Wed May 4 18:08:57 EDT 2011
the lines that David Holmgren has developed probably help to increase
biodiversity, in the context of the more or less degraded and altered former
agricultural landscape within which he is working. On the other hand,
planting of an invasive though useful species on land abutting an area of
relatively intact native vegetation is likely to reduce biodiversity by
out-competing the native flora and causing local extinctions, i.e. it
In all of this, observation is the key. Does the species in question have
hard seeds or burrs which can be transported long distances by birds or
small mammals? What is its growth rate compared to local native plants (or
indeed other exotics) given the same soil and microclimatic conditions? Are
there native predators/diseases which will keep it in check? Will it create
the conditions for its own replacement by other species (successional
processes) in the short to medium term or will it create a monoculture in
the longer term?
An issue which is crucial both in restoration ecology and permaculture, is
diversity at the genetic level. Genetic diversity, whether of agriculturally
useful plants or those which are simply "there" as components of a natural
ecosystem, is the powerhouse of evolution and provides the reservoir of
resilience necessary to enable recovery from human-induced climate change.
So as well as assessing the overall impact of introducing new exotic species
to an area, the specific impact on what may be a unique local variety of an
otherwise widespread native species must be considered. In urban/urban
fringe areas in particular, populations of remnant local natives which have
persisted in the face of overstorey clearing, soil disturbance, changes to
the water table, pollution etc. are likely to have the resilience
necessary to cope with new factors such as global warming.
In summary, the best approach to natives/exotics is not to see it as
either/or, but to apply that good sustainability rule of thumb, the
University of New South Wales
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