Environmental Engineering Reference
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and species-area relationships depend on trophic rank
(Holt
et al
. 1999). Reductions in size and increased
isolation among habitat fragments of different species
can therefore disrupt trophic interactions. We distin-
guish between ecological and evolutionary genetic
consequences of fragmentation for such interactions.
The former refl ects alterations in distribution, abun-
dance and dynamics of potentially interacting species,
affecting their encounter rate or interaction frequency.
The latter, amongst other factors, refl ects alterations in
the genetic constitution of the potentially interacting
species that affects the outcome of their interactions.
Moreover, ecological and genetic consequences inter-
act, that is, changes in encounter rate can drive life
history evolution of organisms involved. In this section,
we discuss some of the ecological consequences
(section 7.4.1), and then, in section 7.4.2, some of the
genetic and evolutionary consequences of habitat frag-
mentation for species interactions.
parasitoid,
Microplitis tristis
, strongly declines with
plant population size (Figure 7.4a). The same is true for
the parasitoid
Eurylabus tristis
, especially in isolated
patches; in order to have a 50% probability of occur-
rence, a plant population size of 10 appears to be
required for closely connected patches but a population
size of more than 700 is required if the nearest neigh-
bouring patch is two kilometers away (Figure 7.4 b).
Accordingly, parasitism rates of the herbivore are lower
in smaller and more isolated plant patches (Figure
7.4c). Presence of the herbivore is thus not affected at
the current scale of fragmentation of the plant's
habitat, but that of the parasitoids is, corroborating the
idea that higher trophic levels are more sensitive to
fragmentation. Release of herbivores from parasitism
in small isolated patches could result in increased her-
bivory. Indeed, the proportion of predated fruits on
plants is much higher in small than in large popula-
tions (Figure 7.4d), but this may also be due to behav-
ioural responses of the herbivore to host density.
Fragmentation not only alters interactions of plants
with their herbivores and associated parasitoids, but
also with diseases. Most airborne diseases have a
threshold host population size below which they
cannot persist; this threshold is higher for pathogens
with low transmission effi ciency. Both small host-
population size and low connectivity may thus decrease
the probability of host populations to be infected, as
shown in several natural systems (Antonovics
et al
.
1997, and references therein). One of the ecological
consequences of corridors designed to increase
con-
nectivity
of target species is that they may also, inad-
vertedly, facilitate the spread of herbivores and diseases
(Hess 1996). Thus, whereas fragmentation may be
detrimental for pollination service, it may reduce the
contact rate with diseases. Since diseases can severely
impact natural populations (McCallum & Dobson
2002), it should be ascertained whether the benefi ts of
corridors
outweigh their risks.
7.4.1
Ecological consequences
The spatial scale at which population processes operate
differs among species, depending on their trophic rank,
degree of habitat specialization, body size and dispersal
ability, among other things. Consequences of altera-
tions in spatial habitat structure such as fragmentation
therefore differ between species, and consequently
change or disrupt interactions such as pollination,
parasitism or predation, and interspecifi c competition.
Habitat fragmentation is most strongly felt by species if
they have larger body size, lower dispersal capacity, are
habitat specialists and are positioned at higher trophic
levels (Tscharntke & Brandl 2004). The presumed
increase in sensitivity to fragmentation with trophic
rank indicates that fragmentation has important con-
sequences for the outcome of multitrophic interac-
tions. An example is the work of Elzinga
et al
. (2005,
2007). They performed a three-year study in a series
of fragmented populations of the white campion (
Silene
latifolia
) along the river Waal (Netherlands), to assess
the effects of plant population size and isolation on
trophic interactions between the plant, one of its spe-
cialist herbivores, the fruit-predating moth
Hadena
bicruris
, and a range of hymenopteran insects that
parasitize the herbivore (Figure 7.4). These studies
have shown that the herbivore is present in virtually
all plant patches, even the smallest and most isolated
ones. However, the incidence of the most common
7.4.2
Evolutionary genetic consequences
As mentioned,
habitat fragmentation
results in loss
of genetic variation, and increased levels of genetic
drift and inbreeding. The genetic consequences of frag-
mentation for species interactions can be profound. A
classic example of the effect of low levels of genetic
variation and heterozygosity on resistance is the African
cheetah,
Acinonyx jubatus
(O ' Brien & Evermann 1988 ).
