> ... Not even Himalayan blackberry
>seems to be competing successfully with the undulating masses of ivy,
>clematis and deadly nightshade. All of the trees have leafy green girdles,
>and many have fluffy clematis seedheads spewing forth from their crowns.

snip...

>My questions are these -- obviously this unusual plant community is
>currently supporting a moderate number of winter city birds

Is it really supporting them, or do they mostly use it for cover? I'd bet
that they eat out a lot. One reason that invasive plants are invasive is
that few or even no bugs are willing to eat them, so there are no controls
on their growth. Most birds eat bugs at least during nesting season, so
the invasives are reducing the total food supply, and certainly the variety
of the food supply, for that patch.

An additional problem is that native plants produce their fruits timed to
when the animals are most likely to need them, the non-natives don't. So
holly and ivy may provide a feast when it isn't really needed, but then
there won't be any late-winter-ripening kinnikkinnik, or early spring
osoberry.


>, but left to
>itself, will these vines kill everything on which they climb?

Ivy will, I don't know about the others. I've seen ivy kill a large big
leaf maple on the UW campus in less than 10 years. Worse, when ivy climbs
up, it's more likely to mature and fruit. Some birds love ivy fruits, so
they spread the damage for miles around, and each new infestation becomes a
new center of invasion.


> Once the
>shrubs and even trees have been completely eliminated, will bird species
>diversity decline?

In the city, I dunno, because of people feeding them (sometimes by the
variety of other non-native ornamentals rather than intent). Cities are
pretty unnatural to start with, and the species which survive tend to be
generalists. But in more natural areas, animal diversity will and does
change--the once living trees which provide fruit/seeds/bugs might for a
time provide snags and different bugs for different bird/mammal species.
But in the longer run, those snags will finally decay and have no
successors. Animal diversity depends directly on plant diversity, so the
more we lose of our complex native plant communities, the worse off all
animals, from invertebrates through humans, are. The effects may not be
immediate, and may show up first as a lack of resiliency--an abnormally
cool winter will have a greater effect on survival, etc.


> Would rats be a problem?

Absolutely. Rats _love_ ivy and similar ground covers. But even they have
to eat out somewhere...

>Given that invasives are here to stay, what level of habitat are they
>providing?

Sometimes good in the short run (years to decades) for a few, usually
obvious, species, though it's always bad for the invertebrates, which are
the truly dominant animals on the planet. Poor for the intermediate run
(decades to thousands of years) even for obvious animals. On the
million-year time scale, things will improve again as mutations and
evolution have time to act. Too bad that's a little late for us!

Allyn Weaks allyn at teleport.com
Seattle, WA USDA zone 7-8
Pacific NW Native Plant Gardening: http://www.teleport.com/~allyn/natives/