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Invasive non-native species have long been recognized as a leading threat to biological diversity, contributing to the decline of nearly half of the imperiled species in the United States for which threat information is available.  Some of the invasive species that threaten imperiled wildlife in the western United States, including endemic birds in San Francisco Bay, sage grouse, and grizzly bears.  Three examples of the difficulties inherent in balancing the need to close pathways of invasive species entry with maintaining active international and interstate trade (wood packing materials, ballast water, and the nursery trade) and I present Defenders of Wildlife’s policy recommendations in these areas.

Furthermore, urbanization negatively affects natural ecosystems in many ways, and aquatic systems in particular. Urbanization is also cited as one of the potential contributors to recent dramatic declines in amphibian populations. From 2000 to 2002 we determined the distribution and abundance of native amphibians and exotic predators and characterized stream habitat and invertebrate communities in 35 streams in an urbanized landscape north of Los Angeles (U.S.A.). We measured watershed development as the percentage of area within each watershed occupied by urban land uses. Streams in more developed watersheds often had exotic crayfish (Procambarus clarkii) and fish, and had fewer native species such as California newts (Taricha torosa) and California treefrogs ( Hyla cadaverina). These effects seemed particularly evident above 8% development, a result coincident with other urban stream studies that show negative impacts beginning at 10-15% urbanization. For Pacific treefrogs (H. regilla.), the most widespread native amphibian, abundance was lower in the presence of exotic crayfish, although direct urbanization effects were not found. Benthic macroinvertebrate communities were also less diverse in urban streams, especially for sensitive species. Faunal community changes in urban streams may be related to changes in physical stream habitat, such as fewer pool and more run habitats and increased water depth and flow, leading to more permanent streams. Variation in stream permanence was particularly evident in 2002, a dry year when many natural streams were dry but urban streams were relatively unchanged. Urbanization has significantly altered stream habitat in this region and may enhance invasion by exotic species and negatively affect diversity and abundance of native amphibians.

Films Media Group. (2008). Invasive species: Surprising findings [H.264]. Found at: http://digital.films.com/PortalPlaylists.aspx?aid=655&xtid=40071. Researched January 22, 2013

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