Agriculture Reference
In-Depth Information
IMPACTS OF EARTHWORM INVASIONS OF
TWO MONTANE FORESTS
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With the advent of European settlers in North America, lumbricid earthworms began to invade the
continent. During early settlements, earthworms were probably spread mainly in soil adhering to
the roots of garden plants and trees. Later, the spread of earthworms increased, presumably because
of the use of machinery in agriculture and forestry. Soil containing earthworm cocoons was probably
transported on tires and human and animal feet, even over long distances. Without accidental long-
distance transport, lumbricid earthworms certainly would not have been able to colonize the entire
North American continent (see
Chapter 5
, this volume).
Colonization of new habitats by earthworms is usually slow and in the horizontal range of 10
m per year (Hoogerkamp et al. 1983; Marinissen and Van den Bosch 1992). Colonization of eastern
North America by earthworms went almost unnoticed by scientists and was well established when
the monitoring of earthworm populations started (Reynolds 1977; see
Chapter 4
, this volume). By
contrast, western Canada was little colonized by European lumbricids until recently. Forests of the
mountain ranges of the Rocky Mountains in southern Alberta were free of earthworms until 1985
(Parkinson unpublished data). Since then, a range of forests has been colonized, and a number of
lumbricid earthworm species have been successful invaders (Scheu and McLean 1993); the most
vigorous is
(Dymond et al. 1997; McLean and Parkinson 1997).
Studying populations of earthworms along transects from forestry trunk roads into the forest,
Dymond et al. (1997) concluded that earthworm colonization usually begins from the roads and
moves into the forest in a wavelike manner. Earthworms may reach exceptionally high populations
along the invasion line, and colonization may be very rapid. In deciduous (aspen) and conifer (pine)
forests,
D. octaedra
(Dymond et
al. 1997; McLean and Parkinson 1997). These invasions completely alter the soil structure and soil
invertebrates and fungal community composition, decomposition, and mineralization processes and
therefore the whole ecosystem.
The climate of the areas where these invasions occurred is continental, with long cold winters
(from −5 to −20AC), during which the organic layers of the forest floor are frozen solid (Parkinson
1988). It was expected, from the work of Holmstrup (1996) and Holmstrup and Zacharias (1996),
that the survival of
D. octaedra
during such harsh conditions would be only as cocoons. However,
sampling of the frozen organic layers, followed by careful thawing and then extraction, yielded not
only a predominance of cocoons, but also small populations of small and large immatures, matures,
and aclitellate earthworm adults, yielding numbers for all growth stages of about 200 m
−2
(McLean
unpublished data).
D. octaedra
may reach populations as high as 2000 to 3000 individuals m
Ï2
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Dendrobaena octaedra
preferentially colonizes the organic layers of forests. Therefore, its effects
on soil structure are usually restricted to the uppermost soil layers. However, in both deciduous
and conifer forests, it strongly changes the structure of organic layers.
Dendrobaena octaedra
feeds
preferentially on F-layer material and transforms it into casts or faecal pellets (Scheu and Parkinson
1994b; McLean et al. 1996; McLean and Parkinson 1997). In addition, some mineral soil material
is incorporated into the organic layers, which eliminates the clear boundaries between organic
layers and mineral soil that typically occur in forest soils of western Canada.
Thorough mixing of the organic and mineral soil layers occurs only in the presence of mineral
soil-dwelling species of earthworms such as
A. caliginosa
and
L. terrestris
. In the first study on
the effects of lumbricid earthworms on the soil structure of Canadian forests, Langmaid (1964)
