Masting and trophic cascades: interplay between rowan trees, apple fruit moth, and their parasitoid in southern Norway
We analyzed berry production in rowan, Sorbus aucuparia L., in southern Norway and examined the ramifying effects of rowan masting on the dynamics of the dominant seed predator and its parasitoid. The apple fruit moth, Argyresthia conjugella Zeller, is a pre-dispersal seed predator of rowan. The larvae of the apple fruit moth rely on rowan berries, which is in turn attacked by the parasitoid wasp, Microgaster politus Marsh. We found typical masting in rowan: berry production varied across years (the mean of coefficient of variation=1.0) and was spatially synchronized at large scale (the averaged correlation coefficient=0.67). Berry production was two-year cycle in western but was three-year cycle in eastern Norway. The abundance of the moth and parasitoid also varied across years and were spatially synchronized. The degree of spatial synchrony decreased and cyclicity in temporal fluctuation became obscure as trophic level goes up. The pattern of seed predation testifies to efficient predator satiation. In a comparison at different locations, rowan trees with more variable berry production are more effective in reducing losses to the seed predator. The parasitoids also exhibited an indication of satiation through the fluctuation of the host abundance. These results show that rowan masting has an adaptive foundation, which greatly impacts the dynamics on these upper trophic levels.
Key words: mast seeding, synchrony, predator satiation, numerical response, trophic cascade.