Leakage and Blockage

Protected areas can have both positive and negative spillover effects on surrounding areas. Sometimes, the establishment of a reserve can displace deforestation into unprotected adjacent land where it would not have otherwise occurred, a phenomenon known as ‘leakage.’ This displacement of land use change can negate any reduction in deforestation within the reserve itself, resulting in a higher level of forest loss over the broader landscape than would be expected without protection and undermining the conservation value of the protected areas. The opposite spillover effect is known as ‘blockage,’ when protected areas reduce deforestation in nearby areas. Blockage can occur when the protection of an area successfully prevents the development of transportation and logging infrastructure in the greater region.

This app uses remotely sensed imagery to explore the spillover effects of protected areas in three different countries. In the left image, each country is divided into four zones. Protected areas are shown in yellow, five- and ten-kilometer buffer zones are shown in purple and blue, and cyan represents the remaining land farther than ten kilometers from a protected area. The image on the right illustrates forest loss from 2000 to 2021, with the dark green background representing intact forest cover in the year 2000 and the color coded pixels representing forest loss by zone. The data was adapted from the Hansen Global Forest Change dataset, which uses Landsat imagery to characterize forest extent, gain, and loss across the world.

Honduras: Ineffective protected areas have no spillover effects

First let’s take a look at Honduras, which has lost more of its forest cover than any other country in Latin America over the past few decades. The primary drivers of deforestation in Honduras are fuelwood harvesting, the illegal timber trade, and clearing for subsistence agriculture and cattle grazing.

Even without the zone map, it is easy to identify the protected areas by the distinct purple and blue rings of heavy deforestation within ten kilometers of their borders. Unfortunately, the prevalence of yellow on the map also indicates a high level of forest loss within the protected areas themselves, and the cyan displays significant deforestation across the remainder of the country. The staggering degree of overall forest loss makes it difficult to discern any spillover effects.

To further examine the relationship between deforestation rates and proximity to a protected area, let’s add a computational element to this visualization. We can multiply the number of forest loss pixels by the pixel area in square meters to calculate the area of forest loss in each zone, and do the same for forest cover in 2000. Dividing the area of forest loss by the area of original forest cover gives us a percent forest loss from 2000 to 2021 that can be compared across zones (Fig. 2).

It is clear that on average, protected areas in Honduras do little to conserve forest cover within their boundaries. Deforestation rates within protected areas are similar to those across the rest of the country. Although there is an indication of some level of leakage, as percent forest loss is slightly higher in five-kilometer buffer zones than in regions more distant from protected areas, this discrepancy is likely negligible. The case of Honduras demonstrates that protected areas must successfully restrict deforestation within their borders in order to exert any spillover effects on neighboring regions.

Ghana: Deforestation leakage threatens to fragment landscapes

Next, we’ll look at Ghana, which has lost almost all of its old-growth forest to fire, mining, and agriculture. The last remaining tracts of primary forest are scattered throughout protected areas in the southern region of the country and comprise part of the Guinean Forests of West Africa, which have been designated as a critical biodiversity hotspot.

Again, most of the protected areas are clearly distinguishable without the zone map by the high degree of forest loss within their buffer zones. Because they are small, numerous, and close together, their buffer zones often overlap. As a result, even though there appears to be relatively less deforestation within protected areas, these islands of intact forest are increasingly separated by their heavily deforested buffer zones.

From Fig. 4, we can see clear evidence for leakage around Ghana’s protected areas. Deforestation rates within ten kilometers of protected areas are more than twice as high as those in the rest of the country. This spillover effect threatens to exacerbate the fragmentation of the remaining forest within preserves, with negative outcomes for biodiversity and wildlife movement. In this case, leakage not only undermines the conservation value of Ghana’s protected areas, but jeopardizes the connectivity of the country’s last remaining old-growth forest fragments.

Cameroon: Blockage enhances the value of effective protected areas

Finally, we will examine spillover effects in Cameroon, home to the second largest area of forest in Africa. All land in Cameroon belongs to the state, which has legislated that 33% of the country be classified as protected areas. The government has also undertaken an ambitious plan to reforest 30 million acres of land by 2030, but Cameroon’s existing forests are experiencing a recent surge in deforestation, primarily due to agricultural expansion.

In contrast to Honduras and Ghana, most of this forest loss has occurred in places distant from protected areas. By comparing the left and right maps, we see ‘halos’ of safeguarded forest around protected areas, a clear indication of blockage. The low prevalence of yellow on the forest loss map indicates that very little forest cover has been lost within protected areas.

Computing percent forest loss in each zone confirms that there is indeed very little deforestation occurring inside protected areas. Not only do Cameroon’s protected areas effectively reduce deforestation within their boundaries, they also slightly decrease forest loss in the unprotected adjacent land within ten kilometers. In this case, protected areas can have positive spillover effects that enhance their conservation value beyond what was originally intended.

Where do we go from here?

These three case studies have demonstrated the range of possible spillover effects around protected areas: leakage in Ghana, blockage in Cameroon, and a lack of any effect in Honduras. Understanding the impact of protected areas on their unprotected adjacent surroundings is critical for conserving larger landscapes as a whole, and can even serve as an early warning sign for the health of a forest reserve. A recent study found that nearby forest cover is a consistent predictor of future deforestation within a protected area’s boundaries; a protected area surrounded by intact forest is unlikely to experience deforestation, but when forest cover in the five-kilometer buffer drops below 20%, the protected area is likely to start being deforested at the same rate as nearby unprotected land.

The model used in this app can be applied to any country, offering important insight into the relationship between forest loss and proximity to protected areas at the national level. However, aggregating the analysis by country erases the impacts of each individual reserve, and a smaller scale analysis is needed to examine and address spillover effects on a case-by-case basis.