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FUNGI - LIFE SUPPORT FOR ECOSYSTEMS
Andrew Cowan N.D.Arb.
Published: essentialARB, Issue 4 - 2001
Fungi are fundamental to the success and health of almost every ecosystem on earth, both terrestrial and aquatic, and essential to the sustainability of biodiversity. However, how often do we consider their existence within a habitat, let alone how conditions could be improved by active encouragement and management of the fungal diversity? |
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Fungi are perhaps the most unappreciated,
under valued and unexplained organisms on earth.
When you ask someone to describe a fungus, you
will get a variety of descriptions ranging from, mouldy
bread and mildew on the bathroom wall, to magicmushrooms
and poisonous toadstools. Some enlightened individuals
will tell you that fungi are essential for things like
bread making, brewing and medicines. However,these are
only some of the more visible supporting roles that
fungi play. Rarely considered, even in general scientific
circles, is that there are many times more fungi than
plants on earth, and that each type plays a crucial
role in the processes supporting the functioning of
major ecosystems.
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Fungi are present almost everywhere, in a spectacular
array of shapes, sizes and colours, and performing a
wide variety of different activities. In 1991 David
Hawksworth, a mycologist at Kew estimated the world’s
fungal diversity at 1.5 million species (equal to the
estimated number of all known other living organisms).
This was thought at the time to be a radical over estimate,
but now other researchers have proposed figures in excess
of 13 million. Fungi perform essential roles in every
terrestrial, and many aquatic, ecosystems, eg. decomposing
dead organic matter to release nutrients, supportingplant life on poor
soils by improving the absorption of nutrients
when they form mycorrhizal associations with roots,
living inside plants as endophytes and forming
symbiotic partnerships with algae to form lichens.
Any deterioration in fungal populations and diversity
can therefore have a considerable impact on ecosystem
health, in fact, the loss of lichens from an area
is often used as an indication of poor air quality.
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What fungi are and how they live provides some
insight into the reasons for their significant
role in ecosystems. The basic structures of most
fungi are microscopic threads called hyphae, which
form the active feeding and growing body of the
fungus. The majority of the world's fungi are
microscopic, and they do not usually produce structures
which are visible to the naked eye, unless the
hyphae form a thick growth (Often referred to
as 'moulds’). However, the most familiar
species are those which produce spore-bearing
fruit bodies, which are clearly visible to the
naked eye. These include puffballs, coral fungi,
earthstars, truffles and other forms of mushrooms
and toadstools These are the so-called 'larger
fungi' or 'macrofungi'.
Some fungi are very adaptable. For example, species
of leaf litter decomposers such as the Parasol
mushrooms (Macrolepicta species) and Funnel Caps (Critoeybe
species) whichdecompose organic matter indiscriminately
regardless of source, while others are far more specific
and occupy a very restricted niche, like the Ear Pick
fungus (Auriscalpium vulgare) which is only found on pine
cones. There are others that are so geographically and
biologically restricted they are considered rare and are
now included on endangered species lists. Some fungi are
known to have rapidly declined due to pollution and loss
of habitat. English Nature is lending its weight to a
Biodiversity Action Plan which aims to conserver 40 species
across England. |
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Decomposition and nutrient recycling
One particularly crucial role of fungi is in the transport,
storage, release and recycling of nutrients. Nutrient
cycling - the continuous supply, capture, replenishment
and distribution of carbon, nitrogen and minerals - is
fundamental for the ongoing health and vitality of all
ecosystems. In woodland ecosystems, a substantial proportion
of the nutrients stored, or in various states of flux,
is in living and dead organisms, both above-ground and
in the soil. Fungi, microbes and fauna may account for
much of this nutrient resource in soil, and these organisms
work together in a soil based food web to recycle the
nutrients.They
expedite crucial transfers and transformations
of nutrients within micro habitats, including
transfer from leaf litter, twigs, branches and
logs into soil, and from soil into plants. As
a result, soil organic matter and nutrient availability
to plants is entirely dependent on the activity
of soil organisms such as fungi. |
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The ability of fungi to decompose major plant
components - particularly lignin and cellulose
- is the basis of their organic recycling role.
Without decomposer fungi, we would soon be buried
in litter and debris. They are particularly important
in litter decomposition, nutrient cycling and
energy flows in woody ecosystems, and are dominant
carbon and organic nutrient recyclers of forest
debris.
Fungi are particularly valuable in acid soils,
where the low pH makes it difficult for the survival
of other organic decomposers such as bacteria.
Bacteria release nitrogen in the form of nitrate
which is easily leached from the soil and therefore
lost to surface roots. However, the fungi that
break down the organic surface litter release
nitrogen into the soil in a form of ammonium nitrate
which is less mobile. This could be very important
to the successful establishment of young trees
and to the sustainability of the ecosystem as
a whole.
Mycorrhiza - ‘fungus-root’
The transformation of nutrients and their transition
from soil into plants is an essential component
of ecosystem nutrient cycling which could not
be achieved without the fungi. ‘Mycorrhizal
associations' form fungus-root systems which are
far superior to roots alone. Many of the world's
plants are partnered by mycorrhizal fungi, both
in natural ecosystems and in agricultural or forestry
crops. The fungi have a mutually beneficial relationship
with the plants, thanks to a two-way exchange
that occurs in modified roots known as mycorrhiza,
(literally 'fungus-roots').
