Biology Reference
In-Depth Information
species became extinct. Currently, reefs are still able to recuperate with highly
variable rates, indicating a differential recovery capacity. But full recovery of reefs
after thermal stress probably needs decades to centuries (Baker et al.
2008
). As
growth of macroalgae is much faster and their temperature tolerance is several
degree Celsius higher than those of corals (Pakker et al.
1995
; Bischoff-Basmann
et al.
1997
), a shift from coral to seaweed dominated coastal ecosystems has been
proposed for future tropical areas (Hoegh-Guldberg et al.
2007
) and has already
been observed during recent decades (Rasher et al.
2011
and references therein).
Current warm-temperate coastlines which will become tropical in future
(Figs.
18.1
and
18.2
) will not be able to compensate for the prospected loss of
reefs as rates needed to establish coral reefs are slow (Baker et al.
2008
). Similarly
as corals, subtidal reef macroalgae potentially face local extinction if temperature
exceeds algal tolerance limits which are firmly set to 30-33
C (Pakker et al.
1995
).
In contrast, eulittoral tropical macroalgae with their higher lethal temperature limits
of up 32-37
C (Bischoff-B
asmann et al.
1997
) will be better able to withstand
future temperature increase in the central tropics. First local extinctions after
warming events have been recorded: In the Galapagos Archipelago six tropical
macroalgal species disappeared after the ENSO warming event in 1982/1983. Here
a transition of the macroalgal and coral habitats to heavily grazed reefs dominated
by crustose coralline “urchin barrens” was observed (Edgar et al.
2010
). Addition-
ally to ocean warming, ocean acidification may decrease coralline abundance in
future which will enhance the cascade effect of decreasing coral recruitment,
opening space for turf algal species, and further inhibition of coral recruitment,
coral fecundity, and coral growth (Hoegh-Guldberg et al.
2007
). Unfortunately, the
functional ecology and thermal traits of coralline red algae which are important
contributors to reef structure and facilitate settlement of corals are virtually
unknown (Nelson
2009
).
As the tropical region will considerably extend polewards at the expense of the
current warm-temperate region (Figs.
18.1
and
18.2
), substantial new transitional
areas will develop along rocky shore coastlines characterized, for example, by
assemblages dominated by tropical to subtropical members of the brown algal
order Dictyotales and not corals such as described for tropical to warm-temperate
transitional areas of the Canary Islands (Sangil et al.
2011
). Locally, other factors
such as wave exposure, local currents, and physical barriers may be more important
than temperature for biogeographic distribution as has been observed along a
tropical to warm-temperate coastal transition region in E-Australia where species
distribution is still stable despite recent rapid warming (Poloczanska et al.
2011
).
€
18.4 Assumptions for Global Seaweed Biodiversity
Global patterns of marine diversity differ from terrestrial habitats where species
numbers decrease moving away from the equator in both hemispheres (Pianka
1966
;
Willig et al.
2003
). Marine coastal taxa also show clear latitudinal trends, but overall
