In spite of a number of devices and data sources, biodiversity still remains a difficult task to execute to quantify it precisely. That is, it is difficult to know the exact number of species on earth. But precise answers are seldom needed to devise an effective understanding of where biodiversity is, how it is changing over space and time, the drivers responsible for such change, the consequences of such change for ecosystem services and human well-being, and the response options available. 

Normally, to assess and know the conditions and direction of biodiversity locally or globally, it it will be important to measure the abundance of all organisms over space and time, considering important factors such as the taxonomy (such as the number of species), functional traits (for example, the ecological type such as nitrogen-fixing plants like legumes versus non-nitrogen-fixing plants), and the interactions among species that affect their abilities and function which includes predation, parasitism, competition, and facilitation such as pollination. Though it is not possible to do this with much accuracy as we lack the resources, it will be very important to estimate the turnover of biodiversity not just only the estimates in space or time.

As we have a number of measures of biodiversity, a very important metric is the species richness which is reffered to as the number of species in a given area. But as important as this metric is, it mut be combined with other metrics for full cover of biodiversity. As a result of challenges the multidimensionality of biodiversity poses to its measurement, a variety of surrogate measures are many times used. These include the species richness of specific taxa, the number of distinct plant functional types (such as grasses, forbs, bushes, or trees), or the diversity of distinct gene sequences in a sample of microbial DNA taken from the soil. Other taxon-based measures of biodiversity, however, rarely capture key attributes such as variability, function, quantity, and distribution—all of which provide insight into the roles of biodiversity.

A scientific way of measuring biodiversity is the use of Ecological indicators which make use of quantitative data. ecosystem condition, services, or drivers of change, but no single ecological indicator captures all the dimensions of biodiversity. These Ecological indicators have now formed a critical component of monitoring, assessment, observing, decision-making and have been designed to communicate information quickly and easily to policy-makers. Economic indicators such as GDP are highly influential and well understood by decision-makers. On the same vein, environmental indicators, such as global mean temperature and atmospheric CO2 concentrations, are becoming widely accepted as measures of anthropogenic effects on global climate. Ecological indicators as they are founded on much of the same principles, carry with them similar characteristics.

Reference

GREENFACTS: Facts on Health and Environment

https://www.greenfacts.org/en/biodiversity/l-3/1-define-biodiversity.html