Severe destruction of habitat and biodiversity loss in the Atlantic Forest biome of South America, which covers parts of Argentina and Brazil, is widely recognized. Only 11.7% of the original vegetation cover still remains, and what does remain is not a single connected forest system, but rather exists as many patches, most of which are smaller than 50 ha. Fewer than 0.03% of the remaining forest cover is larger than 10,000 ha. And the small forest areas are usually surrounded by human settlements and thus easily accessible for hunters.

In this article, researchers explored to what extent population of different classes of mammals have declined in the Atlantic Forest biome of South America. Large mammal species are especially threatened in this environment by hunting, habitat conversion, degradation, and fragmentation. These species play a crucial role in the healthy functioning of ecosystems and their decline has potential negative effects for ecosystem services. For example, tropical forests store 55% of global terrestrial carbon, the driving element of climate change if released in the atmosphere. Large fruit-eating mammals are crucial for dispersing the seeds of forest plants and trees and thus maintaining the carbon stored in the forest.

To estimate the extent of loss of mammal species in the Atlantic Forest, the researchers compiled data from 105 studies which looked at population decline of animals at specific sites or regions. Based on this data, the researchers estimated a defaunation index, to describe the extent to which the population of all mammal species and that of different groups of species have declined relative to historical levels in the entire forest biome. The defaunation index for a species can range from 0.0 (completely intact) to 1.0 (completely defaunated, or removed).

The mean defaunation value for all mammal species covered is 0.57. Large mammal species, (with a body weight above 10 kg) are especially badly affected. Apex-predators, like jaguars, who are on top of the food chain, have the highest defaunation levels (0.79).

Overall the authors conclude:

The systematic defaunation throughout the Atlantic Forest results in a functionally “half-empty” forest ecosystem with subsequent disruptions in the ecological roles performed by several mammal species.

The authors also discuss potential measures to counter the trend of defaunation in the Atlantic Forest. They note that protecting large areas of continuous forest cover is key for sustaining large mammal populations. However, there have been multiple conservation efforts in recent decades, for example, by designating certain forest remnants as protected areas. Unfortunately, one problem with existing protected areas is that they are mostly in mountainous areas, where only around 5% of the whole Atlantic Forest biome lives.

For advocates, these findings represent a reminder of the large extent of the problem of population decline and biodiversity loss in one of the most severely deforested regions in the world. Furthermore, it points towards the need to focus on protecting continuous forest cover as the most effective solution.