Agriculture Reference
In-Depth Information
C
HANGES
IN
E
COSYSTEM
P
ROCESSES
AND
P
LANT
G
ROWTH
C
AUSED
BY
E
ARTHWORMS
Earthworms are generally believed to increase nutrient mineralization and thereby affect plant
growth beneficially. However, most of the information on earthworm-mediated changes in nutrient
mobilization and plant growth comes from agricultural systems, or crop plants, and considerably
less is known about these processes in forest systems and on the effect of earthworms on tree
growth (Scheu 2004). Furthermore, it has been realized only recently that the effects of earthworms
on the aboveground system is not restricted to plants but propagates into the aboveground food
web. As documented by Scheu et al. (1999) and Bonkowski et al. (2001), earthworms can alter the
palatability of plants to herbivores and therefore influence herbivore development. Wurst and Jones
(2003) presented evidence that earthworms may even affect the performance of herbivore predators
such as parasitoid wasps. Overall, there is an increasing awareness that the below- and aboveground
communities are much more closely linked than previously assumed (Scheu 2001; Van der Putten
et al. 2001; Scheu and Setl 2002; Wardle 2002). However, most of the sparse information available
on this subject does not come from forest systems.
Invasion of forest soils in North America by European earthworm species probably not only
affects soil structure and the indigenous soil biota, but also affects plants and therefore the whole
aboveground system (see
Chapter 5
, this volume). Unfortunately, information on how the invasion
of the belowground system by earthworms may affect the aboveground food web is limited. Scheu
and Parkinson (1994c) reported the growth of an understory grass species of aspen forests increased
in presence of the litter-dwelling earthworm species
D. octaedra
. However, overall, they suggested
that the invasion of aspen forests by European earthworms is more likely to change the plant species
composition rather than the overall productivity of these forests. Similarly, Alban and Berry (1994)
concluded that the productivity of aspen forests responded very little to earthworm invasions.
However, in laboratory experiments, Welke and Parkinson (2003) documented that Douglas fir
seedling biomass increased in the presence of populations of
A. trapezoides
, but these results need
to be confirmed in the field.
Gundale (2002) confirmed that earthworm invasions of North American forests altered plant
community composition in the understory. The results of this study are of particular concern because
the invasion of forests resulted in extirpation of a rare fern species,
Botrychium mormo
. This
extirpation was associated with a transformation of moder to mull humus, which presumably was
mainly because of invasion by the epi-endogeic earthworm species
L. rubellus
. Considering the
vast spatial range of the invasion of North American soils by European earthworm species, these
interrelationships need considerable further attention.
The implications of the invasion of North America by European species for ecosystem processes,
plant growth, and plant community composition passed almost unnoticed until recently. Studies
made so far have concentrated on the effects of invasions of deciduous forests; however, there is
now increasing evidence that the invasion of coniferous forests is not restricted to epigeic earthworm
species such as
D. octaedra
(cf.
McLean and Parkinson
1997), which may have few effects on
plant growth and plant community composition. Pine forests in the eastern front ranges of the
Canadian Rocky Mountains recently have been invaded by endogeic earthworm species such as
Aporrectodea
sp. (Migge unpublished data). Moreover, a range of ecological groups of European
earthworms is currently invading the temperate rain forests of western North America. The sites
invaded include national parks with pristine flora and fauna, such as those on the Olympic Peninsula
and Vancouver Island (Scheu personal observation).
REFERENCES
Alban, D.H. and E.C. Berry. 1994. Effects of earthworm invasion on morphology, carbon and nitrogen of a
forest soil,
Appl. Soil Ecol
., 1:243Ï249.
