Agriculture Reference
In-Depth Information
Some of the earthworm distributions in East Asia are more difficult to assess because numerous
species have been transported, perhaps for hundreds of years, and con-
comitant habitat alterations have all but eliminated any chance of locating the ancestral homes of
these peregrine species. Some species are often found mainly in anthropogenic habitats, and others
Amynthas
and
Metaphire
are found only in undisturbed or relatively undisturbed soil systems. Without any other data, it can
be determined that certain earthworm species are at least potentially peregrines, but it may not be
possible now to determine their original ranges (see Chapter 3 , this volume).
CHARACTERISTICS OF EARTHWORMS THAT
MAKE THEM INVASIVE
Four primary features of earthworms are suggested as important for their ability to travel and
establish populations in new areas: a tolerance of environmental variability; suitable ecological
niches; high reproductive potential; and an appropriate reproductive system, which may be bipa-
rental or uniparental (Barois et al. 1999; Fragoso et al. 1999).
To be transported successfully to other regions, an earthworm species or its cocoons must be
able to survive some degree of disturbance, perhaps including fluctuations in temperature and
moisture levels, and be able to survive in a new habitat with different soil conditions, novel sets
of soil organisms, and potentially unaccustomed seasonal patterns of temperature and moisture.
The simplest case is the transport of a body of soil containing the earthworms or cocoons; the
larger the body of soil is, the greater its degree of insulation from environmental changes during
the movement. It would be expected that large amounts of soil would be least challenging to an
earthworm species that lives in it but also less likely to occur than the movement of smaller quantities
of soil. Therefore, the most successful earthworm travelers will be those with the maximum degree
of tolerance for adverse soil conditions.
Among the known tolerance mechanisms of earthworms is their ability to enter dormancy
in response to higher temperatures, low soil moisture, or both (Lee 1985). Many peregrine species
of Lumbricidae, such as the
species, which are common throughout the world,
have this ability. Similarly, the ubiquitous tropical peregrine species
Aporrectodea
P. corethrurus
can become
dormant in drought conditions. On the other hand,
species in the central United
States can enter dormancy during drought periods, but none of these species is known to be
peregrine, although several do well in moderately disturbed anthropogenic habitats such as lawns
and gardens. The only obvious difference suggesting why the
Diplocardia
species may not have
traveled with human aid is the physical robustness of construction among the Lumbricidae, which
appears to be greater than that of
Diplocardia
spp. Yet, robustness is hard to quantify and perhaps
even harder to put to a meaningful test. On the other hand, some
Diplocardia
Diplocardia
species commonly
collected for fishing bait can survive soil temperatures of 35
A
C in small containers, which is
invariably fatal to most peregrine Lumbricidae.
Bouch (1977) outlined several major aspects of earthworm behavior and morphology corre-
lated with ecology and defined the ecological categories now in wide use, that is, epigeic, endogeic,
and anecic species.
are those inhabiting superficial organic matter accumulations
and the soil-litter interface and are typically small bodied, dark colored, and capable of rapid
movements. Many of the common vermicomposting species fall into this category. Their natural
food sources are short lived and widely scattered, so they produce large numbers of offspring or
may even be parthenogenetic and rely on dispersal to locate new resource patches.
Epigeic
species
earthworm species live and feed in the mineral soil layer, are usually lightly pigmented
or unpigmented, range in size from small to very large, do not move quickly, and have lower
reproductive rates. Further division of endogeic species has been made (i.e., polyhumic, mesohumic,
and oligohumic) based on the relative age, concentration, and location of organic matter they utilize
as food. The polyhumic species somewhat resemble epigeic species in coloration but are capable
Endogeic
 
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