Biodiversity is a broad and complex concept. It encompasses the overall variety found in the living world, from genes, to species, and ecosystems. Here, we will focus on species, considering the variety of plant and animal species in a certain area (species richness) and their population sizes (species abundance). Population size is the number of individuals per species, generally expressed as the abundance of a species or briefly "species abundance". The various types of natural regions in the world, also called "biomes", vary greatly in number and composition of species; a tropical rainforest is entirely different from deserts or tundras.

The markedly increased rate of biodiversity loss we face today is the unintentional result of increasing human activities all over the world. The process of biodiversity loss is generally characterized by a decrease in the abundance of many original species and an increase in abundance of a few other, opportunistic species as a result of human activities. Extinction is the last step of what may be a long process of degradation; numerous local extinctions may precede the potentially final global extinction. As a result of human development, many ecosystems that had differed from one another are becoming increasingly alike, more "homogenized". Decreasing populations are as much a signal of biodiversity loss as strongly expanding species, which may sometimes become invasive.

Although we know that global biodiversity is declining at an unprecedented rate (Jenkins 2003), until recently it was difficult to measure the process of biodiversity loss, as species richness appeared to be an insufficient indicator. First, it is hard to monitor the number of species in an area, but more importantly, disturbance may initially increase species richness as original species are gradually replaced by new human-dispersed invasives. For this reason, the Convention on Biological Diversity (CBD) has chosen to use, among other measures, species abundance as an indicator of biological degradation. Thus, in this report and in the GLOBIO model, biodiversity is defined as the whole set of original species and their corresponding abundance. Furthermore, because of the effort involved, thorough mapping and monitoring across large areas is not feasible in most regions. Fortunately, there are numerous thorough peer-reviewed empirical studies that quantitatively link changes in habitat, such as fragmentation, to biodiversity loss. Based on literature for specific habitat types and the extent of the pressures present, we can model the potential loss in biodiversity compared to the undisturbed state by projecting the impact of changes in different pressures over time. By comparing and analyzing historical changes in habitats, including use of satellite imagery, records in changes can be projected using different types of scenarios and assumptions.

Biodiversity loss is expressed here as the average species abundance of the original species compared to the natural or low-impacted state. Increasing exotic populations do not compensate for the loss of decreasing populations in the indicator. If the indicator is 100 per cent then the biodiversity is similar to the natural or low-affected state. If the indicator is 50 per cent then the average abundance of the original species is 50 per cent of the natural or low-affected state, and so on. To avoid masking, significant increased populations of original species are truncated at 100 per cent, although they should have a negative score.