Environmental Engineering Reference
In-Depth Information
to routine inspection. This is not necessarily prob-
lematical, as some attributes (e.g. landscape structure)
may change slowly, unless catastrophically altered, in
which case they become the primary focus of atten-
tion in a restoration programme: that is, putting the
original contours back.
1.3.2 Ecosystem description
Before we can decide a target, we need to know, and
be explicit about, what it is we are aiming for. We
need a clear description of essential attributes of our
target ecosystem. Hobbs and Norton (1996) provided
a set of such attributes to be considered for measure-
ment and manipulation in restoration programmes.
1.3.3 Targets in restoration ecology
1 composition
: species present and their relative
abundances;
2 structure
: vertical arrangement of vegetation and
soil components (living and dead);
3 pattern
: horizontal arrangement of system
components;
4 heterogeneity
: a complex variable made of com-
ponents 1-3;
5 function
: performance of basic ecosystem pro-
cesses (energy, water, nutrient transfers);
6 dynamics and resilience
: successional processes, rate
and amplitude of recovery from disturbance.
Ehrenfeld (2000) suggests three simple types of goal
that we might consider for designing restoration
prescriptions: species, ecosystem functions and eco-
system services. In many ways these are a re-phrasing
of Hobbs and Norton's schema. These all have advant-
ages and disadvantages, many outlined in Ehrenfeld
(2000) and earlier by Ehrenfeld and Toth (1997).
Species
When we take a species-level approach to designing
a restoration scheme we may use a number of targets:
These categories are conceptually clear, but not all of
them are amenable to direct manipulation: essentially
attributes 1- 4 may be, to a greater or lesser degree,
directly manipulated by such processes as civil engin-
eering of soil contours, tree planting, seed sowing, alter-
ing hydrological regime and species re-introduction.
Attributes 5 and 6 are a consequence of successful
restoration, and therefore may be considered as prime
indicators of progress towards a target. We must also
note, however, that some recovering ecosystems will
not be able to attract or sustain key indicator species
(attribute 1 above) without some recovery in all six
attributes: for example, there is no point in re-
introducing predator species without there being prey
present and abundant in the first place, which is depend-
ent upon sufficient, robust, primary productivity.
In a similar vein, Vital Ecosystem Attributes (VEAs)
and Vital Landscape Attributes (VLAs) have been
suggested by Aronson
et al
. (1993a, b) and Aronson
and Le Floc'h (1996a) respectively. These are intended
as an aid to quantitative evaluation of whole ecosys-
tem structure, composition and functional complexity
over time. This entire coherent set of parameters is
readily comprehensible, but not all may be amenable
• keystone species: related to particular functions;
• endangered species: often obscure as to wider
function, but may be charismatic;
• assemblages: most likely to be summative as to total
ecosystem status.
This approach may rescue particular species from
extinction, locally or globally, and could definitely
be said to increase biodiversity, but there are some
potentially serious pitfalls. Landscape and ecosystem-
level interactions may be ignored, with potentially
adverse consequences in the long term, inadvertent
pressure may be put on other species, and schemes
overly focused on one species or group (e.g. birds) may
lead to loss of habitat for other species, resulting in
their decline.
Ecosystem functions
This level of analysis offers the potential to recognize
explicitly the need for different components of eco-
systems to be connected and working effectively. The
types of processes and pools which may be examined
are:
