Environmental Engineering Reference
In-Depth Information
disturbance factors such as
acidifi cation
or
eutroph-
ication
of a soil or a water body, or
climate change
in the atmosphere. Note that an ecosystem under stress
may reveal to be resilient, resistant, or unstable (see
Section 2.4.2 ).
Landscape integrity
can be indicated
the way McIntyre and Hobbs (1999) describe
intact
landscapes
as a
reference system
(see also Chapter 5).
The degree of human intervention associated with
intact landscapes can be extremely low, particularly in
reserves managed for conservation, such as in Antarc-
tica, but they can also be intensively managed, such as
traditional agricultural landscapes in Europe. Distur-
bance factors may result in the fragmentation of habi-
tats of particular species, among other disturbances at
the landscape scale.
going into the details of this debate, we prefer to illus-
trate the unifying concepts of
functional groups
,
keystone species
and
framework species
, which
- rather than species richness - play a signifi cant role
to explain or restore ecosystem functioning.
Functional g roups
Various ecological classifi cations have been assessed to
escape from dealing with individual species lists and to
focus on ecological species groupings, for example life
forms, strategies, adaptive syndromes and guilds. Cur-
rently, the term 'functional groups' is being used to
indicate a grouping of individually known species in
one particular class of functions (e.g. all the nitrogen-
fi xing plants in a community). Functional groups have
been identifi ed by multivariate techniques (often even
without an indication of which type of function is
associated with the species group), and by deductive
methods that are based on the
a priori
statement of the
importance of particular processes or properties in the
functioning of an ecosystem (e.g. C
3
or C
4
grasses, and
N
2
- fi xing Fabaceae, also known as Leguminosae). In
restoration projects that make use of functional groups,
it is often assumed that the effects of increasing species
richness on ecosystem productivity work through
changes in functional diversity. Indeed, ecosystem
functioning in general is probably more related to the
number of, and interactions among, the functional
groups present at a site than to the overall species
number (J. Wright
et al
. 2009 ).
2.4.4
Ecosystem functioning
Before we move on to introduce the concept of 'biodi-
versity', we must touch upon the relationship between
ecosystem stability and the species richness of a biotic
community (see e.g. McCann 2000). Ecosystem stabil-
ity is often measured in terms of
ecological func-
tions
, or functioning or functionality, which includes
rate of primary production, rate of decomposition or
rate of nutrient cycling. Since the early twentieth
century, the application of artifi cial fertilizers to spe-
cies-rich seminatural grasslands, in order to increase
ecosystem productivity, has caused a large decline in
species richness. This in turn has triggered a strong
and sustained investment in research on how produc-
tivity and species richness are related. Among efforts
to explain the relationship between
species richness
and
ecosystem productivity
, we can distinguish two
approaches, differing in perspectives on what is the
cause and what is the effect: (1) 'How does species rich-
ness depend on ecosystem productivity?', a question
that is inspired by a primary interest in determinants
of biodiversity (see Section 2.5.1), and (2) 'How does
ecosystem productivity depend on species richness?'
The latter question is at stake here, as it relates to the
role of
biodiversity
as contributing to
ecosystem
functioning
, for example productivity (see e.g. Naeem
et al
. 2002). Some observations are in support of the
rivet hypothesis
(the majority of species essentially con-
tributing to ecosystem productivity), whereas others
favour the
redundant - species hypothesis
(only a few key-
stone species contribute to the productivity of the eco-
system). For the purpose of this chapter, rather than
Keystone s pecies
Since the seminal review of Paine (1980), biotic inter-
actions have been considered as the main underlying
mechanism explaining the relationship between
species diversity and ecosystem stability. Removal of a
weakly interacting - that is, functionally insignifi cant
- species would yield no or slight change, and removal
of keystone species may have a cascade of effects on
the community composition, transmitted by a chain of
strongly interacting links. A keystone species is a
member of a food web that has a disproportionally
large effect on community structure. A key function
can be due to high abundance of a species in a food
web (e.g. a prey species) relative to other species, or
result from having a large impact relative to the abun-
dance of the species itself (e.g. a top predator).
