Long distance dispersal
The aim of this page is to look at the potential transport of bryophyte propaglues over long distances, in particular across oceans. The term "long distance" is here taken to mean a distance of more than a thousand kilometres. More precisely the page will look at the role of wind. The available evidence indicates that while migratory birds may carry bryophyte propagules over long distances, such birds (and other animals) would play only a very minor role in long distance dispersal. There is also no evidence in favour of ocean currents playing a significant role in long distance bryophyte dispersal.
Wind is the major agent of long distance particulate transport and people in Australia don't need to be bryologists to be aware of that fact. From time to time Australian newspapers and television programs present reports of severe dust storms that have carried Australian soil to New Zealand. Occasionally enough soil is carried across the Tasman Sea to give the snow-covered Southern Alps of New Zealand a noticeably brownish tinge. If soil particles can be carried such distances then so could spores and other propagules of a variety of organisms, including bryophytes. Indeed, it has been estimated that spores with diameters up to 12 micrometres would be capable of carried over 12,000 kilometres by wind. There are many bryophytes with spores in this size range. As a point of interest it's worth noting that there is strong evidence suggesting that spores of a wheat rust (a microfungus) were carried from Africa to Australia by high altitude winds in 1968-69. The distance involved is at least 12,000 kilometres.
Adding support to the potential of long distance wind dispersal is the fact that bryophyte spores have been caught at high altitudes. Also, bryophytes are found on many isolated islands where long distance dispersal is the only possible explanation. For example, over a hundred bryophyte species have been found on sub-Antarctic Marion Island, situated in the "roaring forties". Given the island's history, the bryophytes now there must have arrived during the last 100,000 years, when the island was no longer ice-covered. The period is far too short to invoke a nearby, drifting continent as a source of short-distance dispersal.
There are considerable challenges in wind dispersal. First, a propagule has to get airborne and carried to a high altitude. This may be no problem for a bryophyte growing on an alpine cliff top, but would be close to impossible for a plant growing on a rotting log in a closed forest. Once airborne at a high altitude the propagule would be subject to higher levels of UV radiation and greater cold. Laboratory experiments have shown that the spores of various species are highly resistant to such extremes and would still be viable after travelling thousands of kilometres in a high altitude air stream. On the other hand, spores of various other species are incapable of surviving such extremes. Propagules must avoid being brought down into the ocean, for example by rain. Once a propagule has come to land after travelling many thousand kilometres there are additional challenges. If it is to generate a new plant, the propagule has to land in a suitable habitat and, unless it's barren land, compete successfully with whatever's already there. All this sounds grim, with successful dispersal seemingly a rare event. That may well be true, but remember that propagules have been carried by winds for millions of years. The longer the period of time the more likely that rare events would happen.
It is widely accepted that long distance wind dispersal does occur, though bryologists may hold different views as to the significance of such dispersal in bryogeography. Taking an example such as Marion Island few would doubt the importance of wind dispersal. On the other hand, what about the occurrence of the same species in both South America and Australasia, but nowhere else? Long distance dispersal might be an explanation, but perhaps it's a Gondwanan species and the Australasian and South American populations are simply the scattered descendents (or relicts) of an original Gondwanan population. You might argue against the second hypothesis on the grounds that Australasia and South America have been separated for tens of millions of years. In that time the scattered descendents should have evolved independently. After all, a number of Australasian and South American species-pairs are known. In each such pair the Australasian species is similar to, and obviously closely related to, the South American species – but they are different species that have evolved from a common, Gondwanan ancestor. However, there is evidence that some bryophytes have extremely slow rates of evolution. Perhaps slow enough to leave some life in the Gondwanan hypothesis? The point of all this is that each case for long distance dispersal must be assessed separately, typically using evidence from a variety of sources – for example, geological, meteorological, physiological and genetic.
Long distance dispersal between east and west certainly occurs but transport across the equator happens very rarely, if at all. Consider the mosses Campylopus introflexus and Orthodontium lineare. The two species are widespread in the temperate areas of the Southern Hemisphere but are not native to the Northern Hemisphere. However, they have been introduced to the latter and, once there, have established well and spread considerably, as noted in the CAMPYLOPUS INTROFLEXUS AND ORTHODONTIUM LINEARE CASE STUDY. Obviously it's not a case of there being unsuitable habitats in the Northern Hemisphere. The propagules never got there without human help.