Carbohydrates from the plant are transferred to the fungus, while soil nutrients such as phosphorus are transported from the fungus to the plant. Mycorrhizal fungi are central to the processes of nutrient capture and recycling for most higher plants in low nutrient soils, as they assist in the acquisition of scarce nutrients and improve their absorption by the plant. Networks of fungal hyphae radiate outwards into the soil from mycorrhizal roots, forming a vast mycelial infrastructure capable of absorbing soil nutrients far more efficiently than plant roots alone. |
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The fungi act as an extension of the root system, resulting
in improved nutrient uptake for the plant. This is particularly
important for soil-immobile nutrients such as phosphorus.
In woodland soils, where plants compete for available
nutrients that may be in short supply, this association
can provide a vital support system to help maintain
the stability of the ecosystem.
Mycorrhiza are grouped into two main types. Ectomycorrhizae
occur predominantly in association with woody plants,
including many of the world's major forest trees. The
fungus forms a sheath around the fine roots of plants,
penetrating between the outer cells, forming
a Hartig Net. A diverse range of fungi form ectomycorrhizae,
and most of these produce large fruit bodies.
The second type, endomycorrhiza do not have a
sheath, but the hyphae penetrate both inside and
between the plant root cells. Fewer species of
fungus form endomycorrhiza than ectomycorrhizae,
and endomycorrhizal fungi do not generally produce
large fruit bodies. |
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Among trees, mycorrhizae are a major part of
the strategy for capturing, taking up and recycling
scarce nutrients, and well over 1000 species of
mycorrhizal fungi may be associated with them.
Living and dead fungi, microbes and fauna may
account for much of the soil nutrient resource
in forests and woodlands. Mycorrhizalfungi may
also buffer plants against environmental stresses such
as disease, for example by protecting plants against
pathogens, by increasing host vigour, and by acting
as barriers, actively competing against the intruders.
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The fungus inside - Endophytes
Still unknown and unexplained, the unseen world of fungi
living inside plants as an inconspicuous embroidery
of threadlike filaments, provides yet another dimension
to the fungal support system. Plants are not just single
organisms, they are entire symbiotic systems. Virtually
every plant species researchers have examined has fungal
endophytes including several fossil plants related to
club mosses. We have not even begun to understand the
complexities of their relationships. Some are thought
to help with the storage and distribution of nutrients
and carbohydrates around the plant, while some are pathogens
waiting for the time to strike when the conditions are
right, others may act to defend the plant by producing
toxins that make the plant distasteful to herbivores.
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This fungal world within plant leaves, stems and roots,
went largely unappreciated until 1977, when researchers
found a grass endophyte to be responsible for many livestock
poisonings, in both cattle and horses that eat its host,
a tall fescuegrass. Research in Europe has
found 40-70 species of endophyte in 11 different
trees and a further 400 associated with grasses.
Endophytes have been found to play a crucial
role in the production of extremely beneficial
chemical compounds. For example, the cancer-fighting
compound taxol, which was originally derived from
the Pacific yew, has been found to be a product
of endophytic fungi. Some of the most recent research,
reported in the New Scientist in April 2000, found
not only that multiple endophytes in various yew
species produced taxol, but that other fungi in
wholly unrelated plants do so too. Since taxol
has antifungal properties, particularly against
‘water moulds’ (not true fungi), it
may help keep pathogens at bay and strengthen
the plant’s defence system. However, a lot
more research is needed as taxol may not be the
most effective of organic compounds. The potential
for finding something far better and much more
effective can not and should not be overlooked.
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What now ?
Despite their
central role in ecosystems and their applications in
biotechnology, knowledge about fungi remains at a low
level. For example, it has been estimated that only
5% of the World's fungi have so far been discovered,
and for most of these, little is known about their biology.
If we don't know what they are, how do we know what
they do, and what capabilities we could be harnessing?
Our lack of knowledge may relate to the inconspicuous
nature of many fungi. Most are rarely seen, and those
producing conspicuous structures appear fleetingly,
at unpredictable and irregular intervals.
The masses of fungal hyphae that spread throughout
the soil and into the plants themselves are responsible
for keeping the entire ecosystem in healthy order. In
the deep layers of organic litter found on the surface
of woodland soils, the decomposer fungi and those associated
withroots as mycorrhizae, form an interlocking web of
mycelium which binds this organic horizon together.
Organisms killed by pathogens contribute organic matter
for nutrient cycling. Fungal pathogens of trees produce
gaps,contributing to natural ecosystem
dynamics, creating cavities in trunks and hollow
logs, used by native animals, and accelerating
the return of woody organic matter to the soil.
Furthermore, some pathogenic fungi are used as
biocontrol agents - a good alternative to chemicals
for controlling weeds and pests. |
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Fungi need a constant supply of
organic matter to survive and thrive. The nutrient
cycle relies on the reintroduction of dead material
to provide a constant source for the fungi to
decompose. In an existing woodland the organic
horizon is topped up each year with falling leaves,
but in our parks and gardens, or on new planting
schemes, this source of nutrients is either non-existent
or is removed as over enthusiastic gardeners remove
all the autumn leaves.
In these situations the application of an organic mulch
becomes very important and will improve the quality
and productivity of the soil.
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| The recognition of fungi in ecosystem restoration and
conservation is long-overdue, and accelerated studies
on fungi are now needed, not only so that we may learn
to harness more of them in more ways, but also to gain
a better understanding of how ecosystems operate. Perhaps
most importantly, we need to learn how to lessen human
impact on ecosystems and to implement more efficient
rehabilitation regimes on degraded land.
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