Antony van der Ent Extended Abstract

Evolution of plant life on ultramafic edaphic islands at Mt Kinabalu
A. van der Ent
Centre for Mined Land Rehabilitation, Sustainable Minerals Institute, The University of Queensland, Brisbane,
QLD 4072, Australia. Email: [email protected]
Abstract
The Malaysia state of Sabah in the biogeographical province of Borneo is one of the world’s five plant diversity
hotspots with in excess of 5,000 species per 10,000 km2 (Kier et al., 2005, Barthlott et al., 2007). The epicentre of
local plant diversity is Mt Kinabalu, which is the world’s premier hotspot with over 5,000 species in an area of just
1,200 km2. Three unusual characteristics are the primary precursors for the exceptional diversity in plant species
on Mt Kinabalu: 1) its isolation and recent mountain formation; 2) the occurrence of ultramafic soils; and 3) the
very high altitude and precipitous morphology.
Keywords: edaphic islands, Malaysia, Mount Kinabalu, plant evolution, ultramafics.
Introduction
Mount Kinabalu in Sabah, Malaysia, is the most
biodiverse place on earth in terms of the number of
species density with >5,000 species in 200 families
and 1,000 genera (Beaman et al., 2003) in an area of
1,200 km2 (Beaman, 2005). Over 900 plant species
occur on the 142 km2 of ultramafics that surround
Mount Kinabalu. Soils derived from ultramafic
rocks are unusual because of high magnesium and
nickel concentrations and low levels of major
nutrients (McCoy et al., 1999). Whittaker (1954)
presented three characteristics common among
ultramafic ecosystems: (a) low in stature and
biomass production; (b) high levels of endemism;
and (c) vegetation distinct from surrounding areas.
Ultramafic edaphic islands
Ultramafics are extremely restricting habitats posing
a strong Darwinian challenge for plants to adapt and
survive (Baker et al., 2010). The insularity of such
soils can be considered mainland islands as opposed
to offshore islands. The ‘physical barrier’, however,
is not water, but instead adverse soil properties
making it an edaphic island. Because ultramafic
outcrops at Mt Kinabalu form ‘edaphic islands’ in a
‘sea’ of other ‘normal’ soils, they can be
conceptualised in terms of the ‘Theory of Island
Biogeography’ (MacArthur and Wilson, 1967).
Such islands, when becoming available for plant
colonization, are subjected to two modes of
immigration; on the population level (i.e. those of
dispersion, immigration and extinction), and the
evolutionary level (i.e. those of adaptation and
speciation). Immigration into the habitat is affected
by the ‘distance’, which might be interpreted as
geographical distance, but also as ‘geochemical
distance’ (the difference in the geochemistry)
between the islands (i.e. the ultramafic outcrop), the
‘sea’ (i.e. other ‘normal’ soils surrounding the
island) and the differences between islands
(ultramafic soils differ greatly in geochemistry).
Each of these may be cumulative or antagonistic in
their
effect,
and
the
greater
these
distances/differences, the less likely a particular
island is to receive colonisers. The number of
species present on an island is primarily determined
by the size of the island (lower numbers on smaller
ultramafic islands) and distance between islands
(species number decreases with distance between
ultramafics islands). Larger islands are also more
diverse because they offer greater opportunity for
speciation (Harrison et al., 2006). However,
ultramafic islands are not truly insular, but recruit
species from the surrounding matrix over ecological
and evolutionary time when such other species
adapt to the prevailing edaphic conditions (Harrison
and Inouye, 2002). There is no ‘sea’ as such to be
crossed; rather, ultramafic islands are directly
surrounded by a rich reservoir of species and preadapted genotypes can make the cross relatively
easily. Therefore, as opposed to true islands,
immigration constantly takes place due to the close
proximity of potential immigrants and, among other
entry mechanisms, often follows (local) extinctions.
Types of immigrants (colonizers to the ultramafic
island) include: (a) indifferent species from the
surrounding sea that occur on both substrates, but
with some showing a higher abundance on
ultramafics; (b) species that reach the islands as an
extension of their normal range (‘outliners’); and (c)
endemics to the substrate (either as taxa or as
infraspecific types) (Kruckeberg, 1986, Kruckeberg
and Rabinowitz, 1985). All three types are found on
Mt Kinabalu. Examples of (a) are species that have
a wide ecological amplitude, of (b) are species
normally found on high altitude granites, but have
their lowest altitudinal occurrence on ultramafics,
and (c) encompass a range of species that have high
taxonomic affinity with precursor taxa on ‘normal’
soils (neo-endemics) and paleo-endemics.
The evolution of ultramafic endemics
Darwin’s ‘survival of the fittest’ is a metaphor for
the ‘better adapted for immediate, local
environment’ and phenotypic plasticity allows
individual plants to adapt to the limiting
geochemical conditions to which they are
confronted (Lasat, 2002). Ultimately, such
adaptation leads to the evolution of new species.
Insular soils (ultramafics) provide (genetic)
isolation which, coupled with strong edaphic and
climatic stresses, promotes evolutionary divergence
and speciation, and thus the genesis of endemism
(Kruckeberg, 1985). Some species are paleoendemics, resulting from widespread populations
that have become confined to ultramafics due to
competition elsewhere (Baker and Whiting, 2002),
although most are neo-endemics that have evolved
from closely related species that acted as precursors
(Rajakaruna and Boyd, 2008). The stages that may
lead to the evolution of edaphic endemics begins
with a low incidence of individuals with tolerance to
the prevailing geochemistry in the surrounding
populations (Kruckeberg, 1991), followed by
adaptive radiation due to strong edaphic filtering
(i.e. ultramafics), catastrophic selection due to
frequent El Niño droughts (in line of Lewis, 1962,
Raven, 1964) and genetic drift leading to rapid
speciation (Beaman & Beaman, 1990). The species
pool then becomes more divergent (Kruckeberg,
1986), eventually leading to reproductive isolation
(Rajakaruna, 2004). Interesting examples include
species in Elaeocarpus, Diospyros, Hopea,
Cyrtandra and Diplycosia with a high number of
ultramafic endemics, and many species on
sedimentary and intrusive rock that probably acted
as precursors. The rapid up-rise of Mt Kinabalu
took place in a time span of less than 800,000 years
between 7.85-7.22 million years ago (Cottam et al.,
2010) and up to 9,200 years ago large glaciers up to
5.5 km2 with moraines as low as 3,000m
(Koopmans and Stauffer, 1967) made many of the
current habitats unavailable for plant colonisation.
This means that speciation of species restricted to
those habitats must have been rather rapid. Strong
changes in the local climate created extreme
climatic fluctuations that enabled catastrophic
selection. Furthermore, the precipitous morphology
created a whole range of sharply distinct edaphic
habitats for plants and geographical isolation.
Threats and conservation
Mount Kinabalu and ultramafic sites within national
park boundaries are safeguarded, but land clearing
has approached all park boundaries. Scientific
research and preservation of ultramafics in Sabah
needs to be given utmost priority, particularly in
those areas without legal protection, before further
destruction results in extinction of local endemic
plant species.
Acknowledgements
The author wishes to thank Prof. David Mulligan
and Dr. Peter Erskine from the Centre for Mined
Land Rehabilitation (CMLR) at the University of
Queensland for providing continuous support.
Antony van der Ent is a recipient of an IPRS
scholarship from The University of Queensland.
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