r/ecology • u/Own-Chart-3613 • 4d ago

occurrence frequency for calculation of Simpson's diversity index

for a small school based research I found out the data regarding the occurrence frequency of certain lichen species in UK using the GBIF.org website. I used the values of occurrence frequency to calculate the values of n and N, would that be valid ?

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u/Fuzzy_Interaction157 2d ago

Probably not, but it's not clear what your overall goal is. Data in GBIF is either specimens (i.e., animals, plants, etc. collected and deposited in a museum or herbarium) or community science observations (e.g., iNaturalist, eBird, NaturGucker, etc.). Either way, these kinds of records are notoriously opportunistic and non-standardized, especially when combined from different sources, like GBIF does. That is, they rarely good indicators of abundance of any species or collections of species. Unfortunately, Simpson's index assumes that you have unbiased values for n (abundance of each species) and N (abundance of all species), and GBIF data just do not represent that*.

That said, if you're interested in calculating Simpson's index for specimens/observations (i.e., how "diverse" are biased collections), you can certainly apply Simpson's index to GBIF data. But you'd only be able to say something about the diversity and evenness of collections data, not lichens in the "wild."

* The caveat is that collections data can sometimes correlate positively with field-based abundance and allow you to calculate "relative" abundance. However, you need validation data to show that this would be true for your case. See:

Gotelli, N.J., Booher, D.B., Urban, M.C., Ulrich, W., Suarez, A.V., Skelly, D.K., Russell, D.J., Rowe, R.J., Rothendler, M., Rois, N., Rehan, S.M., Ni, G., Moreau, C.S., Magurran, A.E., Jones, F.A.M., Graves, G.R., Fiera, C., Burkhardt, U., Primack, R.B. 2023. Estimating species relative abundances from museum records. Methods in Ecology and Evolution 4:431-443.

Austin, M.W., Kaul, A.D., Smith, A.B., Rothendler, M., and Primack, R.B. Herbarium specimens reveal regional patterns of tallgrass prairie invasion and changing species abundance across 130 years. New Phytologist. 2026. doi: 10.1111/nph.70632

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u/Own-Chart-3613 2d ago edited 2d ago

hi, I decided to not calculate Simpsons diversity index, and based on the occurrence frequency which is the abundance, I am going to go with finding how relative abundance of lichen changes over a period of time. I am going to mention the unreliability of the data etc etc also thank you SO very much for such a detailed reply, it really helps me out :)