Systematic test of biodiversity indicator performance to understand the reliability of these ecosystem indices as decision-support tools in local to global contexts

Biodiversity indicators measure changes in the environment. They are often used to monitor progress towards global conservation targets, impacts of policies, and guide management actions. Yet they are rarely tested to ensure they reliably depict how biodiversity is changing.

Last year, members of the Red List of Ecosystems Thematic Group, led by Jessica Rowland (Deakin University), tested the reliability of three indices that they recently developed to monitor past and likely future changes across marine, freshwater, and terrestrial ecosystems (Red List Index of Ecosystems - RLIE, Ecosystem Area Index - EAI and the Ecosystem Health Index - EHI).

In the paper recently published in Ecological Indicators, they used a stochastic ecosystem model of the Meso-American coral reef to test how well the indices represent the status of ecosystems. They created six scenarios with differing levels of threat from fishing and mass coral bleaching. For each scenario, the team simulated 100 different reef futures over 200 years and calculated the indices.

They tested three key aspects:

• Sensitivity: are the indices able to distinguish between different levels of threat, such as different levels of fishing pressure?

• Responsiveness: how quickly can the indices detect changes in ecosystems? This can allow for rapid actions to stop declines or signal that conservation actions have been effective.

• Complementarity: do the indices show similar patterns or reveal differences in how different parts of biodiversity are changing?

They also examined the impacts on the sensitivity and responsiveness of the indices of decisions made during their calculation:

• Classification scale: how does the scale at which the ecosystems are defined (local to global scale) affect the ecosystem indices?

• Choice of ecological variable: how do the ecosystem features used to calculate the indices affect their portrayal of ecosystem change? For example, coral cover vs. biomass of herbivorous fish.

• Frequency of calculation: how does the frequency of calculating the indices affect how quickly they can detect ecosystem changes?

This study showed that the ecosystem indices are sensitive, and responsive and offer complementary information on changes in ecosystems. However, the Red List Index of Ecosystems sometimes behaved counter-intuitively due to shifting baseline syndrome. This is where the index “forgets” changes over time. This is also an issue for the Red List Index of species survival. 

The scale of ecosystem classification often affects the size of the ecosystems used in the Red list of Ecosystem assessments. This can influence the output from the indices – smaller ecosystems tended to have lower index values and decline faster (larger risk or declines) than larger ecosystems. Careful choice of ecological variables used to calculate the Red List Index of Ecosystems and Ecosystem Health Index is critical to reliability in detecting ecosystem change.

This study is one of the few examples of a systematic test of biodiversity indicator performance. The team’s findings will help improve our understanding of the reliability of these ecosystem indices as decision-support tools in local to global contexts.