Reproduction and dispersal

Propagule dispersal

Propagules can be dispersed by wind, water, vertebrates or invertebrates and on this page I will give some examples. Much of the information has been taken from the sources given in the following reference button.

Wind or water

Wind is a major disperser of both sexual and asexual propagules and these have been caught at high altitudes or well out at sea. An important determinant of whether a propagule is able to be moved by wind (and, if so, how far) is the propagule's size. Ascospores and basidiospores are small and easily carried aloft by even light breezes. Amongst the vegetative propagules soredia, though larger than sexual spores, are often small and of a size to allow for transport by wind. How far could such propagules be carried by wind? On the subject of spores it's worth noting 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. The fact that wind could carry propagules larger than spores is clear to many Australians who have seen dust storms carry inland soil many hundreds of kilometres to Australia's coastal cities. Many would also know that from time to time Australian soil is carried on to New Zealand. Furthermore, some cryptogamic similarities between various widely separated areas of the southern hemisphere correlate well with the wind connections between those areas. This suggests that long distance propagule dispersal is responsible for the widespread southern hemisphere distributions of various species of lichens, bryophytes and ferns.

Though wind has the potential to carry various propagules hundreds to thousands of kilometres that is only the first step towards the creation of distant colonies. The propagules must survive the journey. Dispersal over thousands of kilometres would require that propagules travel via high altitude winds and so be subject to low temperatures, dryness and high levels of UV radiation. Finally, regardless of how far a propagule is carried, it must land in a suitable habitat and there be able to overcome any competitors or predators. The ability to survive cold, dryness, UV radiation, competitors or predators will vary depending on the species and the specific circumstances of any particular propagule dispersal path. Given such specific dependencies I'll go no further on that topic except to give some illustrative results pertinent to one aspect of survival during long-distance transport. Experiments on desiccation tolerance of ascospores have shown that after one week of desiccated storage 59% of Lecidea macrocarpa spores, 24% of Pertusaria pertusa spores and 0% of Xanthoria parietina spores germinated within eight days of transfer to a humid atmosphere. After four weeks of desiccated storage the percentages were 7, 21 and 0 and after eight weeks 5, 7, 0.

The subject of spore discharge from the basidia of basidiolichens is dealt with on the BASIDIOLICHEN page. In many ascolichens the mature ascospores are forcibly ejected from the asci into the surrounding air. The distance to which the spores are shot out varies with species and ranges from a few millimetres to several centimetres. Such an ejection ensures that the spores are shot beyond the boundary layer of slow moving air near the substrate. Another way of helping spores get beyond the substrate's boundary layer or beyond low obstacles is to raise the asci and you will find many species in which the apothecia are raised on stalks. Further away from the substrate the air currents are likely to have some turbulence and so increase the chances of the spores being lifted further by such currents. Forcible ejection also helps put spores clear of any nearby obstruction such as grasses, rocks, irregularities on tree trunks that could trap the spores. In the mature apothecia of some genera the asci and other hymenial elements (explained on the REPRODUCTIVE STRUCTURES page) break down to leave a powdery mass of spores and debris, called a mazaedium. In many species the mazaedia are held atop short stalks and such lichens are colloquially called pinhead lichens since the mazaedium-stalk combination looks like a small pin at first glance, often no more than a millimetre or two long. The genera Calicium, Chaenotheca and Cyphelium provide many examples of such lichens. The powdery mass of a mazaedium is exposed to the air so allowing winds to carry them away. It has been shown that a wind speed of 18 kilometres per hour would carry off spores from mazaedia of Cyphelium inquinans.

In contrast to sexual spores the vegetative propagules receive no initial forcible ejection. Amongst such propagules the lightweight soredia are most likely to be wind dispersed effectively. Experiments have shown that a wind speed of about 10.5 kilometres per hour can dislodge soredia from a Cladonia podetium. Wind can also liberate soredia from flat thalli as shown in an experiment with Lecanora conizaeoides as the subject. Wind blowing over dry Lecanora conizaeoides thalli would liberate soredia but liberation soon stopped though the wind was still blowing. Initially few soredia were wind-liberated from wet thalli but the liberation rate increased after a short period of drying and, overall, more soredia were liberated from the wet thalli. At a wind speed of about 10.5 kilometres per hour soredia were trapped about 1.5 metres away within a minute. Soredium-like thallus fragments were also liberated by wind from the thalli of several lichen species. It was found that water droplets acting simultaneously with wind improved the liberation in some species but not in all.

