Cat Population Simulator Documentation
Complete guide to parameters, algorithms, and scientific foundations
Legacy model and Hawaiʻi scenarios
This documentation describes the existing educational engine. Its archived audit identifies demographic defects that remain unresolved by the location explorer. Population outputs are not validated Hawaiʻi forecasts.
Every preset is a hypothetical Hawaiʻi scenario with explicit assumptions, including the closed-population case. Presets are not calibrated local colonies. Selecting one resets the complete parameter set.
Mapped environmental evidence, model estimates, management choices and official health advice are separate. The explorer does not infer infection probability or pregnancy-safe areas.
Explore Hawaiʻi watershed and stream contextOverview & Scientific Basis
This simulator uses an individual-based stochastic population model to project feral cat population dynamics under different management scenarios. The model is based on peer-reviewed research, primarily:
- Miller et al. (2014) - Core methodology for open population modeling
- Boone et al. (2019) - Long-term mortality tracking framework
- Nutter et al. (2004) - Baseline demographic parameters
- Gunther et al. (2022) - Compensatory mechanisms in managed populations
Key Model Features
- 6-month timesteps - Aligned with feline breeding cycles
- 1,000 Monte Carlo iterations - Provides confidence intervals
- Individual tracking - Each cat has age, sex, and sterilization status
- Open population dynamics - Immigration, emigration, and abandonment
- Density dependence - Realistic population regulation
Why 6-Month Timesteps?
Cats can reach sexual maturity at 4-6 months and have a gestation period of ~63 days. A 6-month timestep captures one complete breeding cycle while being computationally efficient. This approach is standard in peer-reviewed cat population models.
Why 1,000 Iterations?
Population dynamics are inherently stochastic—random events affect outcomes. Running 1,000 simulations and averaging results provides stable estimates with 95% confidence intervals, showing the range of likely outcomes rather than a single prediction.
Core Parameters
Initial Population
initialPopulationThe starting number of cats in your focal area. This represents the colony or population you want to model.
Scientific basis: Miller et al. (2014) used 50 cats as a typical focal population representing approximately 0.5 km² of urban habitat.
• For a single colony, use actual count if known
• For an area, estimate based on sightings or surveys
• Larger populations take longer to simulate but give more stable results
Simulation Length
simulationYearsHow many years to project the population forward. Longer simulations show long-term trends but have more uncertainty.
Scientific basis: Boone et al. (2019) used 10-year simulations as standard, noting that management effects often take 5+ years to manifest.
• 5 years: Short-term planning
• 10 years: Standard for comparing strategies
• 20+ years: Long-term population trajectory
Management Strategy
strategyThe intervention approach applied to the population.
Scientific basis: Based on strategies modeled in Miller et al. (2014) and Boone et al. (2019).
• None: Baseline—what happens without intervention
• TNR: Trap-Neuter-Return—cats are sterilized and returned
• Removal: Cats are permanently removed from population
• Hybrid: Kittens/juveniles removed, adults sterilized
Intervention Intensity
intensityThe proportion of targetable cats that your program attempts to treat each 6-month period.
Scientific basis: Miller et al. (2014) found 15-20% intensity needed for decline in isolated populations, 30%+ in connected populations.
• 25%: Low-intensity, volunteer-based effort
• 50%: Moderate, consistent program
• 75%+: High-intensity, well-funded program
• Actual cats treated = intensity × trappability (70%)
Carrying Capacity
carryingCapacityThe maximum population the environment can sustain. Population growth slows as this limit is approached.
Scientific basis: Carrying capacity is determined by food availability, shelter, and territory. Urban areas with feeding stations have higher capacity.
• Set to 2x initial population for typical scenarios
• Higher values allow more population growth
• Lower values create more density-dependent effects
Demographic Parameters
These parameters control birth and death rates. They are based on field studies of feral cat populations and have the greatest impact on population dynamics.
Kitten Survival (0-6 months)
kittenSurvivalThe probability that a kitten survives to 6 months of age. This is the most critical demographic parameter.
Scientific basis: Nutter et al. (2004) documented 75% kitten mortality in feral populations. Levy et al. (2003) found similar rates.
• 25%: Typical feral conditions
• 35-40%: Managed colony with feeding
• 45-50%: Exceptional care, low predation
• This parameter is heavily affected by density
Juvenile Survival (6-12 months)
juvenileSurvivalThe probability that a juvenile (6-12 months old) survives to adulthood.
