The necessity to move and adapt within a dynamic landscape the tradeoff of life strategies

One way to avoid exclusion by competition is thus to colonize new habitats. These habitats eventually become unsuitable, either because they are progressively invaded by stronger competitors, or because the habitats themselves vanish. In fact, many habitats are dynamic in the sense that they vanish at one place while being recreated somewhere else. For bryophytes, duration of habitats typically ranges from a few months or even weeks in the case of

Fig. 7.15. Frequency of occurrence of Tetraphis pellucida and Dicranum flagellare on logs varying in degree of isolation by distance from nearest suitable substrate (reproduced from Kimmerer 1994 with permission of the author and The Bryologist).

pioneer species of temporary habitats such as dried-out ponds to thousands of years in the case of forest floors, bogs and rocky habitats (Fig. 7.16). The risk of extinction from these habitats, either due to competition, habitat longevity (Soderstrom & Herben 1997), or herbivory and parasitism (Davey & Currah 2006), constitutes a selective pressure to disperse and ensure population maintenance.

Bryophytes, like any other living organism, should thus ideally exhibit a great longevity through gametophytic adaptations to local environmental conditions (e.g. drought) and competition. At the same time, they should display a high reproductive potential, beginning at the earliest stage of their development and continuing throughout their existence, resulting in numerous offspring with high establishment rates. However, because the resources that are available for each individual are limited, some of these attributes become mutually exclusive. There is therefore a trade-off in the utilization of resources. The most favourable balance in the various trade-offs varies in relation to the nature and stability of the habitat, leading to the evolution of adaptive suites of life history features. In the classical theory of McArthur and Wilson (1967), organisms can be classified into two categories. 'r-species' produce many, small offspring with low chance of survival in

Table 7.1. Bryophyte

life strategies of During (1992)

Life span

Number and

Reproductive

Strategy

(years)

size of spores

effort

Typical habitat

Examples

Fugitive

<1

numerous, < 20 m

high

very temporary (e.g. burnt

Funaria hygrometrica

ground)

Colonist

few

numerous, < 20 m

average

temporary (e.g. bare soil

pioneer species on ground

patches, branches)

(e.g. Bryum spp.),

branches

(e.g. Orthotrichum spp.),

concrete

(e.g. Grimmia pulvinata)

Perennial stayers

many

numerous, < 20 (im

low

stable habitats (e.g. peat

Po lytrich urt i stric tun i

bogs)

Annual shuttle

<1

few, >20(im

high

cyclic, temporary habitats

Are Iridium alternifolium

(e.g. mud flats)

Dominants

many

few, >20(im

low

open, stable habitats

mire Sphagnum species

(e.g. 5. cuspidatum,

5. lindbergii)

5. lindbergii)

Fig. 7.16. Relationships between habitat size and duration of major habitats of temperate and boreal bryophytes (reproduced from Soderstrom & Herben 1997 with permission of Borntraeger).

early successional environments. By contrast, slow-growing, highly competitive 'K-species' reproduce after a long period of development and produce a few, large offspring with a high chance of survival in dense 'climax' environments.

A life strategy can thus be defined as a recurrent combination of life history traits that are predicted to occur prominently in response to particular ecological conditions. The r and K strategies defined above mostly apply to animals. In bryophytes, offspring size is not only related to the risk of high juvenile mortality but also to dispersal ability. Furthermore, reproduction does not specifically occur at a precise stage of the life cycle. For example, many bryophytes produce vegetative propagules and, more exceptionally in the case of neotenic species (Box 1.1), spores at the protonemal stage.

During (1979, 1992) showed how the concept of life strategies can be applied to bryophytes (Table 7.1). During's classification is based on the existence of two major trade-offs. The first of these trade-offs concerns the production of few, large spores or of many, small spores (Fig. 7.17). Large spores are thought to have a low dispersal capacity but better chances of successful establishment and a longer life span in the diaspore bank (Box 7.3). They are therefore prominently produced by shuttle species of unstable habitats that recur predictably at a given site. This is, for example, the case for hornworts in temperate areas, which are well adapted to regular disturbance in arable fields thanks to their diaspore bank (see Section 5.3), or of annual thalloid liverwort communities in xerotropical environments experiencing a severe drought season (see Section 3.3). Small spores, by contrast, are produced in such large numbers that many will reach distant sites. Such a strategy will be prominently adopted by colonist and fugitive species of ephemeral habitats. The classic example of a fugitive bryophyte is the weedy Funaria hygrometrica (Fig. 7.18), a species of habitats that occur unpredictably and are suitable for growth for a very short period of time such as, for example, burnt ground. The colonist strategy is comparable to the fugitive strategy, but colonists tend to occur in habitats (e.g. bare soil patches, branches, etc.) that are available for growth for somewhat longer periods. Colonists therefore have a potential life span of several years but substantially invest in reproductive effort by means of small spores and also often vegetative diaspores as their habitat eventually disappears (Fig. 7.19). Three categories of colonists can be recognized. Typical colonists, such as Bryum dichotomum, often produce both spores and gemmae and are typical of secondary successional communities such as ruderal habitats. Gap-dependent colonists, such as B. rubens, have short-lived gametophytes and assure population maintenance by subterranean tubers. They are typical on bare soils after disturbance of the ground vegetation (e.g. gaps in calcareous grasslands or forest floor caused by herbivores). Finally, pioneers disperse primarily by spores and colonize harsh environments (e.g. epiliths such as Grimmia pulvinata and Tortula muralis on concrete walls or pioneer epiphytes) during primary succession.

