SUMMARY
Evolution, diversity en niche dynamics of
seaweeds
Plants and animals are not random ly d istributed on th e face o f the planet, but exhibit nearly always
well defined ranges and m eet universally recognizable patterns. During the past decades, studies of
species diversity patterns w ere fo r th e m ost p art approached fro m an ecological perspective and
focused largely on finding links betw een environm ental param eters and species richness. Although
this type o f research confirm ed th e strong correlations between clim ate and species diversity, the
fundam ental question how clim ate affects th e num ber o f species in a particular com m unity or
region was n ot satisfactorily answered.
A num ber o f recent studies approached questions relating to species diversity patterns by
investigating species' ecological niches in an evolutionary fram e w o rk. Each species is characterized
by a particular ecological niche, a set o f biotic and abiotic conditions in which th ey can th riv e and
can m aintain stable populations. Statistical inference o f ecological niches, known as niche m odeling,
is also used to determ ine w h e th e r variation in th e macro-ecological e nvironm ent affects
diversification in space and tim e . In o the r words, w h a t e ffe ct has niche evolution in a given group o f
species on its diversification?
U ntil recently, evolutionary niche dynamics w ere only studied fo r te rre stria l organisms. Despite the
fact th a t niche m odeling was already successfully applied fo r m arine organisms, an evolutionary
approach o f studying ecological niches and its relationship w ith diversity patterns was never
a tte m p ted . To achieve this objective, seaweeds (m arine macroalgae) are a well suited group o f
organisms. Previous knowledge o f macroecological niches o f seaweeds arises on the one hand from
ecophysiological experim ents th a t determ ined survival, grow th and reproduction under d iffe re n t
culture conditions, and on th e o th e r hand fro m studies correlating algal d istribu tio n patterns w ith
isotherm s. Evolutionary dynamics o f macroalgal niches never received much a tte n tio n . Im portant
progress in th e fie ld o f niche m odeling and to ols fo r analyzing evolutionary patterns in a
phylogenetic context cu rren tly a llow a m ore global approach to th is issue.
In general, this thesis aims at characterizing macroecological niches and large-scale geographic
diversity patterns o f seaweeds fro m an evolutionary p oint o f view. Our goal is to elucidate how
niches evolved, how niche dynamics can lead to diversification and how th is results in observed
d istribu tio n patterns. For this purpose both a global m arine environm ental data set as well as new
m odeling tools is developed. This in com bination w ith techniques fro m phylogenetic, ecological,
evolutionary and geographical research helps us to clarify these patterns.
Chapter 1 presents Bio-ORACLE (Ocean Rasters fo r Analysis o f CLimate and Environm ent). This data
set aims to boost m arine d istribu tio n and niche m odeling applications by providing the firs t
com prehensive standardized and u niform global
dow nloadable and usable fo r predictive studies.
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m arine
environm ental
dataset,
readily
Bio-ORACLE is constructed fro m a com bination o f satellite imagery and interpolations o f in situ
oceanographic m easurem ents and currently contains 23 raster map layers containing d iffe re n t
dim ensions representative fo r th e m arine environm ent. We dem onstrated global applicability
through a case study o f th e invasive green alga Codium fra g ile subsp. fra g ile , predicting its potential
spread.
The choices made during the ecological niche m odeling process can affect th e tra n sfe ra b ility and
overall predictive perform ance o f th e resulting m odel. Chapter 2 presents a fra m e w o rk designed to
prom ote correlative m odeling o f invasive species. We introduce tw o m ethods th a t have the
p otential to increase the tra n sfe ra b ility o f correlative species d istrib u tio n models (SDMs), more
specifically a to o l th a t reduces geographical bias in occurrence records and one th a t identifies
suitable sets o f predictors. We apply these m ethods to the introduced and highly invasive seaweed
species Caulerpa cylindracea, aim ing to infer a solid niche m odel fo r this species th a t can assist in
predicting areas w ith suitable habitat w orldw ide and assessing the risk o f fu rth e r spreading. Our
results show th a t reducing th e set o f predictors to those anticipated to be o f global significance
yielded a strong im provem ent o f SDM tran sfe ra bility, w ith occurrence thinning, model com plexity
and background choice having relatively m inor effects. If both occurrences fro m the native and
invaded regions are available, they should be com bined, as this results in the best-perform ing
models and reduces the sensitivity to choices made in the m odeling process. The resulting SDM o f
the highly invasive species Caulerpa cylindracea achieves very high predictive power.
In chapter 3, we infer the evolutionary history o f th e green algal genus H alim eda using a tim e
calibrated m olecular phylogeny. W e m odel distribu tio ns and reconstruct th e ancestral niche o f the
cu rren t species w ith in the genus. Results showed th a t the niche o f Halimeda is conserved fo r
tropical, n u trie n t depleted habitats, w hile one section o f th e genus invaded colder w aters several
tim es independently. Since known d istribu tio n ranges are considerably sm aller than m odeled
potential ranges, we conclude th a t restricted geographical ranges are likely th e result o f dispersal
lim ita tion .
In chapter 4, we present an em pirical approach to gain understanding in the biogeography and
evolution o f w arm -tem perate m arine floras. The focus in this chapter lies on th e m arine green algal
Codium decorticatum species complex. W e use clim atic niche m odeling and divergence tim e
estim ation to explore th e historical biogeography and evolution o f therm al variables w ith in the
complex. The clim atic models w ere projected to th e Last Glacial M axim um and th e Pliocene to
examine w he th er th e Indian Ocean served as a therm al barrier fo r dispersal. Our results suggest
th a t during th e late M iocene - early Pliocene species w ith in th e C. decorticatum com plex dispersed
via episodic dispersal events. A fte r geographic isolation, species have adapted to local conditions
and th e ir therm al niches have diverged from th e ir closely related species.
In Chapter 5, we examined the evolution o f th e therm al niche through tim e in relation to the
diversity pattern o f th e brown alga genus D ictyota. The evolutionary history o f this genus was
characterized by means o f a tim e calibrated m olecular phylogeny in com bination w ith th erm al data.
In addition, th e relationship betw een th e evolution o f th e th erm al niches and th e diversification
rate was studied. Results show th a t species diversification w ith in this genus is positively associated
w ith the a bility o f clades to shift th e ir therm al niche. This outcom e m ight have im plications fo r the
in te rp re ta tio n o f large scale m arine diversity patterns.
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