All statistics is about extrapolating from the known to the unknown, estimating what cannot be counted. Demographers may survey a sub-sample of a human population, and use modelling to draw inferences about the population as a whole. But that is just one species - understanding biodiversity, such as in the context of multiple plant and animal species, is much more difficult.
Ecologists therefore sample environments to estimate species diversity and abundance. But when environments are surveyed to assess biodiversity, the only certainty is that not all the individuals and species in the population will have been counted. This is not only because plants and animals can’t fill out forms, but also because some species will be harder to observe than others and different methods of observing will work differently between species.