Modeling ecological traps for the control of feral pigs

Abstract

Ecological traps are habitat sinks that are preferred by dispersing animals but have higher mortality or reduced fecundity compared to source habitats. Theory suggests that if mortality rates are sufficiently high, then ecological traps can result in extinction. An ecological trap may be created when pest animals are controlled in one area, but not in another area of equal habitat quality, and when there is density-dependent immigration from the high-density uncontrolled area to the low-density controlled area. We used a logistic population model to explore how varying the proportion of habitat controlled, control mortality rate, and strength of density-dependent immigration for feral pigs could affect the long-term population abundance and time to extinction. Increasing control mortality, the proportion of habitat controlled and the strength of density-dependent immigration decreased abundance both within and outside the area controlled. At higher levels of these parameters, extinction was achieved for feral pigs. We extended the analysis with a more complex stochastic, interactive model of feral pig dynamics in the Australian rangelands to examine how the same variables as the logistic model affected long-term abundance in the controlled and uncontrolled area and time to extinction. Compared to the logistic model of feral pig dynamics, the stochastic interactive model predicted lower abundances and extinction at lower control mortalities and proportions of habitat controlled. To improve the realism of the stochastic interactive model, we substituted fixed mortality rates with a density-dependent control mortality function, empirically derived from helicopter shooting exercises in Australia. Compared to the stochastic interactive model with fixed mortality rates, the model with the density-dependent control mortality function did not predict as substantial decline in abundance in controlled or uncontrolled areas or extinction for any combination of variables. These models demonstrate that pest eradication is theoretically possible without the pest being controlled throughout its range because of density-dependent immigration into the area controlled. The stronger the density-dependent immigration, the better the overall control in controlled and uncontrolled habitat combined. However, the stronger the density-dependent immigration, the poorer the control in the area controlled. For feral pigs, incorporating environmental stochasticity improves the prospects for eradication, but adding a realistic density-dependent control function eliminates these prospects.

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

“…When we scaled our deterministic models to have the same intrinsic population growth rates as our stochastic model, we found that the general patterns were similar between the two approaches but that the stochastic model tended to have larger abundances for slightly longer, and tended to persist longer. Thus, our results support Dexter & McLoed [ 55 ] because we found that the birth-pulse fluctuations in our stochastic model led to different effects of culling patterns on abundance trajectories relative to our deterministic models. But, in contrast to [ 55 ], we found that the populations with stochasticity tended to be more robust at low abundance because of the potential for higher fecundity at low densities relative to the deterministic models.…”

“…Previous work suggests that the impacts of culling may be underestimated in deterministic models that neglect fluctuations around carrying capacity and lag times in reproductive processes [ 55 ]. When we scaled our deterministic models to have the same intrinsic population growth rates as our stochastic model, we found that the general patterns were similar between the two approaches but that the stochastic model tended to have larger abundances for slightly longer, and tended to persist longer.…”

Movement responses inform effectiveness and consequences of baiting wild pigs for population control

“…First, we did not undergo concentrated efforts to remove wild pigs during our baiting program in 2016, and had minimal removal in 2018, thus most animals remained on the landscape following baiting activities. Previous studies have suggested that wild pigs from surrounding areas will quickly immigrate into areas left vacant by control efforts (Bodenchuk, 2014;Dexter and McLeod, 2015). It is unclear how immigration would further influence movement behaviors of any remaining wild pigs following baiting programs, but could possibly exasperate the negative consequences such as spread of diseases.…”

Modelling seasonal habitat suitability for wide-ranging species: Invasive wild pigs in northern Australia

“…They are most widespread in the tropical north, yet spatial knowledge is either empirical, detailed, and local scale [ 16 – 22 ], or expert-based, coarse, broad scale, and poorly validated [ 23 – 25 ]. Improved regional-scale knowledge of wild pig distribution could be used to delineate management units and limit re-invasion of conservation sites following local eradication [ 13 , 26 ]. It could also help assess the magnitude of environmental and economic impacts or the risk of establishment of infectious animal diseases, especially when abundance estimates are derived [ 27 – 30 ].…”