Forest ecosystems cover one third of Earth’s terrestrial area and are home to nearly two-thirds of all bird species, including more than 70% of globally threatened species (BirdLife International 2023). Intact forest habitats are critical for securing threatened species and ecosystems, but they continue to be lost at high rates, with negative consequences for biodiversity, climate change, and the rights and livelihoods of local communities (Harris et al. 2021). The ultimate drivers of forest loss and degradation vary spatially but are associated with trends around global trade, economic development, and international demand for commodities, leading to clearance for industrial and small-scale agriculture (Leblois et al. 2017), logging, and wildfires (van Wees et al. 2021).

More than one-third of the world’s Key Biodiversity Areas (KBAs, most of which have been identified for birds) and 60% of global KBA extent are forested. Many of these forested KBAs have been identified for their global significance for the conservation of forest species—they support populations of >5,200 such species, almost 70% of which depend on forests for their survival (Crowe at al. 2023). To determine which forested KBAs are under the greatest threat, identify the actions needed, and track the effectiveness of conservation interventions, it is critical to assess the condition of forests within KBAs, as forest integrity is directly linked to the levels of both species and ecosystem endangerment.

The Forest Landscape Integrity Index (FLII) offers an indicator of the status of Earth’s remaining forests (Grantham et al. 2020). It describes the degree of forest modification by integrating data on forest extent, threats, and alterations in connectivity. FLII scores range between 0 (lowest integrity) and 10 (highest integrity) and can be categorised as low (≤6.0), medium (>6.0 and <9.6) or high (≥9.6).

Crowe et al. (2023) overlapped the FLII layers with KBA spatial data to investigate forest integrity in 6,844 KBAs that contain forest and have been identified for forest species. The results reveal that globally, the mean integrity across all such KBAs is classified as low (FLII ≤6). Mean FLII is lowest in Europe (4.0 ± 2.8, i.e. low integrity), and highest in Oceania (8.3 ± 1.6, i.e. medium integrity) (see figure 1). By area, 44% of the forested area in KBAs (1.7 million km²) is classified as high integrity, while 39% is medium integrity and 18% is low integrity. These proportions indicate that the integrity of forests in KBAs is slightly greater than the average for forests globally (Grantham et al. 2020). The region with the highest area of high-integrity forest is South America, followed by Africa and Asia, while forested KBAs in Europe, Central America, and the Caribbean have highest proportions of low-integrity forests (see figure 1).

Data from Crowe et al. (2023)

More than half of all forested KBAs have been identified for one or more globally threatened forest species, with many such sites occurring in Asia (32%), South America (18%) and Africa (17%). The mean integrity of forest sites triggered by globally threatened forest species, at 6.8, is at the lower end of the medium integrity category, reflecting that threats may have reduced the integrity of sites used by these species, perhaps in some cases contributing to their globally threatened status.

Forest integrity is positively correlated with KBA size, the extent of forest in KBAs, and the proximity of other forested KBAs. Therefore, efforts to improve or restore lost or degraded forest habitats in KBAs, connect forest fragments and/or to expand and manage forest habitats beyond the KBA boundaries, where appropriate, might improve the quality of these KBAs for forest species and make a substantive contribution to the conservation of globally threatened species. Conserving the extensive areas of forested KBAs that contain high-integrity forest and that remains unprotected or partially protected should be a priority. Conservation actions needed at these sites may include safeguarding through the establishment of protected areas, ‘other effective area-based conservation measures' (OECMs), broad-scale land-use policies or other approaches (Donald et al. 2019), and/or appropriately targeted conservation measures, such as restoration to increase connectivity and reduce fragmentation and edge effects.

FLII provides important information on the state of KBA forest estate and allows for targeting and prioritising where conservation management is most needed. Future updates in the FLII layer could enable assessments of trends over time in the integrity of KBAs and will help evaluate the success or failure of conservation interventions. These types of remotely sensed efforts will be especially important at sites that are large, remote, with limited access and/ or visitation, and for sites whose triggers are known to be affected by loss of forest extent or condition.