The question of whether rats are detrimental to the environment centers on a few globally widespread invasive pests. Specifically, the Norway rat (Rattus norvegicus) and the black rat (Rattus rattus) are not native to most places they inhabit and cause significant environmental harm. Their success is tied to their opportunistic nature and ability to thrive near human settlements, making them commensal species that traveled the world as stowaways on ships. These introduced rats have had a severe impact on non-urban areas, especially fragile island ecosystems where native species evolved without such predators.
Impact on Native Flora and Fauna
Invasive rat species are recognized as one of the most successful animal groups contributing to the extinction and decline of native species, particularly on islands. Their omnivorous diet and high reproductive rates enable them to quickly dominate an ecosystem, primarily through predation and resource competition. Rats are opportunistic predators that consume a wide variety of vulnerable native wildlife, including the eggs and chicks of ground-nesting birds, small reptiles, invertebrates, and even small mammals.
The black rat, in particular, has been associated with the decline or extinction of numerous indigenous vertebrate species, including land birds and burrowing seabirds. The predation pressure is so intense that simply removing invasive rats from an island can lead to a dramatic recovery of native bird populations in a short time. For example, on one island, the removal of black rats was swiftly followed by the arrival of a seabird colony and the successful hatching of hundreds of chicks, which had been impossible before.
Rats also exert a strong influence through competitive exclusion, rapidly consuming resources that indigenous species rely upon. They eat the seeds and seedlings of native plants, restricting regeneration and potentially causing long-term shifts in forest composition. By consuming insects and fruit, rats directly compete with native herbivores and omnivores for limited food sources, leading to the decline of indigenous populations.
Physical Alteration of Ecosystems
Beyond direct consumption, the physical activities of invasive rats can structurally change the environment, affecting soil stability and plant communities. Norway rats, in particular, are prolific burrowers, and their tunneling networks can destabilize soil and riverbanks. This activity can contribute to erosion, especially in coastal or riparian areas, altering the physical landscape of the ecosystem.
Rodents constantly need to gnaw to maintain their continuously growing incisors, which leads to environmental damage, such as damaging natural woody vegetation. By selectively feeding on certain seeds and seedlings, rats modify the plant species composition of a forest or grassland. This selective pressure can favor non-preferred plant species, fundamentally changing the habitat structure for other native organisms.
The Rat’s Role in Disease Ecology
The presence of high-density rat populations creates an ecological concern because they serve as reservoirs and vectors for zoonotic pathogens that can affect both wildlife and human health. Rats are known to carry and transmit over 60 different diseases, including leptospirosis, hantavirus, and salmonellosis. The environmental spread of these diseases occurs through contamination of water and soil via the rats’ urine and feces.
As disease reservoirs, rats maintain pathogens within their population, allowing them to spill over into other species. Ectoparasites like fleas and ticks that live on rats can transmit diseases such as plague to other wildlife or humans. In environments with high rat densities, the risk of these spillover events increases, amplifying the ecological and public health threat.
Distinguishing Invasive Threats from Native Rodents
It is important to provide context by recognizing that the severe environmental problems discussed are almost exclusively caused by non-native, introduced species. Native rodent species, such as woodrats or native mice, play an integral part in the balance of their indigenous ecosystems. These native species often function as primary food sources for predators, like raptors and snakes, supporting the local food web.
Native rodents also perform beneficial ecological services, such as soil aeration through their burrowing and the dispersal of seeds and fungal spores. The difference in impact is primarily due to the lack of natural population controls for the invasive species, allowing their numbers to reach destructive levels. Therefore, the conservation concern is not with rodents as a group, but with the few highly adaptable, non-native species that aggressively outcompete and prey upon indigenous life.