et al. 2001; Sansón et al. 2001; Brito and Ocaña 2004; Tuya and Haroun 2009). It
has been noted that the main system of currents that affects the archipelago is the
cold Canary Current, a descending branch of the Gulf Current that flows from the
north of the Iberian Peninsula towards the southeast. On its way through the
European Atlantic coast, it becomes progressively richer in propagule and larvae
of different species. Some of these species successfully reach the islands and become
established if conditions are favorable. Therefore, most of the species found in the
Canary Islands have their origin in the Atlantic-Mediterranean region. To a lesser
extent, American tropical and subtropical species have also colonized the Canary
Islands through this current. In many cases, the islands constitute the only region in
the eastern Atlantic Ocean where these species are found. Although there is no clear
system of currents that connects the islands with the tropical southern African coast,
some species from this region have colonized the Canaries. Probably southeast
African weather conditions occasionally counteract the Canary Current, favoring the
arrival of these species. Biota common to the Saharan coast (the region of mainland
Africa nearest to the Canary Islands) are scarce, mainly because the oceanographic
conditions generated by the upwelling of cold waters near the continent area are
very different from the conditions of the Canaries (Aguilera et al. 1994).
This set of variables defines the characteristics displayed by coastal populations, communities and ecosystems. Because the structure, composition and
functioning of ecosystems show characters in between those of temperate and
tropical systems, they are considered subtropical. The richness, diversity, disparity
and originality of the populations are very high and trophic relationships among
species are complex in many cases. The size of populations is usually small, due to
both the low productivity of the waters and the spatial restrictions of habitats. In
this sense, it should be emphasized that all benthic ecosystems, with the exception
of abyssal plain ecosystems, lack of continuity throughout the archipelago because
they are confined to the insular edifices and seamounts. The surface occupied by
benthic ecosystems depends directly on features inherent to the islands’ edifices
(e.g. coastal ecosystems between 0 and 100 m depth are larger on some islands
than in others). In this sense, according to the different availability of habitats and
to the oceanographic conditions at the islands, the population size of a species can
be different in each of them. On the other hand, given that most species are linked
to the bottoms, the non-continuity of ecosystems means that the only way of
exchange between populations is by larval dispersal. Since few species (only some
species of fish and probably also some cephalopods) have adults that are able to
move between islands, genetic connectivity between the islands’ populations in
turn depends on factors such as the distance between islands, the speed and
direction of currents, and characteristics of each species (e.g. dispersal ability of
the larvae). The population size of each island is also directly related to different
levels of human exploitation. The east to west oceanographic gradient across the
archipelago is also responsible for the distribution of biota. In the western islands,
with warmer waters, species with tropical affinities are more abundant, while
species with temperate affinities are more abundant in the eastern islands of
temperate waters (Brito et al. 2001; Sangil et al. 2007). Finally, the proximity of
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J. C. Hernández et al.
has been noted that the main system of currents that affects the archipelago is the
cold Canary Current, a descending branch of the Gulf Current that flows from the
north of the Iberian Peninsula towards the southeast. On its way through the
European Atlantic coast, it becomes progressively richer in propagule and larvae
of different species. Some of these species successfully reach the islands and become
established if conditions are favorable. Therefore, most of the species found in the
Canary Islands have their origin in the Atlantic-Mediterranean region. To a lesser
extent, American tropical and subtropical species have also colonized the Canary
Islands through this current. In many cases, the islands constitute the only region in
the eastern Atlantic Ocean where these species are found. Although there is no clear
system of currents that connects the islands with the tropical southern African coast,
some species from this region have colonized the Canaries. Probably southeast
African weather conditions occasionally counteract the Canary Current, favoring the
arrival of these species. Biota common to the Saharan coast (the region of mainland
Africa nearest to the Canary Islands) are scarce, mainly because the oceanographic
conditions generated by the upwelling of cold waters near the continent area are
very different from the conditions of the Canaries (Aguilera et al. 1994).
This set of variables defines the characteristics displayed by coastal populations, communities and ecosystems. Because the structure, composition and
functioning of ecosystems show characters in between those of temperate and
tropical systems, they are considered subtropical. The richness, diversity, disparity
and originality of the populations are very high and trophic relationships among
species are complex in many cases. The size of populations is usually small, due to
both the low productivity of the waters and the spatial restrictions of habitats. In
this sense, it should be emphasized that all benthic ecosystems, with the exception
of abyssal plain ecosystems, lack of continuity throughout the archipelago because
they are confined to the insular edifices and seamounts. The surface occupied by
benthic ecosystems depends directly on features inherent to the islands’ edifices
(e.g. coastal ecosystems between 0 and 100 m depth are larger on some islands
than in others). In this sense, according to the different availability of habitats and
to the oceanographic conditions at the islands, the population size of a species can
be different in each of them. On the other hand, given that most species are linked
to the bottoms, the non-continuity of ecosystems means that the only way of
exchange between populations is by larval dispersal. Since few species (only some
species of fish and probably also some cephalopods) have adults that are able to
move between islands, genetic connectivity between the islands’ populations in
turn depends on factors such as the distance between islands, the speed and
direction of currents, and characteristics of each species (e.g. dispersal ability of
the larvae). The population size of each island is also directly related to different
levels of human exploitation. The east to west oceanographic gradient across the
archipelago is also responsible for the distribution of biota. In the western islands,
with warmer waters, species with tropical affinities are more abundant, while
species with temperate affinities are more abundant in the eastern islands of
temperate waters (Brito et al. 2001; Sangil et al. 2007). Finally, the proximity of
478
J. C. Hernández et al.