Agriculture Reference
In-Depth Information
layers, fragmented pine needles that were heavily colonized by fungi (i.e., F
layer needles) favored
2
maximum growth of the earthworms.
In coniferous forests, the organic layers contain large communities of saprotrophic fungi
comprising diverse hyphal types, mycelial strands, rhizomorphs, and other fungal structures,
together with ectomycorrhizal fungi on conifer ÑfeederÒ roots and arbuscular mycorrhizal fungi
(AM) fungi associated with roots of shrubs (some of which also have
nodules) and of
herbaceous plants. These fungi represent a large potential food source for the epigeic earthworm
species colonizing these environments. However, it has been considered that there is severe com-
petition between microorganisms (particularly fungi) and earthworms for the available nutrients in
the soil organic matter (Scheu and Schaefer 1998).
Unlike the situation for forest litter, for which fungal-feedingÏpreference studies by collembo-
lans and mites abound, and for attempts to evaluate the potential ecological impacts of these
activities, there have been relatively few such studies for earthworms. Bonkowski et al. (2000)
produced an effective review of such studies, and although few refer to forest ecosystems, some
interesting generalizations could be made. In their study, the five earthworm species they used
included epigeic, anecic, and endogeic species, and all had very similar feeding preferences for the
nine fungal species tested. However, consumption of preferred fungi by detritivorous species was
considerably greater than that by geophagous species.
Basidiomycetes were generally refused as food, a result corresponding with the observation
that cellulose and lignin decomposer basidiomycetes were also rejected as food (Moody et al.
1995). Data from comparable studies with Collembola are not so consistent; for instance, Visser
and Whittaker (1977) showed that
Frankia
spp. and sterile
dark fungal forms to two basidiomycetes, which were generally avoided and appeared to be toxic.
However, P.J.A. Shaw (1988), working with eight ectomycorrhizal and four saprotrophic basidio-
mycete species from forest litter or soil, showed that three ectomycorrhizal fungi were preferred
and were nutritive; two common saprotrophic species were eaten but were not nutritive; and two
other fungal species were dangerously toxic to earthworms. Given this situation and that, as yet,
only a restricted number of microbial species have been tested in earthworm feeding studies, it
seems that a wider study of the palatability of basidiomycetes for earthworms is desirable before
any generalizations can be made. This is particularly important for studies of coniferous forest
ecosystems in which these fungi are of great ecological importance and earthworm invasions and
spread are occurring rapidly.
It has been suggested (Bonkowski et al. 2000) that fungi of early successional stages on decom-
posing plant debris are preferred and partially utilized by earthworms. These fungi serve primarily
as indicators of food quality to earthworms and other soil invertebrates. The choice of species of fungi
for earthworm feeding preference experiments, in the main, seem to focus on species from agricultural
Onychiurus subtenuis
preferred
Cladosporium
or grassland soils or crop residues. The sequence of fungi on such residues (see
Chapter 12
, this
volume) is very different from that occurring during coniferous litter decomposition.
In the L
and F layers of the floor of a montane lodgepole pine forest, dematiaceous hypho-
mycetes, coelomycetes, and sterile forms often represent about 85% of the fungal hyphae. Basid-
iomycete mycelia increase in frequency of occurrence with increasing stages of litter decomposition,
and in the H layer, the fungal community is often dominated by hyaline species of
2
Mortierella
,
Penicillium,
. The F and H organic layers are permeated with ectomycorrhizal
pine roots in which the fungi, mainly basidiomycete species, extend into organic layers. A similar
pattern of succession of saprotrophic fungi has been reported during the decomposition of leaf litter
in a montane aspen poplar forest (Visser and Parkinson 1975).
Several studies have been made of feeding preferences by springtails and mites on saprotrophic
fungi from these two forests (McLean et al. 1996) and have shown a common preference of these
invertebrates for dark-pigmented fungal forms. However, McLean et al. (1996) could not find any
effect of mesofaunal grazing on fungal species richness and diversity. Since the invasion of these
and
Trichoderma
