Welsh Journals

whatever nourishment they can from it. Their activities modify the cowpat and allow further insects to invade it. The whole succession of about 40 insect species is over in a week or two. The best example of a primary succession in Britain is a sand dune system. Here bare sand is colonised firstly by Marram, a grass which spreads by underground rhizomes. The grass forms clumps and is very stiff, so that within the clump the air is still and less exposed to the wind than the bare dune. Also the Marram produces moisture, as do all plants, so we have a "microhabitat" within the clump suitable for invasion by other species. Other grasses, sedges and creeping plants help to stabilise the dunes, allowing woody shrubs and eventually tree species to grow. Often on large sand dune systems the whole range of different seral stages can be seen. A secondary succession does not start from scratch and therefore is normally shorter than a primary succession. The most well known and common succession in this country (Fig 1) goes as follows: Grassland Rough Grassland Scrub Woodland High Forest. The various stages in this succession are of course all around us, and the nature reserves of the Welsh Trusts represent all these different habitats. Figure 1. Diagram showing a typical secondary succession from grassland to high forest in a UK environment.