Modern biological invasions in Japan are having various types of serious impacts on ecosystems and human life, and appropriate countermeasures are urgently needed (Washitani, 2004). In contrast, when historically viewed on a much longer time scale, major factors responsible for the unique indigenous flora and fauna of contemporary Japan included geo-biotic exchanges with the Eurasian Continent accompanied by intermittent isolations of the Japanese Archipelago, as well as its unique environmental conditions.

　In the present essay, as a preface to the second number of the special issue, ‘Biological invasion in Japan,’ I will briefly summarize the natural environmental features, including geological, climatic and biotic, and the geological-ecological histories of the Japanese Archipelago, which have also interacted with human history and are relevant to the biotic exchanges with Eurasian Continent, in order to contrast them with modern biological invasions.

　The small Indian mongoose, Herpestes javanicus (Family Herpestidae, Order Carnivora, Mammalia), is one of the most urgent species to eradicate among invasive mammals in Japan. Recently, the Japanese government has begun full-scale control of the mongoose as a model case for conserving the biodiversity of Japan’s subtropical islands. We review and assess the impacts of the mongoose and control practices on Amami-Ohshima Island and Okinawa Island based on recently acquired information. On Amami-Ohshima Island, a total of 9,960 mongooses were captured under the full-scale project of the Ministry of the Environment which ran for four years from 1999 to October 2003. According to the April 2002 mongoose census taken after three years of full-scale implementation, the population was estimated to be 1/4 (1,500-2,000 mongooses) less than in 1999. However, our investigation revealed that the remaining mongooses in the mountainous area are having negative impacts on native species, especially on the Amami rabbit, Pentalagus furnessi. People who are involved with the full-scale mongoose project should recognize that the investment in the project, at least regarding the number of traps and trapping period, is incredibly low at this stage and funding should be increased to achieve the goal. Further eradication projects should take into consideration the low density and partial distributions of the mongoose population in mountainous areas.

　Irresponsible breeding and release of pet raccoons (Procyon lotor) has caused their naturalization in Japan. The raccoons have already naturalized in 42 of Japan’s 47 prefectures, and there has been a remarkable increase in agricultural (cattle breeding) and urban areas, where the raccoons can find food and hide easily. Their home range size varies widely according to the habitat, ranging from 35 ha in urban areas to 2,219 ha in forests (mean: 299.1 ha). The raccoon avoids energy loss in winter by semi-hibernation. It has opportunistic and omnivorous feeding habits, taking crops and fruit in agricultural areas and native species in forests. It also preys on indigenous species such as the Japanese crayfish (Cambaroides japonicus) and Ezo salamander (Hynobius retardatus). Reproductive observations have shown high productivity (reproductive rate: 66% in yearlings and 96% in adults, litter size: 3.6 in yearlings and 3.9 in adults) and a potentially rapid rate of population growth (0.20 to 0.25). Intensive eradication guided by biological data will be indispensable to controlling invasive alien raccoons.

　The Ryukyu Archipelago is an assemblage of many continental islands and a few oceanic islands scattered between Kyushu of Japan, and Taiwan. Recent changes in geographic ranges were reviewed for feral amphibian and reptile populations in this archipelago, chiefly on the basis of data and informa­tion becoming available after 1995. Six species of amphibians (Rana catesbeiana, Fejervarya sp., Poly­pedates leucomystax, Microhyla ornata, Bufo gargarizans miyakonis, B. marinus) and 14 reptiles (Chinemys reevesii, Cuora flavomarginata, Mauremys mutica, Trachemys scripta elegans, Pelodiscus sinensis, Hemidactylus frenatus, Hemiphyllodactylus typus typus, Lepidodactylus lugubris, Anolis caro­linensis, Ramphotyphlops braminus, Elaphe quadrivirgata, E. taeniura, Protobothrops elegans, P. mucrosquamatus) are considered to have one or more feral populations in the Ryukyu Archipelago at present. Some biological aspects of these populations are also reviewed. Possible effects of such feral am­phibians and reptiles upon the indigenous biota, including a number of endemic and endangered taxa, are briefly discussed.

