Environmental Engineering Reference
In-Depth Information
numerically identified as belonging to a natural or
semi-natural community, which provides them with a
value of naturalness. This is how restoration eco-
logists are capable of distinguishing between native
species, which are worthy of being reintroduced if they
have become extinct, and alien invaders.
b
5.2.4 Abundance, commonness and rarity
a
Within a community, in general a few species are
abundant or dominant and a larger number is rare or
subordinate. This is reflected in so-called dominance-
diversity curves (see Whittaker 1965, 1975), and in the
distinction between dominants, subordinates and
transients (Grime 2001). Similarly, at a regional scale
some species are common and many species are rare.
This is reflected by Preston's (1962) classic lognormal
distribution. Hubbell (2001) provided a nice historical
overview of these two aspects of commonness and
rarity, at the community scale and at a geographic
scale.
Some two decades ago, seven forms of rarity were
identified by Rabinowitz (1981), according to combina-
tions of the geographic range, the habitat specificity
and the local population size of a species (Table 5.1).
According to Gaston (1997) - who proposed to leave
out habitat breadth as a parameter - broad consensus
favours a definition of rarity based on abundance
and/or range size, with species of low abundance or
small range size being regarded as rare. Here range
size is treated as a measure of the area of the spatial
distribution of a species, with geographical range size
being that measure for the full global breadth of
the occurrence of species. Virtually all definitions of
rarity explicitly mention at least one parameter based
on abundance or range size. Gaston (1996b) argued
that species-range-size distributions have received
remarkably little attention, in contrast to species-
abundance distributions. Assuming the lognormal
species-range-size distribution to be real, it can be
viewed in terms of evolutionary and ecological deter-
minants of species occurrences, although their relative
significance remains unclear. The general, positive,
interspecific relationship between local abundance
and range size has been explained in terms of sim-
ilarity of the ecological characteristics that permit
species to become locally more abundant and
Ecosystem process (productivity)
Fig. 5.1
Species richness as a function of ecosystem
productivity: (a) spatial gradient model according to
Grime (1973, 1979) and Tilman & Pacala (1993);
(b) spatial and succession model after Connell (1978).
After van Andel (1998a).
The productivity-diversity pattern is scale-dependent.
In lakes, the relationship is hump-shaped at the local
scale such as within ponds, for example, and linear
at the regional scale, between ponds (see Chase &
Laibold 2002). This dependence on scale results because
dissimilarity in local species composition within
regions increases with productivity. By using this
knowledge, a relatively high species richness could
be achieved in local communities through arranging
an intermediate primary productivity, associated with
intermediate environmental stress and intermediate dis-
turbance, for example by removal of produced plant
biomass, even if knowledge of the mechanisms
behind the relationship is still largely speculative.
Species richness as a neutral concept is interesting
from a scientific point of view, for example in search
of an explanation for the relationship between species
diversity and ecosystem stability (see Chapter 4), but
it is insufficient as a criterion for nature-management
purposes. Indeed, it is not just a high number of species
that is highly valued, but particularly the values of
each of the component species that count. Rare or sub-
ordinate species are often more highly valued as a tool
for nature conservation than common or abundant
species, and aggressive alien invaders are not wanted.
This is why target communities are not only defined
as a species list, but as a syntaxon, a community of
species known to be co-occurring frequently, and
