Rat numbers appear to be increasing across the globe according to a US study.
Perhaps unsurprisingly to pest managers, a recent study has confirmed that rat numbers are on the rise in many cities around the world, and that the rise in numbers is linked to climate warming and increasing urbanisation.
The researchers looked at rat numbers in 13 cities in the US, as well as Tokyo, Amsterdam and Toronto. Over the data period, rat numbers increased significantly in 11 cities, with Washington DC showing the biggest increase. Three cities showed a decline, with New Orleans showing the largest decrease. Approximately 40% of the variation (increased rat numbers) was due to increasing temperature (versus long-term mean average); 35% was due to lower levels of vegetation (i.e. higher levels of urbanisation); and 20% was due to higher population density.
Although the general increase in temperature appears to correlate with increased rat populations, the exact reason for this is not apparent from the data. Certainly, it would seem logical that warmer winters would mean less mortality and therefore larger rat populations, but the data did not show higher increases in rat numbers during the winter months. Further studies, with the inclusion of more tropical and sub-tropical cities, are required to better understand this correlation.
The study correlated cities with less green space (higher levels of urbanisation) with higher rat numbers. Add in the increase in population, this higher level of urbanisation and increase in refuse products provides plenty of nesting sites and food sources for rat populations.
With increasing urbanisation and populations migrating to cities, the rat numbers are only projected to escalate. Although for many cities rat numbers are increasing, cities experiencing extreme heat in the future may see a reduction in rat numbers. It is also important to recognise that the types of rodents present in cities may change. For example, there is evidence that Rattus rattus (roof rat) is expanding its geographic range with the warming climate.
The authors recognise that the solution is not simply to use more control products – which may become even more problematic if resistance levels to anticoagulants continue to increase, even if new control methods become available. The need for integrated rodent management strategies on both a building and city level are required, in particular to eliminate rodent nesting places in buildings and eliminate food sources through improved refuse management and sanitation.
Further reading: Richardson, J. L et al. (2025). Increasing rat numbers in cities are linked to climate warming, urbanization, and human population. Science Advances, 11(5). https://doi.org/10.1126/sciadv.ads6782