Scientific basis: Juveniles have passed the high-mortality kitten stage but haven't fully developed adult survival skills.
• 60%: Harsh conditions, high predation
• 70%: Typical feral conditions
• 80%: Managed colony
Adult Survival (per 6 months)
adultSurvivalThe probability that an adult cat survives each 6-month period. Adults are the most resilient age class.
Scientific basis: Miller et al. (2014) found adult survival has the highest elasticity (0.573)—changes in adult survival have 3x more impact than changes in reproduction.
• 70%: High-risk environment (traffic, predators)
• 80%: Typical feral conditions
• 90%: Protected colony, low threats
Litters Per Year
littersPerYearAverage number of litters a breeding female produces per year.
Scientific basis: Nutter et al. (2004) documented 1.4 litters/year in feral populations. Cats can theoretically have 3 litters/year but rarely achieve this.
• 1.0: Poor conditions, seasonal breeding
• 1.4: Typical feral population
• 2.0+: Excellent conditions, year-round breeding
Kittens Per Litter
kittensPerLitterAverage number of kittens born per litter.
Scientific basis: Median litter size is 3-4 kittens (Nutter et al. 2004). Range is typically 1-8.
• 2-3: Young or older mothers
• 3-4: Prime breeding age
• 5+: Exceptional fertility
Female Ratio
femaleRatioProportion of the population that is female. Only females can reproduce.
Scientific basis: Sex ratio at birth is approximately 1:1. Adult populations may skew slightly due to differential mortality or dispersal.
• 50%: Natural sex ratio
• 55%: Slight female bias (males disperse more)
• 45%: Male-biased (some colonies)
Population Connectivity
Why Connectivity Matters
Miller et al. (2014) showed that immigration from surrounding areas is the primary reason TNR programs fail to reduce populations. Even high-intensity TNR cannot overcome continuous influx of unsterilized cats from neighboring areas.
Population Type
populationTypeWhether cats can enter or leave the focal population.
Scientific basis: Most real populations are 'open'—cats move between areas. Truly closed populations (islands, fenced areas) are rare.
• Open: Realistic for most mainland populations
• Closed: Islands, fenced sanctuaries, or isolated areas
• Open populations are much harder to manage
Neighborhood Size
neighborhoodSizeThe estimated cat population in surrounding areas that could send immigrants to your focal area.
Scientific basis: Miller et al. (2014) used 4x the focal population as neighborhood size, representing the surrounding 2 km².
• Set to 0 for closed populations
• 4x focal population is a reasonable default
• Urban areas may have much larger neighborhoods
Immigration Rate
immigrationRatePercentage of the neighborhood population that immigrates to the focal area each 6-month period.
Scientific basis: Boone et al. (2019) used 2% as default. Young males are primary dispersers (75% male bias in immigrants).
• 0%: Closed population
• 1%: Low connectivity
• 2%: Moderate connectivity (default)
• 5%+: High connectivity, urban areas
Emigration Rate
emigrationRatePercentage of the focal population that leaves each 6-month period.
Scientific basis: Emigration partially offsets immigration but sterilized cats are less likely to leave established territories.
• Usually set equal to immigration rate
• Lower emigration = population accumulates
• Sterilized cats tend to stay in territory
Abandonment Rate
abandonmentRateNumber of pet cats abandoned into the feral population each 6-month period.
Scientific basis: Miller et al. (2014) used 4 kittens/timestep (2 male, 2 female) as default abandonment rate.
• 0: No abandonment (rare)
• 2-4: Typical urban/suburban
• 8+: High abandonment areas
• ~30% of abandoned cats may already be sterilized
Density Dependence
As population density increases, competition for resources intensifies. This creates natural population regulation—and also explains why populations can rebound after management reduces numbers.
How Density Affects Population
Abundant resources. Kitten survival and breeding rates at maximum. Population grows rapidly.
Competition increasing. Kitten mortality rises, breeding rate decreases. Population growth slows.
Severe competition. High kitten mortality, reduced breeding, slight adult mortality increase. Population stabilizes near carrying capacity.
Density-Dependent Kitten Mortality
densityKittenMortalityHow strongly kitten survival decreases as population approaches carrying capacity.
Scientific basis: Miller et al. (2014) documented increased kitten mortality at high densities due to resource competition and disease.
• 0: No density effect (unrealistic)
• 0.5: Moderate effect (default)
• 1.0: Strong effect—kitten survival drops sharply at high density
Density-Dependent Breeding Reduction
densityBreedingReductionHow strongly breeding rate decreases as population approaches carrying capacity.
