Potential effects of incorporating fertility control into typical culling regimes in wild pig populations

<div><p>Effective management of widespread invasive species such as wild pigs (<i>Sus scrofa</i>) is limited by resources available to devote to the effort. Better insight of the effectiveness of different management strategies on population dynamics is important for guiding decisions of resource allocation over space and time. Using a dynamic population model, we quantified effects of culling intensities and time between culling events on population dynamics of wild pigs in the USA using empirical culling patterns and data-based demographic parameters. In simulated populations closed to immigration, substantial population declines (50–100%) occurred within 4 years when 20–60% of the population was culled annually, but when immigration from surrounding areas occurred, there was a maximum of 50% reduction, even with the maximum culling intensity of 60%. Incorporating hypothetical levels of fertility control with realistic culling intensities was most effective in reducing populations when they were closed to immigration and when intrinsic population growth rate was too high (> = 1.78) to be controlled by culling alone. However, substantial benefits from fertility control used in conjunction with culling may only occur over a narrow range of net population growth rates (i.e., where net is the result of intrinsic growth rates and culling) that varies depending on intrinsic population growth rate. The management implications are that the decision to use fertility control in conjunction with culling should rely on concurrent consideration of achievable culling intensity, underlying demographic parameters, and costs of culling and fertility control. The addition of fertility control reduced abundance substantially more than culling alone, however the effects of fertility control were weaker than in populations without immigration. Because these populations were not being reduced substantially by culling alone, fertility control could be an especially helpful enhancement to culling for reducing abundance to target levels in areas where immigration can’t be prevented.</p></div

Effects of 5 culling patterns under different model structures.

<p>X-axis: weekly abundance over 10 years. Five different culling patterns are distinguished using the same color scheme as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0183441#pone.0183441.g001" target="_blank">Fig 1</a>. Black lines indicate conditions with no culling. Top plots are for populations with no immigration; bottom plots are for populations with immigration. For the individual-based model, each line is the mean replicate simulations for runs that led to eradication within 10 years. The probability of eradication shows the proportion of 30 simulations that led to eradication. The mean behavior of runs that did not lead to eradication within 10 years is shown in Fig D in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0183441#pone.0183441.s001" target="_blank">S1 File</a>.</p

Description of parameters.

<p>Description of parameters.</p

Effects of adding fertility control.

<p>Percent reduction in abundance due to sterilant (i.e., relative to culling only) after 4 years as a function of different net population growth rates (note: negative <i>r</i> values due to culling). Each point represents one simulation. Lines are predictions from the model presented in Table C in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0183441#pone.0183441.s002" target="_blank">S2 File</a>. Predictions were truncated outside the range of the data. Colors indicate data from different intrinsic population growth rate parameters: Red: <i>r</i> = 0.26, Blue: <i>r</i> = 0.58, Black: <i>r</i> = 0.89. A, C, E and G columns are for populations closed to immigration, B, D, F and H columns indicate scenarios where immigration from neighboring populations occurs. Predictions are cut-off to avoid predicting outside the range of the data.</p

Relationship of population reduction metrics and culling conditions without (A,C,E) and with (B,D,F) immigration from surrounding areas.

<p>Points in A-D show all the data for the indicated culling intensity and gap period; points in E and F show all the data for all culling intensity and gap periods from similar conditions. Lines are predictions using the models presented in Tables A and B in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0183441#pone.0183441.s002" target="_blank">S2 File</a>. Colors indicate data from different intrinsic population growth rate parameters: Red: <i>r</i> = 0.26, Blue: <i>r</i> = 0.58, Black: <i>r</i> = 0.89. The A,B and C,D give predictions for two different gap lengths: 8.6 days (dotted), 25 days (solid). Predictions are cut-off to avoid predicting outside the range of the data.</p