for the endemic species in various geographical scales, from local endemics (single-island endemics) to endemics of wider
distributions, and (3) for the number of each Family’s species on the islands where they are
distributed.
The occurrence of the small island effect (SIE) was examined. Various factors were
tested for their contribution to the shaping of the vascular plant species number on islets of
the South Aegean. These factors were the maximum elevation, the distance from the nearest
large island, i.e. the nearest island not included in the SIE, the shape index of the islets,
calculated as “maximum length / maximum width” of the islets, the percentage of the islets’
area, which is “protected” by nearby islands against wind and storms, and the “disturbance
penetration distance”.
The environmental heterogeneity was described in two different ways: firstly, using the
land cover types of the “CORINE Land Cover 2000” Program, and secondly, using the
“habitat types” of the European Network of Protected Areas “NATURA 2000”. An
additional approach was used for the description of the environmental heterogeneity in the
South Aegean; habitat diversity was quantified according to the combination of light,
temperature, moisture and soil salinity conditions which prevail at the location where each
plant species grows. Data on South Aegean plants’ requirements in these environmental
factors were derived from the Southern Aegean Indicator Values (SAIVs). The SAIVs system
describes the ecological behaviour of 2,242 South Aegean vascular plant taxa, according to
their requirements in major environmental factors.
The “area per se” and the “habitat hypothesis”, as well as the “Choros model”, which
combines area with environmental heterogeneity in a single variable, were tested for their
effectiveness in interpreting vascular plant species richness.
A strong positive correlation between area and number of species was found for the
197 islands: the increase in area results in an augmentation of the vascular plant species
number. The slope of the SAR (z-value) corresponds to actual geographical and historical
traits of the archipelago.
The three regions of the study area which are “traditionally” considered as
phytogeographically different, namely Central, Eastern and Southern Aegean are actually parts
of a united phytogeographical area. The only difference observed is in the rate of the species
number increase between in Central and Southern Aegean.
The occurrence of the “Kykladenfenster”, i.e. the floristic impoverishment of the
Kyklades complex, compared to the rest of the Aegean islands, is not corroborated. In
contrast, the strong phytogeographical relationship of the Central Aegean with the Eastern and Southern Aegean is supported. This result generally indicates that the phytogeographical
borders of these three island complexes are not strict.
The SIE was detected in the whole study area as well as in the Southern Aegean. The
discontinuous model which combines two linear equations in a single one was successful in
describing species richness’ variations. The SAR for small islands is significantly weaker and
area interprets a low percentage of the variation in species number. On the contrary, the
model for the detection of SIE, which integrates environmental heterogeneity, was not
effective in the study area. In the case of small islands, area remains the most important
parameter in shaping the species richness pattern. The area combined with the distance from
the nearest large island and the disturbance penetration distance, managed to interpret a
higher percentage of the variation in species richness. Maximum elevation, shape index and
the area protected by neighboring islnds were not significant in the evaluation of species
richness in the Southern Aegean.
There is a strong positive correlation between the number of local endemics (singleisland
endemics) and area and the rate of increase is very quick. Therefore, concerning local
endemics, the islands maybe considered as biogeographical regions’ equivalents, except that
most of its local endemics originated mainly through allopatric speciation.
The families’ SARs are significantly different from the pattern of the total vascular flora
and they are shaped by the total number of the families’ species which occur in the study area
as well as by their distribution on the islands. Some of the families’ SARs could be interpreted
based on some traits of the bioform and ecology of their species. The value of the constant
“c” in the familes’ SARs varies, following the total number of species of each family in the
study area, indicating that the variation in c-values depends on family size. The general trend
is that the most species-rich families exhibit the highest c-values. Moreover, the c-value for
total vascular flora is much higher than that of any family. This result can be considered to
support the ecological view of the c-parameter as an indicator of the capacity of the studied
area; the larger the species pool, the higher the number of species that can be found within
the measuring unit of area.
The selection of the measure to quantify environmental heterogeneity is a major
problem. Approaching environmental heterogeneity through CORINE land cover types and
the NATURA 2000 habitat types indicated that area is more effective in interpreting the
variation in species number. Environmental heterogeneity significantly depends on area; as a
result, it is impossible to combine them as independent variables within the same equation.
However, even if they were combined, environmental heterogeneity does not contribute to
the interpretation of significantly higher percentage in the variation of species richness. This conclusion would change, if the environmental heterogeneity approach was based on criteria
which are closer to the actual concept of “habitat”, such as though the “Southern Aegean
Indicator Values”.
The “Choros” model, even if applied using a generalized measure of environmental
heterogeneity, such as the CORINE land cover types and the NATURA 2000 habitat types, is
more capable of interpreting species richness, compared to the classical SAR model, although
this higher capability is not striking. The drawback of this model is that there is no allembracing
or broadly accepted habitat definition can be spotted among various
phytogeographical studies. The estimation of plant habitats using the data on Southern
Aegean Indicator Values of light, temperature, moisture and soil salinity considered some
aspects of the ecological needs of plant species. Environmental and physiographic factors
have been used in other studies to define plant habitats. Despite dissimilar approaches to
habitat diversity, results concerning its role in shaping plant species richness tend to converge,
because most of the habitat definitions used reflect, more or less, topographic and geological
heterogeneity, which creates more habitat types and thus promotes species richness, especially
when the species involved tend to be habitat specialists.
Area and environmental heterogeneity interact and are highly amalgamated, so that
species number constitutes the common result of both of them. The two hypotheses
concerning the effect of area and the effect of environmental heterogeneity complete each
other, thus describing the pattern of increase in species richness more effectively. A standerd
definition of “habitat” and the development of more accurate and broadly accepted methods
of quantifying environmental heterogeneity would further elaborate the answer to the
question “area per se” or “habitat diversity – environmental heterogeneity”.
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