Or, using citizen science to analyse nudibranch biodiversity

Ask any regular False Bay diver where the best place to spot nudibranchs is, and you will almost certainly be directed towards A-Frame. And rightfully so; it is one of our favourite dive sites and where we witnessed the awesome laying of an egg case by a pyjama shark! Not to mention tons of nudis!

We naturally wanted to test this anecdote and thought best to do so with some citizen science data.

GBIF, the Global Biodiversity Information Facility, is an enormous network of data providers that curates open access to global biodiversity data. Many research institutions, countries and organistations share their species occurrence data on the GBIF platform resulting in hundreds of peer-reviewed publications each year. Roughly half of these records come form citizen science initiatives, such as iNaturalist.

We downloaded 1665 records of nudibranch occurrence from both citizen science and academic sources, to put False Bay to the test.

To make a fair comparison of diversities across sites that may vary in size or frequency of visit, biologists typically employ a technique called rarefaction, where measures of species richness are scaled down until they contain the same number of observations as the smallest sample. This is especially necessary for citizen science data, but comes with the consequence of discarding lots of valuable information.

A new technique that rarefies or extrapolates diversities to a common measure of sample completeness avoids discarding data, and is available in the R package iNEXT.

map of nudibranch diversity in False Bay
Nudibranch diversity across dive sites and tide pools of False Bay.

A quick run of our data through iNEXT and lo! and behold! … A-Frame! (and Rocky Bay).

Note here that we are only looking at species richness, that is, the diversity in the number of different species. We ignore other measures of diversity that include abundance, or the relative amount of each species. We can’t rely on divers to take pictures of every single nudibranch they see due to a little thing I like to call “gasflame fatigue.”

Dylan Irion

Dylan is a marine scientist and Save Our Seas Foundation project leader, working towards a PhD that aims to unravel the drivers of white shark population dynamics in South Africa. He is a passionate freediver and SCUBA diver, and volunteers as a Sea Rescue crew member.


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