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low fruit predation and low infection risk differed
between host species, but generally not within host
species. In contrast to the expectation for pollinators
that are also seed predators and vectors of disease, the
authors found that the effects of timing of anthesis
via fruit set on the one hand and via predation and
infection on the other were generally not in oppos-
ing fitness directions within host species; they were
either uncorrelated or affecting reproductive success
in the same direction, allowing for simultaneous
maximization of fruit set and minimization of
damage. This example illustrates the complexity of
the biotic interactions that modulate the effects of
a simple host trait, timing of anthesis, on a host's repro-
ductive success. Thrall
et al.
(1995) were able to
model the disease transmission in the
Silene-Ustilago
host-pathogen system as an epidemiological process.
Plants have evolved an overwhelming diversity of
mechanisms that can act as direct defences against
herbivores (e.g. van der Meijden & Klinkhamer 2000).
Because of much genetic variation in both concentra-
tions and composition of plant chemical defences,
specialist and generalist herbivores and their natural
enemies could be differentially affected, which may
change their effect on plant fitness. In addition,
many studies have demonstrated indirect defence
mechanisms, and here tritrophic interactions come into
play. A wealth of rather recent literature is available
on plants benefiting from the active chemical attrac-
tion of natural enemies of their herbivores (e.g. Dicke
& Vet 1999, Vos
et al.
2001). An example is the para-
sitoid wasp
Cotesia glomerata
which infests caterpil-
lars of
Pieris brassicae
, both herbivore and parasitoid
being chemically attracted by volatiles released from
the leaves of
Brassica oleracea
. Such an indirect
defence by the plants can be considered a com-
bination of bottom-up and top-down effects. It can
involve the provision of shelter or of alternative food,
as floral and extrafloral nectar, or of information that
can be used by carnivores during foraging. Chemical
information on herbivore presence and identity may
indeed be essential for successful location of herbi-
vores by carnivores. Both the production of volatiles
and the responses of arthropods to these volatiles
are subject to variation, and natural selection acts on
both sides of the interaction. Superimposed on this
part of the food web, there is apparently an informa-
tion web, which may even be more complex than the
food web itself, because information conveyance occurs
irrespective of trophic relationships. Vos
et al.
(2001)
examined the effects of herbivore diversity on para-
sitoid community persistence and stability, mediated
by non-specific information from herbivore-infested
plants in a
Brassica-Pieris-Cotesia
system. Parasitoids
were attracted by infochemicals from leaves contain-
ing non-host herbivores. Thus, when information
from the plant is indistinct, herbivore diversity is likely
to weaken interaction strengths between parasitoids
and hosts. In general, more than one herbivore
will feed on a plant, and a single herbivore species is
generally attacked by several carnivore species. The
authors therefore modelled parasitoid-herbivore sys-
tems increasing in complexity, referring to experiments
and field data. A simulated increase in herbivore
diversity promoted the persistence of parasitoid com-
munities. However, at a higher threshold of herbivore
diversity, parasitoids became extinct due to insuffici-
ent parasitism rates. Thus, they concluded, diversity
can potentially drive both persistence and extinctions.
4.3.4 Trophic cascades
Paine (1980) was the first to use the term 'trophic
cascade'. Pace
et al.
(1999) suggested that empirical
studies from a variety of systems indicate that trophic
cascades are widespread, although many factors regu-
late their occurrence. They described trophic cascades
as 'strong interactions within food webs that influ-
ence the properties of the system', thereby including
a much wider spectrum of interactions, and suggested
that cascades are no longer the sole province of lake
and intertidal ecologists but clearly occur in a divers-
ity of ecosystems on land and in the ocean. Polis
et al.
(2000) questioned this generalization and,
referring to the review of 41 studies by Smith
et al.
(2000), suggested distinguishing between species-
level and community-level cascades. Starting from the
plant as a primary producer, species-level cascades
occur within a subset of the community or compart-
ment of a food web, such that changes in predator
numbers affect the success of a subset (one or a few)
of the plant species, whereas community-level cascades
substantially alter the distribution of plant biomass
throughout an entire system, in a manner consist-
ent with the EEH. Although these definitions refer
