Gracilaria: A dynamic habitat

Nick Partington, St. Olaf College

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The findings: In my previous post, I described the methods we would be taking this summer to explore how the biodiversity of fishes differ among dense and sparse patches of the invasive alga Gracilaria vermiculophylla. We followed these methods, and we produced some interesting results!

We finally sorted and identified all of the fishes we collected from our samples this summer, and were able to measure the biodiversity between dense and sparse habitats. In particular, we were interested in four measurements of biodiversity. The first, abundance, is simply the overall number of fishes collected from each habitat type. The second, species evenness, measures how evenly individual fishes

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Some of the fishes we collected this summer, separated by species and sample.

are distributed among the different species collected in each habitat type. Finally, diversity took into account species richness, which counts the total number of species collected, and the Simpson’s Diversity Index, which quantifies diversity based on the number of species and the relative abundance of each of those species.

These measurements provided us with some interesting results. In the end, we collected a greater abundance of individuals in sparse sites than in dense sites. We also saw both greater species evenness and greater species richness in dense sites. Additionally, the Simpson’s Diversity Index showed a greater diversity of fishes in dense sites.

As I mentioned, abundance of individuals and species richness were both calculated by simply counting the overall number of individuals and species, respectively, collected in each site. Species evenness, on the other hand, required a bit more analysis. Figure 1 shows rank abundance curves for both sparse and dense patches of G. vermiculophylla. These curves tell us how evenly individuals are distributed among the species collected from each site. For each habitat type, species are ranked from 1 to 10 in decreasing order of abundance. That rank is then compared with the abundance of each species. The slope of the resulting line is what we are interested in. Basically, the flatter the line, the greater the species evenness. In our analysis, the line representing dense sites had a flatter slope, signaling greater species evenness in dense sites than in sparse sites.

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Figure 1. Rank abundance curves for both dense and sparse habitats. The slope of the curve representing dense habitats is closer to 0, indicating greater species evenness in those sites.

As I mentioned, overall we found a greater abundance of individual fishes in sparse sites, while we had greater species evenness, species richness, and diversity in dense sites. These differences between sites are very interesting in themselves. But what is even more interesting is that these results are the complete opposite of what was concluded after this same study was conducted last summer. Therefore, there must be some factor(s) that changed between these two studies. We’re not exactly sure what these factors are, but nonetheless, this highlights the importance of long term studies, as well as the importance of continuing this study to see how these trends in biodiversity change and pan out in the long run. I think a very interesting takeaway from this project is that invasive species, like G. vermiculohylla, can potentially provide benefits and sustain biodiversity in ecosystems here in Charleston and throughout the world.


Special thanks to Tony Harold and Mary Ann Taylor for their guidance in this research project. This project is funded by the National Science Foundation and is supported by the Fort Johnson REU Program, NSF DBI-1757899.

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