Scientific basis: Gunther et al. (2022) documented reduced breeding in high-density populations due to stress and resource scarcity.
• 0: No effect on breeding
• 0.3: Moderate reduction (default)
• 0.6+: Strong reduction—fewer litters at high density
Compensatory Effects
When population is reduced (by any management strategy), remaining cats have more resources. This leads to improved kitten survival andincreased breeding—partially offsetting management efforts. This is why sustained, high-intensity intervention is required for population reduction.
Management Strategies
No Action
Baseline scenario showing natural population dynamics without intervention.
- • Population regulated by natural mortality
- • High kitten mortality (75%)
- • Population stabilizes at carrying capacity
- • Useful for comparison with management scenarios
TNR (Trap-Neuter-Return)
Cats are trapped, sterilized, and returned to their territory.
- • Sterilized cats remain in population
- • Occupy territory, reducing immigration
- • Requires 70%+ sterilization rate for decline
- • Less effective in open populations
Removal
Cats are permanently removed from the population.
- • Immediate population reduction
- • Opens territory for immigrants
- • Triggers compensatory reproduction
- • Requires sustained effort
Hybrid
Kittens and juveniles removed; adults sterilized and returned.
- • Combines benefits of both approaches
- • Adults maintain territory
- • Removes high-mortality age classes
- • May be more resource-efficient
Strategy Effectiveness (from Miller et al. 2014)
| Strategy | Intensity for Decline (Isolated) | Intensity for Decline (Connected) |
|---|---|---|
| TNR | 15-20% | 30%+ |
| Removal | 15-20% | 25-30% |
| Hybrid | ~15% | ~25% |
Cost Calculations
Sterilization Cost
sterilizationCostCost per cat for spay/neuter surgery, including anesthesia and basic care.
Scientific basis: Costs vary by location. Hawaii costs are typically higher ($100-150) due to limited veterinary resources.
• $25-50: Low-cost clinic, mainland
• $75: Typical subsidized program
• $100-150: Hawaii, full-service
• $150+: Private veterinary clinic
Trapping Cost
trappingCostCost per trapping event, including equipment, bait, and labor.
Scientific basis: Includes trap depreciation, bait, transport, and volunteer/staff time.
• $0: Volunteer-only programs
• $25: Typical with some paid staff
• $50+: Professional trappers
Feeding Cost
feedingCostPerMonthMonthly cost to feed each cat in a managed colony.
Scientific basis: Feeding is often part of TNR programs to monitor colonies and maintain cat health.
• $0: No managed feeding
• $10-15: Basic feeding program
• $25+: Premium food, supplements
Removal Cost
removalCostCost per cat for permanent removal, including trapping, transport, and processing.
Scientific basis: Includes all costs associated with removing a cat from the population.
• $50-100: Shelter intake
• $100-150: Professional removal
• $200+: Remote area removal
Total Cost Calculation
Total Cost =
(Cats Sterilized × Sterilization Cost) +
(Trapping Events × Trapping Cost) +
(Population × Feeding Cost × Months) +
(Cats Removed × Removal Cost)
Cost per Cat Reduced = Total Cost ÷ (Initial Population - Final Population)
Simulation Algorithm
Each simulation runs for the specified number of years, with events processed in a specific order each 6-month timestep. This order is based on the Vortex population modeling software used in peer-reviewed studies.
Reproduction
Intact adult females may produce litters. Litter size is drawn from a Poisson distribution. Breeding probability is affected by density and seasonality.
Mortality
Each cat faces age-specific mortality. Kittens have highest mortality (75%), affected by density. Adults have lowest mortality (~20% per 6 months).
Aging
All surviving cats age by 6 months. Kittens become juveniles, juveniles become adults.
Movement
For open populations: immigrants arrive (Poisson-distributed), emigrants leave, abandoned cats are added.
Intervention
Management actions are applied. Cats are selected for treatment based on intensity and trappability (70%).
Recording
Population counts, costs, and mortality are recorded for this timestep.
Why This Order?
Processing reproduction before mortality means newborn kittens experience mortality in the same timestep they are born. This is biologically appropriate—kittens born at the start of a 6-month period face mortality throughout that period.
Parameter Relationships
Parameters don't act in isolation—they interact in complex ways. Understanding these relationships helps interpret simulation results.
Density → Kitten Survival
NegativeStrongAs population approaches carrying capacity, kitten survival decreases due to resource competition. At 90% capacity, kitten survival may drop from 25% to 15%.