　Information was obtained on changes over a 30-year period in a fish community after invasion of alien fish species in Mizoro-ga-ike, a natural pond of nine hectares in area located in Kyoto City, and then an investi­gation done on changes over five years in populations of two alien fishes, largemouth bass (Micropterus salmoides) and bluegill sunfish (Lepomis macrochirus) while the local government took measures to control their populations. The species richness of the native fish community of the pond decreased from 14 species in the 1970s to six species in the 2000s: i.e., more than half of the native species became extinct during those 30 years. On the other hand, the percentage of alien species increased from 7.7% in 1972 to 50.0% in 2002. A population control program using net fishing and spawning redd destruction was implemented by Kyoto City in 1998. This resulted in an effective decrease in population estimates for M. salmoides from 84 in 1998 to several in 2002. Those for L. macrochirus, on the other hand, showed only a gradual decrease from 9,500 in 1999 to 5,400 in 2002 after the start of population control. Although the population of 1+ cohorts (one year old) of the species did not decrease distinctively after 1999, those of 2+ and 3+ cohorts decreased constantly. Population estimates of Channa argus based on a mark and recapture method resulted in a decrease from 540 in 1998 to 220 in 2002 in spite of no efforts to control on species. Causal relationships of alien fish inva­sion to changes in the fish community in Mizoro-ga-ike and the effects of the control measures on alien fish populations were examined.

　The floodplains on the middle reaches of rapid Japanese rivers are characterized by extensive grav­elly areas with sparse vegetation cover. There are a number of plant species endemic to the gravelly floodplain habitats, including two threatened species, Aster kantoensis and Ixeris tamagawaensiss. The Kinu River is a typical rapid river in central Japan, and the river endemics have remained among the vege­tation of the middle reaches until recently. These unique habitats, however, are rapidly being altered by an aggressive invasion of the alien grass (Eragrostis curvula), which is supposed to have become established through dispersal from upstream stands formed due to active introduction of the species for soil erosion control. The substrate condition of invaded areas tends to be changed from gravelly to sandy/silty, because tussock of E. curvula trap sand and silt during flooding. Thus, the original condition with sparse vegetation cover principally consisting of riparian endemics adapted to gravelly conditions is being lost. E. curvula exerts a strong negative influence on the performance of riparian endemics. A resto­ration project aimed at recovering the original condition of the floodplain by adaptive management techniques was started in 2002. As a ‘first-aid’ minor restoration effort, E. curvula was mechanically removed from a part of the floodplain (1,800 m²), fine sands were washed out by spray the ground with a jet of water to restore the original gravelly condition, and stocked seeds of A. kantoensis were sown. A local population of A. kantoensis was successfully restored, but more extensive exclusion measures are needed in the next stage of management to prevent further negative impact of the invasive species.

　Introduction of alien aquatic plants started in the Meiji Era (late 1800s) and some of them have been naturalized and caused serious weed problems in Japan. Eichhornia crassipes, Elodea nuttallii, Egeria densa and Pistia stratiotes have proven to be typical noxious aquatic weeds. The history of such in­vasive alien aquatic plants is briefly reviewed. Recently, more and more alien aquatic plants have been in­troduced due to a boom in aquarium and water gardening. Some of the newly introduced alien aquatic plants have begun to spread quickly. To date over 40 species have escaped and become naturalized in Japan. They threaten the biodiversity of native aquatic plants in various ways. The social backgrounds of the spread of alien aquatic plants are surveyed and the need for social education as well as legal regu­lation to discourage destructive activities are emphasized.

　A huge amount of alien species have been introduced into Japan and some have become ‘invasive,’ having adverse effects on ecosystems, human safety, or agriculture, forestry and fisheries. Considering that the Japanese regulatory system has not dealt with the issue comprehensively, taking into account various suggestions from many entities concerned, and aiming to materialize the provisions of Article 8 (h) stipulated by the Convention on Biological Diversity, the Japanese Cabinet finalized a draft of the Invasive Alien Species Act and submitted it to the Japanese Diet (= Parliament) on March 10, 2004.

　The framework consists of the following three main points: (1) Invasive Alien Species (IAS) shall be designated in the Cabinet Ordinance of the Act as having adverse effects on ecosystems, human safety, or agriculture, forestry and fisheries. Various actions dealing with IAS such as raising, planting, storing, carrying, and importing shall be prohibited in Japan with the exception of specified cases such as by obtaining permission from the competent ministers. (2) National/local governments, Non-Profit Organizations, and other entities concerned shall take appropriate measures to mitigate the impact of IAS that already exist in Japan. (3) Uncategorized Alien Species (UAS), which may possibly be catego¬rized as IAS through detailed investigation, shall need detailed investigation by the Japanese govern¬ment in order to be allowed to be imported into Japan. The period for determining categorization shall be within six months from requests by importers or exporters of UAS into Japan.

　The Diet passed the draft and the new Invasive Alien Species Act was promulgated as of June 2, 2004. Following the establishment, the Japanese Cabined made a basic policy on October 15, 2004, for effective implementation of the Act. The Act shall be enforced within one year from its promulgation and IAS and UAS shall be designated before the enforcement based on suggestions by academic experts. Tens of IAS and thousands of UAS are expected to be designated in the first step.