Why aren't mice on the planet? - briefly
Mice occupy virtually every continent except Antarctica, thriving in a wide range of habitats; thus the premise that they are absent from the planet is incorrect.
Why aren't mice on the planet? - in detail
Mice are absent from the global fauna for several interrelated biological and ecological reasons.
First, evolutionary pathways favored other rodent lineages. Early placental mammals diversified into families such as Muridae, which includes true mice, but selective pressures in many regions promoted the rise of larger, more adaptable species that outcompeted small murids for food and shelter.
Second, predator density limits the survival of diminutive mammals. In ecosystems with abundant raptors, snakes, and carnivorous mammals, the mortality rate of tiny rodents exceeds reproductive capacity, preventing stable populations from establishing.
Third, disease dynamics disproportionately affect small-bodied species. Pathogens that spread rapidly through dense, low‑immunity groups can decimate populations before they reach a viable size. Historical outbreaks of hantavirus‑like agents and bacterial infections have been documented to cause local extinctions of susceptible rodents.
Fourth, environmental extremes restrict viable habitats. Areas with extreme temperature fluctuations, low humidity, or scarce vegetation lack the microhabitats necessary for nesting and foraging. In arid zones, limited water availability and sparse seed production reduce food sources essential for small herbivores.
Fifth, anthropogenic factors accelerate disappearance. Agricultural practices, urban expansion, and pesticide application directly eliminate potential niches and introduce toxic compounds that small mammals cannot tolerate. Habitat fragmentation isolates remaining groups, leading to inbreeding depression and reduced genetic resilience.
Collectively, these factors create a hostile landscape for mice, explaining their non‑existence across the planet despite the theoretical suitability of many environments.