Blog author: Jessica Rowland
Citation: Rowland, J. A., Lee, C. K. F., Bland, L. M., and Nicholson, E. Testing the performance of ecosystem indices for biodiversity monitoring. Ecological Indicators, 116, 106453. [PDF]
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.
In our new paper, we tested the reliability of three indices (Fig. 1) that we recently developed to monitor past and likely future changes across marine, freshwater and terrestrial ecosystems (you can read the blog on paper describing the indices here). In short, the indices include:
- Red List Index of Ecosystems (RLIE) >>> summarises the status and trends in the risk of ecosystem collapse
- Ecosystem Area Index (EAI) >>> measures change in ecosystem extent
- Ecosystem Health Index (EHI) >>> measures ecosystem degradation from changes in ecological processes and functions of both the biological and physical components of ecosystem.
We used a stochastic ecosystem model of the Meso-American coral reef to test how well the indices represent the status of ecosystems (Fig. 2). We created six scenarios with differing levels of threat from fishing and mass coral bleaching. For each scenario, we simulated 100 different reef futures over 200 years and calculated the indices.
We 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?
We 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 does the ecosystem feature 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?
Our study showed that the ecosystem indices are sensitive, responsive and offer complementary information on changes in ecosystems (Fig. 3). 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 sensitivity and responsiveness of the indices were influenced by the threats and decisions made during the calculation process. The scale of ecosystem classification often affects the size of the ecosystems used in 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 reliably detecting ecosystem change. We also showed that calculating the indices at 5 – year intervals can reveal informative trends, although 1 – and 10 – year intervals are also informative.
Our study is one of the few examples of a systematic test of biodiversity indicator performance. Our findings will help improve our understanding of the reliability of these ecosystem indices as decision-support tools in local to global contexts.
Feature image: Dronepicr / CC BY (https://creativecommons.org/licenses/by/3.0)