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Case StudiesLife strategiesBryophytes are found in such diverse habitats that, overall, they show a very wide variety of adaptations to those different habitats. Amongst these adaptations are different strategies with regard to life span, reproductive effort and spore viability, to list just three points. Bryophytes are commonly classified to six types of life strategies, which are described below. First, there are a few growth form descriptions that you'll need to understand. A dendroid growth form is somewhat tree-like – a main stem and then numerous short branches radiating out from the top of the main stem. Leafy bryophytes may show a mat growth form, when the stems lie flat and are interwoven. Naturally thallose bryophytes can also form mat-like growths. In a turf numerous erect shoots grow together, often very crowded. A weft has loosely interwoven and ascending shoots. Now, here are brief descriptions of the six life strategies. FugitiveFugitive species have a very short life span. Considerable effort is put into sexual reproduction and the age of first sexual reproduction is under a year. A very large proportion of the plants produce sporophytes but there is no production of specialized vegetative propagules. The gametophytes are fairly simple with virtually no branching. The spores are small, less than 20 micrometres in diameter, persistent and long-lived. Fugitive species generally grow as open turfs. The fugitive strategy suits a highly unpredictable habitat that exists for a short time. An example is the barren landscape after a major bushfire. In some areas the barrenness is rather short-lived, with considerable vascular plant regrowth within few months after a fire. ColonistColonists have a moderately short life span and put much effort into reproduction by both sexual spores and specialized vegetative propagules. Vegetative propagules are produced mostly in the early stages of the life cycle and the sporophytes later. Initial vegetative reproduction often occurs at a few months age, but the first sexual reproduction does not occur before the end of the first year, and mostly at an age of 2 to 3 years. The gametophyte normally branches and the spores are small, as in the fugitive species, and mostly very persistent. Colonist species may be short turfs, open turfs or thalloid mats. Colonists often appear in habitats that first appear at unpredictable times and locations, but which then last for several years. Post-fire landscapes can also provide opportunities for colonists because in some areas the regrowth of vascular plants after a major bushfire is very slow, taking many years to reach significant cover. Annual shuttleAnnual shuttle species have a very short life span and put a lot of effort into sexual reproduction, the age of first sexual reproduction being less than a year. A very large proportion of the plants produce sporophytes but there is no production of specialized vegetative propagules The gametophytes are fairly simple, with virtually no branching. The spores are large, mostly from 25 to 50 micrometres in diameter, but sometimes up to 200. The spores remain viable for several years. Annual shuttle species grow typically as open turfs or thalloid mats. The annual shuttle strategy suits a habitat that exists for a relatively short period, say no more than a year or two, but which re-occurs in the same area. In addition, at times of high stress the gametophytes die, but the species persists in the form of spores which will produce fresh gametophytes once the stressful period has come to an end. Examples of annual shuttle habitats are animal dung, steep stream sides (which are often prone to erosion and collapse) and seasonally flooded areas. Examples of the last are roadside depressions and farm dams in dry country. Short-lived shuttleThis strategy is very similar to the previous, though with longer life span. A lot of effort is put into sexual reproduction but little or none into the production of specialized vegetative propagules. Age of first sexual reproduction is typically 2 to 3 years. Gametophytes normally branch. The spores are large, mostly 25-50 micrometres in diameter, but at times up to 100. Spores probably remain viable for several years and the short-lived shuttle species grow as short turfs or thalloid mats. The habitats occupied by short-lived shuttle species are similar to those of annual shuttle species, except that the habitats are longer-lasting, typically 2 to 3 years or even a little longer. Short-lived shuttle species are more resilient and don't show the "persist as spores" stress avoidance strategy of the annual shuttle species. Animal bones, the temporarily open higher areas of salt marshes and some grasslands are other examples of habitats exploited by short-lived shuttle species. Perennial shuttleA perennial shuttle species has a long life span, sexual reproductive effort is moderate to absent and effort put into the production of specialized vegetative propagules is moderate to high. The gametophytes branch. Spores are large (25-200 micrometres), as are the vegetative propagules. In appearance the perennial shuttle species may grow in cushion-like colonies, mats or turfs. Perennial shuttle species occupy habitats that are constant and numerous within an area, but which predictably end after a certain period. Epiphytic habitats are a good example, since the plants on which the epiphytic bryophytes live, will eventually die. Perennial stayersA perennial stayer has a long life span. The effort put into the production of spores or specialized vegetative propagules is relatively low, often localized in small areas. Spores are small, under 20 micrometres in diameter, and length of viability varies considerably in this group. Large cushions, dendroids, mats and wefts are the growth forms shown by perennial stayers. Perennial stayers occupy habitats that are constant, or where the fluctuations in conditions can be tolerated by the gametophytes. Swamps, bogs and forest floors are examples of such habitats. CommentsThe most important comment is that the six categories are pigeon-holes created by humans in an attempt to analyse bryophyte life histories. Not surprisingly, the boundaries between the categories are not black and white – and some bryologists would use a modified categorization. Nevertheless, the six categories are useful as a simple summary of bryophyte behaviours and sufficient as food for thought on this website. The second point is that a particular species need not always follow the same strategy. Bryum argenteum is a cosmopolitan moss that is reported as a long-lived perennial in the Antarctic but as a colonist in temperate areas. References
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