The speed of change in ecosystems ranges from the imperceptible to the abrupt. Rapid, nonlinear changes (referred to as regime shifts) over time scales as short as 1 year are by their nature difficult to study and even more difficult to attribute to specific causes. Nevertheless, the accumulation of data over periods of decades can provide critical tests of mechanistic proposals.
Using time series data from fishery catches, long-term monitoring of plankton and planktivorous fish biomass, and oxygen concentration measurements over the past 50 years, Daskalov et al. describe two major regime shifts and several minor ones in the Black Sea ecosystem. Predatory fishes were heavily depleted in the 1960s, causing a cascade of effects down the food chain in the 1970s whereby top-down consumer control was replaced by bottom-up resource control of the system, which became dominated by planktivorous fishes. A second major shift happened in the early 1990s, when there was a population collapse of planktivorous fishes and an outburst of an alien jellyfish Mnemiopsis leidyi. The time series data suggest overfishing as the driver of both of these shifts, rather than pollution or the alien invasion per se. The top trophic level of predatory fish has not recovered (and seems unlikely to), although the appearance of the jellyfish Beroe ovata, which preys on M. leidyi, may promote the recovery of the next highest trophic layer of planktivorous fish. -- AMS
Proc. Natl. Acad. Sci. U.S.A. 104, 10518 (2007).
