There are many factors, both human and nonhuman, that set the stage for the 1983 mass mortality of Diadema antillarum. The actions of nature and of people were increasing urchin population densities while simultaneously destroying the urchins' habitat.

One of the biggest long-term problems, and arguably the factor over which we have the most control, is chronic overfishing. Research beginning in the 1960s showed that fish biomass had already been reduced by up to 80% from previous decades, and the problem has only grown worse since then (Hughes 1994). It is clear that intensive fishing is a major stress on reef ecosystems (Levitan 1992; McClanahan et al. 1994), but the effects of overfishing on urchin populations are somewhat less obvious.

Simply put, the removal of various fishes gives urchins a relative advantage over the fish. Many of the fish harvested from Caribbean reef systems are caught in simple traps (Hughes 1994), so both carnivorous and herbivorous fishes are being removed. Some of these carnivorous fishes are the previously discussed predators of D. antillarum (Hunte et al. 1986). Some of the herbivorous fishes are the surgeonfishes and parrotfishes that compete with D. antillarum for food resources (Lewis & Wainwright 1985). The removal of both predators and food competitors effectively destroyed the two primary controls on urchin population growth (Hughes 1994). With these limits removed, the urchins began to grow to employ their food resources more fully.

Food limitation alone could not even keep reasonable control of population density, as test size (Levitan 1988b, 1989) and dietary preferences (Carpenter 1981; Levitan 1991) can be modified to allow a large number of individuals even in a food-limited environment. As a result, population density of D. antillarum on Caribbean coral reefs was reaching abnormally high levels (Hughes 1994), and the urchins continued to outcompete herbivorous fishes for food resources, thus suppressing herbivorous fish populations (Hay & Taylor 1985). Furthermore, the densely packed urchins became progressively more food-limited, which leads them to higher predation on living coral organisms (Carpenter 1981). The high density of these urchins caused a vulnerability to contagious disease: tightly packed urchins have closer contact and thus can speed the transfer of pathogens between themselves. This vulnerability was fully exploited in the 1983 mass mortality.

Another major factor in the demise of Diadema antillarum was the ferocity of storms that have struck the Caribbean in recent decades. The most significant of these was Hurricane Allen in 1980, which destroyed large portions of coral reefs, especially the fringing reefs that are more exposed to the destructive effects of wind and waves (Hughes 1994). These reefs, which had taken centuries to build, were dealt a crippling blow over the course of a few days, and they have scarcely had a chance to rebuild since then.

So it was not totally surprising that a major disturbance would occur in the coral reefs in 1983. In fact, some ecologists believe that every population viability analysis should include an allowance for catastrophes (Mangel & Tier 1994), and that these catastrophes can actually be predicted in terms of approximate frequency and severity. According to Peterson & Black (1988), mortality following severe physical stress may increase with population density. This prediction is supported by the conditions of the mass mortality of D. antillarum.