Status of invasive alien species problems in Japan is briefly summarized, with special emphasis on invasions during the period from the 1960s to today, one of the two eras characterizing the modern history of the biological invasion into Japan. A number of cases of invasions are categorized from the viewpoints of route of invasion, i.e., intentional or unintentional introduction, release and escape; and types of impacts of invasive alien species on ecosystems, biodiversity, and human life and health. Examples are given of species and situations exemplifying the individual types of influences.

　There is growing concern about the impact on the habitat of the native Japanese squirrel (Sciurus lis) in relation to the spread of the Formosan squirrel (Callosciurus erythraeus thaiwanensis), an alien species that was introduced and became naturalized in Kamakura City, Kanagawa Prefecture, in the 1950s. We used a logistic regression model to predict the probability of occurrence of the Formosan squirrel throughout Kanagawa Prefecture. Wooded areas, the proportion of cultivated fields surrounding a wood and the proportion of evergreen trees in a wood, factors that influence the presence of Formosan squirrels, were used as environmental variables in the model. As a result, we found that woods suitable for the Formosan squirrel were distributed not only around Kamakura, which Formosan squirrels currently inhabit, but throughout Kanagawa Prefecture. Furthermore, a comparison of its current habitat and its predicted suitable habitat suggested that the assemblage of large woods along the Sagami River and Sagami Bay would be conductive to increasing its population and expanding its distribution to western Kanagawa. Because there are forests in western Kanagawa which are extensive and are expected to be a suitable habitat for the Formosan squirrel, we should prevent Formosan squirrels from invading the west side of the Sagami River.

　The exotic White-browed Laughing Thrush (Garrulax sannio) has been recorded in Gunma Prefecture, central Japan, since the 1990s. We studied the distribution of this alien species, caught one individual in 2000, and found an active nest in 2001. The recorded observations were all in the southeastern region of Gunma Prefecture, except for one in Ibaraki Prefecture. The earliest record was made in 1994, as far as we know. So far, the Laughing Thrushes in Japan have been found in human-modified habitats such as secondary forests, bamboo thickets, cultivated fields, and residential areas. Because such habitats are common in rural areas of Japan, the White-browed Laughing Thrush seems to have no difficulty in expanding its region and may present problems regarding conservation of biological diversity in those areas.

　Introduction of exotic birds has been increasing throughout the world. We conducted inquiry investigations in order to reveal the spread of introduced bird species in Japan. Forty-three exotic species are known to breed or are regarded as breeding. The Feral Pigeon (Columba livia), Chinese Bamboo Partridge (Bambusicola thoracica), Red-billed Leiothrix (Leiothrix lutea), Budgerigar (Melopsittacus undulatus) and Red Avadavat (Amandava amandava) are widely distributed. The Rose-ringed Parakeet (Psittacula krameri manillensis), Common Peafowl (Pavo cristatus), Light-vented Bulbul (Pycnonotus sinensis), Melodious Laughing Thrush (Garrulax canorus), Crested Myna (Acridotheres cristatellus) and Black-billed Magpie (Pica pica sericea) are locally common, though they have limited ranges. Sightings of other species were sporadic and regionally limited. Most of introduced birds in Japan are escaped caged birds. About two thirds are from tropical regions, and show diet generalism as omnivores, granivores and frugivorous or herbivorous insectivores. Light-vented Bulbuls, Feral Pigeons and Common Peafowl exerted crop damage. Although adverse effects by exotic birds are not prominent yet, there is great concern that exotic birds will become a threat to native biota and habitats in future.

　Rainbow trout (Oncorhynchus mykiss), brown trout (Salmo trutta) and brook trout (Salvelinus fontinalis) introduced to Japanese inland waters have significant impacts on native aquatic organisms such as white-spotted charr (Salvelinus leucomaenis), Dolly Varden charr (Salvelinus malma), masu salmon (Oncorhynchus masou) and Sakhalin taimen (Hucho perri), in coldwater habitats. Rainbow and brown trout have successfully established reproducing populations in Japan, and especially expanding their distribution in Hokkaido. Brook trout have shown relatively limited invasion in some spring-fed streams and ponds both in Honshu and Hokkaido. The introduced exotic trout have been implicated in reducing populations of native fishes, especially stream salmonids, through predation, competitive interaction for resources, and interspecific hybridization. Monitoring, management, including eradication, and evaluation of the effectiveness of these measures should be carried out to contain the ecological risks of trout invasion.

