Transportation of plants and animals from one island to another by floating
on logs is hardly a new theory. See Charles Darwin (1859), "On the Origin of
Species", as well as more recent works on island biogeography. Ways that
organisms can travel across water include:

Flying: insects, spiders (on silk), winged seeds, birds, bats

Hitching a ride on a flying organism: mites, parasites, sticky seeds (and
snails!) on ducks' feet, seeds eaten by birds, etc. Note that, in the last
case, the bird effectively brings its lunch with it to its new home. The
snake that has consumed most of Guam's native birdlife may have arrived
coiled on the strut of an airplane.

Floating: waterproof seeds, seashore plants that reproduce vegetatively and
withstand salt water, some reptiles that can float and also withstand salt
water for a time, etc.

Swimming: Mammals such as elephants and deer do swim purposefully from
island to island occasionally, if they can see the other island across the
water.

Rafting: In the tropics, huge masses of plant matter can be swept offshore
during floods and travel very long distances before they land or founder.
Even organisms that are not very resistant to salt water could be rafted in
this way, e.g., some land mammals. Land snails that can shut their shells
tight are particularly apt to travel this way.

Land bridges: Wait for sea level to go down and walk across the strait.
(Obviously, this works only in cases where the sea is shallower than about
200 m.) This is probably how rhinos reached Java, for instance. And some
land and freshwater mollusks.

Introduction by humans: This can be purposeful or unpurposeful, ancient or
modern, and we've discussed a lot of examples on Conch-L of snails and zebra
mussels wreaking havoc in new places. (For this purpose, we can consider
North America as a very large island.)

Most of the time, the few individuals that come to a new island die out
before they reproduce, or die out after a few generations. Consider that
most garden plants brought to Hawaii have not entered the forest and
displaced native plants. But in a few cases, the new immigrant burgeons in
population and becomes a serious pest, like east Asian kudzu and honeysuckle
in the southern US, voracious mongooses in the Caribbean, rats on Polynesian
islands, chestnut blight in North America, etc. It's hard to tell ahead of
time which species will become pests, so it's a good idea to plant your
garden with native plants, and not throw those pretty fish from the aquarium
into the nearest stream when you move. Could be the next walking catfish or
killer bee.

In the long run, if people continue to fly everywhere without taking
adequate precautions, so many species will travel from one place to another
that the world will have much more homogeneous faunas and floras. You will
step off the plane in California and see only Mediterranean grasses, or step
off the plane in Hawaii and see only ornamentals brought in by gardeners.
Oops -- that time has already come. Oh well, there's still native life in
Florida, isn't there?

Andrew K. Rindsberg
Geological Survey of Alabama