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viable population size is for a species (Nunney &
Campbell 1993). It depends on the biology of the species
and on the options that are available regarding size,
number and location of the habitat that can be preserved.
ative prey species are habitat specialists, but a pred-
ator is a habitat generalist, predator colonization can
couple the dynamics of these prey species. This gives
rise to apparent competition or mediation (see Chap-
ter 5). Indeed, most food chains are part of a complex
food web, composed of multiple species on each trophic
level and complex linkage patterns across levels. I will
elaborate with a selection of examples.
6.6 Cascade effects in metapopulations
Habitat fragmentation can be studied at the level of
populations of individual species, but can also be
expected to play a coincidental role for several spe-
cies that co-occur in a particular type of habitat. These
may be communities of coexisting species or any other
type of species assemblage. Tscharntke (1992) provided
qualitative evidence for potential effects of fragmenta-
tion of
Phragmites australis
habitats on a number
of insect and bird species, in view of their depend-
ence on a metapopulation structure. This was shown
to differ for each of the species, but also for the rela-
tionships between the species. Although the concept
of metacommunity (Holt 1997, Hanski 1999) confus-
ingly suggests that all the components of a com-
munity are affected by fragmentation in a similar way,
it does put emphasis on the notion that habitat frag-
mentation affects species groupings as a consequence
of affecting single species. Habitat fragmentation
may change or disrupt interactions such as pollina-
tion, parasitism or predation, and interspecific com-
petition (see Steffan-Dewenter & Tscharntke 2002);
these authors hypothesized that the effects of habitat
fragmentation on species richness and community
interactions would be stronger at higher trophic
levels, and mentioned different
z
values for the slopes
of species-area curves of a number of insects as
related to their specialization on food plants: from 0.07
in polyphagous species to 0.11 in oligophagous, 0.16
in strongly oligophagous and 0.22 in monophagous
species. Theoretical studies on the consequences of
spatial heterogeneity on community structure (Holt
1997) suggest that sparse habitats in a heterogeneous
landscape are likely to sustain a biased array of
species, including habitat specialists with unusually high
colonization or low extinction rates and habitat gen-
eralists sustained via spillover from more abundant
habitats. Trophic specialization would lead to a kind
of magnification of these effects, whereas trophic
generalization would lead to an avenue of direct
interactions among alternative prey species. If altern-
6.6.1 Plants, insect herbivores and
parasites/predators
In a study on patches of
Urtica dioica
, of different
area and degree of isolation, habitat fragmentation was
shown to reduce species richness of phytophagous
Heteroptera, Auchenorrhyncha and Coleoptera (Zabel
& Tscharntke 1998). Monophagous herbivores had a
higher probability of absence from small patches
than all (monophagous and polyphagous) herbivore
species. Species richness of herbivores correlated
positively with habitat area, and species richness of
predators correlated negatively with habitat isolation.
The authors concluded that increasing habitat con-
nectivity in the agricultural landscape should primar-
ily promote predator populations.
Food chains are useful starting points for examin-
ing the implications of metapopulation dynamics
for community structure. Van der Meijden and van
der Veen-van Wijk (1997) analysed whether and to
what extent interactions within a tritrophic system are,
indeed, affected by spatial distribution of habitat
patches. Specifically, they reviewed their long-term
data (two decades) on the relationships between the
plant populations of ragwort (
Senecio jacobaea
), its most
important herbivore, the monophagous cinnabar
moth (
Tyria jacobaeae
), and the specialist parasitoid
of the herbivore,
Cotesia popularis
. Populations of
ragwort showed tremendous fluctuations in biomass
at both local and regional scales, with local fluctu-
ations frequently resulting in extinction. The metapopu-
lation type that fits ragwort best is the source/sink
metapopulation consisting of patches with mostly
negative population growth rate. Herbivory of cinnabar
moth, especially in open habitats, contributes to the
local extinction risk of the plants, but the overall
metapopulation of ragwort does not seem to be in
danger of extinction. On a local scale, the cinnabar
