Agriculture Reference
In-Depth Information
COMPETITION
Earthworm populations can be influenced by any animals with activities that affect their habitat
conditions and food supply. These could include grazing animals in grassland and termites in
tropical and subtropical habitats, both of which can drastically influence the nature and quality of
organic matter available to earthworms. In a broad sense, earthworms can be regarded as in
competition with such animals for food; however, the term
in the strict ecological sense
is usually reserved for interactions between and within closely related species.
Interspecific competition is thought to play a major part in determining the structure of earth-
worm communities (Bouch 1983; Lavelle 1983). Despite the relative paucity of species (1 to 16)
in most earthworm assemblages, a considerable amount of niche separation is apparent, suggesting
ecological adaptation to permit the coexistence of potentially competing species. Phillipson et al.
(1976), for example, found evidence for ecological separation in time and space and differences
in food preferences among the 10 earthworm species present in an English beechwood; they
concluded that these differences were important in reducing interspecific competition. Lavelle
(1983) considered that differences in patterns of vertical distribution and in body size are most
important in achieving earthworm niche separation in tropical soils, whereas horizontal and tem-
poral separation and feeding specialization are the main factors in temperate soils. He concluded
that competition is greater in temperate soils, where populations tend to be concentrated close to
the surface because there is heavy dependence on litter as a food source, and the depth of soil that
can be exploited is reduced by smaller body size. In tropical regions, by contrast, where higher
temperature conditions favor microbial activity and enhanced levels of interaction among fauna,
microflora, and organic matter, more diverse food resources can be utilized, permitting the occur-
rence of a wider range of earthworm sizes, greater depth distribution in the profile, and reduced
competitive pressure.
However, in the absence of supporting evidence, differences among them are not in themselves
indications of ecological niche differentiation resulting from competition (Begon et al. 1990), and
there is little direct evidence for interspecific competition among earthworms in the field. A decline
in native species has been linked with the establishment of introduced lumbricid species in cultivated
areas of Australia, New Zealand, and South Africa (Barley 1959; Martin and Charles 1979; Reinecke
and Visser 1980), and there is evidence from pot experiments that interspecific competition could
be a factor in such a decline (Dalby et al. 1998). Likewise, James (1991) reported a reduction in
native
competition
Diplocardia
spp. following invasion of tallgrass prairie in Kansas by
A. caliginosa
and
Octolasion cyaneum
. When the vegetation and soil have been altered radically, such trends may
largely reflect the inability of native earthworm species to adapt to the new conditions and greater
tolerance by lumbricid species for soil disturbance (Callaham and Blair 1999), but when no such
disturbance has occurred, competitive replacement is a possibility.
Interspecific competition in the field is notoriously difficult to detect, but the apparent absence
of current competition does not mean that it is unimportant as a factor influencing community
structure and population density. Competition may occur only occasionally, or not at all, because
natural selection in response to competition in the past may have favored avoidance of competition
through niche differentiation (the Ñghost of competition pastÒ; Connell 1980). Thus, unsuccessful
competitors may have been eliminated already, and mature, present-day communities may com-
prise earthworm species able to coexist with little or no competition. Furthermore, competition
need not be continuous or intense to be important because competition for scarce resources at
critical stages in the life cycle could have important consequences for the species involved.
Interspecific interactions may not necessarily always have negative effects on one or more of
the interacting species. Temple-Smith et al. (1993) reported that
A. longa
gained more weight in
the presence of
A. caliginosa
than in single-species culture; whereas decomposing leaf bundles at
the entrance of
in
English woodland (Phillipson et al. 1976). However, in the longer term, the litter-burying activities
L. terrestris
burrows provided favorable microsites for
Dendrodribus rubidus
