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Upon first glance, It appears that the oyster bar habitat at Ft. Johnson has seen a shift in species composition over the past 40 years--out of all the species found and identified down to species level, only the slippershell Crepidula plana was noted in both surveys (a complete list of all species found in each survey can be found below). Unfortunately, abundance estimates for each species from both surveys are unknown, and it cannot be said with certainty which species dominated the habitat at each time point. Thus, we are forced to use number of species as a proxy for this measure.
After combining species into "common groups," it appears that the current habitat composition is less diverse than in the early 1970's (blue bars) than in 2011 (red bars) in terms of number of groups, as well as number of species in each group:
While the 1970's surveys found crabs to be the most speciose groups, the 2011 survey did not record any crabs, and it appears that gastropod species now dominate the habitat in terms of number of species:
All unlabeled slices constitute 4.3% of the pie All unlabeled slices constitute 9.1% of the pie
However, it must be noted that the differences in sampling between these two surveys are likely to confounded above results. For example, the winter of 2010/2011 was quite harsh in Charleston, which undoubtedly affected the appearace of many mobile organisms, such as crabs. During the winter, individuals move offshore into deeper, warmer waters. Thus, during an especially cold winter, their return to the intertidal habitat may have been delayed. Additionally, when examining the list of species that was found in 2011, the majority of them are species that are either sessile (i.e., barnicles, bryozoan), or have reduced mobility and spend their entire life in the intertidal zone (i.e., slippersnails, amphipods).
Furthermore, while an official sample was not taken in the spring of 2012, a brief walk through the area revealed oysters (Crassostrea virginica), ribbed mussels (Geukensia demissa), mud crabs, and hermit crabs (Clibanarius vittatus), which were not reported in 2011, but were reported in the 1970's survey. This provides evidence that the habitat likely hasn't changed as much over the past 40 years as the 2011 survey indicated, and that single surveys may not be adequate to make long term comparisons in a given habitat. Surveys should be repeated, both within a single year and over successive years in order to make valid conclusions about changes in habitat compositions. This discrepancy also highlights the importance of recording exact sampling locations, as the 1972, 1973, and 2011 reports were all a bit ambiguous in describing the exact location in which the survey took place.
Oyster bar in 2012
Regardless of any sampling biases that may have occured, it is interesting to note that the sessile bryozoan Schizoporella unicornis was not found in either of the 1970's survey, but it was found in the 2011 survey. This species is native to Japan, but has been unintentionally introduced to the east coast of the US.
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