Native vs. non-native (was Re: all theory thread DESIGN PRINCIPLES).

Toby Hemenway hemenway at jeffnet.org
Wed Aug 30 12:10:56 EDT 2000


on 8/29/00 6:59 PM, mangodance at bmn at iglou.com wrote:

> Exotic invaders taking over systems due to poor human
> decision-making do not equilibrate with natives.

We're in agreement that invasive plants plus stupid management, or
over-exploitation, is a recipe for disaster. Intelligent management can
reduce invasives. I've seen Scotch broom and Himalayan blackberry, two
rampant invasives here in the NW, reduced to insignificance in just a few
years once people stop clearing for pastures. That allows a canopy to form,
shading out the invaders. I could list many other examples, but that's what
I mean by equilibrating: the invasives become restricted to small patches,
even if they don't disappear.
> 
>> Of course, introducing a plant known to be invasive is foolish unless done
>> in the right context.
> 
> Just what is the "right context"?

For example, Russian olive is invasive in many situations. Here,
conservation people hate it, even though the elements that cause its
invasiveness in other areas seem to be missing, and it is a useful pioneer,
soil builder, and nurse plant. Or, yellow flag iris is invasive when planted
in open or flowing water, but I know people who use it in garden ponds, and
I suspect it is not a problem in that context (the root fragments can't
spread). That's what I mean.
 
>> [some plants are] branded as invasive because they interfere
>>with humans' use of
>> an area, by displacing cattle forage or farmland.
> 
> Can you support this?

You bet. Here's a quote from "Impacts of Introduced Species in the United
States" by Daniel Simberloff, a fairly famous biologist writing in
Consequences journal, from a website whose URL I've lost (I have the file,
though): "Best documented are [exotics'] costs to agriculture: about a
fourth of this country's agricultural gross national product is lost each
year to foreign plant pests and the costs of controlling them." He mentions
that "Leafy spurge, an unpalatable European plant that has invaded western
rangelands, caused losses of $110 million in 1990 alone." The literature is
full of statements like this, with no mention of inappropriate land use as
the real cause.  

> The problem with Kudzu is not that it restores cleared land to
> forest.  It is quite the opposite.  It can handily exploit such
> cleared areas but eventually get into forests and turn them into
> graveyards. 

My observation of kudzu in Georgia and South Carolina was that it uses road
margins, abandoned fields, and other deliberate clearings to enter small and
fragmented forest patches that have plenty of human-made edge and thus are
lighted deep into the interior. It seemed not to do well in larger, darker
forests. Again, bad land management coupled with an opportunistic species is
a disaster. I'd ask why it hasn't taken over Japan.
> 
>>  most relatively undisturbed native ecosystems are
>> under no threat from exotics
> 
> This is not supported by fact. [snip] Hawaii has massive exotic problems in
>its natural areas.  The southeast also has severe problems.

Most of the southeast is 2nd (or later) growth forest, farm, or drained
wetland; little is undisturbed, hence its susceptibility. You have a point
with Hawaii, although a lot of the trouble there is with animals, less so
with plants. And islands have species in proportion to the logarithm of
their land area (an exponential drop as islands get smaller), and have no
way to replenish local stocks, so they are far more sensitive to disturbance
than continents. Also, since much of Hawaii was cleared for farming, many
exotics got a good hold there for launching into the forest margins. Again,
short-sighted land use. I stand by my statement, with its restrictions. In
old-growth I only see invasives at road cuts. They need sun.
> 
>[Holmgren's recombinant ecologies are]
> an issue of rehabilitation and not restoration.

To me that distinction seems arbitrary (but I'm not a pro). For example,
when David Wingate used N-fixing, non-native casuarina on Nonesuch Island to
nurse the return of a native cedar forest, allowing native birds to return,
was that rehab or resto? The replanted cedars didn't survive on their own,
so he jump-started the forest with exotics, then took them out. I suspect
most successful restorations involve rehab.

>A single goal of
> "restoring" degraded land being used to champion plants that may
> cause problems?

I don't think that's what Holmgren means, but I'll avoid speaking for him;
he's been on this list. For me, it makes sense to rapidly restore the health
of the soil and attract a diverse fauna via fast-growing non-natives
(especially if they're already there; I believe that's what David was
referring to, and as examples to learn from). These plants then can later be
overtaken either by better-adapted natives (since conditions favor them
again), or by human-useful exotics. Used intelligently, these weedscapes are
self-destructing, like most pioneers. It's using biology rather than
engineering. The alternative, from what I've seen of restoration (forgive me
if I'm oversimplifying), is herbicides, huge amounts of human and machine
labor, and frequent replanting (I'm told herbicide makers often fund
restoration projects; they like repeat customers). Not to sound smug, but
that sounds like the classic "thoughtless and protracted labor" as opposed
to Holmgren's "thoughtful and protracted observation." If we can't eliminate
the conditions that attract invasives--large clearings, low fertility,
etc--they'll just come right back.

I think the nameless mangodance and I are in basic agreement about
goals--curtailing species loss, reducing human impact, enhancing
biodiversity--but we differ on methods. I find fast-growing non-natives an
important tool in disturbed landscapes for achieving those goals. But as
Mollison says, let's stay the hell out of the bush.

Toby  




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