Something else puzzled Cowles. Plant life differed dramatically from dune ridge to dune ridge. Plants growing near the lakefront were not usually found further inland, and inland forest trees could not be found on exposed dunes near the beach. Cowles and his students observed that as plants grow on a site they change it. When these pioneer plants have changed the site sufficiently, the way is prepared for other plants.
New plants can now be more successful and so they crowd out the pioneer species.
Once marram grass becomes established on a dune, for example, that dune changes. As grass blades decompose they begin to build humus. Change sets in. Cowles pieced together the puzzle and formulated basic ecological concepts. At the close of the Ice Age about 10,000 years ago, the landscape was covered with the spruce and fir forests now found in Canada.