macroalgal diversity, resulting in the formation of impoverished sea urchindominated areas denuded of all but encrusting algae (Lawrence 1975; Sangil et al.
2006a, 2006b). Compared to typical urchin barrens elsewhere in the world
(Chapman 1981; Pearse and Hines 1987; Valentine and Johnson 2005; Guidetti
and Dulcic 2007), barrens generated by D. africana in the eastern Atlantic appear
to be maintained by relatively low numbers of sea urchins (starting at 2 ind m
-2 ).
Probably, the oligotrophic condition of waters surrounding the archipelago provides limited nutrient resources that are not sufficient to maintain extremely high
urchin densities. However, species movement and food consumption rates, which
involve grazing large areas of rocky substrate at night (Tuya et al. 2004a), allow
comparatively lower urchin densities to maintain a barren habitat denuded of most
erect macroalgae biomass.
Loss of macroalgal beds due to D. africana grazing activity produces not only
lower species diversity (Herrera 1998; Garrido 2003) but also a lack of habitat
suitable for fish feeding and reproduction (Brito et al. 2004; Tuya et al. 2005;
Clemente 2007). Macroalgae constitute a major energy source that maintains the
food web, and are the main biological engineers on rocky reefs of the Canary
Islands, which lie between tropical and temperate waters and has no coral reefs.
Consequently, algal cover provides the habitat that supports the whole system. It
constitutes a well-known indicator of benthic conservation status (Hernández et al.
2008b). Another good example of a third-order change that occurs as a consequence of the development of urchin barrens is the distribution and abundances of
large holothurians, linked to specific type of habitats across shallow-waters of the
eastern Atlantic (Tuya et al. 2006b). This fact could be related to variations in the
availability of particulate organic matter across habitats. Benthic grazers, such as
sea urchins, can transform up to 70 % of the consumed algae to detrital fecal
matter (Vadas 1977; Koike et al. 1987; Frantzis and Gremare 1992; Lison de Loma
et al. 2000; Mills et al. 2000; Mamelona and Pelletier 2005). High densities of D.
africana on barren grounds likely produce large amounts of fecal pellets, as a
source of fresh particulate organic matter (Lawrence 1975; Wotton 1994; Lawrence and Klinger 2001; Levinton et al. 2002; Mamelona and Pelletier 2005).
Consequently, holothurians were significantly more abundant in D. africana barrens compared with vegetated habitats, a pattern that was strongest for the
numerically dominant species, Holothuria sanctorii (Tuya et al. 2006b).
As a major herbivore, D. africana is recognized as having a key ecological role
that threatens the resilience of marine systems in the eastern Atlantic, with the
subsequent establishment of undesired conservation states (Hernández et al.
2008a). The reported increasing frequency of overgrazing events, concomitant
with loss of associated ecosystem goods and services, generates a serious environmental problem that constitutes a key threat to coastal marine ecosystems and
local communities of the islands. Hence, the necessity of managing the populations
of D. africana to prevent overgrazing management has become apparent, especially in areas of the Canary Islands where algae-associated services (such as
fishing stocks) are important for livelihood sustenance. Algal stands are very
restricted in space in the Canary Islands and have shown a temporal regression
496
J. C. Hernández et al.
2006a, 2006b). Compared to typical urchin barrens elsewhere in the world
(Chapman 1981; Pearse and Hines 1987; Valentine and Johnson 2005; Guidetti
and Dulcic 2007), barrens generated by D. africana in the eastern Atlantic appear
to be maintained by relatively low numbers of sea urchins (starting at 2 ind m
-2 ).
Probably, the oligotrophic condition of waters surrounding the archipelago provides limited nutrient resources that are not sufficient to maintain extremely high
urchin densities. However, species movement and food consumption rates, which
involve grazing large areas of rocky substrate at night (Tuya et al. 2004a), allow
comparatively lower urchin densities to maintain a barren habitat denuded of most
erect macroalgae biomass.
Loss of macroalgal beds due to D. africana grazing activity produces not only
lower species diversity (Herrera 1998; Garrido 2003) but also a lack of habitat
suitable for fish feeding and reproduction (Brito et al. 2004; Tuya et al. 2005;
Clemente 2007). Macroalgae constitute a major energy source that maintains the
food web, and are the main biological engineers on rocky reefs of the Canary
Islands, which lie between tropical and temperate waters and has no coral reefs.
Consequently, algal cover provides the habitat that supports the whole system. It
constitutes a well-known indicator of benthic conservation status (Hernández et al.
2008b). Another good example of a third-order change that occurs as a consequence of the development of urchin barrens is the distribution and abundances of
large holothurians, linked to specific type of habitats across shallow-waters of the
eastern Atlantic (Tuya et al. 2006b). This fact could be related to variations in the
availability of particulate organic matter across habitats. Benthic grazers, such as
sea urchins, can transform up to 70 % of the consumed algae to detrital fecal
matter (Vadas 1977; Koike et al. 1987; Frantzis and Gremare 1992; Lison de Loma
et al. 2000; Mills et al. 2000; Mamelona and Pelletier 2005). High densities of D.
africana on barren grounds likely produce large amounts of fecal pellets, as a
source of fresh particulate organic matter (Lawrence 1975; Wotton 1994; Lawrence and Klinger 2001; Levinton et al. 2002; Mamelona and Pelletier 2005).
Consequently, holothurians were significantly more abundant in D. africana barrens compared with vegetated habitats, a pattern that was strongest for the
numerically dominant species, Holothuria sanctorii (Tuya et al. 2006b).
As a major herbivore, D. africana is recognized as having a key ecological role
that threatens the resilience of marine systems in the eastern Atlantic, with the
subsequent establishment of undesired conservation states (Hernández et al.
2008a). The reported increasing frequency of overgrazing events, concomitant
with loss of associated ecosystem goods and services, generates a serious environmental problem that constitutes a key threat to coastal marine ecosystems and
local communities of the islands. Hence, the necessity of managing the populations
of D. africana to prevent overgrazing management has become apparent, especially in areas of the Canary Islands where algae-associated services (such as
fishing stocks) are important for livelihood sustenance. Algal stands are very
restricted in space in the Canary Islands and have shown a temporal regression
496
J. C. Hernández et al.