The level of biodiversity on Earth is not an immutable constant and instead exists in a state of constant flux

• New species can arise over time as a consequence of evolutionary change leading to eventual speciation
  

• Species numbers may also be diminished when external factors result in the complete eradication of a species (extinction)
  

Evidence from fossils suggest that there may be currently more species alive on Earth today than at any time in the remote past

• Currently, there are roughly 1.2 million species that have been recorded and classified, but estimates suggest there could be a further 7.5 million species yet to be discovered
  

The accuracy of a species diversity estimate is dependent on the mechanisms used to determine species classification

• Taxonomists who are ‘lumpers’ tend to classify species according to shared similarities – resulting in lower estimates

• Taxonomists who are ‘splitters’ tend to focus more on the differences between organisms – resulting in higher estimates

Biodiversity Over Time

Biodiversity Crisis

While more species currently exist than at any time in the remote past, there has also been a significant reduction in species diversity in recent times

• It is estimated that the world has seen a 73% decline in the average size of monitored wildlife populations over the last 50 years
  

Assessments of biodiversity are made by collaborative multinational agencies – such as the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES)

• These assessments are based on repeated surveys and may rely on contributions from both expert scientists and members of the public (citizen scientists)
  

When assessing the biodiversity of a particular region, two key measures are investigated:

• Species richness describes the number of different species present in an area (more species = greater richness)
  

• Species evenness describes the relative abundance of the different species in an area (similar abundance = more evenness)
  

Ecosystem stability requires a high level of both species richness and species evenness in order to resist environmental change

• An ecosystem with many species but few individuals within each population would not be stable (low evenness)
  

• An ecosystem with only a few species consisting of many individuals would similarly lack resilience (low richness)
  

Species Diversity

Both communities have the same species richness but they have different species evenness

Same species richness but different species evenness