A lake in Minnesota was thriving with black bass and bluegill. These particular fish build nests on the sandy bottom of the lake. Now, ecologists are finding that over time a layer of humus has slowly been covering the pond. The ecologists are noting that the populations of black bass and bluegill are declining, and the fish that are able to make nests in humus are rising in numbers. The ecologists can probably conclude that this event is an example of:
commensalism
parasitism
primary succession
succession