First assessment of population size, population dynamics, and carrying capacity of free-roaming dogs in Karaj, Iran

Dogs ( Canis familiaris ) were the first animals domesticated by humans, likely in Eurasia between 20,000–40,000 years ago, and remain deeply integrated into human societies. However, in many countries, free-roaming dog populations have expanded due to uncontrolled reproduction, abandonment, and widespread supplemental feeding. In Iran, scientific assessments of these populations are scarce despite growing ecological, social, and public health concerns. This study provides the first systematic estimation of population size, demographic structure, and environmental carrying capacity of free-roaming dogs in Karaj, Iran. We conducted six surveys across 35 randomly selected blocks between May and August 2024 using a modified WSPA block-survey method. Age class, sex, visible health status, and ear-tag information were collected. Vital demographic rates, including birth, death, survival, and growth, were calculated and incorporated into a Leslie matrix to model long-term population dynamics under sterilization and euthanasia scenarios. Carrying capacity was estimated using a model integrating human support, shelter availability, demographic structure, and ecological variables. The mean estimated population size was 7,459 dogs (95% CI ± 1,530). Human support was present in 94% of surveyed blocks, elevating the carrying capacity to 68,895 dogs. Projection models revealed that sterilization rates below 50% do not meaningfully reduce long-term population growth, whereas 75% sterilization leads to sustained decline. Euthanasia sharply reduces numbers but raises ethical concerns and produces demographic rejuvenation. This study establishes baseline demographic information for Karaj and highlights the key role of anthropogenic support in sustaining dog populations. These findings provide critical insights for developing evidence-based, humane, and sustainable dog population management policies.