Environmental Engineering Reference
In-Depth Information
Keystone species sometimes play a role in reintroduc-
tion programmes, for example in the case of the
reintroduction of European beaver (
Castor fi ber
) in a
river fl ood plain (see Chapter 8). It is often, however,
diffi cult to know what the true keystone species were
or are, if at all, in cases where degradation and trans-
formation have gone very far, especially in species-poor
communities.
2.5
BIODIVERSITY
Thanks to rising concerns about the loss of species
richness and genetic variation within species due
to human impact, the broad topic of
biodiversity
(biological diversity) has received much attention
worldwide since 1992, following the adoption of the
Convention on Biological Diversity (CBD), the Rio
Declaration on Environment and Development, and
Agenda 21 ( http://www.un.org ). ' Restoring biodiver-
sity', an expression that we will not apply in this topic,
is obviously a central issue in the fi eld of restoration
ecology (see Falk
et al
. 1996, 2006; J. Wright
et al
.
2009). What is of importance here is the
reinforce-
ment
or rescue of populations (Chapter 7) and the
reintroduction
of species (Chapter 8) in specifi c eco-
systems. While species diversity and genetic diversity
- measured as numbers or some mathematical formula
- are neutral concepts, it is the task of restoration
ecologists to value and evaluate both types of diversity
in terms of 'naturalness', rarity, or risks. From a socio-
economic perspective, biodiversity can be considered as
a living, evolving biotic component of the stock of
(renewable, and cultivated) natural capital. From an
ecological point of view, however, if a species goes
extinct, it is not renewable. This implies that not only
the current functional aspects of biodiversity
but also its evolutionary potential must be taken into
account.
In the CBD, biological diversity is described as 'the
variability among living organisms within species,
between species and of ecosystems'. Though this
defi nition has been adopted worldwide, we consider
the inclusion of ecosystem diversity in the defi nition
of biodiversity as confusing, because ecosystems
not only are composed of biota but also contain an
abiotic component. Here, following SER (2004), we
focus on concepts related to restoration of (1)
taxo-
nomic diversity
in biotic communities, among species of
plants, animals and/or microorganisms, and (2)
genetic
diversity
among individuals and populations within
species.
Framework s pecies
An interesting alternative approach, increasingly
applied in projects of ecological restoration in tropical
forest areas (see also Chapter 9), is to introduce a
subset of species - called ' framework species ' - to reori-
ent an ecosystem along a desired or targeted
succes-
sional trajectory
, aiming pragmatically for a balance
between the competitive exclusion of undesired species
(e.g. exotics or invasives) and the facilitation of coloni-
zation of desired species. The so-called
Framework
Species Method
fi rst developed in Queensland, Australia
(Goosem & Tucker 1995), and more recently in nor-
thern Thailand (Elliott
et al
. 2003), appears to be a
promising approach to restoring tropical forests. In
this technique, native 'framework species' (also called
foundation species
) are selected from the reference
plant community on the basis of fi eld trials and func-
tional traits such as fast growth, high survival in
exposed areas and rapid production of a dense canopy
and fl eshy fruit. Their relative position in forest succes-
sion (e.g. early, intermediate or advanced) can also
serve as a selection criterion (Rom á n - Da ñ obeytia
et al
.
2011). The goal is to assemble a group of species that
can rapidly 'capture' or occupy a restoration site and
attract seed-dispersing birds and mammals, and other
dispersers, that will introduce seeds of additional
native plant species, thus catalysing a progression
towards a diverse native forest community. At the
same time, this approach helps shade out unwanted
invasive exotics or rapidly expanding populations of
native colonizers, or it prevents their arrival on the
site. Whether or not the community will be identical
or even close to that of the reference system may take
decades to determine. But, given the rapidity with
which tropical forests grow, there at least one can hope
for signifi cant fi ndings in a reasonably short period,
and these results will in turn be of great interest for
restoration in other, slower developing ecosystems
as well.
2.5.1 Species diversity in biotic
communities
It has been empirically shown in various ecosystems
that species richness often exhibits a positive relation-
ship with ecosystem productivity, with peak species
