Dynamic Zoogeography
7
penetrable to eompeting immigrants-may serve as additional plugs in a funnel.
For Udvardy, evidenee of this type of barrier is eireumstantial. In the ease of the
Suez Canal, its role ean be better substantiated.
The direetion of the movement through the zoogeographie pathways of the
filtrating bridge is another problem. The interchange between two provinces is
always unequal: the influenee of one provinee on the other is stronger; one taxon
might spread better in one sense, another may be more suecessful in another. In
the well-known ease of the two Americas the mammals of North Ameriea spread
with numerous species over almost all of South Ameriea, whereas the mammals of
South Ameriea had only a numerieally and geographica1ly limited sueeess in
North Ameriea. The result was a predominant invasion ofthe Nearetie fauna into
the Neotropie region. Darlington (1959) explains these one-sided invasions in
general terms as the result of a eompetitive advantage of the dominant biota
whieh evolved in the large zoogeographie unit (with warmer and more stable
eonditions) over those ofthe small one (in this case the Darlingtonian Megagea).
In the ease of the Suez Canal, the donor of alm ost all of the migrating species
is the tropieal Indo-West-Paeifie region-the biggest and qualitative1y riehest of
all the marine zoogeographieal regions.
Following the work of Andrewartha and Bireh (1954), it is well aeeepted that
every species has an "innate tendeney towards dispersal". Leston (1957) reiterates
this notion and speaks of a specifie "spreading potential" of every animal family.
The sueeess of a migrational advanee is not determined only by ability to
jump the hurdles of the filtrating bridge but also by the ability to find a niehe in
the new eeosystem, and to maintain it through sueeessful reproduetion. Many
faetors and eomponents are eovered by the term "niche", but its aeeepted meaning being an interspecifie web of relations, even if it is indireetly, though no less
fatefully, determined by the abiotie eeologie valenees of the eompeting species.
In the ease of the Lessepsian migration-unlike the example of Krakatauthe new environment was not virgin and empty but inhabited by an eeosystem
with a eertain amount of environmental resistanee and a limited seleetion of
empty niehes. The phenomenon is further eharaeterized by the fact that not only a
few hardy migrants erossed from the Red Sea to the Mediterranean, but hundreds
of species, frequently perhaps whole seetions of an original food web; so that
nothing like the Eltonian explosive populations of invaders eould be witnessed.
Sinee the separation of the Mediterranean from the Indo-West-Paeifie is as
old as the Miocene period, there were extremely few species pairs present on both
sides of the Isthmus of Suez. This is abasie differenee from the ease of the
geologieally mueh younger Central Ameriean Isthmus. Beeause of the lack of
eongenerie pairs, there was and probably is no direet eompetition between aborigines and invaders. Following the more reeent statements of Schminke (1973),
eongenerity means similar feeding meehanism and requirements. In the ease of
Lessepsian migration, to the extent to whieh the presently reeognized taxonomie
genera in different families are reliable, one eannot expeet many instanees of
direet eompetition for the same position in the food web.
The genetieally isolated emigrant diasporas, living under different abiotie and
biotie surroundings, have most probably already started on the way to speciation.
7
penetrable to eompeting immigrants-may serve as additional plugs in a funnel.
For Udvardy, evidenee of this type of barrier is eireumstantial. In the ease of the
Suez Canal, its role ean be better substantiated.
The direetion of the movement through the zoogeographie pathways of the
filtrating bridge is another problem. The interchange between two provinces is
always unequal: the influenee of one provinee on the other is stronger; one taxon
might spread better in one sense, another may be more suecessful in another. In
the well-known ease of the two Americas the mammals of North Ameriea spread
with numerous species over almost all of South Ameriea, whereas the mammals of
South Ameriea had only a numerieally and geographica1ly limited sueeess in
North Ameriea. The result was a predominant invasion ofthe Nearetie fauna into
the Neotropie region. Darlington (1959) explains these one-sided invasions in
general terms as the result of a eompetitive advantage of the dominant biota
whieh evolved in the large zoogeographie unit (with warmer and more stable
eonditions) over those ofthe small one (in this case the Darlingtonian Megagea).
In the ease of the Suez Canal, the donor of alm ost all of the migrating species
is the tropieal Indo-West-Paeifie region-the biggest and qualitative1y riehest of
all the marine zoogeographieal regions.
Following the work of Andrewartha and Bireh (1954), it is well aeeepted that
every species has an "innate tendeney towards dispersal". Leston (1957) reiterates
this notion and speaks of a specifie "spreading potential" of every animal family.
The sueeess of a migrational advanee is not determined only by ability to
jump the hurdles of the filtrating bridge but also by the ability to find a niehe in
the new eeosystem, and to maintain it through sueeessful reproduetion. Many
faetors and eomponents are eovered by the term "niche", but its aeeepted meaning being an interspecifie web of relations, even if it is indireetly, though no less
fatefully, determined by the abiotie eeologie valenees of the eompeting species.
In the ease of the Lessepsian migration-unlike the example of Krakatauthe new environment was not virgin and empty but inhabited by an eeosystem
with a eertain amount of environmental resistanee and a limited seleetion of
empty niehes. The phenomenon is further eharaeterized by the fact that not only a
few hardy migrants erossed from the Red Sea to the Mediterranean, but hundreds
of species, frequently perhaps whole seetions of an original food web; so that
nothing like the Eltonian explosive populations of invaders eould be witnessed.
Sinee the separation of the Mediterranean from the Indo-West-Paeifie is as
old as the Miocene period, there were extremely few species pairs present on both
sides of the Isthmus of Suez. This is abasie differenee from the ease of the
geologieally mueh younger Central Ameriean Isthmus. Beeause of the lack of
eongenerie pairs, there was and probably is no direet eompetition between aborigines and invaders. Following the more reeent statements of Schminke (1973),
eongenerity means similar feeding meehanism and requirements. In the ease of
Lessepsian migration, to the extent to whieh the presently reeognized taxonomie
genera in different families are reliable, one eannot expeet many instanees of
direet eompetition for the same position in the food web.
The genetieally isolated emigrant diasporas, living under different abiotie and
biotie surroundings, have most probably already started on the way to speciation.