Despite the potential for long distance dispersal it is likely that the majority of even the lightweight propagules such as spores would land close to their parent thalli. There is some experimental evidence in support of this conclusion and similar results have been found in studies of spore dispersal in bryophytes and non-lichenized fungi. There are numerous obstacles (such as plants, rocks and buildings) that could block a propagule's path or disrupt wind flows to the extent that propagules are suddenly dropped. In fact you could say that in order for a propagule to be dispersed successfully over a long distance a sequence of fortuitous events needs to occur. The propagule must be carried away from the substrate, it must avoid obstacles, there needs to be enough turbulence to keep the propagule airborne and rising until it can be caught by the winds that can carry it far away, if it rains too soon the propagule is at risk of being washed to the ground before it has travelled very far, the dangers of cold, dryness and UV radiation were mentioned earlier and, finally, the propagule needs to be deposited on land and not in the ocean. Thus, successful dispersal is likely to be a rare event but given enough time and enough attempts a large number of successes are likely.

The Tasman Sea and the Pacific area around New Zealand are home to a number of islands, each at least several hundred kilometres distant from both Australia and New Zealand, that have arisen in recent times, geologically speaking, rather than being continental remnants. Hence the lichens on those islands would have arrived via long distance dispersal and one study suggested the Ramalina species on those islands arrived by means of wind-dispersed propagules, mostly from Australia or New Zealand. For example, in the winter the prevailing winds on Lord Howe and Norfolk Islands come from the direction of Australia, in the summer the prevailing winds are from New Zealand and the two islands have a mix of Australian and New ZealandRamalina species. The Kermadec Islands are 976 kilometres north-east of New Zealand, winter winds come from the direction of New Zealand and the summer winds are north-easterly, coming in from the Pacific. Of the eight Ramalina species found in the Kermadec Islands, six are also found in New Zealand and the other two are Pacific species. A total of 15 Ramalina species were found on the eight islands that were the subject of the study. Ascospores were found on virtually every examined thallus of seven of those species, soredia were found on virtually every examined thallus of six of the species and, in the remaining two species ascospores were common, though absent from some of the examined thalli. Four of the soredium-producing species also produced ascospores, though sometimes rarely. The ascospores and soredia were of sizes theoretically capable of long-distance wind dispersal and the authors of the study pointed out that there was evidence that similar sized (or larger) pollen grains and aphids had been dispersed to some of those islands from either New Zealand or Australia.

The larger symbiotic propagules are less likely to be carried long distances by wind and often the dispersal distances would be measured in centimetres or metres rather than kilometres. It is feasible for larger propagules to be wind-dispersed further in a series of steps, rather than as one continuous movement. However, such further dispersal would take longer and does rely on the consistent movement of fragments away from the parent thallus during each windy period, rather than just a tossing back and forth, with no gain in displacement. At the end of this page there is more about fragment dispersal. Even whole thalli, in particular those of VAGRANT LICHENS, can be blown about by the wind, once again generally over rather short distances. On the subject of lichens mimicking tumbleweeds one lichenologist wrote the following, about the development of thalli of the fruticose species Evernia prunastri, that he'd seen in coastal England:

These developed as ball-like thalli attached to low-growing Prunus spinosa amongst shingle. On reaching a certain size in this exposed habitat they seemed liable to become detached by wind and to be carried away. Evernia prunastri is a sorediate species and when travelling at speed (speeds of approximately 20 m/s were observed) could well scatter both soredia and thallus fragments along its path.

A tumbleweed phenomenon on a smaller scale was reported in 1901 for a new species of perithecial lichen found on tree bark and named then as Sagedia macrospora. The perithecia can become detached from the thallus and the author of the 1901 paper observed that ...

If you moisten such a detached perithecium on a glass slide it absorbs water in a very lively fashion. After about five minutes the hymenium swells so much that it curves strongly outward and the [perithecium's] less flexible outer casing is broken.

