Sergio Rossi – Marine Animal Forests (2017)

Описание

This volume is a multiauthored compilation of established and recent knowledge on
a particular kind of marine epifaunal communities and ecosystems. Having this in
mind, readers might be struck by the unusual term “forest” used in this context. Most
people connect this term with assemblages of trees on land, with tall trunks and a
majestic canopy of branches and leaves as in the case of mahogany trees, oaks,
beeches, or sequoias, often with a second canopy of younger trees and bushes
underneath. There are also other types of complex architecture of lower tiering in
the terrestrial vegetation such as cactus forests, shrubs, savannas, and meadows.
They all have in common a three-dimensional structure, their way of energy assimilation,
and a function of ecosystem services for other biota.
Transferring this image to the marine realm, one would think first of another type
of vegetation: the gigantic kelp forests, which reach similar size, shape, and threedimensionality
and have similar functions for their associated biota. On the faunal
side, the largest architecture is performed by scleractinian corals, gorgonians, and
sponges. Just as on land, there are many other structures of minor tiering but similar
architectural complexity and related functions on the ocean floor, both on the floral
and the faunal side. Some of them such as seagrass meadows and mussel banks are
fairly well studied, whereas others, particularly in deep or remote regions of the
oceans, have remained undetected or have received attention only recently due to the
development of new techniques and facilities, often provided by increased international
cooperation. For convenience and as a unifying concept, all complex epifaunal
shapes and constructions in the sea are assembled under the term “animal forests” in
this book, although many coral reefs and gorgonian “forests” resemble shrubland
rather than trees, some large scleractinian reefs rather appear like buildings, sponges
do not reach tree size and the big ones are not branched, and most of the smaller
members of three-dimensional zoobenthic communities are comparable to herbs
at best.
“Marine animal forests” (MAF) as understood in this compilation are composed
of many epi-zoobenthic taxa and comprise as a common trait their three-dimensional
architecture tiering from a few centimeters to tens of meters; if the fossil parts of
some coral reefs are included, the height of these structures may even reach hundreds
of meters. They occur in all oceans, from the tropics to the poles and from shallow
water to the deep sea. Zoobenthic communities with a marked tiering are very old;
they date back to the Palaeozoic when stalked crinoids (“sea lilies”) dominated the ocean floors. Irrespective of whether these ecosystems and communities do indeed
bear major structural analogies to their terrestrial vegetational counterparts and to
three-dimensional plant architecture in the ocean, they definitely reveal analogies in
many functions. There are, however, decisive differences in many characters not
only between land and sea as well as between animal and plant architects, but also
among the various ecosystems and communities presented in this book. For example,
as a consequence of very different media (air and water), terrestrial vegetation
and MAF differ in energy flow and dispersal opportunities. Tropical reef corals host
unicellular primary producers and depend, like kelp and terrestrial vegetation, on
sunlight, whereas the Mediterranean coralligenous depends like other MAF systems
on the input of allochthonous organic matter.
Awealth of old and recent information on MAF systems has accumulated, which
makes a joint presentation and a distinction of common traits and differences
worthwhile. In the first chapter, an overview of these findings is presented. The
areas from where the information was derived extend over a wide geographical range
of seascapes, including tropical shallow-water coral reefs, shallow communities in
the Mediterranean and in Mesoamerica, mesophotic coral communities on the
continental slope, bathyal cold-water “coral gardens” on Azores seamounts of the
Mid-Atlantic Ridge, and other cold-water ecosystems in the deep sea as well as in
northern and southern cold-temperate and polar regions. Structuring ecosystem
engineers comprise the entire variety of hermatypic and soft corals, notably gorgonians,
other cnidarians, sponges, bryozoans, polychaetes, and other taxa which
emerge from the seafloor. This elevation signifies improved access to food particles,
less siltation and overgrowth, avoidance of shading, improved light conditions,
better dispersal of offspring, and (for cnidarians) protection of the fragile polyps.