The second trade-off involved in During's classification concerns the potential life span of the gametophyte, which is negatively correlated with reproductive effort (Longton 1997). In the British moss flora, for example, all the short-lived fugitive species and annual shuttles commonly produce sporophytes; in colonists and long-lived shuttles, the proportion of species frequently producing sporophytes drops to about 60%, whilst this proportion is lower than 50% in perennial stayers and dominants (Fig. 7.20). Annual shuttle species avoid periods of environmental stress (e.g. severe

Fig. 7.17. Spore diameter and number of spores per capsule in a sample of bryophyte species with different life history strategies (data from Longton 1997). 1 = Dawsonia lativaginata; 2 = Buxbaumia aphylla; 3 = Polytrichastrum formosum; 4 = Scapania undulata; 5 = Polytrichum piliferum; 6 = Diplophyllum albicans; 7 = Orthotrichum cupulatum; 8 = Funaria hygrometrica; 9 = Brachy-thecium rutabulum; 10 = Rhynchostegium confertum; 11 = Pohlia nutans; 12 = Mnium hornum; 13 = Ceratodon purpureus; 14 = Dicranella heteromalla; 15 = Leucobryum glaucum; 16 = Pleurozium schreberi; 17 = Lophocolea biden-tata; 18 = Ptilidium pulcherrimum; 19 = Physcomitrium pyriforme; 20 = Phascum cuspidatum; 21 = Tortula truncata; 22 = Preissia quadrata; 23 = Pellia epiphylla; 24 = Conocephalum conicum; 25 = Reboulia hemisphaerica; 26 = Sphaerocarpos michelii; 27 = Riccia glauca; 28 = Archidium alternifolium.

Fig. 7.17. Spore diameter and number of spores per capsule in a sample of bryophyte species with different life history strategies (data from Longton 1997). 1 = Dawsonia lativaginata; 2 = Buxbaumia aphylla; 3 = Polytrichastrum formosum; 4 = Scapania undulata; 5 = Polytrichum piliferum; 6 = Diplophyllum albicans; 7 = Orthotrichum cupulatum; 8 = Funaria hygrometrica; 9 = Brachy-thecium rutabulum; 10 = Rhynchostegium confertum; 11 = Pohlia nutans; 12 = Mnium hornum; 13 = Ceratodon purpureus; 14 = Dicranella heteromalla; 15 = Leucobryum glaucum; 16 = Pleurozium schreberi; 17 = Lophocolea biden-tata; 18 = Ptilidium pulcherrimum; 19 = Physcomitrium pyriforme; 20 = Phascum cuspidatum; 21 = Tortula truncata; 22 = Preissia quadrata; 23 = Pellia epiphylla; 24 = Conocephalum conicum; 25 = Reboulia hemisphaerica; 26 = Sphaerocarpos michelii; 27 = Riccia glauca; 28 = Archidium alternifolium.

drought or frost) by producing short-lived gametophytes that vanish during these periods (Fig. 7.21). The resource allowance for the reproductive effort is typically high in such species, which tend to produce few, large spores that accumulate in soil pending a new period of favourable growth conditions.

Box 7.3

Ecological significance of bryophyte diaspore banks

Plants have to cope with unstable habitats in time (e.g. seasonal climate variations) and space (e.g. habitat degradation or destruction). When faced with the risk of local extinction, they may either disperse in an attempt to establish new populations or remain in the form of long-lived diaspores, from which new establishment will subsequently be possible under favourable growth conditions. Parts of these diaspores may become buried into the soil, requiring light for germination, constituting a bank of diaspores (During 1997). Because of the vulnerability of their gametophyte, bryophytes are, in particular, likely to rely more on stored propagules for their long-term survival than seed plants.

The diaspore bank of bryophytes allows species to survive unfavourable periods and facilitates rapid colonization after disturbance. The composition of the diaspore bank therefore influences the post-disturbance species composition and diversity (Jonsson 1993).

Not all species present in the actual vegetation are indeed represented in the diaspore bank. In boreal forests, for example, Jonsson (1993) found that typical colonizers of disturbed soil were very common in the diaspore bank, whereas some of the most abundant forest floor species were absent (Box 7.3 Fig.1).

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