Simulating free-roaming cat population management options in open demographic environments

Abstract

Large populations of free-roaming cats (FRCs) generate ongoing concerns for welfare of both individual animals and populations, for human public health, for viability of native wildlife populations, and for local ecological damage. Managing FRC populations is a complex task, without universal agreement on best practices. Previous analyses that use simulation modeling tools to evaluate alternative management methods have focused on relative efficacy of removal (or trap-return, TR), typically involving euthanasia, and sterilization (or trap-neuter-return, TNR) in demographically isolated populations. We used a stochastic demographic simulation approach to evaluate removal, permanent sterilization, and two postulated methods of temporary contraception for FRC population management. Our models include demographic connectivity to neighboring untreated cat populations through natural dispersal in a metapopulation context across urban and rural landscapes, and also feature abandonment of owned animals. Within population type, a given implementation rate of the TR strategy results in the most rapid rate of population decline and (when populations are isolated) the highest probability of population elimination, followed in order of decreasing efficacy by equivalent rates of implementation of TNR and temporary contraception. Even low levels of demographic connectivity significantly reduce the effectiveness of any management intervention, and continued abandonment is similarly problematic. This is the first demographic simulation analysis to consider the use of temporary contraception and account for the realities of FRC dispersal and owned cat abandonment.

Figure 1. Generalized life-cycle diagram depicting free-roaming cat (FRC) population demographics used in simulation models.

Numbers refer to specific age cohorts, separated in age by timestep T = 6 months. Parameter S_i denotes age-specific 6-month survival rates, while F_i denotes reproductive rates across age classes. Abandonment of owned litters, as a contribution to the focal population, is represented by quantity A ₁.

Figure 2. FRC metapopulation structure used in simulation models.

Spatial representation (left) of a focal FRC population in an area surrounded by similar habitat inhabited by untreated cats, and a generalized representation (right) of that same metapopulation configuration used within the Vortex simulation environment. Dispersal rate designated by parameter D.

Figure 3. Impact of population management options on simulated FRC abundance.

(A) Fifty-year mean abundance trajectories for a simulated Large Urban population subject to different population management strategies at a rate of 40% treatment of all untreated individuals each timestep. Simulations include demographic isolation. Uppermost trajectory is the baseline, no-treatment scenario. (B) Abundance trajectories as above but with demographic connectivity (dispersal, litter abandonment). R, Remove; S, Sterilize; ConA, Contracept-A; ConB, Contracept-B.

Figure 4. Comparative performance of simulated FRC management options across population types.

Row headings define the rate of treatment of individuals, as percentage of untreated kittens (K), adults (A), or both (B) treated each 6-month timestep. Column headings identify the inclusion of specific population connectivity characteristics in a given scenario: litter abandonment (Ab), dispersal to the surrounding neighborhood population (D), or population isolation (Iso). Each cell is color-coded based on the combined result of a specific model scenario, defined in terms of the mean stochastic growth rate (r) over the 50-year timeframe of the simulation and the risk of population elimination (P(E)) within that same time period (see color key at bottom of figure). Cells shaded gray represent scenarios that were not evaluated in this analysis.

Figure 5. Impact of gender-specific management strategies on simulated FRC abundance.

Fifty-year mean abundance trajectories for a simulated Large Urban free-roaming cat population subject to 40% Removal (A, top) or Sterilization (B, bottom) of adults each timestep. Separate models feature exclusive trapping of males or females in addition to the standard scenarios featuring indiscriminant trapping across gender. Baseline models feature no management imposed on the population.

Figure 6. Stochastic population growth rate r_s under different FRC management strategies.

(A) Simulated Large Urban populations under conditions of demographic isolation. (B) Simulated Large Urban populations under conditions of demographic connectivity. Treatment rate applies to both kittens and adults. Dashed line indicates the condition where r_s = 0.0.