Bryophytes as food and as shelter

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Anyone who has observed a bryophyte sample under the dissecting microscope realizes the extent to which bryophytes serve as a shelter for a vast array of micro-organisms. These range from myxomycetes of tropical rainforests, which display specialized biotypes on epiphyllous liverworts (Schnittler 2001) to invertebrates, including aphids, nematodes, rotifers and tardigrades (Merrifield & Ingham 1998, Peck 2006). In terrestrial environments, the moss habitat is attractive because it provides buffered temperature and humidity conditions. In rivers, aquatic moss cushions reduce water velocity and act as filters of water-borne particles, accumulating large quantities of detritus and periphyton. Bryophytes are also important as both egg-laying sites and a protective nursery for small larvae, which can obtain shelter during floods and consume the abundant food sources available to them. Aquatic bryophytes are attractive refugia for macro-invertebrates because generalist consumers, such as geese and crayfish, selectively consume angiosperms over aquatic mosses, even when the latter represent the bulk of the biomass (Parker et al. 2000). The moss tissues, in fact, include a suite of chemical compounds that deter herbivores (Box 2.1). As a result, aquatic bryophytes typically support twice as many plant-associated macroinvertebrates as riverweed (Parker et al. 2000) and macroinvertebrate densities are about ten times higher in aquatic bryophytes than on the river bed (Suren 1991).

While bryophytes thus offer appropriate habitat conditions for a variety of invertebrates, direct consumption appears to be remarkably low. Although some invertebrates tolerate or circumvent the chemical defences of bryophytes and feed on them (Longton 1992, Parker et al. 2000), they most often display a strong dietary preference for angiosperms when confronted with a feeding choice (Smith et al. 2001). Snails forced to feed on moss suffer a significant weight loss (Oyesiku & Ogunkolade 2006). Similarly, although accidental ingestion may occur, thereby enhancing dispersal by endozoo-chory (Section 6.2.2.2), vertebrate herbivores have never been shown to graze heavily on mosses, except in Arctic regions. This can result locally in a dominance of bryophytes over grasses in heavily grazed habitats, as shown, for example, in Welsh oakwoods (Rieley et al. 1979).

It is neither the mineral content nor the calorific value of bryophytes that make them unsuitable sources of food. Mineral concentrations of bryophytes and angiosperms are, in fact, similar (Table 2.1) and mosses contain the same sugars as higher plants. Lipid levels are reasonably high (5% of dry weight) in the vegetative parts of mosses. In spores, lipid concentrations can reach up to 30% and this may account for the preference for capsules by lemmings.

Table 2.1. Comparison of the chemical composition of mosses and angiosperms (in % of dry weight) (after Prins 1982)

crude

P

Ca

K

Mg

Na N

proteins

bryophytes

0.02-0.15

0.01-0.95

0.36-0.69

0.05-0.22

0.05-0.51 0.96-1.91

5.1-11.9

angiosperms

0.05-0.26

0.14-1.68

1.13-4.16

0.20-0.73

0.03-0.53 1.32-4.1

8.3-28.1

The caloric value of mosses (3800-4500 cal is also in the same range as that of angiosperms and no species has been reported to be poisonous. In fact, the main reason bryophytes are not valuable sources of energy is that their digestibility is extremely low (Ihl & Barboza 2007). In reindeer, the maximum net energy gain from mosses is only 78 cal g_1 compared to 1281 cal g_1 in grasses. In lemmings, the difference is 70calg_1 in mosses against 552calg_1 in forbs. As a result of the poor digestibility of mosses, their consumption may even result in a net loss of nutrients from the animal (Ihl & Barboza 2007).

These differences in digestibility between bryophytes and angiosperms reflect their different strategies for preventing attacks by micro-organisms. Bryophytes mostly lack a cuticle that provides mechanical protection against infection in vascular plants and therefore entirely rely on the use of 'chemical weapons' to prevent microbial and fungal infection. In bryophytes, the high concentrations of polyphenolic lignin-like compounds (c. 37% of dry weight in mosses against c. 19% of lignin in forbs) provide the cellulose and other polysaccharides with a defence against hydrolytic attack. This also makes the cellulose content less accessible to digestive enzymes. In addition, polyphenols often have an antibiotic action, impeding the digestion of ruminants or hind-gut fermenters. The high affinity of bryophyte cell walls for heavy metals (Section 9.2.2), the presence of other secondary metabolites (Oyesiku & Ogunkolade 2006), and the terpenoid oils contained within the oil bodies of liverworts (Section 3.1.1), may also act as a repellent. Bryophytes therefore appear to be promising 'organic' sources of repellents against snails and other browsing invertebrates (Box 2.1). Only in a few instances have herbivores seemingly adapted specifically to feeding on bryophytes. For example, the relative increase of the rumen weight of Spitsbergen reindeer compared to those of Norway could be seen as an adaptation to a moss diet and the post-caecal spiral in bryophage lemmings may help in the digestion of mosses (Longton 1992).

Quite surprisingly, bryophytes are freely consumed in quantity by Arctic and alpine vertebrate herbivores such as reindeer, caribou, musk ox, most Arctic-breeding geese and lemmings. During the winter, mosses such as

Calliergon, Dicranum and Polytrichum form 30-40% of the diet of the latter (Longton 1992). Why then, if Arctic and alpine herbivores do not feed on bryophytes for energy, do they ingest such large quantities of bryophytes? The presence of a specific chemical compound, arachidonic acid, is a possible reason why these plants are eaten under cold conditions (Prins 1982). Arachidonic acid is also present in algae, ferns and Ginkgo biloba, but is absent from other gymnosperms and all angiosperms. Among the plants in which it has been reported, the concentration of arachidonic acid is highest in mosses (up to 35% of the fatty acids). Arachidonic acid has interesting properties for life under cold conditions. The low melting point of the molecule (—■49.5°C) might contribute to the lowering of the melting point of fats, which aids in maintaining sufficient limb mobility at low temperature. Arachidonic acid also offers protection to cell membranes against cold, enhancing membrane fluidity and thus permitting continued enzyme function at low temperatures. Since animals are not capable of synthesizing arachidonic acid, it is adaptive for those living permanently in cold regions to eat moss.

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Responses

  • diana
    How do bryophytes shelter invertebrates?
    7 years ago
  • sophie
    Is it mosses source of food?
    2 years ago
  • stefanie
    Which bryophytes men feed as food?
    2 years ago
  • declan
    How the bryophytes used in house food chelter?
    9 months ago
  • uwe
    Can bryophytes be used as food sources for humans?
    5 months ago

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