Agriculture Reference
In-Depth Information
Cotton 1983); drainage of water-logged soils in high-rainfall areas of southeastern Australia resulted
in significantly increased abundance of
A. caliginosa
(Baker 1998).
CONCLUSIONS
In undisturbed sites, which support relatively ÑstableÒ earthworm communities, the abundance and
diversity of the earthworm fauna are determined primarily by interactions between climate and
soils. These factors determine the physicochemical parameters of the soil environment, the nature
of the vegetation that can be supported, and the quantity and quality of the litter it produces. On
a global scale, temperature is the climatic variable of greatest significance to earthworms because
it determines their metabolic rates and the diversity of food resources that can be exploited, but on
a more local scale, moisture restrictions often determine local patterns of earthworm distribution
and activity.
Earthworms are often exposed to high rates of predation and are subject to pathogen and parasite
attack; there may be times when predation especially can significantly depress population densities.
However, when populations are not constrained by physicochemical environmental factors, food
supply, notably the quality and quantity of the litter input, is the factor that most frequently limits
earthworm abundance.
Competition for food is generally believed to be important in determining earthworm popula-
tions, but there is little information on the nature of this competition and the frequency with which
it occurs. Available evidence suggests that interspecific competition has been minimized through
niche differentiation; when it occurs, it is probably diffuse in nature and may not have a major
influence on population trends. By contrast, intraspecific competition may be common following
periods of rapid population growth and may operate in a density-dependent manner to adjust
population density to the available food supply.
Most, if not all, earthworm species appear to undertake surface migrations to a greater or lesser
degree (Mather and Christensen 1992), but the significance of this behavior for population processes
is not known. If, as Mather and Christensen suggest, migration is primarily a resource-seeking
activity, it could be important in enabling the population to locate more suitable habitats when
conditions become unfavorable. Migration is undoubtedly significant in the colonization of new
habitats, although active migration may often be less important than passive dispersal in this regard.
There has been little debate about the relative importance of density-dependent and density-
independent factors in determining earthworm abundance. Although there is a lack of critical
information on earthworm population dynamics, some of the conclusions that have emerged from
studies of soil insects such as the garden chafer
Phyllopertha horticola
appear relevant (Milne
1984): Most of the times when populations are low, natural control is caused by the combined
effects of weather, which is always density independent in its action, and interspecific competition
of predators, parasites, and disease, which may be density independent or weakly density dependent
but rarely if ever strongly density dependent in action. Under favorable conditions, when earthworms
are able to ÑescapeÒ from these natural control mechanisms, populations will ultimately be regulated
by intraspecific competition acting in a strongly density-dependent manner.
Virtually any form of human intervention will influence earthworm populations, often adversely,
when the intervention is disruptive, as in mining and mechanical cultivation. One aspect of human
intervention with potentially important consequences is accidental or deliberate introduction of
exotic species, which could dramatically change abundance and species composition, possibly to
the detriment of native species, and influence soils and plant production (see
Chapter 5
, this volume).
However, there is considerable scope for promoting earthworm activity through management
practices that remove constraints such as low pH and unfavorable moisture conditions, which
minimize the adverse effects of cultivation and pesticide use and increase the food supply through
organic amendment and increased crop residue return to the soil.
