Environmental Engineering Reference
In-Depth Information
dispersal. Rapid dispersal has been associated with traits
such as short generation time, long fruiting period,
large seed number, small seed size, prolonged seed
viability and transport by wind or animals. Rejmánek
(1999) mentioned, in addition, phenotypic plasticity,
small genome size and vegetative reproduction (in
aquatic habitats).
None of the afore-mentioned characteristics imply,
a priori
, aggressiveness. Nevertheless, it should be kept
in mind that invasiveness depends not only on the
characteristics of the invading species, but also on the
interactions with the abiotic and biotic factors in
the new habitat. In this respect changing environmen-
tal conditions are of importance. Dukes and Mooney
(1999) suggested that, although climate change might
not directly favour alien plant species over natives,
many invasive plant species share traits, such as
rapid dispersal, short regeneration times and higher
tolerances, that could increase their dominance in a
climate undergoing transition. Sometimes, a species
that has been a native member of plant communities
can start behaving as an aggressive invader. For
example,
Phragmites australis
has been a member
of tidal plant communities in New England, USA, for
thousands of years, and an extensive increase in
its occurrence has been noted only during the last
100 years or so (Orson 1999). Currently, the species
can form dense monocultures with less diverse plant
associations, probably due to large-scale changes in
the hydrology.
In spite of the reality that invasions cannot be pre-
dicted quantitatively because parameter estimation is
technically close to impossible for conditions still
alien to a species (Hengeveld 1999), progress is being
made in predicting the fate of invaders. Studies by
Kolar and Lodge (2001), using quantitative statistics
to identify characteristics that distinguish groups of
species more or less likely to become established or
invasive, revealed in general clear relationships be-
tween the characteristics of releases and the species
involved, and the successful establishment and spread
of invaders. For example, the probability of bird
establishment increased with the number of indi-
viduals released and the number of release events.
Also, the probability of plant invasiveness increased
if the species had a history of invasion and reproduced
vegetatively. Davis
et al.
(2000) formulated a number
of testable predictions for plant invasibility, in brief:
• environments subject to pronounced fluctuation in
resource supply will be more susceptible to inva-
sions than environments with more stable resource
supply rates;
• environments will be more susceptible to invasion
during the period immediately following an abrupt
increase in the rate of supply or a decline in the
rate of uptake of a limiting resource;
• invasibility will increase following disturbances,
disease and pest outbreaks that increase resource
availability;
• invasibility will increase when there is a long
interval between an increase in the supply of
resources and the eventual capture or recapture of
the resources by the resident vegetation;
• the susceptibility of a community to invasion will
increase following the introduction of grazers into
a (nutrient-rich) community;
• there will not necessarily be a relationship between
the species diversity of a plant community and its
susceptibility to invasion;
• there will be no general relationship between the
average productivity of a plant community and its
susceptibility to invasion.
5.2.3 Species richness
According to MacArthur and Wilson's (1967)
dispersal-assembly hypothesis, species richness on
islands results from an equilibrium between immigra-
tion and extinction rates of the species concerned. In
view of this, species richness is a neutral theoretical
concept which implies that species number can be
quantified for theoretical purposes without recognizing
differentiation among them (see Hubbell 2001).
Species-area curves represent the same phenomenon
(Rosenzweig 1995). For nature conservation and restora-
tion purposes, species richness has been valued, often
with the short-cut notion that high species richness is
a goal of the management regime. This was triggered
by the observation that the application of artificial fer-
tilizers since the early 20th century had contributed
to a large extent to the decline in species. It was empir-
ically shown that species richness in general exhibits
a hump-shaped relationship with ecosystem pro-
ductivity (see Fig. 5.1; Grime 1973, Connell 1978,
Rosenzweig & Abramsky 1993, Tilman & Pacala 1993).
