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BLURRING OF BIOGEOGRAPHIC BOUNDARIES: A MULTIVARIATE ANALYSIS OF THE REGIONAL PATTERNS OF NATIVE AND NONINDIGENOUS SPECIES ASSEMBLAGES IN PACIFIC COAST ESTUARIES
Citation:
Lee II, H, W G. Nelson, J O. Lamberson, AND D. Reusser. BLURRING OF BIOGEOGRAPHIC BOUNDARIES: A MULTIVARIATE ANALYSIS OF THE REGIONAL PATTERNS OF NATIVE AND NONINDIGENOUS SPECIES ASSEMBLAGES IN PACIFIC COAST ESTUARIES. Presented at 13th International Conference on Aquatic Invasive Species, Ennis, Ireland, September 19-23, 2004.
Description:
Many, if not most, invaders have wide physiological tolerance limits and generalist habitat requirements. Consequently as a group nonindigenous species should have wider geographic distributions compared to native fauna. In turn, these broader distributions of nonindigenous species should tend to blur regional differences in community composition. To test whether "biogeographic blurring" is occurring, we compared the discreteness of assemblages of native versus nonindigenous estuarine benthos across biogeographic zones on the Pacific Coast of the USA. Each of 187 benthic grabs from the U.S. EPA's EMAP survey of the soft-bottom benthos of small estuaries in Oregon, Washington, and California was assigned to one of three biogeographic provinces and to one of five salinity classes. The three biogeographic provinces were the Californian Province south of Point Conception (34.449 N) to the Mexican border, Columbian Province north of Cape Mendocino (40.440 N) in Northern California to the Canadian border, and a Transition Province between Point Conception and Cape Mendocino. A total of 429 native, 53 nonindigenous, 113 cryptogenic (species of uncertain origin), and 247 indeterminate (species that can not be classified) species were identified in the EMAP survey, though only the native and nonindigenous species are used in the present analysis. Analysis of similarity (ANOSIM) with a two-way crossed design was used to test if the species composition of the native or nonindigenous species assemblages differed among the three biogeographic provinces after accounting for salinity. The "R" statistic was used as a measure of separation among the biogeographic provinces, with larger R values indicating greater separation.
After accounting for salinity, the native species assemblages showed strong regional separation (overall R=0.443). In pairwise tests, the native assemblages of the Columbian and Transition Provinces were significantly separated (R=0.259)t indicating that Cape Mendocino is an ecologically significant break for native fauna. Point Conception in Southern California represented an even more discrete faunal break, with both the Columbian and Transition Provinces strongly differentiated from the Californian Province (R=0.608 and 0.576, respectively). In contrast, the nonindigenous species assemblages did not show a significant difference among the biogeographic provinces (overall R=0.13, NS). Thus, the composition and relative abundances of the nonindigenous species were not sufficiently different among these Pacific Coast estuaries to form discrete biogeographic provinces. Because nonindigenous species constitute only a moderate component of the total benthic assemblages in these small estuaries, the present regional patterns of the composite benthic assemblages are not strongly altered by the occurrence of non indigenous species. However, as the number and abundance of nonindigenous species increases, regional differences in benthic community composition will decrease, blurring the distinctiveness of biogeographic boundaries within these estuaries. One potential ecological consequence of such "homogenization" is the loss of beta diversity, or species turnover, at the biogeographic scale. Another possible consequence are shifts in the taxonomic composition of the "reference" or "control" communities used in assessing pollution impacts. Such shift in the composition of control sites will confound the detection of pollution-related impacts over time.