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Nature Notes July 2018

Nature Notes

Writing this in June, as we rapidly approach the Summer Solstice (longest day), I am experiencing the huge build-up of biomass that results from the increased input of energy from the sun as the days get longer. It follows that the more energy there is entering the northern hemisphere, the more life is regenerated, first from photosynthesis by green organisms such as plants and algae, and then from the animals that feed on these. For example, the circa 250,000 leaves of a deciduous oak tree can in some years be completely defoliated by hundreds of moth caterpillars, each one eating up to 10 leaves before pupating. However, one blue tit can eat from 70 to 100 caterpillars each day, and there can be as many as 18 young blue tits in one nest, plus 2 parents. That is about 2,000 caterpillars eaten by that one family in one day. There can be as many as 4 blue tit families in one hectare of woodland, so it is possible that up to 8,000 caterpillars could be consumed every day for the 18 to 20 days between hatching and fledging (leaving the nest), that is about 160,000 caterpillars consumed at some time between the spring equinox and the summer solstice sustaining up to 80 blue tits per hectare. And this is not all, sparrowhawks time their breeding season to coincide with the fledging of blue tits which they feed to their young. One family of 6 sparrowhawks could consume as many as 40 blue tits in one day! When all this is over, the oak trees produce a second flush of leaves by 1st August, this is called Lammas growth, because it coincides with the ancient feast of Lammas or Loaf Mass.

Have you ever been involved in hay-making? This is when a farmer will cut the grass that has grown and is ripe for cutting in June. The grass has absorbed all the energy from the sun up to the solstice and produced a huge bulk of grass tissue, tasty and energy giving for the cattle and horses that eat it. As many as 8.4 tons of hay can be harvested from one hectare of a good grassy field. If one cow can eat 24 kilograms of hay in a day, this would sustain that cow for about 42 days. That cow can then sustain quite a few people in the form of milk or beef. In ecological terms, this dynamic system is known as environmental services, none of us could survive without them.

Dr.Martyn Stenning