Agriculture Reference
In-Depth Information
reliance on fungi, particularly among litter-feeding epigeic and anecic earthworm species (which
feed on materials extensively colonized by fungi), remains uncertain. During passage through the
earthworm gut, fungal hyphae (except those protected inside root fragments) (Reddell and Spain
1991a) are digested preferentially, whereas many (but not all) types of fungal spores pass through
unharmed. This can have important consequences for the microbial processes and succession in
earthworm casts (Tiunov and Scheu 1999; see
Successional Processes within Casts
,
this chapter).
) of pure cultures of a
range of fungi. Earthworms gained or maintained weight when fed six of the eight fungi tested and
died when fed the other two. Several other species of fungi (primarily toxin- or antibiotic-producing
fungi such as
Morgan (1988) studied the food value (for the epigeic species
E. fetida
spp.) appeared to be detrimental to
earthworms (Edwards and Fletcher 1988; Morgan 1988). Studies using six earthworm species and
more than 10 soil and litter fungal species (Moody et al. 1995; Bonkowski et al. 2000) have shown
that earthworms prefer, and partly digest, the rapid-growing fungi species typically associated with
the early successional stages of decomposition (cellulolytic fungal species and those that consume
soluble carbohydrates, e.g.,
Aspergillus
spp.,
Fusarium
spp. and
Penicillium
spp.). On the other hand, fungal species
appearing later in the succession (characteristically degraders of recalcitrant polymers, e.g., Basidio-
mycetes) were avoided and proved to be a poor food source for the earthworms.
An examination of the survival of fungal spores during passage through the earthworm intestine
showed that survival varies greatly depending on the characteristics of the spore and the earthworm
gut environment (Brown 1995). For instance, Moody et al. (1996) observed that no viable spores of
Fusarium
and
Trichoderma
Fusarium lateritium
survived passage through
L. terrestris
and
A. longa
; spores of
Mucor hiemalis
were more severely affected by passage through the guts of
L. terrestris
than through the guts of
A.
longa
(10 and 28% survival, respectively). The survival of spores of yet another fungal species
(
Chaetomium globosum
) was unaffected by passage through guts of either earthworm species.
Reddell and Spain (1991a,b) demonstrated that spores of an actinomycete,
sp., and
spores of more than 20 mycorrhizal fungal species or groups remained intact after passage through
the gut of
Frankia
Pontoscolex corethrurus
, and most remained viable. On the other hand, spores of other
species, such as
appear to have low germination rates in earthworm casts
(fungistasis) (Striganova et al. 1989b); still others (
Ulocladium botrytis,
) did not germinate after
passage through earthworm guts (Keogh and Christensen 1976). Fungal digestion appears to show
a gradient along the earthworm gut, in which fungi are digested mainly in the anterior and middle
gut regions, with little digestion occurring in the hindgut (Gonzalez 1990; Tiwari et al. 1990). A
similar pattern of digestion of protozoa along the earthworm gut has also been observed (Piearce
and Phillips 1980).
Free-living soil protozoa may also be important dietary elements for some earthworm species.
For example,
Pithomyces chartarum
was able to grow to maturity in sterilized soil recolonized by soil bacteria
and fungi only after the addition of protozoa (Miles 1963). Flack and Hartenstein (1984) recorded
large weight gains in earthworms after adding protozoa to their food. Bonkowski and Schaefer
(1997) showed further that populations of the endogeic earthworm
E. fetida
increased in soils
containing high densities of amoebae, and that earthworm growth rates increased with the addition
of amoebae to soil. Similarly, Doube and Gupta (unpublished data) showed that
A. caliginosa
A. trapezoides
and
E. fetida
ate soil containing flagellate and cilliate protozoans preferentially, and both earthworm
species responded positively to the fluid in which the protozoa had been living, indicating that the
earthworms can respond to the products of protozoan activity, possibly low molecular weight
compounds, as well as to the protozoa themselves. Protozoan protoplasm is highly assimilable by
earthworms, and nonencysted forms (trophozoites) are digested preferentially while encysted forms
seem to survive passage through the gut (Piearce and Phillips 1980; Rouelle 1983; Barois 1987;
Gupta and Doube unpublished). However, not all protozoa are beneficial: Morgan (1988) reported
that two species of protozoa (
Tetrahymena pyriformis
and
Proterioochromonas minuta
) killed
E.
fetida
within 3 days in a restricted culture environment.
