Greetings all! I've been lurking for over a year, and so I feel I kind-of know you good folks already. Actually, that's a bit creepy, so I'll stop there; I'm not a stalker, honest.
I can't contribute much to the meteorological and oceanographic discussions (although I wish I could; but I know when I'm out of my depth!), but I'm a palaeontologist and entomologist with a hand in local community sustainability, so hopefully can add some useful stuff on those lines. This was the thread that finally got me registered.
One thing I've been seeing in the insect life of the UK in recent years is a clear northwards shift in populations. Species are settling in the UK when they only used to be occasional migrants, and southern species are turning up much further north than ever before. In my little patch of mid Wales, there are quite a few species that have suddenly made it home, with no previous records this side of the border. When you see them, they're in sheltered sunny spots that make the most of the weather, but that's just the stop-gap; once they have a foothold, they're only likely to expand.
What does this mean for biodiversity, though..? Well, in the short term it's likely to carry on going up, as more incomers arrive and settle, without driving out the ones at the southern end of their range. After all, biodiversity in general decreases northwards, and the UK is constantly being fed by warmth-loving species from the continent. These species generally don't cause any chaos, either; there are the horror stories like the Harlequin Ladybird ("Eats all the natives!"), but these also reach their own balance over time ("Actually, it only nibbles some of them, and none look to be heading for extinction as a result..."). No, what worries me more is changes in abundance, and boom-and-bust cycles.
Living in the middle of sheep-wreck (the desert of the overgrazed grassland), you get to see how sensitive disturbed ecosystems are too a final push. Stress the community by long-term, intensive grazing, and you'll knock out most of the species locally. A few generalists (or specialists, even) will cope, and then thrive through the lack of competition. They can then form a massive food source for, say, spiders, which under the right conditions will mushroom (unlike mushrooms, alas). The system flips off the deep end when you get an extra push from, say, a mild winter.
Mild winters fail to kill off the predators. These then form ravening hordes that devour as many eggs and spring larvae as they can find, before having to resort to each other. The caterpillar eradication starves the bird population, which should be also feeding on the spiders, and so on. There are a lot of buffers in these systems, and a lot of potential ways that imbalances get evened out over the space of a few years. There are also loads of unexplained patterns in known insect abundance and distribution; why are some species declining, and others thriving? Sometimes, we just have no idea.
What worries me in relation to biodiversity loss at the moment is the wild weather swings; the buffers can only take so much buffeting. Most species aren't going to be actually wiped out for some considerable time, in my opinion, but the thing to watch for is ecosystems going chaotic - apparently random blooms of unusual species, and that sort of thing. They've always happened, given random seasonal anomalies, but of course we're now likely to see a lot more of them...