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The dwarf shrub heather Calluna vulgaris is an important component of the UK landscape, with value for amenity, nature conservation and animal production. A model, called "HeathMod", is described, which investigates the impact of grazing on upland heather. It combines an empirical model of production with a simple process-based model of grazing impacts. Validation from short-term studies in the literature indicated that the model predicts grazing impacts well, although more studies of grazing impacts are required to confirm the model's accuracy. Analysis of the model allowed derivation of a simple general formula for predicting sustainable levels of grazing, given knowledge of local site productivity. Predicted levels of sustainable grazing were lower than previous estimates, mainly because the model predicts the long-term impacts of sustained grazing. Application of the model is most appropriate in areas where heather is in mixed-age stands, because there is inadequate information available to model interactions between grazing and degeneration in even-aged stands of heather. (C) 2002 Elsevier Science Ltd. All rights reserved.

A new spatial simulation model of vegetation dynamics, called ‘VegeTate’, was applied to relationships between expansion of bracken (Pteridium aquilinum(L.) Kuhn) patches and grazing. VegeTate simulates multispecies dynamics within each cell of its grid, including localized apparent competition mediated by grazing and trampling. It can be parameterized to simulate various plant communities and species, includes seasonality and uses a hexagonal grid. Dynamics of individual bracken patches in grassland, and of heathland initially containing sparse grass and bracken were simulated. Mixtures of grass and bracken formed mosaics, in which locally high grass densities were associated with low standing mass and high grazing intensity, whereas high bracken densities were associated with high standing mass and low grazing intensity. Control of bracken by grazing depended on the impact of trampling. In simulations, small bracken patches occupying one or a few cells could be controlled by grazing, but not the expansion of large patches. In a sensitivity analysis, this phenomenon was reproduced after substantial changes in parameter values and environmental conditions, suggesting that it should be observable in the field. Interaction between patch size and grazing impact strongly influenced the spatial dynamics of simulated vegetation. The size of bracken patches became bimodal, so that most of the area of bracken was within a few large patches. Both spatial and temporal vegetation dynamics became non-linear. These results are consistent with observations that some bracken patches contain several genets and some genets occupy several patches. The hexagonal grid and trampling were important factors in the simulation results.

The youngest parts of clonal plants benefit from substantial physiological support from older parts, but the extent to which this physiological dependence persists through time is poorly understood. The development of autonomy among connected subunits was therefore analysed in the clonal species Glechoma hederacea. The stolons of a series of clonal fragments with differing numbers of primary ramets were severed at a fixed point relative to the four oldest primary ramets. The subsequent growth of both parts of the severed fragments was compared with that of a series of intact fragments.

The number of cattle herds placed under movement restrictions in Great Britain (GB) due to the suspected presence of bovine tuberculosis (bTB) has progressively increased over the past 25 years despite an intensive and costly test-and-slaughter control program. Around 38% of herds that clear movement restrictions experience a recurrent incident (breakdown) within 24 months, suggesting that infection may be persisting within herds. Reactivity to tuberculin, the basis of diagnostic testing, is dependent on the time from infection. Thus, testing efficiency varies between outbreaks, depending on weight of transmission and cannot be directly estimated. In this paper, we use Approximate Bayesian Computation (ABC) to parameterize two within-herd transmission models within a rigorous inferential framework. Previous within-herd models of bTB have relied on ad-hoc methods of parameterization and used a single model structure (SORI) where animals are assumed to become detectable by testing before they become infectious. We study such a conventional within-herd model of bTB and an alternative model, motivated by recent animal challenge studies, where there is no period of epidemiological latency before animals become infectious (SOR). Under both models we estimate that cattle-to-cattle transmission rates are non-linearly density dependent. The basic reproductive ratio for our conventional within-herd model, estimated for scenarios with no statutory controls, increases from 1.5 (0.26–4.9; 95% CI) in a herd of 30 cattle up to 4.9 (0.99–14.0) in a herd of 400. Under this model we estimate that 50% (33–67) of recurrent breakdowns in Britain can be attributed to infection missed by tuberculin testing. However this figure falls to 24% (11–42) of recurrent breakdowns under our alternative model. Under both models the estimated extrinsic force of infection increases with the burden of missed infection. Hence, improved herd-level testing is unlikely to reduce recurrence unless this extrinsic infectious pressure is simultaneously addressed., Author Summary Epidemic models are commonly used to assess the impact of alternative management strategies. The efficacy of controls is typically assumed from “expert opinion” rather than estimated from data. Managed endemic diseases such as bovine tuberculosis offer the potential to estimate the efficiency of control directly from epidemiological data. Our methodology constitutes a shift in the level of statistical rigor applied to “policy” models and offers insights into the epidemiology of Bovine tuberculosis in Great Britain. bTB continues to persist and spread relentlessly in Britain, despite extensive testing and control programs. Cattle farmers question the efficacy of cattle controls, blaming the badger wildlife reservoir. Contrary to much public perception, we demonstrate the importance of cattle-to-cattle transmission, especially in larger herds. We estimate that in the worst case scenario up to 21% of herds may be harboring infection after they clear restrictions. However, we also estimate that there is a high rate of re-introduction of infection into herds, particularly in high incidence areas. Eliminating the hidden burden of infection alone is unlikely to be sufficient to prevent recurrent breakdowns. Rather, the high rate of external infection, both through cattle movements and environmental sources, must be addressed if recurrence is to be reduced.

