Ecosystems are very finely tuned. Species depend on each other trophically and symbiotically. Sometimes the smallest change can have huge implications, as the proverb quoted above demonstrates.
For want of a nail the shoe was lost.
For want of a shoe the horse was lost.
For want of a horse the rider was lost.
For want of a rider the message was lost.
For want of a message the battle was lost.
For want of a battle the kingdom was lost.
And all for the want of a horseshoe nail.
We have a cooking apple tree in the back garden. It's sandwiched between a "tree of heaven" (Ailanthus altissima) and some privet (Ligustrum vulgare), but has always done pretty well. In 2006, when we moved in, we had a bumper harvest. Same in 2008 and 2010. The odd-number years were usually half as good - I remember reading that apple trees often alternated years. So, being 2012, I was expecting lots of apples.
We have one. And not a particularly impressive-looking one at that. We had a bad apple harvest predicted at the start of the year, due to the mild winter. So the buds started forming early, only to be destroyed by colder, wetter weather during the spring. So hardly any blossoms. The lack of blossoms meant fewer bees were attracted to the garden anyway. Some were able to pollinate the blossoms, but only one fruit developed. Normally by July we have windfalls all over the garden - there has been nothing.
One consequence of windfalls is an increase in the number of wasps. On one hand, it has been nice to sit out in the garden with Paul without wondering whether he's going to knock down plant pots in his bid to escape the next time a wasp appears. On the other hand, wasps are jolly useful for getting rid of pests. They eat aphids. A lack of windfalls means few wasps, which means loads of aphids. Aphids appear to attract the ants, who are busy harvesting them off my Tetrapanax.
So, though the full picture is much more complicated, our dodgy winter and spring has been responsible for loads of aphids around here.