Rodents, such as rats, can be the cause of many diseases that affect public health. Hence, rodent prevention (e.g., sanitation, rodent-proofing, etc.) and control should be included in integrated pest management programs that are provided by pest management professionals. The United States Fish and Wildlife Service provides additional information about invasive species (plants and animals) in the US, including rats.
Why should we be concerned about rats invading new areas, such as islands?
A previous assessment showed that rats have invaded approximately 80 percent of the world’s islands (Towns et al. 2006). There are three rat species (Rattus: black rat/ship rat, Rattus norvegicus: Norway rat/brown rat, Rattus exulans: Pacific/Polynesian rat) that are considered important invasive rat species on continents and island groups due to the damage they can cause to ecosystems (Duron et al. 2016). Rats are generally distributed by trade routes (ships and other means of cargo transport). Many scientists believe that Norway rats were inadvertently brought to Europe from China and then distributed to other locations (Song et al. 2014). Black rats were brought to Europe from India (Aplin et al. 2011).
Wildlife conservationists work to reduce or remove invasive rat populations on islands to protect ecosystems, including native plant and animal populations. The success of this work depends on the methods used for rat removal, the frequency and duration of surveillance and control measures, the size of the island environment and the sustained willingness/support of island residents to help with the process (Duron et al. 2016).
In the United States, there are three rat species that are considered invasive to the Hawaiian Islands. Brought by Polynesians, Polynesian rats invaded in approximately 400 A.D. (while black and Norway rats were likely introduced in the late 1700s by visitors from the West. In addition to damaging production of local crops such as sugar cane, bananas, coconuts, coffee, and other crops, rats may eat eggs, nesting birds (such as native nene geese), and nesting sea turtles, hence damaging native wildlife populations. Hawaii’s Department of Land and Natural Resources provides additional information about the state’s continued struggle against rats (and mongooses). There are examples of small islands, such as Huelo, off of the coast of some larger Hawaiian Islands where no rats currently reside. In the case of Huelo, there are fan palm plants (called loulu palms) that only exist on that islet since the plant cannot survive scavenging rats and other rodents found on the larger islands that strip its bark and chew its leaves.
A global study (Duron et al. 2016) reviewed 136 published rat control projects. These projects occurred primarily in Australia and New Zealand; however, some studies took place on islands from the United States, such as Hawaii. Some of the studies reported controlling other organisms (in addition to rats) such as opossums, mice, and even invasive plants. In general, studies initiated rat control programs based on negative connotations (e.g., risks that rats posed to endangered animals, especially birds) from other areas where rats had invaded, rather than from evidence-based studies showing damage from rats on the specific location of interest (Duron et al. 2016).
Some islands or other areas may not have an abundance of predators to naturally control rodent populations, hence rat populations often flourish in these environments. Rat control seems to be more successful in temperate island environments compared to tropical island environments, likely due to year-round breeding and abundance of food sources (among other variables) in tropical environments compared to temperate environments. Rat management and control remains important in protecting public health and susceptible environments, hence, continued work must be done to prevent and/or manage infestations.References
Aplin KP, Suzuki H, Chinen AA, Chesser RT, Ten Have J, Donnellan SC, Austin J, Frost A, Gonzale JP, Herbreteau V, Catzeflis F, Soubriere J, Yin-Ping Fang YP, Robins J, Matisoo-Smith E, Bastos ADS, Maryanto I, Sinaga MH, Denys C, Ronald A, Van Den Bussche RA, Conroy C, Rowe C., Cooper A (2011) Multiple geographic origins of commensalism and complex dispersal history of black rats. PloS One 6:e26357
Duron Q, Shiels AB, Vidal E (2016) Control of invasive rats on islands and priorities for future action. Conservation Biology 31:761-771. Song Y, Lan Z, Kohn KH (2014) Mitochondrial DNA phylogeography of the Norway Rat. PLoS ONE 9:e88425
Towns DR, Atkinson IAE, Daugherty CH (2006) Have the harmful effects of introduced rats on islands been exaggerated? Biological Invasions 8:863–891.