Plant life differs dramatically from dune ridge to dune ridge. Plants growing near the lake front are not usually found farther inland, and inland trees cannot be found on dunes near the beach. He wondered why. Cowles observed that as plants grow on a site they change it. When pioneer plants have changed the site sufficiently, the way is prepared for other plant species.
New plants can now be more successful, so they crowd out the pioneers.
Once marram grass becomes established on a dune, for example, that dune changes from an area with full sun to one with partial shade. As grass blades decompose, they build humus. Where there was only sand, now there is the beginning of soil. Change sets in. Cowles pieced together the puzzle and formulated basic ecological concepts.