Eastern Pacific Coral Reef Ecosystem
291
Island is a collection of small islands and rocks whose steep sides descend
below the photic zone. While this site supports abundant coral populations, the absence of shallow shelves has prevented the accumulation of
reef frameworks. Coral frameworks were also present at Cocos and the
Galapagos Islands before the 1982-1983 ENSO, but most of these were
severely damaged and subsequently have become greatly reduced in size,
even disappearing in some instances, due to the grazing and boring activities of echinoids and other bioeroders.
19.4 Ecological Processes
Most studies on the ecology of interacting species and, where known, the influence of such processes on coral community structure (i.e., species composition, relative abundances, zonation and other spatial distributions)
have been conducted on only one or a few coral reefs; therefore, the generality of these ecological processes on eastern Pacific reefs needs to be
substantiated. The nutrient-rich eastern Pacific waters favor not only phytoplankton, which blocks downwelling light utilized by corals, but also internal bioeroders, and benthic filamentous and macroalgae, which compete
directly with corals for space (Birkeland 1977). Under conditions of extremely high nutrient availability, e. g. during upwelling pulses, coral tissues
are often invaded or overgrown by benthic algae, resulting in partial to total
colony mortality. Dinoflagellate blooms have also been observed to cause
coral mortality in Costa Rica and Panama, presumably from large amounts
of mucus produced by dinoflagellates, which adheres to and smothers the
corals. Reef-associated mollusks, crustaceans and fishes also die in large
numbers, perhaps from toxins and oxygen depletion.
In spite of generally high abundances of fish and sea urchin herbivores
on many reefs, benthic algae often grow in profusion and compete with
corals for space. This might be one of the reasons why pocilloporid reef
frames are so common in the eastern Pacific region. Pocilloporid species
are among the strongest coral competitors with relatively high rates of calcification. Colonies in close proximity typically grow vertically, forming
rigid meter-sized blocks that are not easily invaded by other benthos.
Diadematid sea urchins, notably Diadema mexican urn A. Agassiz, graze on
algae and are often abundant on reefs, and a cidarid pencil sea urchin in
the Galapagos Islands, Eucidaris galapagensis Doderlein, grazes on algae
as well as live corals. Sea urchins can attain extraordinarily high abundances on coral reefs, with population densities commonly ranging between 50 and 100m- 2 in Panama and the Galapagos Islands following the
291
Island is a collection of small islands and rocks whose steep sides descend
below the photic zone. While this site supports abundant coral populations, the absence of shallow shelves has prevented the accumulation of
reef frameworks. Coral frameworks were also present at Cocos and the
Galapagos Islands before the 1982-1983 ENSO, but most of these were
severely damaged and subsequently have become greatly reduced in size,
even disappearing in some instances, due to the grazing and boring activities of echinoids and other bioeroders.
19.4 Ecological Processes
Most studies on the ecology of interacting species and, where known, the influence of such processes on coral community structure (i.e., species composition, relative abundances, zonation and other spatial distributions)
have been conducted on only one or a few coral reefs; therefore, the generality of these ecological processes on eastern Pacific reefs needs to be
substantiated. The nutrient-rich eastern Pacific waters favor not only phytoplankton, which blocks downwelling light utilized by corals, but also internal bioeroders, and benthic filamentous and macroalgae, which compete
directly with corals for space (Birkeland 1977). Under conditions of extremely high nutrient availability, e. g. during upwelling pulses, coral tissues
are often invaded or overgrown by benthic algae, resulting in partial to total
colony mortality. Dinoflagellate blooms have also been observed to cause
coral mortality in Costa Rica and Panama, presumably from large amounts
of mucus produced by dinoflagellates, which adheres to and smothers the
corals. Reef-associated mollusks, crustaceans and fishes also die in large
numbers, perhaps from toxins and oxygen depletion.
In spite of generally high abundances of fish and sea urchin herbivores
on many reefs, benthic algae often grow in profusion and compete with
corals for space. This might be one of the reasons why pocilloporid reef
frames are so common in the eastern Pacific region. Pocilloporid species
are among the strongest coral competitors with relatively high rates of calcification. Colonies in close proximity typically grow vertically, forming
rigid meter-sized blocks that are not easily invaded by other benthos.
Diadematid sea urchins, notably Diadema mexican urn A. Agassiz, graze on
algae and are often abundant on reefs, and a cidarid pencil sea urchin in
the Galapagos Islands, Eucidaris galapagensis Doderlein, grazes on algae
as well as live corals. Sea urchins can attain extraordinarily high abundances on coral reefs, with population densities commonly ranging between 50 and 100m- 2 in Panama and the Galapagos Islands following the