The gelatinous mass of spores and perithecial contents are thereby ejected. The author noted that the tiny perithecia, no more than a millimetre in diameter, could be blown or tumbled by the wind and, once wetted, release the spores in a new location. He also thought the viscosity of the moistened contents would help spores attach to wood. The lichen did not rely on detached, rupturing perithecia but could also release spores in the ordinary perithecial way - through a tiny opening at the apex of an intact perithecium.

Various symbiotic propagules are produced on thallus surfaces. Water flowing over a thallus may carry loosened or weakly attached propagules away and droplets striking a thallus may dislodge such propagules. Water trickling down a rock face or a tree trunk can help colonization downhill from a parent thallus and one researcher reported that...

...soredia can be liberated from thalli of Lecanora conizaeoides and Lepraria incana by water trickles under laboratory conditions approximating to those in the field and that soredia, probably those of Lepraria incana, were found in rainwater draining down trunks of Fraxinus excelsior. Soredia in such trickles could easily lodge upon irregularities of the bark. Sorediate lichens on vertical surfaces, for example on churchyard memorials, can be seen in downward streaks suggesting rainwash. Observations on these streaks do indicate that extension downwards is more rapid than extension upwards or sideways...

Laboratory experiments have shown that droplets, similar in size and speed to those of raindrops, can splash soredia from the cup-like podetia of Cladonia thalli to a distance of up to a metre. Droplets can also splash soredia from flat thalli but to distances not as great as those of soredia ejected from cup-like receptacles. Spores of Cladonia coccifera are often discharged into a watery film over the apothecial surface and carried into the cuplike podetia. Conceivably the spores could be further dispersed by water droplets. It is interesting to note that similar cup-like structures have evolved independently in several non-lichenized fungi and bryophytes, where splash dispersal is also efficient at transporting propagules to distances of a metre or more. Wind dispersal from the powdery mazaedia was mentioned a little while ago but it is also possible that raindrops could splash, or flowing water could wash, the spores away from the mazaedia.

Animate dispersers

The LICHENS, VERTEBRATES AND INVERTEBRATES page gives examples of various vertebrate or invertebrate species that use lichens in various ways. As a consequence many such animal species are likely to act as propagule dispersers, albeit unintentionally. At one extreme, a large, furry mammal might pick up thalli or thallus fragments as it brushes past lichen-covered branches and later dislodge those propagules elsewhere as it scratches itself against a tree trunk. At the other size extreme an almost microscopic invertebrate could pick up soredia on its bristly body as it crosses a lichen thallus and later dislodge them during grooming. On the subject of invertebrates, propagule dispersal need not be external. Various invertebrates eat lichens and viable Xanthoria parietina ascospores have been collected from rotifer faeces. In fact tests of spores swallowed by rotifers showed a 15% germination rate of spores excreted from rotifers that had swallowed 20 or more spores but rotifers that swallowed 8 or fewer spores excreted broken spores which failed to germinate. In between the size extremes of large mammal and tiny invertebrate, think of the various birds that nest in the dappled light within shrubs and incorporate lichen fragments in their nests to help camouflage them. Such fragments may well continue to grow. Moreover, fragments could be dropped during transport and so generate new thalli wherever they come to land. As birds walk over lichen thalli on branches or on the ground their feet may pick up soredia (and they have been found on birds' feet) which can be deposited elsewhere. Finally, an animal can help with dispersal even if it carries no propagules away. That furry mammal mentioned earlier might walk across a lichen colony and break up many thalli, fail to pick up anything but leave behind numerous small fragments each of which could be dispersed by agents such as wind, water or other animals. An insectivorous bird, pecking around a lichen thallus in search of food might break off isidia or dislodge soredia and release them into the surrounding air. Some lichenivorous invertebrates are messy eaters and leave 'crumbs' (ie, potential propagules) scattered about.

The rest of this section will be devoted to some more examples of dispersal by invertebrates. The larvae of various lacewing species carry 'rubbish packets' on their backs. In the United States the larvae of Nodita pavida have been seen from Maryland to Texas and at least one observer reported never seeing the packets to contain anything but lichen fragments. A packet is about six millimetres across and, at first glance, looks very much like a small lichen thallus but, as noted in the paper listed in the following reference button, the packets "are readily distinguished by the collector of lichens - especially when they move". The fragments may well grow and establish new thalli wherever the packet is discarded thus, inevitably, the larvae would help disperse some lichens over short distances.

