Food web complexity and variability in leaf beetle population size in managed and natural willow stands Peter Dalin Department of Entomology Swedish University of Agricultural Sciences Uppsala, Sweden

Managed willow system Natural willow system Natural enemies Leaf beetles Host plant

Predation on eggs of the leaf beetle Phratora vulgatissima L.

Egg predators Anthocoris nemorum (Anthocoridae) Orthotylus marginalis (Miridae) Calocoris fulvomaculatus (Miridae)

Stochastic exponential growth model N t+1 = min(λ t * N t ; K) λ t = F t * S(F t = 113) F t = f b * f e (f e = 19.3) S = s 1 * s 2 Extreme predators: * Sedentary predator P e = 1 * Active predator P b = 1 Total no. eggs laid = N t * F t Total no. egg predated is determined by P b and P e

Coefficiant of variation (CV)

Outbreak risk

Extinction risk

Conclusion: Systems dominated by predators with a sedentary behaviour can cause a higher population variability (CV) and increased extinction risk of host populations.