Invasive Species

Riparian systems are being increasingly invaded and modified by non-native (exotic) plant and animal species. Invasive exotic plants, which have taken over thousands of acres of streamside habitat in California, are a topic of increasing concern for landowners and conservationists (Dudley and Collins 1995). These invasive species spread rapidly, exclude native vegetation, may increase fire danger, and often use large amounts of water. Disturbance, both anthropomorphic (human induced) and natural, may facilitate invasion by exotic plants. It has been suggested that areas experiencing regular disturbance, such as flood events in riparian habitat, may be more prone to invasion by non-native plants due to the availability of resources, and the ability of floodwaters to distribute seeds and propagules. The non-native invaders are able to take advantage of the same mechanisms that facilitate the growth of lush riparian forests, but they are able to out-compete native plants. Exotic plants usually do not support the same diversity of wildlife found in native riparian forests. When plants such as giant reed or periwinkle dominate the riparian zone, native riparian plant establishment is compromised. When this happens, the habitat values provided by native riparian forests are lost.

Arundo donax dominating riparian habitat along the Russian River in Sonoma County, California.

For example, invasive grasses have been shown to have deleterious effects on a wide variety of chemical, physical, and biological  properties of an aquatic ecosystem. If an invasive grass such as giant reed (Arundo donax) becomes established  in a riparian area (out-competing native trees such as sycamore,  bay laurel, cottonwood, and big leaf maple) the long-term consequence  may be that the shade, large woody debris, and leaf litter provided  by these native species will be lost, resulting in changes in  stream temperature and modification of in-stream structure and  the aquatic food chain. For a list of other invasive, non-native species, see the Riparian Plants section.

A once complex riparian forest that is providing shade, food, and structure for salmonids and other species may be transformed  into a monoculture with very little habitat value after invasion  by an exotic species. Furthermore, native riparian species are not especially long-lived (20 - 100 years is typical) so invasive species can have extremely negative effects on riparian corridors  in a relatively short timeframe.

Reference
Dudley, T., and B. Collins. 1995.  "Biological Invasions in California Wetlands. The Impacts and Control of Non-Indigenous Species in Natural Areas." Pacific Institute for Studies in Development, Environment and Security.