Concern among scientists and environmentalists prompted an attempt to control the animals' proliferation; many were killed by injection with formaldehyde, while others were simply removed from the reefs and destroyed. In the late 1970s, however, new research data indicated that similar expansions, or blooms, had occurred previously, followed by periods of decline. Thus, it seemed likely that the sudden growth of the starfish population during the 1960s represented a phase in the organism's natural cycle. Most outbreaks last one to two years, although some have persisted for as long as five years. What causes these dramatic population explosions is unknown; however, authorities hypothesize that the periodic input of high-nutrient loads from land sources and removal of species that prey on adult starfish may be responsible.