MAF ecosystems are of paramount importance for hydrodynamic and biogeochemical
cycles, food webs, and sedimentation levels. Tropical reef corals export nitrogen
and organic matter into the surrounding environment and produce and cement
carbonate. Many suspension feeders act as carbon sinks, retaining part of the organic
matter involved in pelagic-benthic coupling in their long-lived structures. They
adapt to changes in seston availability by increased food intake during phytoplankton
blooms and consumption of stored food at times of low seston presence. Bivalve
filter feeders in shallow waters increase the seston flux due to mixing caused by the
exhalant jets from their siphons. There are many more interactions between suspension
feeders and their environment presented in this volume, which have to be
omitted here for reasons of space.
As a unifying concept, both in the ocean and on land complex structures of key
species (“engineers”) usually support high species diversity and density, sometimes
also biomass, of associated fauna. These engineers shape their environment, increase
habitat heterogeneity, create a multitude of niches and cryptic habitats, and provide
food, shelter, and reproductive facilities for an associated mobile fauna. On land,
arthropods, birds, reptiles, and mammals are the main users; in the ocean, this fauna
consists of a huge variety of invertebrates, many fish, and some other vertebrates. However, vertical exchange is much more pronounced in the sea: planktonic invertebrates
and suprabenthos undergo diel vertical migrations into and out of MAF.
Many invertebrates and fish use the “second floor” in these communities as “watchtowers”
for improved outlook, greater food availability, and better dispersal of eggs
and larvae.
All these properties and services contribute to the ecosystem services provided by
MAF to their associated fauna and the entire marine ecosystem. Another important
part of these services are those granted to man. Without going into much detail (all
these services are mentioned exhaustively in this book) some of the principal human
activities using MAF ecosystems include, e.g., fishing (fish, invertebrates, turtles),
collecting (precious corals, other corals and seashells, fish and invertebrates for
aquaria, species for pharmaceutic and medical use, construction materials), and
tourism (diving, sightseeing). They involve occupation and income for many coastal
populations and are the basis for many goods, trades, and commercial activities
worldwide. Both facets of the ecosystem services provided by MAF represent
enormous values, as is stated by one of the authors of this book, although some
managers seem to consider only that part which is of direct use to humans.
A second unifying property between terrestrial forest, kelp, and marine animal
forest ecosystems is that all of them are exposed to natural perturbations and
threatened by human impacts and global climate change (which after all is a
human impact as well). Natural disturbances (e.g., hurricanes, floods, occasional
warming caused by El Niño) are an innate property of ecosystems, and, as long as
they are not frequent or chronic, most ecosystems tend to recover from their effects
fairly well. As for forest fires in some terrestrial ecosystems, the medium term effects
of some major perturbations on marine ecosystems can even be positive, but
continuous disturbance will lead to biodiversity degradation, destruction of complex
ecosystem structures, simplification of functions, and loss of ecosystem services. In
recent times, anthropogenic impacts have increased enormously: destructive fishing
practices, other forms of harmful or exaggerated harvesting, inconsiderate aquaculture,
contamination, pollution, eutrophication, oil and gas exploitation, mining,
urbanization, and increased use of coastal areas are posing severe threats to many
MAF ecosystems. Simultaneously, global warming is proceeding, increasing the sea
level, seawater acidity, iceberg scour and the frequency of hurricanes, and worsening
the bleaching effects caused by El Niño.
In these days, a compilation of review chapters highlighting the richness and
vulnerability of animal structures in the sea is a timely and badly needed venture. This
book contains the experience and joint knowledge of a large number of marine scientists
who have been involved in one way or another in the exploration, investigation, or
conservation of marine epifaunal communities. Their take home message in this
volume is: Let us take special care of these ecosystems! Not only to save their
commercial services to man, to continue exploiting fish, seafood, and precious corals
in the future. There is yet another, innate wealth in these ecosystems: Like terrestrial
complex systems, above all tropical rain forests, they are part of nature’s evolutionary and genetic heritage which makes man’s life worthwhile and enjoyable.
These ecosystems have taken a very long time to develop from the stalked crinoid
assemblages in the Palaeozoic to present times. If we do not take precautions, they
might disappear in a few decades, just as some of the kelp forests and coldwater coral
reefs which were destroyed by trawling. Probably, not forever – ecosystems tend to
recover if they are given sufficient time – but as part of man’s vital environment.