Climate change and the loss of marine ecosystem engineers
Carlo Cerrano
DIPTERIS, Univeristy of Genoa
cerrano@dipteris.unige.it
During recent years, many researchers have predicted that diseases in terrestrial and marine
ecosystems could increase due to climate warming. Thermal stress may directly affect the
physiology of organisms or reduce their resistance, resulting in numerous diseases affecting
natural populations. Mortality is a natural process but recent diseases seem to be ever more
important in regulating population dynamics of several organisms (Hayes and Goreau 1998;
Epstein et al. 1998; Harvell et al. 1999). Sometimes, at a regional scale, sudden and diffused
diseases give rise to a mass mortality of one or more species. Generally, these mass mortalities
do not lead to extinction, although local populations, with their associated fauna, may disappear
(Carlton et al. 1999). True documented extinction of benthic species are very rare and generally
related to the destruction of their habitat. There is recent evidence of an increase in the frequency
and intensity of mass mortalities of several species over the past 30–40 years in tropical and
temperate areas (Cerrano and Bavestrello 2009) even if an increased awareness could also be
partially responsible for this trend.
Seagrass meadows, kelp forests, coral reefs, mussel and scallop beds, xenophyophore and
sponge fields, typically modify their own habitat increasing the fitness of associated species (Bruno
and Bertness 2001). These organisms are defined foundation species (Dayton 1972) or ecosystem
engineers (Jones et al. 1994). The presence of foundation species can often reduce water flow
velocity as demonstrated for algal canopies (Jackson and Winant 1983) and seagrass meadows
(Gambi et al. 1990) and this physical effect can reduce re-suspension (Gacia and Duarte 2001)
thus stabilizing the substrate, increasing the fine sediment fraction (Eckman et al. 1981) and the
quantity of sediment organic matter (Pusceddu et al. 2007). As a consequence, organisms living in
habitats characterised by the presence of these “engineers” can experience a sort of “buffer zone”
where environmental modifications occur slower and within narrower ranges with respect to the
surrounding ambient. Into a land forest several environmental factors such as light, temperature
and humidity are more stable during the day and during the year respect to outside, where wind,
sun and rain can rapidly change (Allen et al. 1972). In seagrass or seafan forests or among coral
colonies, for example, water movement and particles transport decrease (Nepf and Ghisalberti
2008, Scinto et al, 2009). Such a process of current baffling and sand entrapment has been
observed and described also in deep coral banks (Wheeler et al., 2008) and has a positive effects
on meiofauna abundance and diversity. When climatic anomalies lead to the decrease of
ecosystem engineers abundance the negative effects on their habitat may be very wide.
Recent mass mortalities of sponges and sea fans have dramatically changed the landscape down
to 45 m depth, although it is difficult to evaluate the effect of the almost complete loss of these
organisms on the entire coralligenous assemblages. The widespread strong reduction of species
of economic interest—e.g. bath sponges and coral—has impaired commercial fishery activities.
Several years after the mass mortality, there are as yet no sign of recovery of shallow populations
of bath sponges. The partial recover of shallow sea fans is probably due to larvae influx from deep
populations (Cerrano and Bavestrello 2008). Mass mortalities determine sudden fragmentation of
affected populations. In the case of populations of benthic sessile species, recruitment could be
strongly compromised. For spawning organisms, the absolute distances that sperms can travel are
limited, so that eggs of isolated individuals may remain unfertilized. Allee effect represents a
threshold in numbers below which rates of population growth become negative, rather than
positive. When recruitment of the affected species continued after the mass mortality, a possible
explanation could be the occurrence of pockets of populations with sufficient density continuing to
seed other areas. In this way, communities living between 60 and 100–120 m depth may represent
key areas for the recovery of affected species, and would need to be included in management
programs. The preservation of these deep populations is crucial for the conservation of these
characteristic elements of the Mediterranean landscape. Unfortunately, still today these
environments are too poorly explored and known to be adequately considered for protection.
Benthic cnidarians are considered among the most efficient suspension feeders in the world ocean
in extracting and processing energy from water column, for this reason the wide variety of
disturbances that affected this group are supposed to have a deep impact on the entire benthic
assemblage (Cerrano et al. 2005; Garrabou et al. 2009). Some of these disturbances are due to
human activities such as pollution, fishing and anchorage, and others are related to climate
anomalies that lead to more frequent mass mortality episodes. Considering the ecological
importance of benthic filter-feeders in regulating littoral food chains (Gili and Coma 1998), and the
slow growth of many important species, mass mortalities may induce long-term effects, not only in
benthic communities but also in the water column (Sala and Knowlton 2006). To correctly evaluate
these complex dynamics, it is important to develop long-term monitoring projects of the most
vulnerable species to evaluate when and where populations dip below threshold densities, which
may compromise recovery and lead to local or global extinction
In the Mediterranean gorgonian mass mortalities, etiological agents have been found only recently
(Bally & Garrabou 2007); Vibrio coralliilyticus has been considered one of the most possible agents
of the diseases affecting Paramuricea clavata. The negative impact of high seawater temperature
on benthic organisms has been highlighted also by several other field observations allowed
describing a possible future tropical scenario for the coastal seascape of the Mediterranean Sea
(Bianchi 2007). Among consequences, there is the decrease of the bathymetric upper limit of
several boreal species such as some sponge and gorgonian species and the loss of structural
complexity.
The knowledge on the effects of mesophotic coral forests on ecosystem processes is still quite
limited and information on the role of their deeper counterparts, such as deep-water cold corals,
oyster beds and sponge fields, is even poorer (Byers et al. 2006).
These organisms deserve protection not only because they can be rare, endemic or endangered
by human impacts, but also because they play a key ecological role in the mesophotic zone. These
results highlight the need of censusing the presence and distribution of these species in order to
promote effective measures of protection. Moreover, this overview underlines the need to improve
the knowledge about restoration techniques such as transplantation as a possible tool to
rehabilitate degraded habitats like coralligenous community.
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