Environmental Engineering Reference
In-Depth Information
the plant community and the pollinator community
(e.g. Bosch
et al
. 2009), but the type of network differs
from the network of food webs (see section 6.4.1).
When pollinators visit fl owers of several plant species,
this may result in a reduction of seed set for each of the
species. Only a few studies on visitation of pollinators
include consequences for the fi tness of plants in terms
of seed set and reproduction (e.g. Kwak & Bekker 2006;
Morales & Traveset 2009 ).
heterogeneity
, transient non - equilibrium conditions
or more complex food web interactions. Since the
1990s, Tilman and coworkers initiated large-scale
experiments on the coexistence of competing species,
particularly on the relationship between
ecosystem
functioning
and
species diversity
(e.g. Kinzig
et al
. 2001 ;
see also Chapter 2). This type of knowledge helps
understanding the problematic relation between eco-
system productivity and plant species richness, also
relevant in the context of ecological restoration.
6.3.2 Indirect effects on community
structure and succession
Effects of p arasitism
Pathogens and parasites may have a huge impact on
ecosystem structure
and
succession
. In recent
times, fungal blights decimated the American chestnut
(
Castanea dentata
) from the eastern deciduous forests of
the United States, mountain hemlock (
Tsuga mertensi-
ana
) from the Cascadia region of North America,
various elms (
Ulmus
spp.) from much of western
Europe and a whole range of
Eucalyptus
species from
the forests of western Australia (Dobson & Crawley
1994). In each of these cases, the removal of a domi-
nant, late-succession species, as a result of a fungal
infestation, led to the development of forests domi-
nated by less competitive species from earlier succes-
sional stages. Even more spectacular is the classic
example in southern England of the indirect effect on
vegetation change due to the viral infection of rabbits
(
Oryctolagus cuniculus
), also referred to by Dobson and
Crawley (1994). Myxomatosis, caused by the
Myxoma
virus, was accidentally introduced into France in
1952, from where it reached Britain in 1953. In
Silwood Park, the result was a 99% reduction in the
rabbit population within a few years, which resulted in
the transformation of a formerly open, grassy parkland
to an oak woodland dominated by pedunculate or
common oak (
Quercus robur
). This change has to date
proved to be irreversible, even after the recovery of
the rabbit population in the 1970s. Apparently, a
threshold of irreversibility
had been crossed (see
Chapter 2 ).
In many cases, it is not so much the direct interactions,
but rather the indirect effects of resource consump-
tion, parasitism, ecosystem engineering and mutual-
ism that are most infl uential to community structure
and succession, and ecosystem functioning. Here we
illustrate a number of such consequences, in the same
order as before. Thereafter, we refl ect on implications
for restoration ecology.
Effects of c onsumption: c ompetition
for r esources
Theory of competition is too extensive to be treated in
its entirety in this chapter. Readers should consult
basic ecology textbooks, such as Krebs (2008), for
further ideas and information. Here we focus on com-
petition between plants, as this is the kind of informa-
tion most relevant for restoration ecologists.
Competition for shared resources, and its impact on
plant communities, has received much attention from
ecologists since the seminal work on the mechanisms
of resource competition by Tilman (1982, 1988).
Tilman developed a theory based on experiments with
micro-algae competing for limiting nutrients until they
reached a steady state, and applied it to plants compet-
ing for resources, such as nutrients and light. He
argued that, if a plant species is able to reduce the
availability of a particular resource to a lower level
than that required by competing species, it may
increase its chances to become the dominant species of
its kind or functional group in the community. Two
species could coexist if there were two limiting
resources, and so on, but there could never be more
species than there were limiting resources. In reality,
however, many more species can coexist than there are
limiting resources, for example due to environmental
Effects of h abitat m odifi cation
While the study of
competition
for shared resources and
its effects on community structure has a relatively long
history, the interest in the effects of habitat modifi ca-
tion in terms of
facilitation
of other species has only
recently gained momentum (e.g. Bruno
et al
. 2003 ;
