Diversity and stability, was: Re: all theory thread (long)

[Toby Hemenway <hemenway@jeffnet.org>]

 Mikal wrote:

>> We have been wrong in the past (as in ³diversity creates stability,"
>> which isnıt true)...
> 
> Could someone fill me in on the context of this line?

Since I said it, I'll field this. Early, simple ecology studies (Elton,
MacArthur) concluded that diversity promotes stability. But more rigorous
studies in the fifties and sixties done by theoretical biologists like May,
Ashby, and Gardner showed that increasing numbers of species reduced the
chance of arriving at a stable ensemble. This work stimulated more detailed
field studies whose results have been contradictory. Redwood forests, for
example, contain relatively few species yet are highly stable and
productive. Eastern (US) hardwood forests contain many species yet undergo
rapid, unpredictable, and spontaneous changes, but are also very productive.
Salt marshes have few species yet are hard to invade or disrupt. In some
cases, ecosystems with many species resist invaders, or recover from
disturbance (fire, windstorm, disease) faster than species-poor systems. In
other cases, they donıt. Hawaii, very species-rich, has been devastated by
exotic species, even in relative wilderness.

The story is much more complex than ³diversity begets stability.² For one
thing, what do the words mean? Is diversity high species richness, high
variance among individuals, number of interractions, the strength of the
interractions, or length of the food chain? Is stability resilience
(recovery from disturbance), persistence (how hard a system is to disturb),
resistance (how much one change ripples through the system), ease of
invasion by another species, or how much the whole system varies over time?
And there are lots of other things that affect stability besides diversity.
Increasing the strength of interractions and the number of species increases
stability in ecosystems with a high density of predators, but reduces it in
systems with low predator densities.

My point is that weıre extracting permacultureıs ³principles² from our
observations of natural systems, and we hope that we choose important rules
rather than trivial ones (like ³all tree trunks are brown²) and describe
them correctly. We have to be prepared for the possibility that we may
choose unimportant principles or get them wrong, and be ready to modify our
ideas. But simplistic ideas like ³diversity promotes stability² are deeply
entrenched, and we donıt give them up or modify them or see their limits
very readily. That's a problem. This may be part of what David Holmgren sees
when he says itıs time for revised principles. Slogans like "everything is
connected to everything else," a catchy little meme, are politically useful
but factually inaccurate (we should add, "but some things are more connected
than others").

Hope this helps. Lots more where this came from, but I'll shut up.
Toby