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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 
precautionary principle.

Paul Osmond,
University of New South Wales

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