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from the flowering plants.
Habitat size and life form
In a study of rare mosses of Alberta (Canada), Vitt and Bellard (1997) concluded that rarity
depends on life form, life strategy, and habitat preference. Long-lived perennials, pleu-
rocarpous mosses forming large mats are more common since they are more competitive.
Rare species occur in small niches, for instance, rock fissures in cliffs. The smaller the
habitat, the rarer the species. The authors studied, however, local rarity. Rarity is here
understood as overall rarity. Therefore many reasons for rarity cannot be applied (occur-
rence at the borders of the range, outside the main range).
Age of taxa
We have to face that species have a limited time for existence. This time is quite high in
bryophytes, as shown by fossil evidences. A high percentage of mosses known from Baltic
and Saxon amber is still existing. They are 45 mio years old. Nevertheless, some species
approach extinction. The most common moss in the Eocene amber forest is Hypnodont-
opsis mexicana (Frahm 2005). It is presently known only from the type locality (collected
in 1928 in Mexico) and a recent collection in Uganda.
Local extinction
Especially the Pleistocene ice ages have led to a considerable extinction of taxa. This is
shown by the effect that species known as fossil from the Tertiary of Europe are found
today on the Macaronesian Islands (Echinodium spp., Andoa bertholetiana) or in E-Asia
(Trachycystis spp.). Some may have survived the climatic changes in special areas such as
the southern Alps, where some species (e.g. Braunia alopecura) can be interpreted as
Tertiary relics that did not expand into the Holocene.
Local, disjunct occurrence
Disjunct occurrences can be the result of either long distance dispersal or a relic. Today,
molecular studies allow to solve this question by determination of genetic distances
between the populations. In such studies it turned out that both possibilities are realized,
even within the same species. For instance, Isothecium holtii occurs in the high oceanic
parts of Western Europe, but it is also found in some sites of Central Europe. These
populations could be the result of dispersal or relics of a former continuous range. A
molecular study revealed that the populations in two valleys of the Harz Mountains are
derived from Wales and Ireland, and are thus a result of long distance dispersal, whereas
the population in the Rur valley of the Eifel Mountains is a relic of a former larger range,
presumably the Atlantic climate period 8,000 years bp (Sabovljevic et al. 2005).
Highly specialized habitat preferences
Cosmopolitan species are usually ubiquitous, that means they have a wide ecological range
and are not much specialized. Rarity may depend on rarely available habitats. Heavy metal
48 W. Foissner et al. (eds)
bryophytes can serve as an example. Habitats (mine deposits) are only found very scat-
tered. Nevertheless obligate heavy metal bryophyte species are found on many of them.
The moss Ditrichum plumbicola is only known from Britain and Germany. It has no
special means of vegetative propagation, but is known in Germany from six places which
are in 50–100 km air distances. The occurrence of the moss Rhynchostegium rotundifolium
is often associated with the surroundings of medieval ruins. It grows below the walls of
castles, where the litter has accumulated. It could be that the species has a preference for
habitats rich in nitrate or phosphate. Dung mosses (Splachnaceae) grow only on substrate
highly enriched with nutrients, most of them solely on dung of animals. Their occurrence is
restricted by the availability of such habitats.
Lack of one sex
There are cases of rare dioiceous species, which are present on a continent in only one sex.
For example, only male plants of the liverwort Plagiochila corniculata are found in North
America, while females are restricted to Europe (Schuster 1984). Sexual propagation is
thus inhibited.
Lack of ability for long distance dispersal
Long distance dispersal happens at higher altitudes in the atmosphere. Diaspores must (a)
be able to get into these altitudes (difficult for species from forest floor) and (b) able to
survive the harsh conditions of high altitudes with strong UV radiation and temperatures
much below the freezing point. In spectacular experiments, van Zanten (1978) exposed
spores of mosses from New Zealand, which occur also in Chile, to UV radiation and low
temperatures. Conspicuously, the spores of species were able to germinate after this
treatment. The species endemic to New Zealand failed to germinate. Endemism is in this
case a matter of lacking UV and frost tolerance of spores.
Bryophytes in the context of the current debate on protist distribution
Spore plants such as bryophytes should have an overall distribution or at least very large
ranges. Vegetative propagation is not a limiting factor but has the same effects. It has been
developed as an alternative to sexual reproduction, for which water is necessary. It can
result in similarly large ranges. As already outlined by Foissner (2006), the small size of
diaspores of bryophytes does not correlate with wide ranges, which concern only part of
the bryophyte species. Many species have limited to very limited ranges, which seems to
be a contradiction to the easy vegetative or generative propagation. Ubiquity is thus
theoretically possible amongst bryophytes (concerning the means and effectiveness of
dispersal), but realized only to a small extent. There are many examples of very small
ranges or local endemism. This discrepancy can be explained by exogenous effects such as
narrow ecological requirements (climatic, habitat). This explanation does not work for all
cases. Molecular causes such as genetic aging leading to extinction may be one possible
explanation. Limitations are not the diaspores (even sterile species can have wide ranges)
but climatic or ecological boundaries. Mountains and oceans are not, boundaries. If a
species has a limited range in spite of existing dispersal facilities, there seems to be an
Protist Diversity and Geographical Distribution 49
endogenous barrier for dispersal. The lacking ability for dispersal seems to be especially
expressed in sterile species. As shown by the wide ranges of other sterile species, sterility
itself cannot be the limiting factor. A factor could, however, be the clonal reproduction of
bryophyte species, which results in a loss of ability for dispersal.
References
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Frahm J-P (2005) The genus Hypnodontopsis (Bryopsida, Rhachitheciaceae) in Baltic and Saxon amber.
Bryologist 108:228–235
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Sabovljevic

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