June 2016

How lovely are the fresh new green leaves on our deciduous trees in the spring time!  How extraordinary too are the animals that live among them.  Each tree is an ecosystem in its own right.  Full of the web of life and each animal is using the tree as its home.  First there is the water drawn up from the roots and filling the veins of the leaves making them turgid and juicy.  Much of this water is then secreted from the stomata holes on the underside of the leaves.  Many hundreds of litres can be moved by one mature tree in a single day.  This is a reason why trees are so useful in a flood-plain, also why flood-plains devoid of trees flood so badly.  The animals in the trees make use of this moisture and many of them, like moth and sawfly caterpillars, eat some of the juicy leaves.  Many of these caterpillars will be eaten by larger insects, spiders and birds such as blue tits, which in turn, un-fit blue tits will be eaten by other animals such as meat eating birds such as sparrowhawks.

Sparrowhawks are what we call top-predators.  Nothing much can catch and eat them.  They fly and perch, often in dense woodland, and frequently take small birds on the wing.  They too will have a family of 3 – 5 nestlings to feed at this time of the year.

The pyramid of numbers is startling.  In some years, one mature oak tree can lose all of its thousands of first flush of leaves to caterpillars.  One family of blue tits can consume 18,000 caterpillars in about 20 days.  One family of sparrowhawks will consume about 10 blue tit sized birds in one day, and do so for about 28 days!  However, when you think of the sheer quantity of lettuce, chickens, sheep and cattle that we humans eat, it kind of puts it in context.  Also, the oak trees always produce another flush of leaves (Lammas growth), many caterpillars do survive to become beautiful moths each of which lay large numbers of new eggs on the trees, and sufficient blue tits survive to make this bird one of the most successful species’ in Europe.  However, sparrowhawks will always be rather thin on the ground and rarely seen.

Meanwhile, if you happen to walk through oak woodland during a warm sunny day during May and early June, and the woodland floor is covered with dry leaf litter, listen carefully for a sound like softly falling rain.  This is the sound of thousands of caterpillar droppings falling on the dry leaves.