low abundances and restricted to northern sites. Balistids were rare at all sites,
although labrids were relatively abundant. A significant inverse relationship
between the abundance of Labridae and urchin densities was found. Parrotfish and
urchin densities were not related. Calcium carbonate comprised approximately
90 % of the urchin gut content, and followed by organic material. Urchin bioerosion rates ranged from lowest at Hol Chan Reserve (*0.2 kg CaCO 3 m
-2 yr
-1 )
to highest at Coral Gardens (*1.4 kg CaCO 3 m
-2 yr
-1 ), an unprotected area close
to Hol Chan. Small urchins and low proportion of organic material in gut contents
were found at low algae covered sites, suggesting that the herbivors were food
limited. Taken together their results indicate that both top-down and bottom-up
factors contribute to the control of sea urchin populations in the lagoons off Belize.
They also suggested that Marine Protected Areas (MPA) may provide the means
for limiting sea urchin densities and bioerosion of coral reefs and result in reef
recovery, returning the calcium carbonate cycle to a more balanced state.
Pacific
Fischer (1990) described the importance of D. mexicanum as a bioeroder in Costa
Rica. This species is found in very shallow waters (down to 8 m), and produces
stronger bioerosion on the limestone. At Caño Island, Guzman (1988) found low
densities of this sea urchin, but nevertheless it caused bioerosion at the base of
massive corals. Also, at Cocos Island grazing activities of this sea urchin caused
high levels of bioerosion and reduced coral recruitment (Guzman and Cortés
1992). After the 1982-83 El Niño event, Guzman and Cortés (1992) stated that
coral recovery would take centuries due the high densities of bioeroders and low
coral reproduction rates. Guzman and Cortés (1992, 2007) indicated that part of
the deterioration of the reef structures at Cocos Island was due to the bioerosive
action of D. mexicanum.
In 2002, there was a five-fold increase of coral cover and a notable sea urchin
reduction (Guzman and Cortés 2007). Guzman and Cortés (2007) determined that
D. mexicanum was not playing a important role on the reef bioerosion. However,
the urchin could still fulfill its key herbivorous role in assisting the recruitment of
corals, as has been observed in other reefs on the Caribbean (Sammarco et al.
1974; Sammarco 1980, 1982a, b; Mumby et al. 2006). Lessios et al. (1996)
reported the presence of an Indo-Pacific echinoid (Echinothrix diadema and
Echinothrix calamaris; Fig. 3.5) at Cocos Island, and that probably arrived to the
island during the 1982–1983 El Niño (Lessios et al. 1996, 1998). These two
species are well known bioeroders in the Indo-Pacific (Bak 1990; Peyrot-Clausade
et al. 2000; Carreiro-Silva and McClanahan 2001). However, their impact in
Cocos Island is unknown. But due to their current densities (0.03–0.02 ind m
-2
;
Alvarado and Chiriboga 2008) it could be predicted that they would be less
compared to that of D. mexicanum.
88
J. J. Alvarado et al.
although labrids were relatively abundant. A significant inverse relationship
between the abundance of Labridae and urchin densities was found. Parrotfish and
urchin densities were not related. Calcium carbonate comprised approximately
90 % of the urchin gut content, and followed by organic material. Urchin bioerosion rates ranged from lowest at Hol Chan Reserve (*0.2 kg CaCO 3 m
-2 yr
-1 )
to highest at Coral Gardens (*1.4 kg CaCO 3 m
-2 yr
-1 ), an unprotected area close
to Hol Chan. Small urchins and low proportion of organic material in gut contents
were found at low algae covered sites, suggesting that the herbivors were food
limited. Taken together their results indicate that both top-down and bottom-up
factors contribute to the control of sea urchin populations in the lagoons off Belize.
They also suggested that Marine Protected Areas (MPA) may provide the means
for limiting sea urchin densities and bioerosion of coral reefs and result in reef
recovery, returning the calcium carbonate cycle to a more balanced state.
Pacific
Fischer (1990) described the importance of D. mexicanum as a bioeroder in Costa
Rica. This species is found in very shallow waters (down to 8 m), and produces
stronger bioerosion on the limestone. At Caño Island, Guzman (1988) found low
densities of this sea urchin, but nevertheless it caused bioerosion at the base of
massive corals. Also, at Cocos Island grazing activities of this sea urchin caused
high levels of bioerosion and reduced coral recruitment (Guzman and Cortés
1992). After the 1982-83 El Niño event, Guzman and Cortés (1992) stated that
coral recovery would take centuries due the high densities of bioeroders and low
coral reproduction rates. Guzman and Cortés (1992, 2007) indicated that part of
the deterioration of the reef structures at Cocos Island was due to the bioerosive
action of D. mexicanum.
In 2002, there was a five-fold increase of coral cover and a notable sea urchin
reduction (Guzman and Cortés 2007). Guzman and Cortés (2007) determined that
D. mexicanum was not playing a important role on the reef bioerosion. However,
the urchin could still fulfill its key herbivorous role in assisting the recruitment of
corals, as has been observed in other reefs on the Caribbean (Sammarco et al.
1974; Sammarco 1980, 1982a, b; Mumby et al. 2006). Lessios et al. (1996)
reported the presence of an Indo-Pacific echinoid (Echinothrix diadema and
Echinothrix calamaris; Fig. 3.5) at Cocos Island, and that probably arrived to the
island during the 1982–1983 El Niño (Lessios et al. 1996, 1998). These two
species are well known bioeroders in the Indo-Pacific (Bak 1990; Peyrot-Clausade
et al. 2000; Carreiro-Silva and McClanahan 2001). However, their impact in
Cocos Island is unknown. But due to their current densities (0.03–0.02 ind m
-2
;
Alvarado and Chiriboga 2008) it could be predicted that they would be less
compared to that of D. mexicanum.
88
J. J. Alvarado et al.