The vacuum effect in free-roaming dog management: Conceptual foundations, boundary conditions, and One Health implications.

Management of free-roaming animal populations in human-dominated environments presents persistent ecological, ethical, and public-health challenges worldwide. Removal of animals from open populations is often assumed to reduce population size and associated risks. However, ecological theory suggests that removal may be followed by compensatory responses through immigration, increased survival, or enhanced reproduction among the remaining animals. This phenomenon, commonly referred to as the "vacuum effect", has been widely invoked in debates on the management of free-roaming dogs and other urban animals, but its empirical basis and the conditions under which it operates remain incompletely understood. Here, we review the conceptual foundations and available evidence for replacement dynamics in free-roaming dog populations, with particular attention to urban systems where food, shelter, and human tolerance support high dog densities. We show that although the "vacuum effect" is ecologically plausible, its empirical manifestations in free-roaming dog populations are limited. The outcome of removal of the free-roaming dogs from a particular area depends on several factors, including the spatial scale and continuity of removal, recruitment through reproduction and abandonment, environmental carrying capacity shaped by human-provided resources, and the strength of institutional regulation. We propose a conceptual framework in which removal does not inevitably lead to rapid replacement. Instead, outcomes vary according to the characteristics of the system and the way interventions are implemented. This interpretation helps to reconcile the opposing positions and highlights the need for integrated approaches combining population management, regulation of resources and recruitment, and disease prevention. Understanding the conditions that influence replacement is important for developing sustainable and humane approaches to free-roaming dog management and control of dog-mediated rabies.