related to the demographic explosion of the sea urchin during recent years (Clemente 2007; Hernández et al. 2008a). High percentage cover is usually attained
only in shallow water stands at exposed sites where hydrodynamics prevent urchin
activity (Alves et al. 2001; Tuya and Haroun 2006), at certain marine protected
areas (Clemente 2007; Hernández et al. 2008a), or at barely perturbed areas
associated with a delayed anthropogenic development such as isolated and less
populated islands (Clemente et al. 2009).
Many factors are thought to be involved in the recent expansion of D. africana
populations and regression of algal beds at the eastern Atlantic Archipelagos. We
can indicate an array of environmental factors that promote high urchin densities,
such as substrate topography and diversity and certain oceanographic events that
can influence sea urchin settlement, recruitment and survival (Hernández et al.
2008a). In addition, several anthropogenic factors, such as the number of islanders
and tourists per coastal perimeter, the number of operational fishing boats and the
overfishing of the sea urchins’ natural predators have been associated with
increased urchin abundances (Hernández et al. 2008a; Clemente et al. 2009).
Diadema africana is a termophilus species, which probably constitutes a
competitive advantage for its proliferation at the sublittoral latitude of the Canary
Islands in contrast to sea urchins of temperate affinities such as P. lividus. This has
been especially relevant in the last decades, when a trend of progressive sea water
warming has occurred in the region as a consequence of global climate change
processes (Brito 2008). In fact, a long-term study has detected a clear relationship
between sea surface temperature and D. africana settlement (Hernández et al.
2010a). Urchin settlement was higher during warmer years. Temperatures above
24 °C produced major settlement pulses and higher post-settlement survival rates
in barren areas. However, some variability across habitats indicates resilience of
the systems. While sea water temperature is a major factor influencing D. africana
density in urchin barrens because it enhances post-settlement survival, specific
factors in healthy macroalgal beds constrain settlement, increases post-settlement
mortality and controls sea urchin populations (Hernández et al. 2010a). The study
shows that warmer summer temperatures (temperatures at settlement) have been
more frequent since the early 1980
0 s with a parallel increase in urchin population
densities (Hernández et al. 2008a). Assuming climate change predictions (IPCC
2007) for the future are correct, the strong positive relationship between thermal
history and post-settlement survival of this ecologically important species suggests
that barren grounds will become more extensive in the future as seawater temperatures warm in relation to anthropogenic activities. Nonetheless, mass mortality
events, as found for P. lividus in 2003 (Girard et al. in press), could became more
frequent due to warmer waters. In this sense, we would like to point out that
researchers have observed mortality events of D. africana in Madeira and Gran
Canaria islands in 2010 (F. Alves, M. Kaufman and F. Espino pers. com.,
respectively). However, it is too soon to know the cause of this recent phenomenon
that, initially, seems to be very spatially restricted. In both cases, urchins were
infected by the bald sea urchin disease, and possibly originated by the bacteria
Vibrio alginolitycus (Hernández et al. 2010b).
15 Echinoderms of the Canary Islands, Spain
497
only in shallow water stands at exposed sites where hydrodynamics prevent urchin
activity (Alves et al. 2001; Tuya and Haroun 2006), at certain marine protected
areas (Clemente 2007; Hernández et al. 2008a), or at barely perturbed areas
associated with a delayed anthropogenic development such as isolated and less
populated islands (Clemente et al. 2009).
Many factors are thought to be involved in the recent expansion of D. africana
populations and regression of algal beds at the eastern Atlantic Archipelagos. We
can indicate an array of environmental factors that promote high urchin densities,
such as substrate topography and diversity and certain oceanographic events that
can influence sea urchin settlement, recruitment and survival (Hernández et al.
2008a). In addition, several anthropogenic factors, such as the number of islanders
and tourists per coastal perimeter, the number of operational fishing boats and the
overfishing of the sea urchins’ natural predators have been associated with
increased urchin abundances (Hernández et al. 2008a; Clemente et al. 2009).
Diadema africana is a termophilus species, which probably constitutes a
competitive advantage for its proliferation at the sublittoral latitude of the Canary
Islands in contrast to sea urchins of temperate affinities such as P. lividus. This has
been especially relevant in the last decades, when a trend of progressive sea water
warming has occurred in the region as a consequence of global climate change
processes (Brito 2008). In fact, a long-term study has detected a clear relationship
between sea surface temperature and D. africana settlement (Hernández et al.
2010a). Urchin settlement was higher during warmer years. Temperatures above
24 °C produced major settlement pulses and higher post-settlement survival rates
in barren areas. However, some variability across habitats indicates resilience of
the systems. While sea water temperature is a major factor influencing D. africana
density in urchin barrens because it enhances post-settlement survival, specific
factors in healthy macroalgal beds constrain settlement, increases post-settlement
mortality and controls sea urchin populations (Hernández et al. 2010a). The study
shows that warmer summer temperatures (temperatures at settlement) have been
more frequent since the early 1980
0 s with a parallel increase in urchin population
densities (Hernández et al. 2008a). Assuming climate change predictions (IPCC
2007) for the future are correct, the strong positive relationship between thermal
history and post-settlement survival of this ecologically important species suggests
that barren grounds will become more extensive in the future as seawater temperatures warm in relation to anthropogenic activities. Nonetheless, mass mortality
events, as found for P. lividus in 2003 (Girard et al. in press), could became more
frequent due to warmer waters. In this sense, we would like to point out that
researchers have observed mortality events of D. africana in Madeira and Gran
Canaria islands in 2010 (F. Alves, M. Kaufman and F. Espino pers. com.,
respectively). However, it is too soon to know the cause of this recent phenomenon
that, initially, seems to be very spatially restricted. In both cases, urchins were
infected by the bald sea urchin disease, and possibly originated by the bacteria
Vibrio alginolitycus (Hernández et al. 2010b).
15 Echinoderms of the Canary Islands, Spain
497