26
3 Biology of Fronts
3.4 Diversity
Studies focused on the effects of marine fronts on diversity are scarce, and their conclusions are somewhat contradictory. Most reports indicate that fronts play a role
in setting up diversity patterns, while other studies suggest that these patterns occur
on large scales not necessarily associated with typical cross-front scales (Stemmann
et al. 2008; Mauna et al. 2011). Several studies ascribe diversity patterns to front
occurrence; involving different groups such as phytoplankton (Ortner et al. 1979;
Jeffrey and Hallegraeff 1980); zooplankton (Ortner et al. 1978; Tranter et al. 1983;
Gaard et al. 2008; Hosia et al. 2008); hyperbenthos (Dewicke et al. 2002); fish larvae
(John et al. 2001; Sánchez-Velasco et al. 2012); cephalopods (Brandt 1983); rays and
sharks (Lucifora et al. 2012); midwater fishes (Olson and Backus 1985); demersal
fishes (Alemany et al. 2009); tunas and billfishes (Worm et al. 2003); and seabirds
(Haney 1986). These findings refer to several types of fronts, and the diversity patterns were expressed as divergences in species composition (e.g. β-diversity or different assemblages); or as diversity in absolute terms (measured as α-diversity (e.g.
species richness), or in defining hotspots). High mean species richness and diversity
of whales and seabirds are consistently associated with fronts at the Southern Ocean
(Bost et al. 2009). In the case of predators, high diversity at fronts is in general attributed to the high biological production and better feeding opportunities because an
abundant prey supply acts as an attractor to individual species and, at the same time,
allows for the coexistence of a high number of predator species (Lucifora et al. 2012).
Fronts concentrate high biological activity but from a conceptual point of view,
this does not necessarily imply that they show higher diversity of species as compared with neighboring environments. Several forms for the relationship between
species richness and productivity have been proposed, but none are generally
accepted. A greater variety of species may be expected in very productive environments because more resources can allow more species to coexist (Wright et al.
1993). However, hump-shaped patterns have also been described whereby as productivity rises, diversity first increases and then declines (Rosenzweig and Abramsky
1993; Gaston 2000) due to increased competitive exclusion (Abrams 1995).
Therefore, high ecosystem productivity can lead to either an increase or decrease in
species richness, or a combination of both (e.g. a hump-shaped distribution).
3.5 Life Histories Traits in Relation to Fronts
A major problem in evolutionary biology is to explain the amount and structure of
biodiversity in widely connected environments like the open ocean. The effects of
fronts on certain organisms could partially explain specific and population diversity. Even though in most cases fronts do not appear to be an absolute barrier to
pelagic organisms present on either side (Sournia 1994), strong temperature gradients across fronts could act to uncouple life-cycle events, including reproduction
(Gaard 1996). In planktonic species whose range spans a strong hydrographic front,
3 Biology of Fronts
3.4 Diversity
Studies focused on the effects of marine fronts on diversity are scarce, and their conclusions are somewhat contradictory. Most reports indicate that fronts play a role
in setting up diversity patterns, while other studies suggest that these patterns occur
on large scales not necessarily associated with typical cross-front scales (Stemmann
et al. 2008; Mauna et al. 2011). Several studies ascribe diversity patterns to front
occurrence; involving different groups such as phytoplankton (Ortner et al. 1979;
Jeffrey and Hallegraeff 1980); zooplankton (Ortner et al. 1978; Tranter et al. 1983;
Gaard et al. 2008; Hosia et al. 2008); hyperbenthos (Dewicke et al. 2002); fish larvae
(John et al. 2001; Sánchez-Velasco et al. 2012); cephalopods (Brandt 1983); rays and
sharks (Lucifora et al. 2012); midwater fishes (Olson and Backus 1985); demersal
fishes (Alemany et al. 2009); tunas and billfishes (Worm et al. 2003); and seabirds
(Haney 1986). These findings refer to several types of fronts, and the diversity patterns were expressed as divergences in species composition (e.g. β-diversity or different assemblages); or as diversity in absolute terms (measured as α-diversity (e.g.
species richness), or in defining hotspots). High mean species richness and diversity
of whales and seabirds are consistently associated with fronts at the Southern Ocean
(Bost et al. 2009). In the case of predators, high diversity at fronts is in general attributed to the high biological production and better feeding opportunities because an
abundant prey supply acts as an attractor to individual species and, at the same time,
allows for the coexistence of a high number of predator species (Lucifora et al. 2012).
Fronts concentrate high biological activity but from a conceptual point of view,
this does not necessarily imply that they show higher diversity of species as compared with neighboring environments. Several forms for the relationship between
species richness and productivity have been proposed, but none are generally
accepted. A greater variety of species may be expected in very productive environments because more resources can allow more species to coexist (Wright et al.
1993). However, hump-shaped patterns have also been described whereby as productivity rises, diversity first increases and then declines (Rosenzweig and Abramsky
1993; Gaston 2000) due to increased competitive exclusion (Abrams 1995).
Therefore, high ecosystem productivity can lead to either an increase or decrease in
species richness, or a combination of both (e.g. a hump-shaped distribution).
3.5 Life Histories Traits in Relation to Fronts
A major problem in evolutionary biology is to explain the amount and structure of
biodiversity in widely connected environments like the open ocean. The effects of
fronts on certain organisms could partially explain specific and population diversity. Even though in most cases fronts do not appear to be an absolute barrier to
pelagic organisms present on either side (Sournia 1994), strong temperature gradients across fronts could act to uncouple life-cycle events, including reproduction
(Gaard 1996). In planktonic species whose range spans a strong hydrographic front,