Â鶹ÒùÔº

Deforestation is a well-known issue affecting biodiversity and carbon storage, but the fragmentation of forests is also a driving factor in the reduction of the world's biodiversity and carbon-storing abilities.

Fragmentation occurs when larger forests are broken into smaller, isolated patches, thereby reducing habitat connectivity and size. Fragmentation is worsened when these patches shrink, split, become more complex in shape, or grow more distant from each other.

However, scientists are still trying to figure out the best way to quantify habitat fragmentation and the rate at which it occurs. A new study, in Science, suggests fragmentation is increasing in over half of the world's forests, particularly tropical forests, despite a suggesting a decline in fragmentation over the last 20 years.

The new study's authors believe the discrepancy lies in the metrics considered. The 2023 study relied on only structure-based metrics—those involving the patch number, size, and edge length. Yet, other studies focusing on connectivity and aggregation metrics have indicated increasing fragmentation, especially in the tropics.

"Aggregation-focused metrics assess how clustered patches are but may also overlook overall extent. Connectivity-focused metrics incorporate both patch area and spatial configuration, offering a more ecologically relevant perspective. Because each captures different aspects of fragmentation, selecting ecologically meaningful metrics is critical to accurately track progress toward conservation goals," the authors explain.

To incorporate all relevant factors, the team used nine fragmentation metrics grouped into structure, aggregation, and connectivity and developed composite indexes for each metric group. Using high-resolution satellite data, they analyzed global forest cover from 2000 to 2020 and also compared trends inside and outside protected areas.

The connectivity-based metrics showed increased fragmentation in 51%–67% of global forests and 58%–80% of tropical forests from 2000 to 2020, and the aggregation metrics showed that 57%–83% of forests became more fragmented. Meanwhile, the team's structure-based metrics only indicated a 30% to 35% fragmentation over the same period.

The study also looked at the drivers of fragmentation, which included shifting agriculture (37% globally, 61% in tropics), forestry (34% globally), and wildfires and commodity-driven deforestation to a lesser degree. They also found that protected areas experienced 82% less fragmentation than unprotected areas, largely from reduced agricultural activity. This hints that much of the increased fragmentation is potentially reversible, as it is often driven by reversible land uses, like agriculture, logging, or natural causes, like wildfires. If protected, these areas may be restored over time.

"These results highlight the effectiveness and importance of tropical protected areas in limiting human-driven fragmentation and underscore the urgent need to expand protection across tropical regions," the study authors say.

The researchers also note possible limitations due to satellite data resolution and difficulties detecting narrow roads, distinguishing natural forests from agroforestry—possibly underestimating natural forest loss, and also difficulties in capturing forest regrowth below 5m in height.

The study underscores the need for better fragmentation metrics in future studies. "Our study reveals widespread declines in forest ecological integrity over the past two decades, driven largely by human activity. The stark divergence among fragmentation metrics underscores the urgent need for ecologically relevant tools to accurately assess and address these changes."