Geoscience Reference
In-Depth Information
upon a reduction or loss of species in response to
environmental variation or other factors (Loreau
et al.
2001 ).
Field studies in terrestrial and marine ecosystems
over the past two decades have provided both sup-
port (e.g. Steneck
et al.
2002 ) and moderate contro-
versy concerning the diversity-stability concept
( Loreau
et al.
2001 ; Cardinale
et al.
2006 ). The contro-
versy relates, in part, to the disproportionate role of
key species in many systems studied, without which
stability is reduced. This issue may be important in
several marine ecosystems where key prey species
(e.g. pteropods, krill, anchovies, and squid in coastal
systems) or taxa that play an important structural
role (e.g. habitat-forming corals) can be critical
resources for other taxa. Thus, reduced biodiversity
due to the loss of prey taxa or habitat-forming spe-
cies may not have large effects on energy l ow or
ecosystem function and stability, so long as key spe-
cies that maintain the functional diversity of the
system are relatively unaffected (Tilman
et al.
1997 ).
Although this debate continues, a recent examina-
tion of experimental studies of nearshore marine
communities (Duffy 2009) and long-term studies of
large marine ecosystems (Worm
et al.
2006 ) provides
fairly strong support for the role of biodiversity in
ecosystem function and stability. For many marine
systems, however, the current understanding of the
natural history and functional roles of most species
is poor; thus it will be challenging to predict how
ocean acidii cation may affect ecosystem function.
ture) are intolerable. Range shifts along other
gradients in pH or carbonate saturation (e.g. depth-
related or horizontally in coastal regions) may also
be possible.
Whether marine organisms, from microbes to
long-lived megafauna, will be able to acclimatize or
adapt to future ocean acidii cation is an important,
but unresolved, question. Acclimatization is the
process by which individuals adjust to environmen-
tal changes (i.e. physiological adjustment). This
may result in a change in energy costs (positive or
negative) associated with living. Adaptation is the
adjustment of species to environmental change
between generations, through natural selection of
individuals tolerant of new conditions. Tolerance or
acclimatization by at least some individuals in the
population allows for adaptation, assuming that the
tolerant traits are heritable and sufi cient time is
available for selection to increase the frequency of
tolerant genotypes through multiple generations. If
individuals in most populations are able to acclima-
tize to reduced ocean pH or carbonate saturation
through the adjustment of physiological homeosta-
sis, changes in ocean biodiversity could be mild—
perhaps only a contraction in genetic diversity
within species with minor effects on ecosystem
functions.
Although it is expected that there will be 'win-
ners' and 'losers' in response to ocean acidii cation,
many vulnerable species (such as corals; see Kleypas
and Yates 2009) may suffer from reduced perform-
ance and survival, and have limited scope for adap-
tation due to the expected pace and magnitude of
ocean acidii cation in the future. Taxa with short
generation times and immense population sizes
such as phytoplankton and microbes (e.g. with one
to three generations per day) have perhaps the
greatest capacity to adapt, given that upwards of 35
000 to 100 000 generations are possible over the next
100 years as ocean acidii cation intensii es. Collins
and Bell (2006), however, found little evidence of
adaptation to high CO
2
levels in a pond alga
(
Chlamydomonas
) after 1000 generations. In contrast,
the scope for adaptation by species with long gen-
eration times (e.g. 10 to 30 yr for some i shes) and
relatively small population sizes is expected to be
limited when selection for tolerant genotypes is
constrained to just a few generations.
10.3
Acclimatization and adaptation
Organisms and species faced with ocean acidii ca-
tion or other environmental changes have four
options—migration, acclimatization (i.e. tolerance),
adaptation, or extinction. Migration, by individu-
als or by a population successively through genera-
tions, may be possible in some cases, but the global
nature of ocean acidii cation coupled with range
limitations imposed by other environmental
parameters (e.g. temperature) may limit this option.
For example, meridional gradients in aragonite
saturation (see Fig. 14.5 in Chapter 14) may allow
the ranges of some mid-latitude taxa to shift toward
more saturated waters of the tropics as acidii ca-
tion intensii es, unless other factors (e.g. tempera-
