Environmental Engineering Reference
In-Depth Information
of ecosystems have been reviewed by Dobson and
Crawley (1994). These authors mentioned the phenom-
enon that fungal blights removed
Castanea dentata
from the eastern deciduous forests of the United
States,
Tsuga mertensiana
from the Pacific north-
west of Canada and the USA, and
Ulmus
species from
much of western Europe, and a whole range of
species from
Eucalyptus
forests of western Australia.
In each of these cases, the removal of a dominant
species led to the development of forests dominated
by less competitive species from earlier successional
stages, or opened up the canopy for colonization by
earlier successional species.
Brown and Gange (1989) were among the first
to pay attention to the effects of both above-
and below-ground plant consumers (herbivores and
pathogens) on plant succession. In an early stage of
grassland succession, plant species richness and
diversity increased as a result of the application of soil
insecticide and, by the second season, were depressed
by foliar insecticide. Three major life-history groupings
- annual and perennial herbs, and perennial grasses
- responded differently to herbivory, with a consider-
able effect on the pattern of early succession. Soil-
borne diseases appeared to be also involved in the
degeneration of
Ammophila arenaria
and
Hippophaë
rhamnoides
, two plant species that dominate the
coastal foredunes of Europe and which are widely
planted for sand stabilization (van der Putten
et al.
1993). Endoparasitic nematodes appeared to be
responsible for reduced vitality of
A. arenaria
, thus
favouring
Festuca rubra
ssp.
arenaria
. Nematodes of
the genus
Longidorus
are capable of damaging the
root system of
H. rhamnoides
, including nitrogen-
fixing nodules, and the related mycorrhizal system, thus
reducing the uptake of phosphate and other nutrients.
This damage may result in acceleration of succession
to, for example,
Sambucus nigra
,
Ligustrum vulgare
and
Rosa rubiginosa
on calcareous soils, or to
Empe-
trum nigrum
on acid soils. In general, the spatial and
temporal dynamics of above- and below-ground
herbivores, plant pathogens and their antagonists
can differ in space and time. This affects the temporal
interaction strengths and impacts of above- and
below-ground higher-trophic-level organisms on plants
(van der Putten
et al.
2001).
A world-famous example of the indirect effect on
vegetation caused by an animal disease is provided
by the infection of rabbits (
Oryctolagus cuniculus
) by
the
Myxoma
virus in southern England (see Dobson
& Crawley 1994). Myxomatosis was introduced into
Australia in 1950 and into France in 1952, from
where it spread throughout western Europe, reaching
Britain in 1953. The initial
Myxoma
virus in 1953 was
a highly virulent strain and the 1950s rabbit popula-
tion was reduced by about 99% within a few years.
The rabbits remained extremely scarce for the following
15 years. Once rabbits had almost disappeared, acorns
buried in grassland by jays had a vastly greater
chance of producing seedlings and becoming estab-
lished. The reduced rabbit grazing was responsible for
the transformation of Silwood Park from an open grass
parkland in 1955 into an oak woodland (
Quercus robur
)
with occasional clearings within 15-20 years. This
change was irreversible, even after the recovery of the
rabbit population in the 1970s.
4.3.3 Tritrophic interactions
In the plant species
Silene alba
and
Silene dioica
,
flowers are the site of infection by the host-sterilizing
anther smut fungus
Ustilago violacea
, as well as the
site of oviposition by the noctuid
Hadena bicruris
.
In diseased populations of the two host species, the
noctuid can act as a pollinator, fruit predator and
vector of fungal spores. Hence, host traits that enhance
or reduce visitation rates by the noctuid could affect
fruit set, fruit predation and infection probability.
Timing of the production of susceptible host stages
can have a large impact on a plant's probability of
infection by pathogens and on the extent of damage
caused by herbivores. Biere and Honders (1996)
investigated the impact of intraspecific differences in
the timing of anthesis in these two closely related host
species on susceptibility to the herbivore and the
pathogen. The fungal pathogen and the seed pred-
ator both had a large impact on female fecundity in
the two
Silene
species, although the fungal infection
had little impact on host reproduction in the first year
of infection. Effects of flowering phenology on the
probability of infection differed between the host
species, but the intraspecific differences in the timing
of anthesis can significantly affect fruit set, fruit preda-
tion and infection risk in natural plant populations.
The phenological rank required for high fruit set,
