 How many species of organisms are distributed on Earth? That question has plagued scientists since Charles Darwin first proposed his theory of evolution. Why is it important to pin down that number? Well, because it's more than just a number. The number of species and individuals distributed across the globe provides a full picture of Earth's biodiversity and offers hints on how organisms have evolved and diversified over millions of years. The evolutionary diversification of organisms is triggered by the mutation of newly born individuals within a certain population. When these mutants reach a critical mass, they're recognized as an entirely new species. This process is called speciation, but no species can maintain its numbers forever. Over geological timescales, decreases in populations seal a species' fate, and they eventually become extinct. A look through fossil record reveals countless species that have succumbed to this rule of nature. The rise and fall of species can be analyzed based on the number of individuals of various species that exist on Earth. So, if scientists can get a handle on that figure, they can begin to understand the fundamental processes that have shaped the biodiversity patterns found in nature. Essentially, the question of how many species are distributed on Earth becomes how is biodiversity on Earth sculpted through evolution? Answering that question, however, is extremely difficult. That's because it's physically impossible to count the number of species on the planet and the total number of various individuals. This is what's known as the shortfall problem on biodiversity information. In many ways, it's like an event horizon in cosmology. The expansion of the universe at faster-than-light speed means there are events that an observer simply cannot see. With the growing number of organisms vastly outpacing the physical means to count them, biologists would likely never approach the general theory that explains biodiversity, not by brute force anyway. Researchers from Japan have developed a statistical model they believe could help overcome the looming shortfall problem. The model begins with existing data on where species of interest has or hasn't been detected over a certain area or plot. Plots form individual cells, with the assumption that individuals are distributed independently of one another within each cell. The assumption yields a probability function that links the probability of finding a species within a plot to the density of that species in the grid cell. Stitching those results across numerous cells produces a map of abundance of a species, along with maps detailing properties such as species diversity and community size. That information is valuable for understanding the facets of regional biodiversity and, importantly, for determining how conservation efforts should be deployed. The team applied their model to a large data set of woody plant communities found in Japan. The results revealed that 20 billion trees are distributed throughout the Japanese archipelago, the largest species being Uraya japonica, or Hisakagi in Japan, with about 36 million individuals. Based on the biogeological settings of the archipelago, the team divided Japan into four eco-regions and estimated the parameters governing the micro-evolution of species in these zones. In this analysis, they applied several different models, including one based on protracted speciation where a new species arises less frequently over time. This model allows researchers to capture not only observable species, but also unobservable species, a biological dark matter of sorts that, though invisible, still affects the dynamics of biodiversity. That's a huge step. The ability to see beyond what current data shows means that researchers could obtain reliable estimates of species abundance across large areas, therefore begin to tackle the shortfall problem. More work is still needed, though. The current model operates on several assumptions that make it hard to calibrate, and the model does not account for changes in species abundance over time. But with further refinement, the model could help researchers grasp the extent of biodiversity on Earth like never before, providing new clues to how evolution has shaped the world we live in today.