COS 200-2 - Unveling the distribution of life on Earth

Abstract: That some species are rare and others are common is one of the oldest observations in ecology. But the exact shape of the distribution of species abundances on Earth has remained elusive. Some have argued that nature shows a bias towards rare species, while others have proposed that most species have intermediate abundances. We use more than 1 billion observations from the Global Biodiversity Information Facility (GBIF) to assess the global species abundance distributions (gSADs) of 39 taxonomic classes of organisms, stratifying our analysis temporally from 1900 to 2019. Our analysis shows that as sampling effort increases through time, the shape of the gSAD is unveiled, i.e., the shape of the sampled gSAD changes, revealing the underlying gSAD. The best statistical fit for almost all classes was the Poisson log-normal distribution, with negative binomial and log-series distributions having poor-fit to the data. The fraction of species unveiled for each class decreases with the total number of species in that class and increases with the number of individuals sampled, with some groups such as birds being fully unveiled. We also found that the percent of the gSAD that is unveiled is strongly dependent on observed species richness, where more speciose classes are less well-sampled, as well as sampling effort where increased number of occurrences allows for a higher likelihood of the class being fully sampled. The results presented here illustrate the importance of considering SADs in a global context, but also highlight the importance of future work to better understand the shape of global species abundance distributions. This evidence for a universal pattern of gSADs across classes suggests that there are general ecological or evolutionary mechanisms governing the commonness and rarity of life on Earth.