Geology Reference
In-Depth Information
Fungal parasites and predators
If plant biodiversity is important in promoting the weathering of rocks, then parasites
must not be ignored for they can enhance biodiversity by preventing any one species
from taking over too much ecological space. Parasitic fungi are no exception. Perhaps,
in this way, parasitic fungi increase the structural diversity of root systems and therefore
the rate of rock weathering, which leads to greater drawdown of carbon dioxide from the
atmosphere. The tree-killing honey fungus,
Armillaria,
so hated by foresters, plays an
important role in the ecology of wild forests. An individual honey fungus of moderate
size has the biomass of a small elephant, but its behaviour is much more reminiscent
of a true predator. A honey fungus monitors its victims much as a pride of lions keeps
watch over zebra herds for weak individuals to hunt and eat. The mycelium of an aver-
age honey fungus parasitizes sugars from the roots of twenty to thirty healthy trees of
different species and kills and digests any that show signs of stress, perhaps because a
branch has been lost during a thunder storm, or because part of the tree has been shaded
out by its neighbours. Although fatal to individual trees, honey fungus has a benign ef-
fect on the forest. It helps to prevent overcrowding and gives rarer species a chance to
thrive by thinning out tree species that are becoming overly abundant.
Certain predators can also play an indirect role in weathering rocks by enhancing
the growth of plants. Unlikely as it may seem, some wooddecomposing fungi reveal a
predatory side to their natures when essential nutrients become scarce. Their staple diet
of lignin and cellulose contains plenty of energy but precious little nitrogen. This must
have made life difficult until they developed the novel and rather macabre solution of
obtaining nitrogen by capturing, killing and digesting some of the nitrogen-rich organ-
isms that seethe in the wood around them. The prime victims to this day are bacteria,
small invertebrates and nematodes, those infamous thread worms that sometimes infest
the roots of plants and occasionally irritate the human gut. Fungi use several ingenious
techniques to capture nematodes, including constricting ring-like snares, sticky knobs,
and adhesive nets. Fungal snares are particularly adroit at capturing their prey. The my-
celium first detects the chemical presence of nematodes and reacts by growing little
loops of hyphal tissue in the immediate vicinity of the hapless animals. A nematode un-
fortunate enough to pass even momentarily through a loop suddenly finds itself in a tight
spot as the mycelium rapidly pumps protoplasm into the snare, making it swell rapidly
like a balloon. The nematode is immediately trapped and digested alive through the re-
lease of powerful extra-cellular digestive enzymes. The resultant nitrogen-rich broth is
absorbed, assimilated and distributed to the rest of the fungus.
At least thirty-five species of fungi have been demonstrated to have predatory in-
clinations including the delicious oyster mushroom,
Pleurotus ostreatus
, which until re-
