Biodiversity Intactness Index (BII)

The Biodiversity Intactness Index (BII) is a scientific measure that estimates the average abundance of originally present species in a given area, compared to their abundance before significant human impacts. It serves as a key indicator of how much of a region’s natural biodiversity remains intact despite land-use change, habitat modification, and other anthropogenic pressures. The BII provides an objective, quantifiable way to monitor biodiversity loss and assess the effectiveness of conservation efforts on both national and global scales.

Concept and Definition

The Biodiversity Intactness Index was developed in the early 2000s by researchers led by the Natural History Museum, London, and has since been refined through global biodiversity assessments. It is defined as the mean abundance of native terrestrial species in an area relative to their abundances in undisturbed ecosystems.
In essence, the BII compares the current state of biodiversity with a baseline or reference condition representing an intact, pre-industrial ecosystem. The index is expressed as a percentage value, where:

• 100% indicates an ecosystem that retains its full, original biodiversity, and
• Values below 100% signify biodiversity loss due to human influence.

For example, a BII of 90% implies that the abundance of native species in the area is, on average, 90% of what it would have been under natural conditions.

Methodology and Calculation

The calculation of the Biodiversity Intactness Index involves integrating extensive data on species populations, habitat types, and land-use patterns. It typically relies on large-scale datasets, such as the PREDICTS (Projecting Responses of Ecological Diversity In Changing Terrestrial Systems) database, which compiles ecological surveys from across the globe.
The general formula can be simplified as follows:
BII=Average abundance of native species in modified habitatsAverage abundance in undisturbed habitats×100BII = \frac{\text{Average abundance of native species in modified habitats}}{\text{Average abundance in undisturbed habitats}} \times 100BII=Average abundance in undisturbed habitatsAverage abundance of native species in modified habitats​×100
Key components used in BII modelling include:

• Land-use types: agriculture, forestry, urban areas, rangelands, and primary vegetation.
• Land-use intensity: low, medium, or high intensity of human modification.
• Species response data: how populations of different taxa respond to various land uses.
• Spatial resolution: data mapped globally, typically at 1 km² to 50 km² grid scales.

Models estimate the average abundance of native species across all land-use categories and compare it to baseline conditions to derive the percentage intactness value.

Interpretation and Scale

The Biodiversity Intactness Index is typically interpreted within these broad ranges:

• Above 90%: Ecosystems largely intact, with minor human impacts.
• 70–90%: Moderate loss; ecosystems under stress but still functionally diverse.
• 50–70%: Significant biodiversity reduction; major declines in native species abundance.
• Below 50%: Severely altered ecosystems with high extinction risks and degraded ecological functions.

Scientists generally consider a BII threshold of 90% as the safe ecological limit, below which ecosystem functions such as pollination, nutrient cycling, and pest control begin to decline markedly.

Global Findings

Global assessments using the BII reveal concerning trends:

• The average global BII is estimated to be around 75%, meaning that, on average, species populations have declined by about 25% compared to pre-industrial levels.
• Tropical regions, especially in South America, Africa, and Southeast Asia, have some of the lowest BII values due to deforestation, agricultural expansion, and mining.
• Temperate regions show moderate losses, whereas arid and polar zones remain relatively intact.

These findings indicate that the world has already crossed the safe planetary boundary for biodiversity integrity in many regions.

Regional and National Assessments

Several countries and research institutions have begun incorporating the BII into national biodiversity monitoring frameworks.

• United Kingdom: Uses BII as an official indicator of environmental performance within the Biodiversity Indicators Partnership framework.
• India: Although national BII assessments are still emerging, regional studies in the Western Ghats and northeastern states show notable biodiversity decline in agricultural and urban landscapes.
• Africa and Latin America: Projects supported by the UN Environment Programme and biodiversity research networks have mapped BII changes to guide conservation priorities.

Importance and Applications

The Biodiversity Intactness Index serves multiple scientific, policy, and conservation purposes:

1. Monitoring Biodiversity Change: Provides a consistent measure of biodiversity status across space and time, allowing for long-term tracking of human impacts.
2. Policy Assessment: Supports evaluation of progress towards international goals such as the Kunming-Montreal Global Biodiversity Framework (2022), Convention on Biological Diversity (CBD) targets, and Sustainable Development Goal (SDG) 15 – Life on Land.
3. Land-use Planning: Helps policymakers identify priority areas for conservation and restoration based on current biodiversity conditions.
4. Ecosystem Services Evaluation: Since biodiversity underpins key ecological functions, the BII informs assessments of ecosystem health, resilience, and service provision.
5. Corporate and Environmental Accounting: Used by industries and financial institutions to evaluate the biodiversity impact of land-based operations.

Limitations and Criticisms

While the BII is widely regarded as one of the most comprehensive measures of biodiversity condition, it has some limitations:

• Data Gaps: Many regions, especially in the tropics, lack sufficient species abundance data for accurate estimation.
• Simplification of Complexity: Averaging species abundance across ecosystems may obscure the loss of specific species or functions.
• Baseline Uncertainty: The definition of “pre-impact” or “intact” ecosystems can vary geographically, affecting comparability.
• Taxonomic Bias: The index relies heavily on terrestrial vertebrates and plants, while invertebrates and microorganisms remain underrepresented.
• Inapplicability to Marine Systems: The BII framework primarily focuses on terrestrial ecosystems, though adaptations for marine environments are under study.

Despite these challenges, the BII remains an invaluable composite metric for summarising large-scale biodiversity changes.

Relationship with Other Biodiversity Indicators

The BII complements other global biodiversity indicators such as:

• Living Planet Index (LPI): Measures population trends across species but does not quantify local ecosystem intactness.
• Red List Index (RLI): Focuses on species extinction risk rather than abundance.
• Mean Species Abundance (MSA): Conceptually similar to BII, estimating species abundance relative to pristine conditions, but BII incorporates more extensive spatial and ecological data.

Together, these indices offer a holistic picture of biodiversity trends and threats at multiple scales.

Policy and Conservation Relevance

The Biodiversity Intactness Index has been increasingly integrated into international conservation frameworks to track the health of ecosystems under global targets such as:

• Convention on Biological Diversity (CBD) – for monitoring progress toward halting biodiversity loss.
• IPBES (Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services) – for global assessments on nature’s status and trends.
• UN Sustainable Development Goals (SDG 14 and 15) – for evaluating terrestrial and marine ecosystem sustainability.

By quantifying biodiversity intactness in measurable terms, the BII bridges the gap between scientific data and policy action, guiding evidence-based decision-making in land management and conservation planning.

Future Developments

Advancements in remote sensing, artificial intelligence, and citizen science are expected to improve the accuracy and resolution of BII assessments. Integration of satellite-based habitat mapping, species distribution models, and eDNA (environmental DNA) data can help overcome data gaps and provide near real-time biodiversity monitoring.
Researchers are also developing marine and freshwater adaptations of the BII to cover the full spectrum of global ecosystems. Furthermore, the incorporation of climate change parameters and ecosystem functionality into future versions of the index will enhance its relevance to global environmental policy.