Environmental Engineering Reference
In-Depth Information
the composition of the organic matter may determine
the chances of germination of early colonizers.
flow, but the degree of dependence varies; some spe-
cies can be grown to flower and set seed without a
host, whereas others do not even germinate without
a host stimulus; root exudates of the host stimulate
germination. The effect on the host is variable, too;
it can be dramatic or hardly measurable and difficult
to detect in other cases. Strict host specificity does not
seem to exist.
The role of interplant parasitism in determining plant
community structure is poorly understood. Pennings
and Callaway (1996) investigated the impact of
Cuscuta
salina
, a common and widespread obligate parasitic
annual in saline locations on the west coast of North
America. Their results suggested that the parasite is an
important agent affecting the dynamics and diversity
of vegetation. Because
C. salina
prefers to parasitize
the marsh-dominant
Salicornia virginica
, it indirectly
facilitates the rare species
Limonium californicum
and
Frankenia salina
, thus increasing plant diversity,
and possibly initiating plant vegetation cycles. For
hemiparasites such as species of
Rhinanthus
,
Odontites
,
Euphrasia
and
Melampyrum
, it is clear that the para-
sites depend on host vegetation to some extent, but
in which way do they affect the vegetation? Is the
vegetation open because of the presence of the para-
site, or is the parasite present because the vegetation
is rather low and open? The data on the role of
vegetation structure show that the latter certainly has
some effect (ter Borg 1985), which may be negative,
neutral or positive (Pennings & Callaway 2002).
5.3.4 Parasitism
Calow (1998) defined parasitism as 'an intimate and
usually obligate relationship between two organisms
in which, essentially, one organism (the parasite) is
exploiting resources from the other organism (the
host) to the latter's disadvantage'. It is, therefore, an
interaction between individuals of different species,
brought about by a consumer-resource relationship,
leading to a gain in fitness of the consumer species
and a decrease in fitness of the resource species. Here
I focus on parasitism at the level of plant communit-
ies; parasitism in biotic communities has been dealt
with in Chapter 4.
Parasitism between plants is a widespread phe-
nomenon (Kuijt 1969), with over 3000 species of
parasitic plants occurring worldwide. Parasitism in
the plant kingdom does occur among trees, shrubs,
long-lived perennials and annuals, and all plant
parasites are dicots in only a few lineages. Parasitic
plant species that are thought of as weeds belong to
the families Cuscutaceae, Loranthaceae, Viscaceae,
Lauraceae, Orobanchaceae and Scrophulariaceae.
San-
talum album
(sandalwood) is one of the well-known
woody parasitic species. A parasite depends on a host
for its fitness, whereas the host can live without the
relationship; it only suffers from the parasite if it is
present. It is not in the interest of the parasite to kill
its host, but it can occur, for example in the case of
Cuscuta
species (Devil's guts). Among plants, holo-
parasites, such as species of
Orobanche
(broomrapes)
and several orchid species exploit both root and
shoot products from the host, while hemiparasites, such
as species of
Rhinanthus
(hay rattles) and
Striga
(witchweed) exploit the root products only and are
capable of photosynthesis themselves. Holoparasites do
not contain chlorophyll and are heterotrophic; they
not only depend on the host for water and minerals,
but also require its organic compounds. All holo- and
hemiparasites are connected with roots or shoots of
host plants by means of a haustorium. Water, min-
erals and a wide variety of organic substances are
transported through this organ. It is always a one-way
5.3.5 Facilitation
Facilitation can be defined as an interaction between
individuals of different species, where one species
changes the environment in such a way that it is
beneficial to the other, either in space or in time. It
does not seem useful to distinguish between direct and
indirect facilitation, as in the case of competition,
because the effects are always indirect, via an impact
on the environment. Plants transform the physical or
chemical soil conditions, for example by aerating a
wet, anoxic soil, increasing the moisture content of a
dry soil or by soil formation due to nitrogen-fixing
plants, or act as a shelter against harsh above-ground
conditions. Large herbivores can pave the way for
