Model Predictions on the Effect of Control Measures

Before the start of control, 80% of the populations starting with a low number of individuals became established and invaded the grid. Those populations failing to become sustainable usually died out in the initial phase (before t = 10) and were not considered in the evaluation of control measures.

Without any control (0% intensity), occupation of the complete grid was reached after approximately 85 time steps. Due to saturation of the grid, the growth rate of populations then approximated R = 1. At the time control started (t = 45) the average growth rate was R = 1.16 + 0.31.

Figure 18.1 compares different management options applied over a period of 5 years with different intensity in their effect on population growth rate R. Only when management intensity was 100% was a local eradication of

Control intensity (%)

Fig. 18.1. Comparison between different management options for H. mantegazzianum. Management was applied to the populations over 5 years at different intensities. Growth rates R were averaged over the 5-year treatment period. Reduction of growth rate R below the threshold value R = 1 is a necessary condition for successful control.

Control intensity (%)

Fig. 18.1. Comparison between different management options for H. mantegazzianum. Management was applied to the populations over 5 years at different intensities. Growth rates R were averaged over the 5-year treatment period. Reduction of growth rate R below the threshold value R = 1 is a necessary condition for successful control.

1.75

100% management intensity 80% management intensity 50% management intensity

1.50

cc o

0.00

100% management intensity 80% management intensity 50% management intensity

Fig. 18.2. Example results for the long-term effect of different intensities of grazing for H. mantegazzianum. Only 100% management led to an eradication of the population within a 5-year time period. The two other options only slightly reduced growth rates. In the 80% treatment demographic, stochasticity led to extinction after approximately 44 years of control.

Fig. 18.2. Example results for the long-term effect of different intensities of grazing for H. mantegazzianum. Only 100% management led to an eradication of the population within a 5-year time period. The two other options only slightly reduced growth rates. In the 80% treatment demographic, stochasticity led to extinction after approximately 44 years of control.

the populations possible within a 5-year period. A reduction of growth rates below the threshold value R = 1 is a necessary, but not yet sufficient, condition for successful local eradication of the populations in the longer term. Control with only 90% intensity fulfilled this condition for all management options, except for habitat reduction. In the case of grazing, even management intensities of 80% and less reduced growth rate below R = 1. In all other scenarios, population densities were reduced, but growth rates remained above R = 1, thus enabling populations to increase steadily.

Even in cases where management intensity was high enough to reduce R to values smaller than 1, this might not lead to extinction of the population within a reasonable time. In an example comparing 'grazing' at 80% and 50% control intensity, both reduced R to values < 1 (Fig. 18.2). The former treatment reduced individual numbers markedly (not shown) and thus led to increased influence of demographic stochasticity, which resulted in large fluctuations and finally to extinction of the population, in this example after approximately 44 years. The lowest control intensity allowed the population to survive at a lower growth rate fluctuating around R = 1.

The reduction of population growth-rate over time clarified the different effects of control options on population growth rates (Fig. 18.3). Grazed populations showed the strongest response to control over the management period (Fig. 18.3A). The second strongest response was to umbel cutting (Fig. 18.3B), followed by seedling establishment (Fig. 18.3C). For control intensities < 60% the prevention of seedling establishment became more reliable than umbel cutting due to lower annual fluctuations in growth rate. Habitat reduction was the least effective measure in both scenarios (Fig. 18.3D).

A) Grazing B) Umbel cutting

A) Grazing B) Umbel cutting

Fig. 18.3. Effect of different management options on growth rate R of H. mantegazzianum over time. The four different management strategies started at t = 45 years and were applied for 5 years.

The second scenario investigated the effect of duration of control. Incomplete management (90%) was applied over 55 years. The qualitative ranking of management success for the different methods was not altered by lower impact but was by prolonged treatment. Survival probability P for the populations was successfully reduced to P = 0 in three of the treatments. Only habitat reduction was not able to affect population survival rates within this time span (Fig. 18.4). Grazing again was the most successful strategy, reaching P = 0 after 38 years of treatment, while umbel cutting achieved P = 0 after 54 years of treatment. However, seedling reduction was not able to reduce P fully to zero in the management period, as the lowest value reached in 55 years of treatment was P = 0.07. Habitat reduction had the least effect and led to only one extinction (P = 0.98) within the simulation period.

Grazing Umbel cutting Seedling establishment Habitat reduction

Grazing Umbel cutting Seedling establishment Habitat reduction

Time (years)

Fig. 18.4. Development of population-survival probability P of H. mantegazzianum under continuous treatment of 90% control, starting in year t = 45 of population development. Grazing and umbel cutting reached P = 0 for the populations most quickly, after 38 (t = 83) and 54 years (t = 99) respectively. Prevention of seedling establishment and habitat reduction did not reach P = 0 within a management period of 55 years.

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