Migration is commonly thought of as a mechanism by which organisms
avoid unfavorable environments for more auspicious ones. Migration can
be the seasonal movement of organisms from place to place or a more
permanent dispersal from one region to another; either way, changing
environmental conditions provide the impetus. But why do some
organisms migrate and not others? How do the benefits of migration
compare with the metabolic costs and hazards of the journey? Do
differences between terrestrial and marine species in their interface with
the environment have different effects on migration? What are the trade-
offs between migration and evolutionary adaptation to local conditions?
Different species respond to environmental changes in different ways,
partly based on their individual adaptations and range of tolerance. This
phenomenon is most easily seen when comparing terrestrial and marine
organisms, which have different relations to their environment. Terrestrial
organisms are surrounded by low-density atmosphere and expend
considerable energy moving and keeping themselves upright; marine
organisms, in contrast, are surrounded by seawater, which has a density
close to their own, making support and movement much less costly
(Sverdrup et al. 2005). Thus, migration is likely to be more metabolically
costly to a terrestrial animal than a marine one, all other things being
equal. The factors affecting migration of land and sea species may
therefore differ considerably, and one geologic episode may affect the two
realms differently.
Terrestrial and marine organisms also differ in the environmental factors
that affect them the most. Temperature and rainfall are major determinants
on the land, but salinity is one of the most important in the sea. The
difference in the salt content of seawater and bodily fluids affects the
transport of water molecules through cell membranes in a process called
osmosis; changes in salinity can have disastrous effects on the
physiological balance of an organism. Temperature variation affects
avoid unfavorable environments for more auspicious ones. Migration can
be the seasonal movement of organisms from place to place or a more
permanent dispersal from one region to another; either way, changing
environmental conditions provide the impetus. But why do some
organisms migrate and not others? How do the benefits of migration
compare with the metabolic costs and hazards of the journey? Do
differences between terrestrial and marine species in their interface with
the environment have different effects on migration? What are the trade-
offs between migration and evolutionary adaptation to local conditions?
Different species respond to environmental changes in different ways,
partly based on their individual adaptations and range of tolerance. This
phenomenon is most easily seen when comparing terrestrial and marine
organisms, which have different relations to their environment. Terrestrial
organisms are surrounded by low-density atmosphere and expend
considerable energy moving and keeping themselves upright; marine
organisms, in contrast, are surrounded by seawater, which has a density
close to their own, making support and movement much less costly
(Sverdrup et al. 2005). Thus, migration is likely to be more metabolically
costly to a terrestrial animal than a marine one, all other things being
equal. The factors affecting migration of land and sea species may
therefore differ considerably, and one geologic episode may affect the two
realms differently.
Terrestrial and marine organisms also differ in the environmental factors
that affect them the most. Temperature and rainfall are major determinants
on the land, but salinity is one of the most important in the sea. The
difference in the salt content of seawater and bodily fluids affects the
transport of water molecules through cell membranes in a process called
osmosis; changes in salinity can have disastrous effects on the
physiological balance of an organism. Temperature variation affects
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