Foissner_Protist Diversity and Geographical Distribution_9048128005

an example, the moss
Tortula pagorum is dioiceous. It exists only in female plants in Europe, while male plants
occur in North America. It propagates by terminal clusters of brood bodies with a length of
about 100 lm. Nevertheless, it has a wide range on both continents.
Bryophytes are able to propagate by a large variety of vegetative ways such as rhizoidal
gemmae, axillary gemmae, brood bodies, detaching leaves or buds, leaf fragments etc.
Although these organs for vegetative propagation are much larger than spores, they have
similar effects for the dispersal over large distances. Recent molecular studies on disjunct
populations of sterile mosses revealed that dispersal is possible even without any specialized
means of vegetative propagation. The moss Campylopus oerstedianus is worldwide known
only in sterile condition. It is distributed from Costa Rica in an arc over Jamaica, Georgia to
southern Europe (Pyrenees, southern Alps, Chalkidike). It is found in addition in the Massif
Central and the Vosges in France (Fig. 1). Determinations of the genetic distances between
the disjunct populations in Europe show that the populations in the Massif Central and the
Vosges are derived from that in the Pyrenees (Sabovljevic and Frahm in press). Therefore,
dispersal over a distance of several 100 km (in the direction of the prevailing winds) is
possible even for a sterile species. The effectiveness of both vegetative and generative
propagation is best demonstrated by the re-colonization of Europe and North America after
the glaciations in the Pleistocene from refugia in southern latitudes.
Fig. 1 Distribution of the moss Campylopus oerstedianus in Europe. The species is only known in sterile
condition and therefore the range may indicate a relictual status of the populations. However, molecular data
indicate a closer genetic relationship of the populations in the Pyrenees, the Massiv Central and the Vosges
in France, suggesting a recent dispersal during the Pleistocene
Protist Diversity and Geographical Distribution 45
Dispersal is not only possible by wind but also by animals. The aquatic liverwort
Ricciocarpos natans has an almost cosmopolitan range. It is presumably dispersed by
water fowl.
Large ranges: relics vs. long distance dispersal
Transoceanic disjunctions of flowering plants are usually only found at genus level.
In contrast, this distribution pattern is frequent in bryophytes. This includes not only
Laurasian and Gondwanan ranges, but also Pangaean ranges. These distribution patterns
can be the result of long distance dispersal but also of relic origin. Species equally found in
North America and Europe are either dispersed (presumably from west to east with the
prevailing wind system) but can also be remnants of former continuous ranges before the
breakup of the Laurasian continent. The question cannot be fully clarified at present.
Records of sterile bryophyte species occurring in North America and Europe in different
sexes in microhabitats such as rock fissures support the relic theory. These species must
have an age of 50 mio years and have not altered morphologically or anatomically.
Although this might be difficult to understand by phanerogamists (since species of flow-
ering plants are distinctly younger), this hypothesis is supported by fossil records of
bryophytes in Eocene amber, which still exist and therefore must have an age of 45 mio
years. On the other hand, bryophyte species common in North America but rarely found in
Europe (also in different habitats) may indicate long distance dispersal. Thus the answer on
this question may not be either—or but as well as.
Tropical African–South American disjunctions would be even more difficult to explain,
since the break up of both continents started 100 mio years b.p. and such an age of a
bryophyte species would be more difficult to believe. Even more problematic to understand
are Pangaean ranges, species which are found in North America as well as in Europe,
South America and Africa. In this case the species must have an age of 180 mio years and
more. This hypothesis was favoured by Frey (1990). This question can be solved today by
molecular studies comparing the genetic distances between the disjunct populations but
have not yet been performed on such a scale.
As spore plants, bryophytes show a much smaller extent of endemism than flowering
plants. Endemism on the species level is found in North America in 16% (liverworts) and 18%
(mosses) of species, respectively. This is especially true for islands: in New Zealand, 86% of
the flowering plants are endemic but only 28% of the mosses (van Zanten and Po´cs 1981).
The dispersal strategy by small uni- or multicellular units should cause that all bryo-
phytes have large ranges because the diaspores can easily be dispersed over long distances.
In fact, however, only few species are really ubiquitous, such as Funaria hygrometrica and
Bryum argenteum. They are like weeds and seem to have been distributed by man. Naturally
wide ranges are found in Laurasian and Gondwanan species. A reason seems to be that the
Inner Tropical Convergence (ITC) forms a barrier for spore dispersal across the equator and
limits gene exchange across the equator. In the Americas, only 65 bryophyte species have a
bipolar range (Ochyra 1992). They have disjunct ranges between Alaska and Chile and seem
to be dispersed by migrating birds. Within the northern and southern hemisphere, many
species have large transcontinental ranges: 65% of all mosses in North America are also
found in Europe (Frahm and Vitt 1993). This is the reason for the lack of neophytic
bryophytes from North America in Europe, as is the case in flowering plants: all species
which could be dispersed by man from North America are already present in Europe. In
Europe, all neophytic bryophytes come from the southern hemisphere. Species such as
46 W. Foissner et al. (eds)
Orthodontium lineare or Campylopus introflexus have then enlarged their secondary ranges
in Europe and spread over large areas in a few decades. Transcontinental ranges can be
explained by either long distance dispersal (=gene exchange) or disjunctions caused by
continental drift (=relics). In fact, both arguments are supported by molecular studies: for
example, Campylopus introflexus, a moss widespread in temperate regions of the southern
hemisphere, is genetically homogenous. This species is fertile and has presumably gene
exchange by the circumglobal air currents (Stech and Dohrmann 2004). Other disjunct
species, such as Lopidium concinnum in New Zealand, Tasmania and Chile seems to be
relics of a former continuous Gondwanan range (Frey et al. 1999).
Biogeography
The fact that many bryophyte species (as well as macrofungi and ferns, see Foissner 2006)
have limited ranges in spite of their high potential of dispersal seems paradox. For
example, the moss Leptodontium gemmascens produces abundantly gemmae that are
80–100 lm long. In spite of this, L. gemmascens is confined to southern England, northern
France, Belgium, Luxemburg and neighbouring parts of Germany and the Netherlands plus
a disjunct occurrence in Denmark (Fig. 2). Possible reasons for restricted ranges are
discussed in the following paragraphs.
Endemism
Within Laurasia, endemism amongst bryophytes is found north of the Pleistocene ice
shield. Species like Bryobrittonia longipes are exclusively found in North America, in
northern Alaska, and in the ice free corridor east of the Rocky Mountains and the Kola
Fig. 2 Distribution of the moss Leptodontium gemmascens. The species produces gemmae abundantly
which allows an easy dispersal (as shown by a disjunct occurrence near Copenhagen), but is confined to a
(for bryophytes) comparably very small range
Protist Diversity and Geographical Distribution 47
Peninsula in Northeast Europe. This species has apparently lost the ability to extend after
glaciation, similarly to the Nunatak effect described