Snell and Snell (personal communication) studied the population of land iguanas (Conolophus subcristatus) on S-Plaza, an islet of about 11.7 ha. Their study covered the whole El Niño event. The qualitative results of this study are so interesting that they will be mentioned here although they are not yet published in detail. 

   Land iguanas on Plazas dig burrows and lay their eggs in January and February, and the young hatch in May to June after about four months of incubation. During El Niño the unusually large rainfall led to massive growth of the vegetation providing the iguanas with plentiful food. This was reflected in excellent body condition of all animals in the population and consequent massive, successful reproduction of the iguanas on Plazas in 1983. Hatching success was high and hatchlings were unusually heavy. High water potential of the soil has been shown (Snell and Tracy, 1985) to result in larger hatchlings. The unusually big El Niño hatchlings emerged into a situation of plentiful food, resulting in high growth rate and low mortality of juveniles. Thus population size rapidly increased during the period of El Niño. 

   At the same time the Opuntia (Opuntia echios), which is one major food item of iguanas during dry seasons, became water-logged and 50% of the trees fell. Usually leaves of fallen trees will resprout and vegetative reproduction can occur. However, under the conditions of high iguana population density after El Niño this did not happen. The reasons are two-fold: immediately after El Niño a period of intense drought followed which reduced the amount of edible vegetation on the low-lying Plaza island very quickly. This secondarily led to preferential feeding of the iguanas on the fallen Opuntias and their resprouting leaves. The iguanas ate all regrowth and reduced Opuntia density to 50% of its former value. This process has been continuing ever since, to the extent that Opuntia reproduction through fruit cannot take place any more. That the iguanas were indeed responsible for this partial destruction of their food base is proven by a natural control. N-Plaza lies only about 50 m north of S-Plaza, has the same physical properties, is covered with the same type of vegetation, but devoid of land iguanas. On this island Opuntias also fell during El Niño, but resprouted normally, resulting in a fast recovery of the local Opuntia population. 

   With the partial demise of the Opuntias on S-Plaza the staple food of iguanas during the dry season has become very scarce, and as more and more of the old Opuntia trees fall and are eaten by the almost starving iguana population the resource base is increasingly vanishing. This has led to reduced reproduction, increased mortality among adults and almost 100% mortality of juveniles in their first year. In 1988 many females that reproduced were so spent that they did not recover from their reproductive effort and died. Presumably, this population will continue to decrease until the Opuntias finally get a chance to regrow, so that the resource base of the iguana population recovers. When this will happen, and through how serious a bottleneck the population will go in between, is presently impossible to say. But given the initial relatively small size of the iguana population on S-Plaza, with 226 adult females only, and 102 adult males (Snell and Christian, 1985), it is conceivable that effective population size could drop well below levels where random drift and founder effects could have a strong impact on the evolutionary trajectory of this population. 

   Similar dramatic effects of El Niño have been observed in other large vertebrates that depend for their food on the marine environment. Here the effects mirrored the ones found in terrestrial organisms: the period of El Niño was disastrous while populations recovered rapidly during the rebound period.