title: Dispersal variability and associated population-level consequences in tree-killing bark beetles.
abstract: BACKGROUND
  Dispersal is a key process in the response of insect populations to rapidly changing environmental conditions. Variability among individuals, regarding the timing of dispersal initiation and travelled distance from source, is assumed to contribute to increased population success through risk spreading. However, experiments are often limited in studying complex dispersal interactions over space and time. By applying a local-scaled individual-based simulation model we studied dispersal and emerging infestation patterns in a host - bark beetle system (Picea abies - Ips typgraphus). More specifically, we (i) investigated the effect of individual variability in beetle physiology (flight capacity) and environmental heterogeneity (host susceptibility level) on population-level dispersal success, and (ii) elucidated patterns of spatial and/or temporal variability in individual dispersal success, host selectivity, and the resulting beetle density within colonized hosts in differently susceptible environments.
  
  
  RESULTS
  Individual variability in flight capacity of bark beetles causes predominantly positive effects on population-level dispersal success, yet these effects are strongly environment-dependent: Variability is most beneficial in purely resistant habitats, while positive effects are less pronounced in purely susceptible habitats, and largely absent in habitats where host susceptibility is spatially scattered. Despite success rates being highest in purely susceptible habitats, scattered host susceptibility appeared most suitable for dispersing bark beetle populations as it ensures population spread without drastically reducing success rates. At the individual level, dispersal success generally decreases with distance to source and is lowest in early flight cohorts, while host selectivity increased and colonization density decreased with increasing distance across all environments.
  
  
  CONCLUSIONS
  Our modelling approach is demonstrated to be a powerful tool for studying movement ecology in bark beetles. Dispersal variability largely contributes to risk spreading among individuals, and facilitates the response of populations to changing environmental conditions. Higher mortality risk suffered by a small part of the dispersing population (long-distance dispersers, pioneers) is likely paid off by reduced deferred costs resulting in fitness benefits for subsequent generations. Both, dispersal variability in space and time, and environmental heterogeneity are characterized as key features which require particular emphasis when investigating dispersal and infestation patterns in tree-killing bark beetles.
authors.affiliation: Department of Ecology and Ecosystem Management, Technische Universität München, Freising, 85354 Germany ; Present address: Institute of Meteorology and Climate Research - Atmospheric Environmental Research (IMK-IFU), Karlsruhe Institute of Technology (KIT), Garmisch-Partenkirchen, 82467 Germany.
authors.forename: Markus
authors.identifier: 0000-0001-8763-3262
authors.initials: M
authors.lastname: Kautz
authors.affiliation: Department of Forest Resource Conservation, Gadjah Mada University, Yogyakarta, 55281 Indonesia.
authors.forename: Muhammad Ali
authors.initials: MA
authors.lastname: Imron
authors.affiliation: Department of Ecology and Ecosystem Management, Technische Universität München, Freising, 85354 Germany.
authors.forename: Kai
authors.initials: K
authors.lastname: Dworschak
authors.affiliation: Department of Ecology and Ecosystem Management, Technische Universität München, Freising, 85354 Germany.
authors.forename: Reinhard
authors.initials: R
authors.lastname: Schopf
keywords: Dispersal mortality; European spruce bark beetle; Foraging; Host selection; IPS-model; Individual-based modelling; Movement ecology; Pioneering; Population plasticity; Risk spreading