High-amplitude fluctuations and alternative dynamical states of midges in Lake Myvatn

Feast or famine Iceland's Lake Myvatn ecosystem is remarkable in that it is so dominated by a single species. Myvatn translates as 'midge lake', and it is the midge Tanytarsus gracilentus that dominates, comprising two-thirds of the lake's secondary productivity. Midge numbers undergo extreme fluctuations of almost six orders of magnitude with an irregular period of 4 to 7 years. A new analysis of 25 years of population monitoring shows that this phenomenon can be explained by alternative dynamical states with the amplitude of the fluctuations set by small subsidies of food entering the habitat. Small decreases in food supply due to human disturbances could explain recent increases in midge fluctuations. In conservation terms, midge populations are inherently unpredictable and are much more vulnerable to small disturbances in the lake than was expected. The Lake Myvatn midges illustrate the fundamental complexities of natural ecosystems and the difficulties in managing them. The cover shows mating swarms of male midges waiting for females to join them. An example of complex population dynamics (the midge Tanytarsus gracilentus in Lake Myvatn, Iceland) is described and modelled. Complex dynamics are often shown by simple ecological models 1 , 2 and have been clearly demonstrated in laboratory 3 , 4 and natural systems 5 , 6 , 7 , 8 , 9 . Yet many classes of theoretically possible dynamics are still poorly documented in nature. Here we study long-term time-series data of a midge, Tanytarsus gracilentus (Diptera: Chironomidae), in Lake Myvatn, Iceland. The midge undergoes density fluctuations of almost six orders of magnitude. Rather than regular cycles, however, these fluctuations have irregular periods of 4–7 years, indicating complex dynamics. We fit three consumer–resource models capable of qualitatively distinct dynamics to the data. Of these, the best-fitting model shows alternative dynamical states in the absence of environmental variability; depending on the initial midge densities, the model shows either fluctuations around a fixed point or high-amplitude cycles. This explains the observed complex population dynamics: high-amplitude but irregular fluctuations occur because stochastic variability causes the dynamics to switch between domains of attraction to the alternative states. In the model, the amplitude of fluctuations depends strongly on minute resource subsidies into the midge habitat. These resource subsidies may be sensitive to human-caused