49
6 Comparisons of Fronts with Terrestrial Boundaries …
compared with neighboring environments. 1 For instance, highly fluctuating
boundaries (in space or time) will be relatively poor in species diversity (di Castri
and Hansen 1992). Several measures of biodiversity have been proposed
(Whittaker 1972); alpha diversity refers to the diversity within a particular area or
ecosystem; gamma diversity is a measure of the overall diversity for the different
ecosystems within a region; and beta diversity represents the turnover of species
between habitats or localities. Gamma and alpha diversity would not necessarily
exhibit a pattern relative to ecotones, even though the species identities would
clearly change across an ecotone; in these cases beta diversity should increase
toward the ecotone (Neilson et al. 1992). It has been shown that copepods living in
an area of high oceanographic complexity present peaks in beta diversity associated with fronts (Berasategui et al. 2006). Although the effect of fronts on larger,
free-swimming animals is more difficult to quantify, it was shown that the presence of fronts influences fish diversity, with dissimilar effects for different frontal
types (Alemany et al. 2009). Fronts could also be distributional barriers for some
species but at the same time to concentrate others increasing diversity.
Thermoregulation, optimal foraging, and behavioral bioenergetics could each lead
to fish aggregations at fronts or could cause a front to appear as a barrier to the
distribution of a fish species (Brandt 1993). It has also been proposed that the patterns of biodiversity across ecotones may become clearer if attributes of ecotones
structure (e.g. width; contrast) are normalized by a relevant spatial scale (body
size, territory size, or species range), or time scale (age until first breeding,
longevity, dispersal rate) (Hansen et al. 1992).
It has been predicted that conditions in the ecotones may be a simple average of
conditions in the patches on either side, or alternatively that they may reflect interactions that occur along the boundary (Strayer et al. 2003). The physical matrix of
most fronts matches the first proposition (e.g. horizontal pattern of temperature in
a tidal front, or that of salinity in an estuarine front); however ecological properties of fronts such as biological production, beta diversity or species abundances
are clearly better reflected by the second prediction. Ecotones are places of tension
where stresses of a genetic character, important in evolution, are at work (Margalef
1997). This could be the case of some frontal types, for example estuarine fronts:
because life originated in the sea, the colonization of the freshwaters must have
occurred, partially at least, through fronts separating the freshwaters and the estuarine waters, so the new capabilities and forms have evolved in response to those
fronts.
Because the effects of a boundary may be cumulative, the age and history of
an ecotone may determine its functional properties and the local ecological conditions around the boundary (Strayer et al. 2003). The biological importance of
fronts depends on a matching of physical and biological time scales. Noting the
longer times scales at higher trophic levels and the successive nature of increasing
1 In the case of fronts the issue of the relationship between diversity and productivity also needs
to be considered. See the Diversity section.
6 Comparisons of Fronts with Terrestrial Boundaries …
compared with neighboring environments. 1 For instance, highly fluctuating
boundaries (in space or time) will be relatively poor in species diversity (di Castri
and Hansen 1992). Several measures of biodiversity have been proposed
(Whittaker 1972); alpha diversity refers to the diversity within a particular area or
ecosystem; gamma diversity is a measure of the overall diversity for the different
ecosystems within a region; and beta diversity represents the turnover of species
between habitats or localities. Gamma and alpha diversity would not necessarily
exhibit a pattern relative to ecotones, even though the species identities would
clearly change across an ecotone; in these cases beta diversity should increase
toward the ecotone (Neilson et al. 1992). It has been shown that copepods living in
an area of high oceanographic complexity present peaks in beta diversity associated with fronts (Berasategui et al. 2006). Although the effect of fronts on larger,
free-swimming animals is more difficult to quantify, it was shown that the presence of fronts influences fish diversity, with dissimilar effects for different frontal
types (Alemany et al. 2009). Fronts could also be distributional barriers for some
species but at the same time to concentrate others increasing diversity.
Thermoregulation, optimal foraging, and behavioral bioenergetics could each lead
to fish aggregations at fronts or could cause a front to appear as a barrier to the
distribution of a fish species (Brandt 1993). It has also been proposed that the patterns of biodiversity across ecotones may become clearer if attributes of ecotones
structure (e.g. width; contrast) are normalized by a relevant spatial scale (body
size, territory size, or species range), or time scale (age until first breeding,
longevity, dispersal rate) (Hansen et al. 1992).
It has been predicted that conditions in the ecotones may be a simple average of
conditions in the patches on either side, or alternatively that they may reflect interactions that occur along the boundary (Strayer et al. 2003). The physical matrix of
most fronts matches the first proposition (e.g. horizontal pattern of temperature in
a tidal front, or that of salinity in an estuarine front); however ecological properties of fronts such as biological production, beta diversity or species abundances
are clearly better reflected by the second prediction. Ecotones are places of tension
where stresses of a genetic character, important in evolution, are at work (Margalef
1997). This could be the case of some frontal types, for example estuarine fronts:
because life originated in the sea, the colonization of the freshwaters must have
occurred, partially at least, through fronts separating the freshwaters and the estuarine waters, so the new capabilities and forms have evolved in response to those
fronts.
Because the effects of a boundary may be cumulative, the age and history of
an ecotone may determine its functional properties and the local ecological conditions around the boundary (Strayer et al. 2003). The biological importance of
fronts depends on a matching of physical and biological time scales. Noting the
longer times scales at higher trophic levels and the successive nature of increasing
1 In the case of fronts the issue of the relationship between diversity and productivity also needs
to be considered. See the Diversity section.