While soredia may be gathered and dispersed by a variety of invertebrates there is the question as to how effective is the dispersal by invertebrates. For dispersal to be effective soredia need to be deposited where they can grow. MICROHABITATS are important and of two soredia deposited only a centimetre or two apart one could find itself in a microhabitat conducive to growth and the formation of a new thallus while the other fails to survive. On this subject it is worth referring to a study of soredial dispersal by arboreal oribatid mites of the species Humerobates arborea. Adults of this species are about 0.7 millimetres long and their dispersal of Lepraria lobificans soredia was observed. A number of soredia-bearing mites were observed on a tree trunk, each mite being tracked until it had deposited four soredia (each deposition point marked with a pin) or until the mite disappeared from sight. Deposition distances ranged from one to 71 millimetres. The greatest observed carrying distance was 473 millimetres (in 16.4 minutes), at which point the mite disappeared under a bark fragment, not yet having deposited any soredia. Given observations of where on the tree trunk the lichen grew it was possible to categorize areas of the trunk as either suitable or unsuitable for Lepraria lobificans growth. About 11% of the deposited soredia were deposited in areas suitable for Lepraria growth. Mites could also carry propagules internally. At least two species of lichen-eating oribatid mites, Trhypochtonius tectorum and Trichoribates trimaculatus, are found on the bright orange-yellow, foliose thalli of Xanthoria parietina. That lichen does not reproduce vegetatively (except possibly by unpredictable thallus fragmentation) so new thalli generally need to be formed by ascospores uniting with the alga Trebouxia arboricola. The ascospores can be wind-dispersed but viable ascospores and viable photobiont cells have been collected from the faecal pellets of both mite species. Clearly the mites put spores and photobiont cells conveniently close to each other, very useful given that species of Trebouxia appear to be rarely found in the free-living state.

Fragments, wind and animals

Where winds are strong and landscapes fairly uncluttered lichen fragments, seeds, soil or plant debris could be blown considerable distances. But in many places there is clutter in the form of trees, shrubs, grasses, boulders or surface irregularities which impede the wind and I'll finish this page with some observations from a study of fragment dispersal in three such habitats. In 1998 a European lichenologist carried out an experiment involving seven soil-inhabiting, fruticose species that could fragment easily. Three cushions of each species were dyed with rain-resistant stain that would have caused no noticeable change to thallus weight, consistency or water uptake. The cushions were fixed to wooden plates and then placed into three different natural habitats: dry sand grassland, open pioneer pine forest and closed, old-growth pine forest. The cushions were fragmented by being stepped on twice by a human and then left for 15 days. Then all dyed thallus fragments within an area of 15 square metres around each cushion were collected, distances from the cushion noted and an area of 300 square metres around each cushion was searched for larger fragments, again with distances noted. In five cases there were clear signs of animal disturbance (for example, displaced wooden plates) and from three of those there was considerable losses of cushion weight. The greatest fragment displacement was 970 centimetres from one of the grassland cushions that had been disturbed by an animal. Otherwise in that habitat the maximum displacement was 57 centimetres. In the pioneer forest the maximum displacement, again from an animal-disturbed cushion, was 84 centimetres and 68 centimetres from a cushion that showed no sign of animal disturbance. In the old-growth forest the displacements were 62 centimetres from an animal-disturbed cushion and 13 centimetres from a cushion showing no sign of animal disturbance. Where there was no sign of animal disturbance it is likely that wind was responsible for at least some of the fragment dispersal and fragments dispersed from undisturbed cushions mostly moved no further than 25 centimetres away - and in closed forest, not surprisingly, wind makes a negligible contribution to dispersal. On the other hand the results indicate that wind could disperse many fragments up to about a metre within a few weeks in grasslands or semi-open habitats and so help in colonization of the neighbourhood. The results also indicate that, in the habitats of the study, animals are a means of helping colonize a larger neighbourhood. Within the study area rabbits were abundant and were likely to have been the most important disturbers of the lichen cushions, though voles, deer, wild boar and birds also could have contributed.