extinct in Floreana and the remnant non-pocilloporid corals, with less than 0.1 %
of cover, were avoided by the urchins. Glynn (1988) suggested that the destructive
effect of sea urchins were the result of erosion of reef structures rather than direct
mortality of surviving corals. The impact of E. galapagensis on habitats other than
coral reefs was described by Brandt (2003). This study was done at a depth of 5 m
off Caamaño Islet (immediately to the south of Santa Cruz Island, outside Academy Bay). It consisted on removing all E. galapagensis from a 6 9 6 m treatment
plot. After 12 weeks macroalgal biomass was 20 times higher in the treatment than
in the control plot, which had densities of *40 ind m
-2 . In addition, benthic
diversity, including sessile invertebrates and algal species, was higher in the plot
devoid of E. galapagensis, while plots with high densities of the sea urchin were
characterized by crustose coralline algae and bare rock. Similar to Glynn (1988),
who reported that coral erosion rates by urchins were slower when the damselfishes Stegastes beebei and Stegastes arcifrons were present, Brandt (2003) also
noted that the abundance of the green foliose algae Ulva sp. was maintained in
plots where both urchins and damselfishes were abundant. The role of damselfishes
in mediating the impact of urchins thus seems important. Irving and Witman
(2009) explored the interactions between the yellow-tail damselfish Stegastes
arcifrons with E. galapagensis and with the ‘‘green’’ sea urchin L. semituberculatus. They found that the ability of sea urchins to switch habitats by removing
algal turf to create coralline barrens was dependent on the absence of damselfish as
well as the species of urchin. In the presence of damselfish, neither species of
urchin caused any significant change to algal assemblages. However, in their
absence, only L. semituberculatus was able to switch the habitat towards greater
cover of encrusting algae by grazing filamentous turfs. In addition, one of their
results contrasts with previous findings about the impact of E. galapagensis on the
abundance of macroalgae described above. Irving and Witman (2009) found that
removal of E. galapagensis from algal communities dominated by encrusting
coralline algae had no effect on algal community structure and abundance, while
the removal of L. semituberculatus resulted in a significant decrease of encrusting
coralline algae and an increase in the abundance of green and other erect algae.
Since the densities of E. galapagensis and L. semituberculatus manipulated by
Irving and Witman (2009) were different, they concluded that relatively high
densities of L. semituberculatus impact algal habitats, but relatively low densities
of E. galapagensis do not. Nevertheless they noted that the densities manipulated
were the same as natural densities at the study site. The study of the impacts of sea
urchins on benthic communities has greatly improved our understanding about the
effects of their grazing and bioerosion in the Galápagos. However more research is
needed with other species of sea urchins, such as T. depressus and D. mexicanum
in the Galápagos, and also with sea urchin species on the coast of Ecuador.
216
J. Sonnenholzner et al.
of cover, were avoided by the urchins. Glynn (1988) suggested that the destructive
effect of sea urchins were the result of erosion of reef structures rather than direct
mortality of surviving corals. The impact of E. galapagensis on habitats other than
coral reefs was described by Brandt (2003). This study was done at a depth of 5 m
off Caamaño Islet (immediately to the south of Santa Cruz Island, outside Academy Bay). It consisted on removing all E. galapagensis from a 6 9 6 m treatment
plot. After 12 weeks macroalgal biomass was 20 times higher in the treatment than
in the control plot, which had densities of *40 ind m
-2 . In addition, benthic
diversity, including sessile invertebrates and algal species, was higher in the plot
devoid of E. galapagensis, while plots with high densities of the sea urchin were
characterized by crustose coralline algae and bare rock. Similar to Glynn (1988),
who reported that coral erosion rates by urchins were slower when the damselfishes Stegastes beebei and Stegastes arcifrons were present, Brandt (2003) also
noted that the abundance of the green foliose algae Ulva sp. was maintained in
plots where both urchins and damselfishes were abundant. The role of damselfishes
in mediating the impact of urchins thus seems important. Irving and Witman
(2009) explored the interactions between the yellow-tail damselfish Stegastes
arcifrons with E. galapagensis and with the ‘‘green’’ sea urchin L. semituberculatus. They found that the ability of sea urchins to switch habitats by removing
algal turf to create coralline barrens was dependent on the absence of damselfish as
well as the species of urchin. In the presence of damselfish, neither species of
urchin caused any significant change to algal assemblages. However, in their
absence, only L. semituberculatus was able to switch the habitat towards greater
cover of encrusting algae by grazing filamentous turfs. In addition, one of their
results contrasts with previous findings about the impact of E. galapagensis on the
abundance of macroalgae described above. Irving and Witman (2009) found that
removal of E. galapagensis from algal communities dominated by encrusting
coralline algae had no effect on algal community structure and abundance, while
the removal of L. semituberculatus resulted in a significant decrease of encrusting
coralline algae and an increase in the abundance of green and other erect algae.
Since the densities of E. galapagensis and L. semituberculatus manipulated by
Irving and Witman (2009) were different, they concluded that relatively high
densities of L. semituberculatus impact algal habitats, but relatively low densities
of E. galapagensis do not. Nevertheless they noted that the densities manipulated
were the same as natural densities at the study site. The study of the impacts of sea
urchins on benthic communities has greatly improved our understanding about the
effects of their grazing and bioerosion in the Galápagos. However more research is
needed with other species of sea urchins, such as T. depressus and D. mexicanum
in the Galápagos, and also with sea urchin species on the coast of Ecuador.
216
J. Sonnenholzner et al.