Concepts in restoration ecology - Restoration Ecology - page 26

not very difficult to prove. In most cases, however,
indicator species are used to point at a complex set
of environmental conditions, either at the ecosystem
scale (soil pH, eutrophic conditions) or extrapolated
to indicate regional-scale processes (seepage water,
atmospheric deposition). As indicator values are
always based on field experience and often difficult
to test quantitatively, it should be recognized that
the values are of a qualitative nature; they can be used
in a comparative way, and only within a certain
well-defined region (e.g., Ellenberg et al . 1991,
Wheeler & Shaw 1995a, Kotowski et al . 1998).
Indicator species may indicate different sets of envir-
onmental conditions even at different sites within a
region; several examples were elaborated by Everts and
de Vries (1991) for plant species along the courses of
brook valleys.
For aquatic systems, Cairns and McCormick (1992)
and Cairns et al . (1993) recognized different types
of indicator for applied purposes, once the goals have
been established: (i) early-warning indicators, sig-
nalling impending deleterious changes in environmental
conditions before unacceptable conditions actually
occur, (ii) compliance indicators, assessing the degree
to which previously stated environmental conditions
are maintained and deviations from acceptable limits,
(iii) diagnostic indicators, determining causes of devi-
ations outside the limits of unacceptable conditions,
and (iv) biogeochemical and socio-economic indic-
ators, which are mutually affected by environmental
degradation and restoration as well.
With regard to restoration of damaged soil eco-
systems, Bentham et al . (1992) have pointed out the
possibility of using microbiological indices to measure
damage and restoration, far superior to conventional
measurements. They used the following character-
istics: (i) size, or the amount of biomass in the sys-
tem, (ii) activity, or the rate of turnover of materials
within the system and export/import of nutrients,
and (iii) the degree of biodiversity within the system.
These parameters are thought to indicate successional
changes in the quality and quantity of organic mat-
erial available to the soil microbial community (see
also Harris et al . 1996).
Whereas keystone species are considered to be
essential for proper ecosystem functioning, indicator
species have a role in human quality control of the
environmental conditions. They can be used in the pro-

cess of piloting a system along a target trajectory that
often is defined in terms of restoring species richness.

2.4.2 Biodiversity at the species level

In Europe and elsewhere many plant and animal
species of concern to conservation and restoration
ecologists have to bridge larger and larger distances
between local populations in an increasingly frag-
mented landscape (see also Chapters 2 and 6).
Dispersability properties that have evolved and been
selected for in earlier times may not be sufficiently
adapted to present requirements in the human-
dominated landscapes. Strykstra et al . (2002) suggested
that species may have adapted their dispersal capab-
ilities to formerly existing environmental conditions
and safe-site availability in communities, incompat-
ible with today's situation of generating new habitats
for restoration purposes. Re-introductions are meant,
and sometimes executed, to repair an ecosystem by
restoring species that - according to ecologists -
were lost but could still play a role in the ecosystem
involved. The aim is to restore a degenerated popula-
tion and to improve ecosystem quality (see Chapter 7).

Re-introduction as compared to invasion

Intended re-introduction should be distinguished
clearly from invasions of alien species. Biotic invaders
are species that establish a new range in which they
proliferate, spread and persist (Elton 1958), in many
cases to the detriment of the environment (Mack et al .
2000), whereas a re-introduced species is supposed
to have disappeared from a community that it still
belongs to. So, we are not at all dealing with the
problem of alien species or even unwanted invaders,
but we may have to cope with alien populations. A
problem that should also be discussed here is whether
re-introduction of native species in their natural
environment (from which they had been lost)
includes the risk of introducing species with charac-
teristics of invasive (alien) species. Should risky
effects, such as those observed from the introduction,
for economic purposes, of Nile perch into Lake
Victoria (Oguto-Ohwayo 1999), be taken into account
when ecological re-introduction of species is consid-
ered? Should population differentiation (intraspecific