The black to purple berries of deadly nightshade (Atropa belladonna)
are poisonous to people but European rabbits eat them safely. The
rabbits have an enzyme, atropinesterase, that inactivates the
nightshade toxin called atropine.

Some fruit hang on for awhile until animals get desperate enough to
eat them, or maybe because the fruit have to "age" or migrating birds
arrive later to eat them. Your observation that the fruit are
withering and would presumably drop might give mice and other small
animals a chance to eat them.

There is a hypothesis that there are many plants whose preferred seed
dispersers went extinct relatively recently. The so-called megabeasts,
such as mastodons, giant ground sloths, and ancestors of camels and
hippos, were all killed off in America about 13,000 years ago. Some
ecologists believe many trees coevolved with megabeasts and therefore
have fruit or thorns that are anachronistic. Examples include the
large orange-like fruit of Osage orange (Maclura pomifera) and the
honeylocust (Gleditsia triacanthos) with large thorns on its trunk and
large pods with seeds that have very hard seed coats. Supposedly,
mastodons may have eaten such fruit. The thorns would have protected
the bark.

More recently in North America, Carolina parakeet and the massive
flocks of passenger pigeons became extinct. They both probably left a
void in seed dispersal of native plants. Then again, people have also
greatly reduced the number of native plants.

References

Why are some fruits poisonous?:
http://www.cnf.ca/naturecanada/autumn00/nc_f00_qa5.html

Barlow, Connie. 2001. Ghost Stories from the Ice Age. Natural History
110(9):62-67 (Sept.)

"The Ghosts of Evolution: Nonsensical Fruit, Missing Partners, and
Other Ecological Anachronisms" by Connie Barlow:
http://btobsearch.barnesandnoble.com...uthorInterview

Maclura pomifera (osage orange) dispersal:
http://groups.google.com/groups?hl=e...ing.google.com



(DSmith) wrote in message . com...
Last week, my wife and I were walking through the eastern Iowa woods
and noticed a few small trees with lustrous red berries. They were
beginning to wither, and most of the other deciduous trees had dropped
their leaves. This got me to thinking:

What function do the poisonous berries serve? I would assume there's
some animal that can digest them and disseminate the seed, but then
why were the berries withering on the vine? Is this a sign of a
missing bird species (e.g., a bird species fallen prey to the West
Nile virus)? Against that idea, I seem to recall often seeing such
bushes full of untouched fruit. What gives?

Thanks in advance,
A nonbotanist dilettante