Overall Assemblage Comparisons

How different are the field sites with respect to their dominant faunas?

Number of Species
Percent of
Total Species
Percent Unique
to Site
Eagle Cove
Cobble Beach
Garrison Bay


Cattle Point
False Bay
Argyle Creek
Total number of species
A breakdown of the number of species that occur at each field site, the proportion of the total species count that is represented at each site, and the proportion that are exclusive to each site.

This chart reflects some of the ecological differences among sites: the species richness (from the student counts) varies by nearly a factor of three between the most (Eagle Cove) and least species rich (False Bay) habitats. It is important to keep in mind that the sampling effort may have been different across the six sites. For example, some student groups may have focused carefully on finding a large number of different species, while other groups focused on examining the natural history of the most abundant or prominent species. The percentages of species exclusive to each site was strongly correlated with the overall species diversity, especially at Eagle Cove which had the largest number of species and the largest proportion of species not found elsewhere.

79 of the 169 total species reported were found at more than one site: Below is a chart of the number of species shared by pairs of field sites.

Reference Habitat Total Species Eagle Cove Cobble Beach Garrison Bay Cattle Point False Bay Argyle Creek
Eagle Cove 109   39 22 39 16 35
Cobble Beach 61 39   11 27 7 22
Garrison Bay 36 22 11   12 15 19
Cattle Point 52 39 27 12   9 11
False Bay 35 16 7 15 9   11
Argyle Creek 45 35 22 19 22 11  
Total number of species in the reference habitat.

These data may be converted to a measure of overall similarity:

  Eagle Cove Cobble Beach Garrison Bay Cattle Point False Bay
Eagle Cove          
Cobble Beach 45.88        
Garrison Bay 30.34 22.68      
Cattle Point 49.68 47.78 27.27    
False Bay 22.22 14.58 42.25 20.68  
Argyle Creek 45.45 41.50 46.91 45.36 27.5
Similarity matrix describing the relationship among species assemblages at the six sites. Units should be read as percent similarity. Generated using Primer5 software package.

These similarity figures may then be used to graphically describe the relative similarity among the six field sites using a branching tree diagram (like a genealogy): sites that are closely related to each other on the tree (such as Eagle Cove and Cattle Point) arise from a branch point with a high similarity score, while sites that are not alike (such as Garrison Bay and Cobble Beach) must trace their similarity back to a branch point with a relatively low similarity score:


Bray-Curtis similarity dendrogram of the six habitats using presence/absence data for 169 species.
Generated using Primer5 software package.

This tree drawing (called a dendrogram) is especially interesting because it reflects the quantitative differences among the six sites, which many of the student researchers also noted qualitatively: the three rocky sites are the most similar to one another, each being more exposed to wave action than the other three sites, and having a similar slope with respect to tidal height. Garrison Bay and Argyle Creek, though they differ in terms of substrate type (mud versus rocky creek), are both protected and far flatter than the rocky sites. Finally, False Bay is the most different from any other field site with respect to its fauna, as it is a very broad, soft-bottom habitat that contains a large variety of sediment types.

The survey and this statistical analysis of its results did not indicate strongly similar faunas at the two soft sediment sites (Garrison Bay and False Bay). This seems surprising because the two locations look similar (mud) and seem to have many similar faunal elements (many burrowing bivalves and polychaete worms). In part, the differences between them could reflect some more particular differences between the two sediments: False Bay includes more sand, and is flushed more vigorously by tidal action than Garrison Bay, which has more very fine mud and clay plus a very rich upper layer of diatoms. Perhaps as a result of such physical differences, the two bays have some striking faunal differences in what are superficially similar habitats (e.g., burrowing thallassinid shrimp in False Bay but not Garrison Bay). The differences could also reflect different sampling efforts by the student groups who studied the two sites. At Garrison Bay, the specific mud site chosen for study was just a few meters from a rocky outcropping where some typical rocky shore animals (e.g., Nucella snails) were found and included in the faunal survey. At False Bay, many similar rocky outcroppings are present at the margins of the bay but were not included by that student group because the rocks and their animals were hundreds of meters away from the sediment areas that those students focused on.

Comparative Assemblage Discussion

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