　In 1996, a naturally occurring nest of an introduced bumblebee, Bombus terrestris L., was found in Monbetsu in the Hidaka region of southern Hokkaido, indicating that it had become naturalized in Japan. In this region, monitoring of B. terrestris has been continued at two sites. The numbers of B. terrestris captured or observed have increased rapidly during the eight years since the evidence of its naturalization was found. Seasonally, the queens of naturalized B. terrestris have appeared as early as mid-April each spring. Queens were observed continuously during the period from April to October, workers from May to October, and males from July to October. B. terrestris was shown to forage among various flowering plants, 40%−70% of the species of flowering plants upon which native bumblebees fed. Nine of ten natural nests of B. terrestris discovered during the study were found in abandoned underground rodent nests. The nest sites of this species were similar to those of B. hypocrita sapporoensis Cockerell, and B. diversus tersatus Smith. B. terrestris clearly has the potential to compete with native bumblebees for floral resources and nest sites. The mean number of new queens born in the colonies of B. terrestris was 4.4 times larger than that of the native bumblebees. This strongly suggested superior reproductive ability of B. terrestris compared with native bumblebees in the region.

　The breeding of stag beetles has come to be very popular in Japan, and over one million exotic stag beetles are imported into Japan every year. This unusual boom has even led to criminal smuggling of stag beetles from the countries prohibiting the collecting of any wild organisms. There is real concern that ecological risks will result from this unprecedented business in insects. Native stag beetles will probably suffer from serious and direct impacts because of the similarity of ecological niches. It is thought that the first impact will be competition for food and habitat, the second, genetic introgression as a consequence of hybridization between the exotic and the native beetles, and the third, invasion of imported parasites. We are studying the second and third impacts. Our molecular genetic analysis of populations of Dorcus titanus, which is a major native species, indicates signs of genetic introgression by exotic strains imported from South-east Asia. Moreover, our investigation of stag beetles imported and sold shows that a lot of mites are coming into Japan attached to the beetles. We have to reconsider and be more wary about importation of exotic stag beetles.

　Of 447 alien insect, phytophagous mite, spider and nematode species in Japan, 415 are insect species. Most were introduced after the end of the Edo period (1859). Coleoptera, Hemiptera, Lepidoptera, Diptera and Hymenoptera represent the major taxonomic groups in decreasing order of number of species. They include 58 insect and nematode species from the United States. Among them, the following three have been the most devastating North American species: the pine wood nematode, Bursaphelenchus xylophilus (the causal agent of pine wilt diseases transmitted by the Japanese pine sawyer, Monochamus alternatus), which was introduced in 1905; the fall webworm, Hyphantria cunea (introduced in 1945); and the rice water weevil, Lissorhoptrus oryzophilus (1976). Their range expansions and population dynamics after their establishment in Japan have been elucidated. Explosive range expansions have always been associated with outbreaks of invasive insects. Changes in future distribution ranges under global warming are discussed. Currently those introduced species are becoming endemic as a result of various eco-physiological processes occurring after their establishment.

　In Japanese riparian habitats, invasion by alien plants constitutes one of the most serious threats to biodiversity through the displacement of native plants. This paper presents a list of “invasive alien plant species” in Japanese riparian habitats. Invasive alien species, defined here as dominant or codominant species in certain plant communities, can be categorized as “agricultural weeds,” “revegetation species,” or “aquacultural species.” In riparian areas, the category of agricultural weeds contains significantly more invasive alien species than would be expected by chance. The same result was found for introduced revegetation species. To prevent the introduction of new invaders into riparian areas, appropriate alien plant management is needed for both natural habitats and agricultural areas of the catchments. The use of alien pasture species for revegetation or control of soil erosion should be avoided in order to conserve biodiversity and native ecosystems.