A group of biologists working in Africa observed that some of the Acacia trees growing in their study plots had clearings around them, so that no other vegetation grew within a few meters around each tree. These trees were also covered with ants. The researchers hypothesized that the ants must have cleared the vegetation around each tree. To test this, the researchers decided to remove the ants from some of the trees by spraying with an insecticide and see if vegetation grow in the clearings with the ants gone. They located 100 Acacia trees with clearings and ants present, and sprayed half of them with insecticide. The trees tended to exist in two general habitats: the first habitat was much higher in elevation and was moisture, cooler, and had more wind exposure than the second habitat. Because the insecticide spray would have drifted in the windy conditions of the higher elevation habitat, the researchers only sprayed trees in the low elevation habitat. They visited their research location once a month for two years to record whether vegetation re-grew in the clearings. Which element of experimental design is missing or flawed in this experiment