PhD Thesis:

Price, B., 2005. Spatio-temporal modelling and analysis of 
        larch bud moth population dynamics in the European Alps. Diss. ETH No. 16423, 
        Swiss Federal Institute of Technology: Zurich, Switzerland, 220pp.
	doi: 10.3929/ethz-a-005151619

Abstract:

The larch bud moth (Zeiraphera diniana GN.; Lep., Tortricidae) has caused large-scale defoliation of larch trees across the entire Alpine Arc every 8-10 years since at least Roman times. The temporal dynamics of the larch bud moth and its population cycles have been researched intensively, however the spatial patterns of larch bud moth population have received less attention. Some studies have noted the synchrony of population cycles within valleys, or, at the Alpine arc scale, travelling waves of larch bud moth (as a special case of synchrony). However, accurate quantification of these patterns has not previously been shown and the reasons for such patterns remain unclear. Recent research into synchrony suggests dispersal or regional environmental correlation (the Moran Effect) could result in such patterns of synchrony. Knowledge of spatiotemporal patterns of population dynamics is of general interest to ecologists and important for management and conservation purposes. As ecological field studies with high spatial and temporal resolution and extent are usually prohibitively expensive and time consuming, modelling studies are necessary for the management and understanding of systems over wide spatial and temporal grains and extents. However, knowledge of the appropriate grains and extents at which to model is necessary to achieve usable results.

The main aims of this research were to 1) to determine the spatio-temporal dynamics of larch bud moth populations at differing scales and 2) to investigate the influence of spatial data resolution on modelling larch bud moth dynamics and determine an optimum resolution for modelling larch bud moth dynamics in the Upper Engadine valley allowing for a balance between model complexity, output accuracy and simulation time.

Through time series analysis, in particular cross-correlation and cross-spectral analysis, we were able to confirm that patterns of synchrony at the valley scale and travelling waves at the Alpine arc scale in population cycles of larch bud moth are present but not consistent across all sites in the Upper Engadine valley. At the Alpine arc scale, waves of larch bud moth travel from west to east across the Alpine arc. Wind-driven dispersal mechanisms in conjunction with a gradient in habitat quality (possibly habitat connectivity) provide a feasible explanation for this phenomenon, whereas the Moran effect does not.

At the scale of the Upper Engadine valley, populations of larch bud moth are in close synchrony with one another with the exception of populations in areas to which migration is restricted due to orographical effects. This finding also confirms the hypothesis that migration is driving synchrony at the valley extent rather than the Moran effect.

We modelled larch bud moth population dynamics and migration under the same process models at the spatial extent of a single valley (The Upper Engadine valley) but with differing spatial grains: 1. that of the entire valley, 2. that of areas known as 'sites' which have an average area of 3.7km2 and are homogeneous with respect to altitude, forest type and aspect, and 3. that of the forest compartment, which have an average area of 25 hectares. We revealed that for the larch bud moth, from these spatial grains, optimum modelling spatial grain is that of the 'site'. However, dispersal appears to be more sensitive to wind conditions as influenced by orography at a higher spatial resolution than has been assumed previously. Thus, while larch bud moth populations should only be considered distinct at the 'site' level and therefore local dynamics modelled at this spatial grain, modelling of migration processes between the 'sites' taking into account orography at a higher resolution would produce more accurate predictions. Comparison of our time series analysis and modelling results revealed that migration is an important causal mechanism for observed patterns of synchrony in larch bud moth populations at the valley scale.

Full text 7.9 MB       doi: 10.3929/ethz-a-005151619