Jökull - 01.01.2004, Síða 11

Theories on migration and history of the North-Atlantic flora: a review
INTERPRETING ENDEMISM IN THE
ICELANDIC FLORA: A
BIOGEOGRAPHICAL COMPARISON
Our present picture of the North Atlantic biota derives
from the climatic and glaciation history of the region,
species distribution patterns and floristic affinities, ap-
parent speciation patterns and endemisms, with the
most recent advances coming from molecular biol-
ogy. A comparison with the Antarctic has rarely been
employed and is mostly of limited value because of
the totally different distribution of landmasses in the
southern hemisphere. However, for Iceland in partic-
ular, a comparative approach may be informative. A
survey of the world’s oceanic islands soon confirms
that there are no comparable islands for all three im-
portant criteria, size, degree of isolation and climate
(Table 1). Svalbard is most similar in terms of size
and isolation but its climate is much harsher. Ice-
land’s closest neighbours, the Faroe Islands, are prob-
ably most similar in climate but their area is less than
2% that of Iceland. Some of the subantarctic islands
have a comparable climate, although more strongly
oceanic. Several have the same degree of isolation,
but again they are all much smaller than Iceland.
The most striking pattern to emerge from the ta-
ble is the difference between the arctic-subarctic, and
the subantarctic islands in degree of endemism. En-
demism is low (<2% of vascular plant species) in the
North Atlantic islands but >10% in all the subantarctic
islands. Glaciation history is incompletely known for
the subantarctic islands but some appear to have been
at least partly glaciated. The difference in endemism
between the arctic and subantarctic islands may be a
product of the different ages of these floras, or they
may reflect much lower probabilities of seed disper-
sal from the much smaller continental landmasses of
the southern hemisphere, even with similar dispersal
distances.
Greenland and Svalbard both have some gener-
ally acknowledged endemic taxa of vascular plants
although the precise number is not agreed upon. Of
all the islands, Iceland has the lowest percentage of
endemic vascular species, with only one weak claim
at present, the apomictic Alchemilla faeröensis, also
found in the Faroe Islands. Considering its size, iso-
lation and the presence of endemic taxa both to the
east and west, the figure may be lower than expected.
However the expectation might be defined and what-
ever the cause of the lack of endemism, it is at least
clear that these data do not support a long and iso-
lated evolutionary history of the Icelandic flora. A
similar point was made by Brochmann and colleagues
(2003), who remarked on the low degree of endemism
in North Atlantic regions compared to areas outside
the main ice sheets, and concluded that this did not
support the in situ Pleistocene survival of the flora.
CONCLUSIONS
In spite of numerous studies and discussions on the
glacial and migratory history of the North Atlantic
flora for over a century, conclusive answers have not
yet emerged.
Discussion has been limited to the pros and cons
of the two contrasting theories. Alternative and less
exclusive explanations have been little considered but
may be more fruitful. The age, origins and history
of the biota of the North Atlantic are likely to vary
regionally. While molecular methods have recently
opened exciting new avenues for exploring migration
history, their power may not suffice to provide a gen-
eral answer. For example, can we interpret results
which indicate a large genetic distance between pop-
ulations as evidence for glacial survival or should it
only be interpreted as an indication of a long time iso-
lation? Will a Pleistocene origin be evident after small
relict glacial populations have mixed with perhaps re-
peated Holocene immigrations? Pollen and macrofos-
sil analyses offer the most direct access to site history.
A problem with the interpretation of pollen records
is that very high temporal resolution may be needed
to distinguish between glacial survival and postglacial
immigration, if most immigration took place during
unusual conditions prevailing for a short or very short
period immediately following deglaciation.
When considering two contrasting hypotheses, the
principle of parsimony (Occam’s razor) should be em-
ployed, e.g. the simplest explanation of a phenomenon
(requiring the fewest leaps of logic) should be ex-
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