In Colorado subalpine meadows, nonnative dandelions co-occur with a native flower, the larkspur. Bumblebees visit both species, creating the potential for interactions between the two species with respect to pollination. In a recent study, researchers selected 16 plots containing both species; all dandelions were removed from eight plots; the remaining eight control plots were left undisturbed. The control plots yielded significantly more larkspur seeds than the dandelion-free plots, leading the researchers to conclude that the presence of dandelions facilitates pollination (and hence seed production) in the native species by attracting more pollinators to the mixed plots. Which of the following, if true, most seriously undermines the researchers' reasoning? ['Bumblebees preferentially visit dandelions over larkspurs in mixed plots.', 'Soil disturbances can result in fewer blooms, and hence lower seed production.', 'If left unchecked, nonnative species like dandelions quickly crowd out native species.', 'In mixed plots, pollinators can transfer pollen from one species to another to augment seed production.']