In the late 1960s, Robert Paine conducted landmark studies on diversity in the rocky intertidal zone, comparing the species diversity in control plots with diversity in experimental plots from which he removed the top predator, sea stars. After 5 years, 15 species of intertidal invertebrates lived in the control plots, while the experimental plots were dominated by only two species, one mussel and one barnacle. The process MOST likely responsible for the loss of species diversity in the experimental plots was:

predation.
parasitism.
competitive exclusion.
resource partitioning.
mutualism.