In a spatially explicit simulation model of vegetation dynamics (‘VegeTate’), I labelled the initial mass of Pteridium aquilinum in each of 225 cells as a single, unique genet or clone. The physical environment was homogeneous and all genets shared the same phenotype. The aim was to discover whether and how the success of each genet was affected by its initial position relative to other genets and a competing grass species. In a scenario in which grazing generated a grass-bracken mosaic with complex spatial dynamics, the amount of growth of each genet ranged widely, from frequent extinction to mass increase by over 300 times. The main factor in the impact of position on genet growth was shown to be a benefit from the initial presence or proximity of a large mass of P. aquilinum. This was because a high density of P. aquilinum reduced local grazing intensity, allowing plant mass to accumulate and shifting the balance of competition in favour of P. aquilinum. Thus variations between cells in initial mass of P. aquilinum were greatly amplified. The implications of this amplification of initial differences between sites for population genetics are briefly discussed. Qualitative features of the spatial distribution of genets at the end of simulations matched reported observations on patchily distributed field populations of P. aquilinum. These features included dominance of a large population by a small minority of genets, widespread mixtures of a dominant genet and one or more subordinate genets, and the presence of patches of P. aquilinum formed both by agglomeration from neighbouring foci and by spread of dominant genets. Under less intense grazing, which allowed little or no development of vegetation mosaics, genet growth varied relatively little and initial variations in relative mass between genets were little changed. Based on this study, I hypothesize that any processes that generate non-linear spatial dynamics will also generate complex genet dynamics.

Resistance against pesticides is a widespread and increasing problem. The control of pesticide dose and the mixing of pesticides have been proposed as methods to reduce the rate at which pesticide resistance develops. A mathematical model is developed to analyse the relationship between pesticide dose and the rate of development of pesticide resistance. The model is applicable to pesticides generally, including fungicides, herbicides and insecticides. The model measures dose in terms of its impact on a specific, sensitive pest phenotype. This measure depends both on the amount of pesticide applied and the application regime. The model separates the impact of a pesticide into a part that differs between pest phenotypes with different levels of resistance and a part that is similar for all phenotypes. Cases in which pesticide resistance can be delayed by reducing pesticide impact are defined algebraically. For these cases a simple relationship is presented between the rate at which pesticide resistance builds up, the variance of resistance in the pest population and the size of the part of the pesticide impact that differs between pest phenotypes. If pesticide resistance can be delayed by reducing pesticide impact, the part of the impact of a pesticide that does not differ between pest phenotypes determines whether it is suitable for use in mixtures. If it is positive (i.e. causes a reduction in pest growth), the pesticide is suitable for use in mixtures. Mixing only reduces the build-up of pesticide resistance by reducing the required doses of the pesticides that are mixed. Although the development of resistance against two different pesticides is delayed when resistance against one is negatively correlated with resistance against the other, mixing them is not necessarily preferable to using them in rotation. The decision whether to mix the two pesticides should still be based on their individual suitabilities for mixing, as defined above.

1 To allow assessment of the ability of plants to exploit heterogeneous habitats, the growth of the stoloniferous herb Glechoma hederacea was compared in three artificial soil environments. In the control treatment named 'uniform', potting compost was evenly mixed with sand; the treatment named 'patchy' contained the same quantity of potting compost, but half was concentrated in a central resource-rich circle in each box. In the treatment named 'low', compost was distributed homogeneously as in the first treatment, but the total quantity of potting compost was halved. 2 The biomass of G. hederacea produced in the patchy treatment was over two and a half times that produced in the uniform treatment, and over ten times that produced in the low treatment. In addition, more leaves were produced in the patchy treatment than in the other two treatments. Almost twice as many stolons reached the perimeters of the boxes. in the patchy treatment as in the uniform treatment. However, the biomass of the portions of the clones that had no basipetal connections via stolons into the central circle did not differ between treatments. 3 There was little evidence that above-ground biomass or ramets were concentrated inside the central circle of the patchy treatment relative to the other two treatments. 4 In contrast, 80% of the root biomass of plants in the patchy treatment was concentrated within the resource-rich central circle. Contrary to the situation in plants grown in homogeneous conditions, root:shoot ratio was highest where soil-derived resources were locally most abundant. Inside the central circle, ramets developed roots earlier in the patchy treatment than in the uniform treatment; outside the central circle, ramets developed roots later in the patchy treatment than in the uniform treatment. 5 In a supplementary experiment the length of young root systems increased exponentially from whatever time their growth was initiated. Thus, local delay or advancement of the initiation of root growth, in response to variations in soil characteristics, could have generated the observed concentration of root biomass inside the central circle of the patchy treatment. 6 G. hederacea can exploit environmental heterogeneity through local functional specialization of plant parts. In many habitats, species that have this ability may outcompete those adapted solely to resource-poor conditions.