Oceanography Climate and the Gulf Stream

Perhaps more intriguing, when inedible algae were absent, only some of the experimental cultures displayed predator–prey cycles of high amplitude. Others cycled with lower amplitude, and with phasing of adults and juveniles suggesting demographic cycles. This result demonstrates ‘alternative attractors’ — that is, two dynamical states that characterize the long-term behaviour of Daphnia populations, with subtle differences in initial conditions determining which state is reached. The nonlinear mathematical models used to study population cycles often predict such phenomena, but most ecologists have probably despaired of verifying their existence in real populations. McCauley and colleagues also reveal a biological mechanism governing the switch between the two types of population cycles. Daphnia are parthenogenetic — they reproduce without sex — and their normal eggs quickly hatch into juveniles. Daphnia also produce, by sexual reproduction, resting eggs for long-term survival (Fig. 1). This diverts energy from normal reproduction and could prevent population explosions and over-exploitation, thus turning off the predator–prey cycle. McCauley and colleagues tested this possibility by replacing mothers carrying resting eggs one-for-one with mothers carrying normal eggs. This nearly doubled the amplitude of population cycles. Production of resting eggs essentially disperses offspring to the future, and is analogous to dispersing them in space. In theory, spatial dispersal could stabilize or destabilize population dynamics, and the range of possibilities for temporal dispersal requires further study. Long-term population dynamics could depend on the hatching of resting eggs. Indeed, others have drawn attention to similarities between the ‘egg banks’ of aquatic animals and the ‘seed banks’ of plants. Dispersal to the future is a widespread feature among animals, in the form of resting eggs, diapausing stages (in which animals go into a kind of suspended animation), and longlived adults. It could stretch out the timescale of ecological dynamics, blurring the distinction with evolutionary dynamics. The study by McCauley et al. should do much to stimulate research on its implications in many types of organisms. ■