Geoscience Reference
In-Depth Information
2006 ), may also inl uence biodiversity and ecosys-
tem function due to the loss of critical habitat for
various coral-associated taxa.
Evidence for shifts in biodiversity during periods
of environmental change is common in the fossil
record. Mayhew
et al.
( 2008 ) report signii cant posi-
tive correlations between variation in global tem-
perature and the rates of origination and extinction
of families and genera through the Phanerozoic.
Correlations were higher for originations, but not
for extinctions, when diversity lagged behind tem-
perature by 10 Myr, suggesting that diversii cation
occurs mainly after a period of extinction driven by
global warming. For marine genera, the effect of
variation in CO
2
levels on extinction rates was
stronger than that of temperature, suggesting that
ocean carbon levels have inl uenced marine species
diversity throughout the Phanerozoic. Five major
mass extinction events that caused the loss of 75%
or more of all species (Jablonski and Chaloner 1994;
see also Chapter 4) are the most striking features of
the fossil record. For many (if not all) of these events,
rapid change in environmental parameters, such as
temperature, oxygen levels, and ocean pH, coupled
with ecological factors, appear to have played a
large role in the high extinction rates (Knoll
et al.
2007 ; Chapter 4 ). Marine life recovered following
each extinction event, but required millions of years,
potentially due to slow rates of evolutionary diver-
sii cation or persistently unfavourable environmen-
tal conditions, or both (Knoll
et al.
2007 ).
Is it likely that ocean acidii cation will reduce the
biodiversity of marine ecosystems and drive signii -
cant shifts in their function? The response of marine
ecosystems will be linked to the rate and magnitude
of changes in ocean chemistry in relation to the
potential rates of acclimatization, adaptation, and
evolution of marine organisms, from microbes to
vertebrates. The ongoing large and rapid changes
in ocean pH and carbonate saturation are expected
to drive environmental changes unseen in the
recent evolutionary history of marine organisms,
posing an evolutionary challenge to acclimatize or
adapt. At a minimum, the genetic diversity of vari-
ous marine taxa is likely to change. It remains
unknown whether ocean acidii cation will drive
species to extinction, but it is possible, based on
the growing literature concerning the sensitivity
10.4
Effects of environmental change
Environmental variation over space or time can
have positive and negative effects on biodiversity
and ecosystem function related to the rate, magni-
tude, duration, and spatial scale of environmental
change (Knoll
et al.
2007). Habitats with greater spa-
tial heterogeneity provide variable environmental
conditions that typically support higher biodiver-
sity than relatively homogeneous habitats. Temporal
environmental variation also plays a key role in
regulating local diversity. Some level of change in
environmental factors (e.g. physical disturbance or
variability in temperature, oxygen, or other para-
meters) or biological factors (e.g. variation in the
abundance of predators or competitors) can lead to
enhanced local diversity (Connell 1978). Moreover,
environmental variability can promote genetic
diversity by selecting for a broad range of geno-
types to match environmental patterns, or allowing
for adaptive radiation as species emerge to i ll new
ecological space. Species, populations, or genotypes
originate through evolutionary divergence to
exploit novel habitats, or through specialization in
response to environmental variation (temperature,
habitat complexity, oxygen concentration, light,
etc.) and biological interactions (trophic, competi-
tive, or mutualistic).
Coral reef ecosystems, typii ed by high topo-
graphic complexity that promotes species-packing,
have been shown to have been cradles of diversii -
cation throughout the Phanerozoic, with high rates
of species origination that are often exported to off-
shore and deeper regions (Kiessling
et al.
2010 ; see
Chapter 4). For example, habitat complexity gener-
ated by highly branched scleractinian corals such as
Acropora
spp. provide habitat for a remarkably
diverse array of i shes and invertebrates. Such
branching corals are keystone functional groups,
and are threatened by ocean acidii cation and other
environmental changes (Bellwood
et al.
2004 ).
Reduced coral growth and weaker carbonate
cementation will increase the probability of damage
to most structurally complex corals during storms,
probably leading to reef l attening and reduced reef
biodiversity (Hofmann
et al.
2010 ). Similar impacts
on scleractinian corals, and perhaps other structure-
forming groups in deep-sea systems (Guinotte
et al.
