Zoogeographical Consequences and the Future of Lessepsian Migration
153
3.8 Zoogeographical Consequences
and the Future of Lessepsian Migration
First of all I shall try to summarize the zoogeographical impact of the Lessepsian migration as it now stands. It is important to be able to give a mathematical
expression of the new situation created in the Mediterranean fauna after a century
of Lessepsian migration. In the discussions above, more than 200 species of
Lessepsian migrants have been mentioned. Taking into account the incompleteness of our knowledge, I would suggest 500 as the probable number of Lessepsian
species actually present in the Mediterranean. Riedl (1963) estimates a total of
15,000 species of animals in the Mediterranean. If so, then Lessepsian migration
has added about 3% to the Mediterranean faunal inventory.
Fredj (1972, 1974) analyzes the distribution of 1244 species of Mediterranean
benthic invertebrates. He chooses a number of phyla rather arbitrarily; for instance, he refers to Lamellibranchiata, but not to Gastropoda. Fredj mentions
only 40 Lessepsian migrants from among the Echinodermata, Polychaeta, Decapoda, and Lamellibranchiata. These constitute 3.21 % of his list. Fredj hirnself
speaks of 4.5% of the species which have a Mediterranean-Indo-Pacific distribution; however, his criteria are not similar to those used by me.
Including the other 37 Lessepsian migrants not mentioned by Fredj in the four
taxa he deals with, it totals 77 out of 1281 species (1244+37). That brings the
weight ofthe Lessepsian migrants to 6.01 % ofthe taxa treated by Fredj.
Let us now use the figure of 48.8% given by Fredj for the percentage of those
species out of the 1244 which live in the Eastern Mediterranean. The Eastern
Mediterranean is by about a half poorer in species than the Mediterranean taken
as a wh oie. Returning to Riedl's figure, one can, therefore, assurne that some 7500
species live in the eastern part of the Sea, out of which the approximated number
of 500 Lessepsian migrants make up 6.6% ofthe total.
However, the term Eastern Mediterranean, as generally used, is too broad,
and sometimes includes the Aegean Sea. Considering the Levant Sea alone, the
proportion ofthe Lessepsian migrants is even higher. Ben-Tuvia (1973) calculates
that the immigrants constitute 11.6% of the fish-fauna of the Israel coast. Out of
the Decapoda known in the Levant Basin the Lessepsian migrants form about
21 % (Holthuis and Gottlieb, 1958; Lewinsohn and Holthuis, 1964). The percentage is perhaps of the same order of magnitude in Polychaeta and probably lower
in Mollusca.
It seems to me justified to suggest that at present the Lessepsian migrants
constitute about 10% of the biota of the Levant Sea. Does this percentage justify
the separation of a special zoogeographical subdivision, a "Lessepsian province"
or subregion within the Atlanto-Mediterranean region? The Black Sea, for instance, which has a Ponto-Caspian admixture of species to a majority of Mediterraneans, has frequently been considered as aseparate zoogeographical province. There are, however, no generally accepted criteria for deciding at what point
of the similarity-dissimilarity relation two areas should be considered different,
and at what level. Banarescu (1970) suggests that "regions" are characterized by
153
3.8 Zoogeographical Consequences
and the Future of Lessepsian Migration
First of all I shall try to summarize the zoogeographical impact of the Lessepsian migration as it now stands. It is important to be able to give a mathematical
expression of the new situation created in the Mediterranean fauna after a century
of Lessepsian migration. In the discussions above, more than 200 species of
Lessepsian migrants have been mentioned. Taking into account the incompleteness of our knowledge, I would suggest 500 as the probable number of Lessepsian
species actually present in the Mediterranean. Riedl (1963) estimates a total of
15,000 species of animals in the Mediterranean. If so, then Lessepsian migration
has added about 3% to the Mediterranean faunal inventory.
Fredj (1972, 1974) analyzes the distribution of 1244 species of Mediterranean
benthic invertebrates. He chooses a number of phyla rather arbitrarily; for instance, he refers to Lamellibranchiata, but not to Gastropoda. Fredj mentions
only 40 Lessepsian migrants from among the Echinodermata, Polychaeta, Decapoda, and Lamellibranchiata. These constitute 3.21 % of his list. Fredj hirnself
speaks of 4.5% of the species which have a Mediterranean-Indo-Pacific distribution; however, his criteria are not similar to those used by me.
Including the other 37 Lessepsian migrants not mentioned by Fredj in the four
taxa he deals with, it totals 77 out of 1281 species (1244+37). That brings the
weight ofthe Lessepsian migrants to 6.01 % ofthe taxa treated by Fredj.
Let us now use the figure of 48.8% given by Fredj for the percentage of those
species out of the 1244 which live in the Eastern Mediterranean. The Eastern
Mediterranean is by about a half poorer in species than the Mediterranean taken
as a wh oie. Returning to Riedl's figure, one can, therefore, assurne that some 7500
species live in the eastern part of the Sea, out of which the approximated number
of 500 Lessepsian migrants make up 6.6% ofthe total.
However, the term Eastern Mediterranean, as generally used, is too broad,
and sometimes includes the Aegean Sea. Considering the Levant Sea alone, the
proportion ofthe Lessepsian migrants is even higher. Ben-Tuvia (1973) calculates
that the immigrants constitute 11.6% of the fish-fauna of the Israel coast. Out of
the Decapoda known in the Levant Basin the Lessepsian migrants form about
21 % (Holthuis and Gottlieb, 1958; Lewinsohn and Holthuis, 1964). The percentage is perhaps of the same order of magnitude in Polychaeta and probably lower
in Mollusca.
It seems to me justified to suggest that at present the Lessepsian migrants
constitute about 10% of the biota of the Levant Sea. Does this percentage justify
the separation of a special zoogeographical subdivision, a "Lessepsian province"
or subregion within the Atlanto-Mediterranean region? The Black Sea, for instance, which has a Ponto-Caspian admixture of species to a majority of Mediterraneans, has frequently been considered as aseparate zoogeographical province. There are, however, no generally accepted criteria for deciding at what point
of the similarity-dissimilarity relation two areas should be considered different,
and at what level. Banarescu (1970) suggests that "regions" are characterized by