Rafael Molina Venegas. University of Seville. December 2014. Supervisors: Juan Arroyo Marín, Abelardo Aparicio y Sébastien Lavergne.
Summary:
The evolutionary and ecological processes that shape
species diversity shift as spatial and temporal scales change and
diversity patterns in assemblages may thus reflect these shifts. In this
PhD Thesis, I explore taxonomic, phylogenetic and functional diversity
patterns in plant assemblages on different geographical scales in the
Baetic-Rifan range, a western Mediterranean biodiversity hotspot, as a
means of inferring the relative contribution of each of the evolutionary
and ecological processes responsible for the emergence and maintenance
of plant biodiversity in this region. The Baetic-Rifan biodiversity
hotspot is an ideal study system for exploring the interaction between
such processes due to its extraordinarily diverse and well-known
regional species pool, which includes both recent narrow endemics and
the extant representatives of past floras, and its heterogeneous
mosaic-like landscapes characterized by great geomorphological and
lithological complexity.
In order to validate the results and conclusions reached in this PhD
Thesis, I tested the influence of phylogenetic resolution and
branch-length information on the quantification of phylogenetic
structure, and explored the impact of tree shape (‘steminess’) on the
loss of accuracy in phylogenetic structure quantification caused by
phylogenetic resolution. Directional biases in phylogenetic structure
quantification due to phylogenetic resolution and dating methods
generally tended towards type II errors. Interestingly, I detected that
the tree shape strongly influenced the loss of accuracy derived from the
lack of phylogenetic resolution, which means that phylogenetic structure
estimates based on poorly resolved phylogenies across biogeographical
regions should be interpreted with caution.
I found that West Rifan plant assemblages resembled Andalusian
ecoregions more than those of nearby northern Morocco ecoregions, which
thus highlights the role played by the Strait of Gibraltar in the past
as a migration route for plants from both sides of this strait. However,
phylogenetic turnover in terminal lineages tends to occur right across
the two landmasses of this hotspot, which thus suggests a common
scenario of allopatric speciation among disparate angiosperm lineages
that may have been facilitated by the intermittent joining of the two
continents. Phylogenetic clustering reported on the eastern margins of
the hotspot may be in large part due to syndrome-driven local
diversification occurring in shrublands in the southeastern Iberian
Peninsula. Diversity patterns of endemic taxa do not mirror the
diversity patterns of the regional pool in which they are included.
Specifically, although taxonomic turnover in regional endemics
throughout the Baetic ranges was related to lithological conditions and
geographical distance, climate and/or elevation explain better the
turnover of non- endemic taxa. By contrast, the phylogenetic turnover of
endemics was weakly related to lithological conditions, which suggests
that edaphic specialist endemics have evolved from multiple lineages and
highlights the role of the substrate in promoting differentiation in the
Baetic ranges. I also found strong structuring of plant lineages along
elevational gradients in the Baetic range, probably due to habitat
filtering acting on life forms and character syndromes with strong
phylogenetic signals. These results indicate that lineages belonging to
the sclerophyllous syndrome, many of which are the extant
representatives of the flora that once existed under the tropical-like
Tertiary climatic conditions, seem to be ecologically restricted to the
lowlands of milder climate, whereas non-sclerophyllous lineages are able
to expand across higher elevation ranges where colder climatic
conditions prevail. At community scale, elevation was the main factor in
predicting phylogenetic and functional alpha diversity (maximum height
and blooming time) in plant communities for both incidence-based and
abundance data. However, their trends were opposing since phylogenetic
diversity increases and functional alpha diversity decreases with
greater elevation. I also detected a shift in deviances in local species
abundances towards larger seed size and phylogenetic clustering at low
elevation and divergent distributions and phylogenetic overdispersion at
greater elevation. By contrast, turnover in incidence-based specific
leaf area and leaf carbon:nitrogen ratio values were related to a pH and
micronutrient gradient. These results highlight the multidimensional
nature of the functional niche of species and suggest that different
trait-mediated mechanisms are simultaneously at work in shaping the
assemblage of local plant communities.