Prologue: Commensal Rodent Problems Across the Globe

*Tatsuo Yabe*

## **1. Introduction**

Black rats *Rattus rattus* have become a rare species in northern and central Europe [1–5]. Most of the USA and Canada are also free of the rats [6]. In Australia, house mice *Mus musculus* pose particular problems in high-rise buildings and skyscrapers [7]. In Japan, house mice and Norway rats *R. norvegicus* are rare in buildings in the centers of big cities. Whereas black rat infestations have been experienced even in modern buildings in almost all big cities since the 1970s except those in the northern area [8–11].

However, black rat problems in big buildings tend to be reduced in these years by structural improvement (T. Tanikawa, pers. comm.).

Instead, black rat problems in residential areas in the centers of big cities have become problems since the mid-1990s [12]. Norway rats living outside buildings in busy streets also have become notable problems [13]. Commensal rat problems in islands in Japan have been focused on from the viewpoint of conservation of ecosystems since early in the 2000s [14, 15].

### **2. Changes in species composition**

From the end of World War II to the 1960s, Norway rats appeared to overtake black rats with urbanization, though total number of both species decreased [16, 17] (**Figure 1**). At that time, sanitation was relatively poor, and 1–2 story buildings occupied even the centers of big cities. Catering establishments such as restaurants and drinking houses were generally in such small buildings and were commonly invaded by Norway rats.

However, the situation changed from about the 1970s. In buildings in the central area of Tokyo black rats became dominant, though Norway rats remained in sewers and in parks and gardens in the area. They rarely entered buildings. In an area in which a pest control company (PCO) operated, black rats infested 66% of all types of buildings, with Norway rats or mixed species of commensal rodents infesting the rest [18]. A similar situation was reported widely in Japan, including Sapporo, a city in the northern area [8–11]. Later, however, the black rat problem disappeared in Sapporo. Causes of the disappearance is not known yet, but I suppose one of the causes is the lower temperature of Sapporo [8, 9]. Accordingly, black rats are dominant in buildings in all major cities except Sapporo.

From around the 1970s, the Japanese economy grew and big buildings with three floors or more rapidly increased in commercial districts [9–11] (**Figure 2**); many were connected with the catering industry. At the same time, though PCOs rapidly multiplied, black rats became successful in these areas. From the mid-1990s, these rats scattered to residential areas. Questionnaires showed that 23% of 322 residences in a ward in Tokyo experienced black rat invasions within the past two years [12].

*Rodents*

**Figure 1.**

*Changes in species composition of commensal rats collected with snap traps in Osaka city and the Tokyo Metropolis during control campaigns [16, 17].*

**Figure 2.**

*Yearly changes in percentage of basal area of buildings >10 m high in commercial districts in Sapporo, Sendai, Shinjuku, Yokohama, and Nagoya [9–11].*
