Environmental Engineering Reference
In-Depth Information
planning for nature conservation (Schröder & Richter
2000).
The blue-winged grasshopper (
Oedipoda caerul-
escens
) has been used as an example to demonstrate
how highly resolved data on the habitat factors can
be provided for an entire landscape (Kuhn & Kleyer
2000). The first step was to develop a regionally valid
statistical habitat model, in terms of probability of
occurrence, which quantified the habitat quality of
all single areas in a landscape and predicted potential
habitats. This habitat model, derived from presence/
absence data and habitat factors (temperature, vege-
tation cover and structure, all mapped on a landscape
scale), produced the potential habitat map of the
study area. The relationship between the presence
of the species and the spatial configuration of the
habitats was then presented in a second model con-
sidering those habitats exclusively. In this second
model, presence/absence data were recorded from the
potential habitats and their isolation and size were
computed as independent variables. The authors used
iterative statistical habitat models to predict the
incidence of
O. caerulescens
.
Strykstra
et al.
(2002) developed a framework for
the relative importance of dispersal, seed-bank
longevity and life-span spectra as related to an estim-
ated reliability of safe-site dynamics in a number of
plant communities. Their analysis brought them to con-
clude that (i) a higher percentage of species with a
specialization in persistent seed-bank formation cor-
responds with a lower reliability in time, (ii) a higher
percentage of species with a specialization in long-
range dispersal corresponds with a lower reliability in
space and (iii) a higher percentage of species with a
long life span corresponds with a lower level of site
disturbance. This kind of analysis may result in estim-
ates of the potential establishment of particular types
of plant communities, rather than plant species. For
individual plant species it is often quite difficult or
even impossible to define suitable patches
a priori
(Freckleton & Watkinson 2002). Even if seed banks
(dispersal in time) are present and would be taken into
account, in addition to dispersal in space, it is not easy
to determine whether a site that is occupied by
living seeds is suitable for recolonization by seedlings
and survival of adults. The site may dramatically
change in the meantime, and established plants are
not capable of moving.
6.4 Connectivity between habitat
patches
A central issue in the analysis of metapopulations is
the frequency of migration, or demographic con-
nectivity, among component populations (see also
Chapter 3 in this volume). Habitat patches are parts
of a landscape mosaic, and the presence of a given
species in a patch may be a function not only of patch
size and isolation, but also of the kind of neighbour-
ing habitat (Andrén 1994) and of the species com-
position in the patches (Kwak
et al
. 1998). I refer to
three definitions of what is called landscape connect-
ivity as a feature of a landscape:
1
the degree to which the landscape facilitates or
impedes movement among resource patches (Taylor
et al
. 1993);
2
the functional linkage among habitat patches, either
because habitat is physically adjacent or because the
dispersal abilities of the organisms effectively con-
nect patches across the landscape (With
et al
. 1997);
3
a property of locations to maintain spatial or func-
tional relationships with other locations in terms of
flows of entities such as materials, energy, informa-
tion, people and animals (van Langevelde
et al
.
1998).
Based on simulated dispersal across heterogeneous
landscapes, Tischendorf and Fahrig (2000) compared
the responses of three connectivity measures -
dispersal success, search time and cell immigration -
to habitat fragmentation. From their analysis they
concluded: (i) two common measures of landscape
connectivity, dispersal success and search time, both
averaged over all patches in the landscape, indicate
higher connectivity in more-fragmented landscapes;
(ii) landscape connectivity measured as immigration
into all habitat cells in the landscape predicts higher
connectivity in less-fragmented landscapes and (iii) the
three connectivity measurements respond differently
to landscape structure and dispersal characteristics.
Consistent measurement of landscape connectivity is
crucial to ease comparisons across different studies.
Dispersal traits of species are crucial to cope with
habitat fragmentation. Krebs (2001) adopted the three
modes of dispersal as proposed by Pielou (1979):
