Everywhere in the water there is animal life, forms that are attached, free-swimming, crawling on the bottom, and clinging to or swimming on the surface film. The gray, slimy encrustation on a sunken log looks like a covering of lichen but is really a freshwater sponge, a colonial animal that feeds by filtering minute plankton from the water. Another attached creature is the barely visible hydra; this twig-shaped predator, related to marine jellyfish, captures water fleas and other small animals in its several poisonous tentacles. Water beetles, backswimmers, water boatmen, and many other creatures move about more or less freely in the water, propelling themselves along with jerky movements. Suspended between surface and bottom are the zooplankton, the tiny water fleas, cyclops, daphnia, and others, which feed by filtering minute algae. On the bottom and below live scavenging worms. Water striders skate on the surface film. Along the shore, frogs, salamanders, garter snakes, and water shrews are hunting. Dabbling and diving ducks patrol about, tipping or submerging for the bottom plants. Moose tracks circle the muddy shore. Because it produces vegetation abundantly, John’s Lake sustains a great diversity of animal life. Beaver Ponds Fully 10 percent of all the present meadow area in the Rocky Mountains is estimated to have been created by beaver, the only animal besides man that engineers extensive changes in the environment to suit its own needs. When beavers dam a stream, they set in motion another form of succession. If the resulting backwater floods a forest area, the trees are soon killed, creating a broad opening in the forest canopy. Water-associated plants and shrubs quickly invade the pond and shoreline, creating favorable habitat for waterfowl, moose, blackbirds, amphibians, wading birds, warblers, marsh hawks, and a score of other animals. After many years the water becomes shallow, filling in with silt and plant debris. When the beavers abandon the site, the dam may rupture for lack of maintenance and the pond will rapidly drain. Or it may continue to hold, delaying for several more years its slow conversion to meadow. Stimulated by the nutrient-rich mud, the water grasses, sedges, and shrubs finally choke the water with their accumulating debris, transforming the area into a bog.