Density → Breeding Rate
NegativeModerateHigh density causes stress and resource scarcity, reducing breeding frequency. Females may skip breeding cycles or resorb litters.
Low Density → Survival (Compensatory)
PositiveModerateWhen population is reduced, remaining cats have more resources. Kitten survival can increase by up to 30%, partially offsetting management efforts.
Sterilization Rate → Population Growth
NegativeStrongSterilized cats can't reproduce but still occupy territory. At 70%+ sterilization, population typically declines.
Sterilization Rate → Immigration Effect
NegativeModerateSterilized cats maintain territory, reducing space for immigrants. This 'buffer effect' is a key benefit of TNR.
Immigration Rate × Neighborhood Size → Immigrants
MultiplicativeHighActual immigrants = neighborhood size × immigration rate. A 200-cat neighborhood with 2% immigration = 4 immigrants per timestep.
Adult Survival → Population Growth
PositiveHighestAdult survival has 3x more impact on population growth than reproduction (elasticity = 0.573). This is because adults breed multiple times over their lifespan.
Understanding Outputs
Population Statistics
- Final Population (Mean)
- Average population at end of simulation across all 1,000 iterations.
- 95% Confidence Interval
- Range containing 95% of simulation outcomes. Wider intervals indicate more uncertainty.
- Probability of Decline
- Percentage of iterations where final population was lower than initial. Higher is better for management.
- Sterilization Rate
- Proportion of final population that is sterilized. 70%+ typically needed for decline.
Mortality Breakdown
- Total Deaths
- All cat deaths during the simulation period.
- Kitten Deaths
- Deaths of cats 0-6 months old. Typically the largest category.
- Natural Deaths
- Deaths from age, disease, environment, and density effects.
- Intervention Deaths
- Cats removed through management (removal strategy only).
Interpreting Mortality Data
High mortality is not necessarily "bad"—it's a natural part of population dynamics. Without intervention, populations are regulated by high kitten mortality. TNR reduces births, which reduces kitten deaths. Removal shifts mortality from natural to intervention. The simulator presents this data neutrally for users to interpret based on their own values and goals.
Limitations & Caveats
Simplified Age Structure
The model uses 4 age classes (kitten, juvenile, adult, senior) rather than continuous aging. This is sufficient for population-level predictions but doesn't capture individual variation.
No Spatial Structure
The model assumes a well-mixed population. It doesn't account for territory boundaries, movement patterns, or spatial clustering. Use for single colonies or defined areas.
Constant Immigration
Immigration rate is constant throughout the simulation. In reality, immigration may vary seasonally or in response to local population changes.
No Disease Dynamics
Disease is modeled as a constant mortality risk, not as epidemics that spread through populations. This is adequate for most scenarios but may underestimate mortality during disease outbreaks.
Stochastic Uncertainty
Even with 1,000 iterations, results have uncertainty. Use confidence intervals and probability of decline rather than focusing on exact numbers.
When NOT to Use This Model
- • Individual cat outcomes - Model predicts population trends, not individual fates
- • Very small colonies (<10 cats) - Stochastic effects dominate, high uncertainty
- • Short timeframes (<1 year) - Seasonal variation may dominate
- • Precise predictions - Use for relative comparisons, not exact numbers
Scientific References
Miller PS, Boone JD, Briggs JR, et al. (2014). Simulating Free-Roaming Cat Population Management Options in Open Demographic Environments. PLOS ONE.DOI
Boone JD, Miller PS, Briggs JR, et al. (2019). A Long-Term Lens: Cumulative Impacts of Free-Roaming Cat Management Strategy and Intensity on Preventable Cat Mortalities. Frontiers in Veterinary Science.DOI
Nutter FB, Levine JF, Stoskopf MK (2004). Reproductive capacity of free-roaming domestic cats and kitten survival rate. Journal of the American Veterinary Medical Association.DOI
Gunther I, Hawlena H, Azriel L, et al. (2022). Reduction of free-roaming cat population requires high-intensity neutering in spatial contiguity to mitigate compensatory effects. Proceedings of the National Academy of Sciences.DOI
Lohr CA, Lepczyk CA (2014). Desires and Management Preferences of Stakeholders Regarding Feral Cats in the Hawaiian Islands. Conservation Biology.DOI
Levy JK, Gale DW, Gale LA (2003). Evaluation of the effect of a long-term trap-neuter-return and adoption program on a free-roaming cat population. Journal of the American Veterinary Medical Association.