Environmental Engineering Reference
In-Depth Information
moth has an ephemeral existence of 1 or only a few
years. Its survival is closely linked with the presence
of ragwort. By numerically overshooting the local
carrying capacity, larvae are subject to scramble
competition for food, which frequently leads to mass
starvation and local extinction. Even on the regional
scale, the cinnabar metapopulation seems to be in dan-
ger of extinction during such events. The positive, and
often high, correlations between temporal fluctuations
in local plant populations and the metapopulation of
cinnabar moth indicated that the searching capacity
of the female moth is high and apparently not much
hampered by interpatch plant population distances. In
this respect, the cinnabar moth may be considered in
one, albeit patchy, population and not in a classical
metapopulation. Habitat heterogeneity in local ragwort
populations even adds to the probability of cinnabar
moth survival. The parasitoid
C. popularis
, with its
supposed limited power of dispersal between local
populations, causes a delay in the local recovery of
the cinnabar moth and consequently enables some of
the local ragwort populations to grow for one season
without herbivory.
heterospecific pollen deposition on the stigmas. This
often results in a reduction of seed set and greater
inbreeding in the plant population, which can poten-
tially be counteracted through gene flow between
local populations. The fitness consequences of frag-
mentation depend on the amount of gene flow still
possible between local populations, and within popu-
lations as well.
6.7 Concluding remarks: the interface
between populations and
communities
The theory of island biogeography deals primarily with
communities, whereas the metapopulation approach
focuses on species populations (see section 6.1). Here
I will comment on a few aspects dealing with the inter-
face between populations and communities.
6.7.1 Fractal geometry
A recent development stems from the application of
fractal geometry (spatial scaling laws) as a useful tool
in the quantification of spatial patterns in ecology and
for a causal analysis of species interactions in spa-
tially structured habitats (Ritchie & Olff 1999b). For
the spatial distribution of large herbivores at a global
scale, Olff
et al.
(2002) have shown that all patterns
can arise from simple constraints on how organisms
acquire resources in space. They used spatial scaling
laws to describe how species of different sizes find food
in patches of varying size and resource concentration
and derived a mathematical rule for the minimum
similarity in size of species that share resources. This
packing rule yielded a theory of species diversity that
predicts relations between diversity and productivity
more effectively than previous models. Size and divers-
ity patterns for locally coexisting East African grazing
mammals and North American savanna plants strongly
support their predictions. In India, however, their
predictions about potential herbivore species densities
could no longer be checked owing to the current in-
tensive land use. The theory also predicts relations
between diversity and area and between diversity and
habitat fragmentation, which in a modified form may
also be applicable to the scale of the landscape.
6.6.2 Plants and insect pollinators
Habitat fragmentation for pollinators can be char-
acterized by spatial heterogeneity in the number of
flowers to be visited. Steffan-Dewenter and Tscharntke
(1999) studied experimental patches of
Sinapis
arvensis
and
Raphanus sativus
, visited by a number
of bee species. Number of seeds per fruit and per
plant decreased significantly with increasing distance
from the nearest grassland for both mustard and radish.
The number of seeds set per plant was positively
correlated with the number of flower-visiting bees. In
general, changes in the species composition of a plant
community may have a great impact on pollination
and pollen flow due to the differences in pollination
efficiency and flight distances; see Kwak
et al.
(1998)
for a review. A reduction in local flower population
size of all or several component plant species causes
changes in a decrease in the richness of the assem-
blage of insect pollinators as well, which affects
pollination quantity and pollination quality. Indeed,
insects must visit several plant species to meet their
energy demands, thus increasing the chance of
