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DEVELOPMENT OF AN INDEX OF ALIEN SPECIES INVASIVENESS: AN AID TO ASSESSING RIPARIAN VEGETATION CONDITION
Magee, T. K., P L. Ringold, AND M. Bollman. DEVELOPMENT OF AN INDEX OF ALIEN SPECIES INVASIVENESS: AN AID TO ASSESSING RIPARIAN VEGETATION CONDITION. Presented at 7th International Conference on the Ecology and Management of Alien Plant Invasions, Ft. Lauderdale, FL, November 3-7, 2003.
Many riparian areas are invaded by alien plant species that negatively affect native species composition, community dynamics and ecosystem properties. We sampled vegetation along reaches of 31 low order streams in eastern Oregon, and characterized species assemblages at patch and reach scales using ordination, clustering, and indicator species analyses. Assemblages represented shrub-dominated streamside fringes in arid shrub steppe, ponderosa pine forests, spruce/fir forests, and wet meadows. Relative alien abundance was used as a metric of the extent of alien invasion. Because some alien species spread more aggressively or have greater influence on ecosystem function than others, we devised an Index of Alien Invasiveness (IAI) to aid in assessing riparian vegetation condition. The IAI incorporates abundance and a ranking score of invasion potential (Ri) for each alien species occurring in an assemblage. Ri was determined for each alien by considering life history attributes associated with invasiveness, evidence of wide ecological amplitude in riparian habitats, and ability to alter ecosystem processes. The IAI functions as a descriptor of the threat to native vegetation from the complement of alien species currently occurring within a species assemblage. We used both the IAI and relative alien abundance values to compare assemblages of markedly different structure and composition. Initial results indicated drier, low elevation riparian communities were more strongly influenced by alien species than those at moister, higher elevation locations. Closed forest assemblages had lower relative alien abundance and IAI values than open forest, shrub or herb dominated vegetation; and aridland shrub assemblages had greater IAI than wet meadow assemblages (95 % CI). Spearman correlations indicated significant negative relationships (p < 0.01) of the IAI and relative alien abundance with total canopy cover, elevation, precipitation, and positive relationships with stream order (p < 0.01). Differences in the IAI between assemblages with similar relative alien cover may aid in prioritizing conservation or restoration efforts.