Resource limitation of diving ducks at Myvatn: Food limits production

We monitored numbers and production of diving ducks on Lake Myvatn and its outlet river, the Laxa, over a period of 25 years, 1975-1999. Aquatic insects, a major food source, were monitored for 23 years, 1977-1999, using window traps. In this paper, we focus on the relationship between the demography of five species of diving ducks and the abundance of aquatic insect species, mainly Chironomidae and Simuliidae. Production of ducks was estimated when most young were 2-4 weeks old, either as young per female (Aythya marila and Aythya fuligula) or as total young found (Melanitta nigra, Bucephala islandica, Histrionicus histrionicus). Variation between years in production of young ducks was strongly correlated with variation in the abundance of aquatic insects. Procladius islandicus and Psectrocladius barbimanus, both of which are univoltine with peak emergence in late July (when many young ducks hatch), as well as the bivoltine Orthocladius consobrinus, showed particularly high correlations with duckling numbers. In one year, 1992, catastrophic weather depressed production. Food appeared to determine production, primarily through its availability to the young. Changes in spring density of breeding ducks of all five species were negatively associated with density in the year before. Resources, estimated as food availability or reproductive success in the year before, were positively correlated with change in breeding numbers in A. marila, A. fuligula and B. islandica. Production three years earlier had a weak but significant effect on change in numbers of M. nigra.