Complex dynamics are often shown by simple ecological models and have been clearly demonstrated in laboratory and natural systems. 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 changes in the hydrology of the lake, with human impacts such as dredging leading to higher-amplitude fluctuations. Tanytarsus gracilentus is a key component of the Myvatn ecosystem, representing two-thirds of the secondary productivity of the lake and providing vital food resources to fish and to breeding bird populations. Therefore the high-amplitude, irregular fluctuations in midge densities generated by alternative dynamical states dominate much of the ecology of the lake.
Summary 1.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 secondorder autoregressive process. 3. We tested the alternative hypotheses that the cyclic dynamics of T. gracilentus were driven by consumer-resource interactions in which T. gracilentus is the consumer, or predator-prey interactions in which T. gracilentus is the prey. We analysed autoregressive models including both consumer-resource interactions and predator-prey interactions. 4. Wing length of T. gracilentus was 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 of Micropsectra lindrothi , a species ecologically similar to T. gracilentus , fluctuated synchronously with T. 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. gracilentus included midges in the genera Procladius and Macropelopia , and the fish Gasterosteus 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. gracilentus abundance. Consumer-resource interactions amplified the exogenous variability affecting T. 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.
Changes in bird populations and their phenology (i.e. timing of nesting and migration) are increasingly linked to global climatic changes, particularly at temperate and Arctic latitudes. These patterns arise from local-and regional-scale effects of weather on demography but long-term time-series data necessary to explore these relationships are rarely available. Colonies of the common eider Somateria mollissima are often monitored annually for nest-down harvesting. We use long-term data from 2 nesting colonies in northwest Iceland (Bíldsey, Brei7afjör7ur: 29 yr and Laekur, Dýrafjör7ur: 55 yr) to examine the effects of weather conditions in each season on breeding numbers, arrival dates and clutch sizes. Numbers of nests in Bíldsey increased following warm, wet winters and first nests were produced later following windy and wet winters at Laekur. In spring, windy conditions tended to be followed by earlier female arrivals at Bíldsey. Warm, wet springs were positively correlated with larger clutch sizes at Laekur, and clutch sizes at Laekur decreased following especially wet and warm autumns. The overall population trends vary among sites and are likely to be influenced by both climatic and management conditions. These findings indicate that the effects of global climatic changes will depend on the details of changes in local weather conditions within each season. The strong effects on nesting dates and clutch sizes suggest that accumulation of body reserves, which sustain females during incubation and into brood rearing, is a key mechanism that could determine how altered distribution and frequencies of depressions within each season will affect future numbers of this species.KEY WORDS: Climate · Phenology · Temperature · Winter harshness · Breeding · Arctic · Waterfowl · Capital breeder Resale or republication not permitted without written consent of the publisherClim Res 38: [237][238][239][240][241][242][243][244][245][246][247][248] 2009 Arctic latitudes (66°N and higher), the short summer predisposes birds to complete breeding in a particularly short time (Poussart et al. 2000). A number of studies have shown how initiation of breeding is related to trends in climatic change, particularly through earlier spring migration and subsequent initiation of breeding (Durant et al. 2004, Frederiksen et al. 2004, Sparks et al. 2005, Jonzen et al. 2006, see also Robinson et al. 2007 and citations therein).Early nesters are generally individuals in relatively good body condition, which often consequently lay the largest clutches and attain the highest breeding success (Erikstad et al. 1993, Arnold et al. 2004, Bêty et al. 2004. Clutch size correlates with body condition in Arctic-breeding geese (genera Anser, Chen and Branta) and in ducks such as the common eider Somateria mollissima (Ankney & MacInnes 1978, Erikstad et al. 1993, Öst et al. 2008. Many birds exhibit a seasonal decline in clutch size (Arnold et al. 2004) and chick survival may decrease with advanced hatching date (Traylor & Alisauskas 2006)...
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