Consumer–resource interactions and cyclic population dynamics of Tanytarsus gracilentus (Diptera: Chironomidae)

Summary1 Tanytarsus gracilentus population dynamics in Lake Myvatn show a tendency to cycle, with three oscillations occurring between 1977 and 1999 having periods of roughly 7 years. The population abundance fluctuated over four orders of magnitude. 2 A partial autocorrelation function (PACF) accounting for measurement error revealed a strong positive lag‐1 autocorrelation and a moderate negative lag‐2 partial autocorrelation. This suggests that the dynamics can be explained by a simple second‐order autoregressive process. 3 We tested the alternative hypotheses that the cyclic dynamics of T. gracilentuswere driven by consumer–resource interactions in whichT. gracilentusis the consumer, or predator–prey interactions in whichT. gracilentus is the prey. We analysed autoregressive models including both consumer–resource interactions and predator–prey interactions. 4 Wing length of T. gracilentuswas used as a surrogate for resource abundance and/or quality, because body size is known to fluctuate with resource abundance and quality in dipterans. Furthermore, the wing lengths ofMicropsectra lindrothi, a species ecologically similar toT. gracilentus, fluctuated synchronously withT. gracilentus wing lengths, thereby indicating that the shared resources of these two species were indeed cycling. Wing lengths of other chironomid species were not synchronized. 5 The predators of T. gracilentusincluded midges in the generaProcladiusandMacropelopia, and the fishGasterosteus aculeatus (three‐spined stickleback). 6 The autoregressive models supported the hypothesis that T. gracilentus dynamics were driven by consumer–resource interactions, and rejected the hypothesis that the dynamics were driven by predator–prey interactions. 7 The models also revealed the consequences of consumer–resource interactions for the magnitude of fluctuations in T. gracilentusabundance. Consumer–resource interactions amplified the exogenous variability affectingT. gracilentus per capita population growth rates (e.g. temperature, rainfall, etc.), leading to variability in abundance more than two orders of magnitude greater than the exogenous variability.