Modelling collective foraging in endemic bark beetle populations (original) (raw)
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Royal Society open science, 2018
Bark beetles use aggregation pheromones to promote group foraging, thus increasing the chances of an individual to find a host and, when relevant, to overwhelm the defences of healthy trees. When a male beetle finds a suitable host, it releases pheromones that attract potential mates as well as other 'spying' males, which result in aggregations on the new host. To date, most studies have been concerned with the use of aggregation pheromones by bark beetles to overcome the defences of living, well-protected trees. How insects behave when facing undefended or poorly defended hosts remains largely unknown. The spatio-temporal pattern of resource colonization by the European eight-toothed spruce bark beetle, , was quantified when weakly defended hosts (fallen trees) were attacked. In many of the replicates, colonization began with the insects rapidly scattering over the available surface and then randomly filling the gaps until a regular distribution was established, which resul...
Trees Wanted---Dead or Alive! Host Selection and Population Dynamics in Tree-Killing Bark Beetles
PLOS One, 2011
Bark beetles (Coleoptera: Curculionidae, Scolytinae) feed and breed in dead or severely weakened host trees. When their population densities are high, some species aggregate on healthy host trees so that their defences may be exhausted and the inner bark successfully colonized, killing the tree in the process. Here we investigate under what conditions participating with unrelated conspecifics in risky mass attacks on living trees is an adaptive strategy, and what this can tell us about bark beetle outbreak dynamics. We find that the outcome of individual host selection may deviate from the ideal free distribution in a way that facilitates the emergence of tree-killing (aggressive) behavior, and that any heritability on traits governing aggressiveness seems likely to exist in a state of flux or cycles consistent with variability observed in natural populations. This may have implications for how economically and ecologically important species respond to environmental changes in climate and landscape (forest) structure. The population dynamics emerging from individual behavior are complex, capable of switching between ''endemic'' and ''epidemic'' regimes spontaneously or following changes in host availability or resistance. Model predictions are compared to empirical observations, and we identify some factors determining the occurrence and self-limitation of epidemics.
Host Suitability, Predation, and Bark Beetle Population Dynamics
Population Dynamics, 1995
Tree-killing bark beetles are the most economically important insects in conifer forests worldwide. However, despite N200 years of research, the drivers of population eruptions and crashes are still not fully understood and the existing knowledge is thus insufficient to face the challenges posed by the Anthropocene. We critically analyze potential biotic and abiotic drivers of population dynamics of an exemplary species, the European spruce bark beetle (ESBB) (Ips typographus) and present a multivariate approach that integrates the many drivers governing this bark beetle system. We call for hypothesis-driven, large-scale collaborative research efforts to improve our understanding of the population dynamics of this and other bark beetle pests. Our approach can serve as a blueprint for tackling other eruptive forest insects. Population Dynamics of Forest Insects The abundance of an organism is determined by a variety of factors related to intra-and interspecific biotic interactions as well as abiotic conditions [1]. In forest ecology, researchers have been fascinated and challenged by the diversity of drivers that govern the eruptive population dynamics of foliage-feeding moths (various Lepidoptera families) and tree-killing bark beetles (see Glossary) (Coleoptera: Scolytinae), including the influence of host trees, symbionts, natural enemies, and competitors (Table 1) as well as climate and land use [2-7]. Given that the joint effect of these biotic and abiotic drivers as well as their interactions are still not well understood for many of these insects (Table 1) and their tree hosts [2,4,5,7-9], it is questionable whether we are prepared to deal with the challenges our forests face in the Anthropocene (i.e., climatic changes and intensification of forest management) [10]. Studies on the population dynamics of eruptive forest insects, and particularly bark beetles, currently focus on variables that can be easily measured over large geographic and temporal scales, like insect and tree host abundance, tree host connectivity, and abiotic climatic factors [2,4,8,9,11]. By contrast, other biotic factors are more difficult to measure at large scales and thus there is a lack of understanding of their roles in regulating insect abundances. Support for their importance comes from small-scale studies on the effects of antagonistic symbionts [12,13], natural enemies [6,7,14-17], and insect genotype [18,19] on the population dynamics of bark beetles, moths, and other eruptive forest insects (Table 1). Moreover, studies usually focus on examining the factors driving outbreaks but largely neglect the equally important causes of population collapse. For example, in cases with abundant but healthy host trees, collapse is often attributed to the absence of factors known to facilitate outbreaks (e.g., poor tree health [20,21]). This is an oversimplification, however, because factors regulating non-outbreak populations are typically different from the ones regulating outbreak populations (Table 1; [4,5,7-9,14,22,23]). Highlights Bark beetles are currently causing unprecedented damage to European and North American forests.
Site condition and predation influence a bark beetle's success: a spatially realistic approach
Agricultural and Forest Entomology, 2003
1 Spatial pattern in abundance of Dendroctonus micans was studied in a 600-ha spruce stand in the Massif Central (LozeÁ re, France). The proportion of trees attacked was measured in 38 plots and these data were used to estimate spatial pattern of attack density in the stand and to identify a transect of decreasing attack density (80% to 30%) over less than 1000 m. 2 Spatial variation in attack density was analysed in relation to (i) data on site and stand characteristics (altitude, slope, tree density, tree average height, yield class and average age) collected from 63 points in the stand and (ii) the releases of the predator Rhizophagus grandis (localization and number of beetles released). 3 The proportion of attacked trees was analysed using geostatistics and showed a strong spatial structure reflecting the spatial scale of interaction of D. micans with its environment. The spatial structure was modelled in order to estimate the spatial distribution of attack density at unsampled locations. 4 A linear model relating interpolated attack density to the number of predators released 6±10 years before the survey in a 300-m radius and to the average slope over a 250-m radius explained 67% of the observed variability. Spatial autocorrelation was taken into account in a spatial regression model.
Dispersal variability and associated population-level consequences in tree-killing bark beetles
Movement Ecology, 2016
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.
Past attacks influence host selection by the solitary bark beetle Dendroctonus micans
Ecological Entomology, 2001
1. A spatio-temporal study of host selection and local spread of a solitary bark beetle attacking live spruce Dendroctonus micans (Kugelann) was carried out using a combination of standard statistical methods, geostatistical analyses, and modelling. The study was based on data from three plots (150±300 trees, 0.3±1 ha) from 1978 to 1993. All trees were mapped and successful and abortive bark-beetle attacks on each tree were counted annually. Because the attacked trees usually survived, temporal attack patterns as well as spatial patterns could be analysed.
Simulating bark beetle population dynamics in response to windthrow events
Ecological Complexity, 2017
The relationship between windthrow disturbance and outbreaks of European spruce bark beetle Ips typographus L. in European Norway spruce forests has been the focus of recent studies. However, the nature in which the spatial characteristics of windthrow events influence bark beetle population dynamics is rarely examined. This represents a significant gap in the literature, as our understanding of how spatial windthrow patterns influence bark beetles can be useful for management efforts to help mitigate large-scale bark beetle disturbance. The objective of this study is to simulate how windthrow events facilitate bark beetle population state transitions from endemic and epidemic levels using a spatially explicit agent-based model. We examined how the spatial extent of windthrow events and the size of tree clusters impacted by windthrow influence this state transition. The results show that the beetle population transition slows with increasing spatial extent of a windthrow event and with larger clusters of windthrown trees, while scattered patterns of windthrown trees accelerate the timing of this transition. This study contributes to our understanding of the role of large-scale wind disturbance in European bark beetle outbreaks. Moreover, it provides a basis for further research to discover the impact of potential forest management applications aiming to mitigate the risk of bark beetle outbreaks.