As with the metric based on relative species richness on nonindigenous species, 
15% of the samples contained no nonindigenous species (Figure 3.3.5). Another 46% 
of the sites were “highly invaded” as defined by nonindigenous species constituting 
>50% of the individuals. The pattern of the relative abundance of non-native species 
(Figure 3.3.5) differs from that based on the relative species richness of invaders 
(Figure 3.3.3) by having peaks at both “low to moderate” levels of invasion (>0 and 
<25%) and another at “very high” levels of invasion (>75%). This bimodal pattern 
reflects, at least in part, apparent geographical and habitat differences in the extent of 
invasion (Figure 3.3.6). The significance of these geographical/habitat differences was 
tested using a Kruskal-Wallis one-way Analysis of Variance on ranks, which found a 
highly significant difference (p < 0.01) in the median values of %NIS A bun among the six 
geographical areas or habitat types. The benthic assemblages in San Francisco 
exclusive of the high marsh were the most invaded, with an average of 61% of the 
individuals per sample consisting of nonindigenous species. The coastal estuaries of 
Oregon and Washington were also highly invaded with about 50% of the individuals per 
sample consisting of nonindigenous species. In comparison, nonindigenous species 
constituted less than 25% of the individuals in samples from Puget Sound, and less 
than 40% in samples from California other than San Francisco and in the San Francisco 
high marsh. Again the lower extent of invasion in the San Francisco high marsh may 
partially reflect that the oligochaetes and insects were not identified to species. 
Based both on relative species richness and relative abundance, it is apparent 
that the community composition and structure of the intertidal assemblages of 
California, Oregon, and Washington have been substantially altered by the invasion of 
nonindigenous species. These alterations are likely to continue as existing 
nonindigenous species increase their range and/or abundance. For example, the 
nonindigenous amphipod Grandidierella japonica has expanded its range from its first 
sighting in San Francisco in 1966 (Chapman and Dorman, 1975) to 46 Northeast Pacific 
estuaries ranging from Tijuana Estuary to Puget Sound by 2002 (Lee and Reusser, 
2007). After a major flood event in 1996, G. japonica became one of the numerically 
dominant amphipods in the Yaquina Estuary, Oregon (Lee et al., submitted) and it has 
become the most abundant intertidal amphipod on the West Coast (Table 3.3.1). 
Intertidal assemblages will also continue to change in response to new invasions. As 
recently as July, 2007, a “major new snail invasion” (“ Assiminea " sp) was reported for 
Coos Bay, Oregon (J. Carlton, August 31,2007 email). The present probabilistic survey 
provides a baseline of the structure of benthic assemblages as of 2002, and it will be 
important to conduct similar regional surveys in the future to assess the extent and 
nature of changes due to invasion as well as other regional drivers such as climate 
change, habitat alteration, and increased nutrient loading. 
36 
