Agriculture Reference
In-Depth Information
Summarizing the available results on earthworms as pests of crops, Lee (1985) and Edwards
and Bohlen (1996) stated that, although earthworms occasionally damage healthy plants, more
commonly they attack very tender or moribund plants already damaged by some other mechanism,
and that there is no reason to regard earthworms as serious pests of plants. However, there are
clearly some instances when earthworms can damage plants either directly or indirectly (Edwards
and Bohlen, 1996; Brown et al. 1999). Care should be taken to prevent these situations from
occurring whenever possible.
5. I
NTERACTIONS
OF
E
ARTHWORMS
WITH
S
EEDS
From the moment a seed germinates, it comes into contact with the soil, a physicochemical environment
and a wide range of soil organisms, all of which may have variable degrees of influence on its growth
and success as a plant. Moreover, even before a seed germinates, some of these factors may already
be influencing its fate. For example, some earthworm species (e.g.,
L. terrestris
) appear to show a
preference for ingesting the seeds of certain plant species, depending on their size, shape, texture, and
taste (Piearce et al. 1994; Shumway and Koide 1994). Observations made more than a century ago by
Hensen (1877) and Darwin (1881) demonstrated the potential importance of surface-feeding anecic
and endogeic earthworms in ingesting, transporting, and distributing seeds in the soil. Moreover, seed
germination may be slower or more rapid in egested earthworm castings than in surrounding soils
(McRill 1974; Atlavinyt and Zimkuviene 1985; Piearce et al. 1994). For example, Grant (1983) and
Decans et al. (2001) observed lower germination rates and slower germination of the seeds of several
weed species in earthworm casts. Furthermore, many seeds are damaged by passage through the
earthworm gut, often affecting their germination success or vigor (Grant 1983).
In view of the selective consumption and the digestive processes of earthworms, the preferential
germination of different seed species in earthworm-linked structures, the dispersal of seeds through
the soil, and the physical-chemical effects of earthworms on the soil environment, it has been
suggested that earthworms may influence plant recruitment and the composition of plant commu-
nities considerably (Piearce et al. 1994; Willems and Huijsmans 1994). Some authors have suggested
that earthworms seem to favor the proportion, and often biomass, of clover in pastures (Stebler et
al. 1904; Bates 1933; Hopp and Slater 1948; Nielson 1953; Satchell 1955; Thompson et al. 1994;
Nuutinen et al. 1998). Positive associations of earthworm casts with the frequency and distribution
of the weeds
Plantago
spp.,
Trifolium,
and
Ranunculus
were also observed in meadows in the U.K.
(Bates 1933; Piearce et al. 1994).
The effect of earthworms on the soil weed seed bank, particularly the influence of anecic species
that preferentially ingest seeds, should not be underrated. Decens et al. (2001) estimated that 1 to 13%
of the total germinatable soil seed bank of a native savanna and two pastures were deposited in the
surface casts of the anecic earthworm species
Martiodrilus carimaguensis
from the Colombian Eastern
Plains. However, if there is preferential ingestion of weed seeds and differential growth of weed
seedlings in earthworm casts or earthworm-worked soils (Piearce et al. 1994), this may eventually
increase the level of weed infestations of crop fields or grasslands, potentially increasing competition
of weeds with the crops or desired plants (Edwards and Bohlen 1996; Stinner et al. 1997).
6. C
HANGES
IN
S
OIL
S
TRUCTURE
C
AUSED
BY
E
ARTHWORMS
The activities of earthworms in the physical ÑengineeringÒ of soils can modify a wide range of
chemical and biological properties and processes influenced by soil structure (see
Chapters 10
and
11
this volume). Earthworm pedoturbation of soils can change soil structure by affecting aggregation
(mostly by casting) and porosity to water and air (by burrowing and casting), thereby affecting soil
physical functions important in root growth and penetration, such as aeration, gaseous exchange,
water infiltration. and water-holding capacity (
Figure 2.5
)
. Earthworm burrowing creates mostly
macropores (pores larger than 30 Hm), and casting affects mainly the meso- and microporosity in
