Ecological complexity and colonization (was: disease)

Gerold Firl (geroldf@sdd.hp.com)
28 Jun 1996 22:08:35 GMT

In this discussion of the enormously destructive post-contact
epidemics in the americas, we've gone over numerous factors which led
to the asymmetry between old world and new world responses to novel
epidemiological environments. Old World populations had co-evolved
with a large number of pathogens over the previous millenia of
civilization, producing a human population which had acquired a viable
set of immunities to some of the diseases, an endemic truce with
others, and various (incompletely understood) coping mechanisms for
the least tractible, such as bubonic plague.

The american population, in contrast, had no history of exposure to
these diseases. When they were suddenly exposed to _all_ of them, the
past few thousand years of eurasian epidemiology was compressed into a
couple of brief centuries. Particularly in the temperates zones, the
die-off was massive.

The question was posed as to why there was no comparible reverse flow;
why didn't american diseases devastate eurasian populations to the
same extent? Again, numerous factors have been discussed, but in this
post I will take a step back and examine the issue from a more
globalperspective, and describe the importance of what might be called
ecological complexity in the population dynamics of previously
isolated biota.

We are all familiar with the consequences of unleashing organisms which
evolved in a relatively more complex ecology in one which is
"simpler"; think of rabbits in australia. The same phenomenon has
occured over and over again, not just with rabbits, but with numerous
old world animals; cattle, horses, sheep, goats, mongooses, snakes (in
guam), starlings, sparrows, dogs, cats, rats, honeybees - and, of
course, humans. European silors and explorers would "seed" remote
islands with pigs or goats, in the confidant expectation that in a few
years the place would be teeming with easy meat. The same process
occured, whether by accident or design, in all the remote areas
touched by european exploration.

Each one of these species has been able to displace more established
lifeforms in isolated ecologies, often reproducing explosively and
creating severe population imbalances. When released in simple
ecologies, they find that they have no serious competitors, and no
effective predators, and hence are able to increase their numbers
dramatically. Results have probably been even more dramatic in the
case of plants; temperate north america, for instance, has been
extensivley colonized by old world plants. Many "natural looking"
landscapes in the eastern US don't contain a single native plant, and
all are heavily infiltrated. (See _ecological imperialism_, alfred
crosby, for copious documentation and examples)

There have been a few minor examples of counter-flow from simpler
ecosystems to the eurasian heartland; muskrats, for example, have been
able to naturalize in european rivers, and european vintners were
nearly wiped out by american insect pests in the 1880's, but these few
and comparitively minor examples strand in stark contrast to the
common pattern of a colonial gradient from eurasia to peripheral
regions. In general, eurasian lifeforms are able to colonize peripheral
areas, but the reverse is not found to be true. (Note: africa is
intermediate between eurasia and the peripheral islands and island
continents in terms of ability to colonize and resistance to
colonization.)

The same process which occurs as an ecological law in the case of
animals and plants also takes place at the two extremes of the
eco-cultural heirarchy: humans and pathogens. When humans first
reached the hitherto uninhabited islands of crete and majorca they
feasted on the pygmy elephants which lived there, quickly wiping them
out. The same thing happened in hawaii with the flightless goose, in
new zealand with the giant moa, in australia with the giant kangaroo
and the marsupial wolf. And finally, the same thing happened in the
americas with the elephants, rhinos, horse, camel, giant bison and
ground sloth, along with the great predators which fed on them. In
each case, when man (the quintessential old world opportunist) suddenly
entered previously isolated territory, they lived high on the hog,
until the easy meat was gone. The pattern is pervasive and universal.

Between the time of the bering land bridge which brought human
predators from the old world to the american eden, and post-columbian
contact some 10,000 years later, new world humans changed from being
an old world colonial species to an established, isolated, and therefore
vulnurable, new world species. When contact was re-established with
old world humans and their diseases, the viral and bacterial pathogens
tore through the virgin territory just like rabbits in australia - or
like paleolithic folsom points through the shaggy hide of a giant
ground sloth.

It's interesting that this pattern of the colonial gradient, from the
population center to the periphery, should hold so consistantly. It
provides a useful perspective from which to view post-contact
epidemiology.

-- 

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=-=-=-=-=-=-=-=-=-=-=-=-=-=----   Gerold Firl @ ..hplabs!hp-sdd!geroldf