Agriculture Reference
In-Depth Information
in soils in temperate countries are dominated by lumbricids and tend to be considerably less diverse
than in soils with other earthworm families in warmer latitudes (Lavelle et al. 1999). However,
even in the most complex soil systems, the diversity of earthworm species does not seem to be
very great, rarely exceeding ten, and there are usually only three to five species in any particular
site. There is some evidence that species that fill the same ecological niche do not normally occur
in the same degree of abundance at a particular site (Edwards and Lofty 1982a,b; Edwards and
Bohlen 1996).
The activity of earthworms differs greatly between seasons in temperate regions, where earth-
worms are active mainly in the spring and autumn. During the winter, they penetrate deeper into
soil, where they are much more protected from the adverse winter cold temperatures. In dry summer
periods, they also burrow deeper into soil and sometimes construct cells lined with mucus in which
they estivate in a coiled position until environmental conditions become favorable again.
Although cocoons may be produced at almost any time of the year, cocoon production is usually
seasonal. In temperate regions, the most cocoons are produced in spring or early summer, with a
second, much smaller peak in autumn. Numbers of cocoons range from 1 to 20 per mating,
depending on species.
The life cycles
of many species of earthworms have not been well studied. There probably is
adequate information on about 12 species of temperate lumbricid earthworms, 7 species from Africa
(Lavelle et al. 1999), and 20 species of earthworms common in tropical agroecosystems (Barois
et al. 1999). Earthworms have potential for very long life cycles of up to 10 to 12 years, although
in the field, many species may live only 1 or 2 seasons because of their susceptibility to a wide
range of predators (Edwards and Bohlen 1996). Indeed, their potential longevity, combined with
their fecundity, means that very large populations could build up rapidly in the absence of predation
or adverse environmental conditions. In addition, some species can produce cocoons parthenoge-
netically without mating, which increases their potential to spread to new sites.
Their moisture and temperature relationships have major effects on their ability to populate
new sites. Earthworms lose moisture through their cuticles, so they are very dependent on soil
moisture, and their activities are linked closely with rainfall patterns in both temperate and tropical
environments. However, for some reason, in periods of intense precipitation, some species may
emerge from their burrows, and they are often found in large numbers on the soil surface, where
they may die. Cocoon production and the growth of earthworms are correlated positively with
temperature, but the cocoon incubation period, percentage hatching, and number of hatchlings
produced per cocoon are correlated negatively with temperature (Edwards 1998). Many species
cannot survive below 0
A
C, and most species cannot survive above 30 to 35
A
C (Edwards 1983).
Nevertheless, they have behavioral patterns and resistant cocoons that enable them to survive
adverse climate conditions.
EARTHWORMS AND SOIL FERTILITY
S
F
OIL
ORMATION
Earthworms are extremely important in soil formation, principally through activities in consuming
organic matter, fragmenting it, and mixing it intimately with soil mineral particles to form water-
stable aggregates. During feeding, earthworms promote microbial activity by an order of magnitude,
which in turn also accelerates the rates of breakdown and stabilization of humic fractions of organic
matter. Different species of earthworms do not affect soil formation in the same way because of
very different behavior patterns. Some species consume mainly inorganic fractions of soil, whereas
others feed almost exclusively on decaying organic matter (see
Chapters 8
and
9
, this volume).
They can deposit their feces as casts either on the soil surface or in their burrows, depending on
the species concerned, but all earthworm species contribute in different degrees to the comminution
