| Summary: | Patterns of spatial and temporal changes in biodiversity, natural or anthropogenic, and
their importance in understanding the dynamics and restoration of natural ecosystems,
have long been among the major questions in ecology. These changes are rather
important to be understood in the early stages of communities’ development. Young
oceanic islands offer the unique opportunity of a natural laboratory for the investigation
of such processes. The interest in using different facets of biodiversity, in order to
understand ecological processes rose over the past years, since there is an increasing
awareness that species’ traits influence species coexistence and ecosystem function.
Ecologists now quantify trait variation or multivariate trait differences within a
community, generally referred to as ‘functional diversity’ (FD). This diversity facet
technically represents the diversity of traits but is taken to represent the diversity of
species’ niches or functions.
This thesis consists an endeavor to investigate small spatial scale patterns as well as
short- and long-term temporal patterns of vegetation, functional diversity and
community assembly on two young islands (Palea Kameni, PK and Nea Kameni, NK)
of Santorini Archipelagos in Greece, which consist the result of subaquatic volcanic
activity. These two islands differ on geological age, history of volcanic activity and
total area. For this purpose three data sets were compiled: i) a vegetation data set
comprised of species abundance data, collected on both islands in three consecutive
years (from 2010 to 2012) using different spatial scales, ii) a data set comprised of
presence-absence data collected from 1911 to 1990 (four censuses conducted in 1911,
1933, 1967 and 1990) on NK and iii) a functional traits data set comprised of 26
functional traits (vegetative, habitat-related and reproductive) for each recorded taxon,
collected from databases and bibliography.
More specifically, this thesis examines the distribution of both plant communities and
habitat types as well as temporal changes in vegetation patterns form 1984 to 2015 on
both islands. Seven plant communities in total have been defined on both islands (four
on PK and three on NK). Four of these communities comprise the data set used in the
current thesis’ analysis. Six habitat types were recognized and described on both islands
in 2015 (five on PK and three on NK). PK is geologically more than 1800 yrs. older
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than NK and no eruptions have disturbed the island’s vegetation since 726 AD. PK's
actual vegetation consists partly of an evergreen sclerophyllous shrub community, most
probably representing the climax vegetation on the island, thus no significant changes
were noted since 1984. After several outbursts of the island’s volcano in the past
century (the last one recorded in 1950), the re-establishment of NK's plant cover has
reached different stages in different parts of the island, according to age and ashes
deposition. Significate variations have been identified in the island’s vegetation even
after 1984, the most tricking in the floristic composition of one of the two therophytic
communities, which has changed substantially.
The exploration of both taxonomic and functional aspects of biodiversity
simultaneously can lead to a better understanding of the ecological processes. The
analysis of functional beta diversity represents a relatively new approach in community
ecology. The aim of this thesis is to track possible temporal changes in taxonomic and
functional alpha and beta diversity within the short time period from 2010 to 2012 and
to investigate, if the temporal patterns change with spatial scale. No evidence for a
general temporal variation trend was found in the two facets of diversity according to
the results, neither for the alpha nor for the beta component. Low functional turnover
among communities as well as a small change in functional beta diversity was observed.
Thus, it is important for ecologists studying biotic homogenization to quantify also
changes in functional beta diversity. Positive correlation among different components
of diversity was detected among some of their facets. It is indeed crucial to determine
whether the observed species replacement and corresponding changes in taxonomic
richness induce functional turnover (i.e. whether ‘loser’ and ‘winner’ species are
functionally different) and/or changes in functional richness. Moreover, the results
highlight the importance of spatial scale when studying variation in the functional
composition among a set of communities, since changes are more evident at fine spatial
scales, where only a part of the total community’s species pool is recorded.
There are only a few recent studies investigating the relationship between functional
diversity and area, the Functional Diversity - Area Relationship (FDAR). In this thesis
the pattern of the relationship between functional diversity and area is examined, along
with the question, whether this pattern differ, when different facets of FD are used, as
well as different trait groups (in order to examine the effect of the data set used in the
analysis on these patterns). A typology of FDARs was identified, depending on the
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facet of FD analyzed: (i) strong positive for indices that quantify the range of functional
traits in the community, (ii) negative correlation for indices quantifying the evenness in
the distribution of abundance in the trait space, (iii) no clear pattern for indices
reflecting the functional similarity of species and (iv) idiosyncratic patterns with area
(either positive or negative or non-significant correlations) for the index that reflects
the divergence of individuals in the functional space occupied by the species
assemblage. As area increases, the range of traits observed in the community increases,
but the abundance of traits does not increase proportionally and some traits become
dominant, implying a reliance on some functions that may be located in either the center
or the periphery of the trait space. This pattern is not dependent on the group of
functional traits used or the type of SAR (island species-area curve or speciesaccumulation
curve).
The two sea-born islands offer the opportunity to study community assembly at two
different phases: a very early and a late stage of vegetation succession and to determine
whether: i) the mechanisms driving community assembly differ between two islands of
different age and history of vegetation development, ii) sampling scale affect the
strength that each assembly mechanism poses on the assembly of plant communities
and iii) the intensity of the mechanisms of assembly change during short time periods.
Null model analysis was used to test for two different assembly mechanisms: habitat
filtering and limiting similarity against random assembly. Functional diversity of the
communities on the younger island implied no specific assembly mechanism; perhaps
due to its age, the community is still at the early stages of colonization (i.e. stochastic
events, such as arrival of new species, prevail). On the older island, the lack of
disturbances for a long time period allowed the establishment of communities driven
by certain assembly mechanisms, such as competition and habitat filtering. Changes in
communities, regarding species composition, species richness and species abundances
during short time periods do not reflect changes in the mechanisms of the communities’
assembly.
The existence of a long-temporal vegetation data series for NK provides the opportunity
to study colonization (immigration and extinction events) as well as the biotic assembly
and the successional processes following the destruction of vegetation after a volcanic
eruption, using alpha and beta functional diversity and null model analysis to test for
two different assembly mechanisms. The results indicate that alpha diversity increases
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with time interval since disturbance, while beta diversity decreases. The patterns of
temporal changes in taxonomic and functional dissimilarity do reveal functional
redundancy, at least to some extent. Long – term temporal changes in assembly
processes revealed that the effect of abiotic filtering decreases over time.
Overall, the use of functional diversity at different facets and components provided
valuable insights into biodiversity patterns and proved to be able to provide important
complementary information to the taxonomic component of diversity. Spatial scale,
even when small local scales are examined, affected all patterns investigated. The two
sea-born islands proved to be a valuable tool for the investigation of different stages in
succession and the assembly of communities, despite the lack of repeatability.
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