Restoration Ecology and the Path to Sustainability - Restoration Ecology: The New Frontier - page 311

since that time by a method entirely independent of
human activity (Cowling et al . 1997 ). In practice,
these terms are often used arbitrarily or subjectively,
as are the terms alien species and invasive species .
Further complexity arises when subspecifi c varia-
tion is considered.
Natural capital An economic metaphor for the
limited stocks of physical and biological natural ele-
ments found on Earth, some of which are of direct
use to society (then called resources) and some of
which are not. According to Rees (1995) and MA
(2005), there are four, partially overlapping types:
renewable (living species and ecosystems), non-
renewable (subsoil assets, e.g. petroleum, coal and
diamonds), replenishable (e.g. the atmosphere,
potable water and fertile soils) and cultivated (e.g.
crops and forest plantations). Natural capital pro-
vides the basis for all life. Following on with the
metaphor, if natural capital is a stock or an asset,
then the 'dividend' is the fl ow in ecosystem goods and
services derived from the assets, for example from
renewable natural capital, called ecosystems and bio-
diversity , and from cultivated natural capital, such as
that found in agroecosystems and other types of pro-
duction systems . But the concept of renewable
natural capital as an asset does include all elements
of ecosystems, not just the obviously marketable bits
and pieces.
Natural goods and services - See ecosystem goods
and services .
Novel ecosystem (synonym: no-analogue ecosystem)
An ecosystem without analogues that has developed
in response to radically altered environmental and
biotic conditions resulting from human activities in
the past century or two (after Hobbs et al . 2006 ); cf.
Seastedt et al . (2008) and Hobbs et al. (2009) . Cf.
emerging ecosystem , designer ecosystem .
Passive restoration A term used by many authors
to indicate the autonomous or autogenic recovery of
a degraded ecosystem by means of the unassisted
processes of resilience , succession or natural regen-
eration. A more precise and evocative term is assisted
natural regeneration (cf. Clewell & McDonald 2009 ).
Potential natural vegetation (PNV) An image or
notion of the vegetation type that would theoreti-
cally arise in an area if all direct human infl uences
were removed. This is not necessarily the 'original'
or ' pre - disturbance ' vegetation, since the biophysical
environment and climate may have been altered.
PNV is useful as an expression of the environmental

site conditions per se at a certain point in time (cf.
T ü xen 1956 ).
Primary forest - See forest.
Production system Land or landscape unit, or sea-
scape unit, allocated to the production of food, fi ber,
pasturage, aquaculture and other marketable com-
modities or subsistence items, and which is usually
maintained with external inputs of energy (e.g. fossil
fuels) and materials (e.g. lime and agrichemicals).
Notable exceptions are agroforestry systems found
in some woodlands of the Mediterranean region, and
in some parts of Indonesia and other tropical forest
regions.
Propagule Seed, fruit, egg or detachable part of a
plant that serves as the source of new individuals.
Provenance The geographic place of origin of
plants or animals, or source of seeds or other prop-
agules , used for experiments or for reintroductions .
Reclamation Conversion of portions of land,
wetland or shallow seas, which are perceived as being
unutilizable, to a productive condition, commonly
for agriculture, aquaculture or silviculture. Is some-
times erroneously used as a synonym for rehabilita-
tion , particularly with respect to mine sites (NRC
1974 ; Whisenant 1999 ).
Reference ecosystem (or reference model) One or
more natural or seminatural ecosystems , ecological
descriptions thereof, or, if these are unavailable,
assemblages of characteristics of presumed natural
or historic seminatural ecosystems which are chosen
to serve as models, benchmarks or targets for plan-
ning in ecological restoration and rehabilitation pro-
jects (White & Walker 1997; Egan & Howell 2001;
SER 2004 ). .
Reforestation The re - establishment of forest on
recently forested or wooded land that was recently
cleared, using native or non - native species by means
of artifi cial, natural or assisted natural regeneration
techniques. See also afforestation and revegetation .
Rehabilitation The improvement of functionality
of an ecosystem, especially a seminatural one,
without necessarily achieving a return to 'pre-
disturbance' conditions. Commonly undertaken in
cultural landscape settings. Emphasis is generally on
restoring ecosystem processes and functions so as to
increase the fl ow of services and benefi ts to people
(SER 2004; Clewell & Aronson 2007). Care must be
paid not to so heavily favour one process or function
with the result of rendering the ecosystem more
fragile or vulnerable than it was before. However,