Northeast Pacific (e.g., Nelson et al., 2004; Ferraro and Cole, 2007, Lee and Reusser, 
2007). 
The three abundant amphipods (> 20 individuals per 0.09 m 2 sample) were 
Grandidierella japonica, Monocorophium insidiosum, and Americorophium salmonis, the 
first two of which are nonindigenous species. Both nonindigenous amphipods were 
widely distributed, ranging from Southern California up into Puget Sound. In 
comparison, the native A. salmonis was not found in California, though the 1999 EMAP 
survey found it as far south as the San Luis Obispo Bay (Latitude = 35.17) in California. 
The only bivalve with a high average abundance was the nonindigenous Gemma 
gemma, an East Coast species introduced with importation of Atlantic oysters (Cohen 
and Carlton, 1995). Gemma gemma has a limited distribution in the Northeast Pacific 
and has only been reported from nine California estuaries (Lee and Reusser, 2007). 
Gemma gemma was only found in 6% of the samples (Table 3.3.1), and the high 
average West wide abundance reflects its high densities in a few locations in San 
Francisco which reached 141,400/m 2 . 
With non-native species constituting the most abundant polychaete, bivalve, and 
amphipod, an obvious alteration to the intertidal benthic communities on the West Coast 
is the proliferation of nonindigenous species. On a regional scale, one measure of the 
extent of invasion is that 42 nonindigenous species were collected, in addition to 
another 32 cryptogenic, or possible nonindigenous species. Not only was a large 
number of nonindigenous species collected but they were widespread. Eighty-five 
percent of the samples contained at least one nonindigenous species (Figure 3.3.3). 
While nonindigenous species were widespread, the extent of invasion appeared to vary 
among sites. To evaluate patterns in invasion, we propose the following metric for the 
relative species richness of nonindigenous species on a per sample basis: 
%NIS Sp p = NIS S pp/(NIS S pp & Natspp) *100 (Equation 3.3.1) 
where: 
%NISs P p = relative species richness of nonindigenous species per sample 
NISspp = number of nonindigenous species in sample 
Natspp = number of native species in sample 
Only native and nonindigenous species are included so as to limit the analysis to 
species with “known” classifications. Inclusion of the cryptogenic species, unclassified 
species, and indeterminate taxa would increase the level of uncertainty, and make 
interpretation more difficult. By normalizing the number of nonindigenous species to the 
sum of nonindigenous and native species, the index is “well behaved” and scales 
between 0 (no NIS) and 100 (all NIS and no natives), though the metric is undefined if 
there are no nonindigenous or native species. Because the index is based on relative 
species richness rather than absolute numbers of nonindigenous species, the 
differences in sample size will not substantially affect the value of the index assuming 
that the relationship between sample area and number of species collected is similar for 
native and nonindigenous species. Over the small areas of the samples, this 
34 
