Pervasive defaunation of forest remnants in a tropical biodiversity hotspot

Abstract

Tropical deforestation and forest fragmentation are among the most important biodiversity conservation issues worldwide, yet local extinctions of millions of animal and plant populations stranded in unprotected forest remnants remain poorly explained. Here, we report unprecedented rates of local extinctions of medium to large-bodied mammals in one of the world's most important tropical biodiversity hotspots. We scrutinized 8,846 person-years of local knowledge to derive patch occupancy data for 18 mammal species within 196 forest patches across a 252,669-km(2) study region of the Brazilian Atlantic Forest. We uncovered a staggering rate of local extinctions in the mammal fauna, with only 767 from a possible 3,528 populations still persisting. On average, forest patches retained 3.9 out of 18 potential species occupancies, and geographic ranges had contracted to 0-14.4% of their former distributions, including five large-bodied species that had been extirpated at a regional scale. Forest fragments were highly accessible to hunters and exposed to edge effects and fires, thereby severely diminishing the predictive power of species-area relationships, with the power model explaining only ~9% of the variation in species richness per patch. Hence, conventional species-area curves provided over-optimistic estimates of species persistence in that most forest fragments had lost species at a much faster rate than predicted by habitat loss alone.

Figure 1. Distribution of remaining forest patches across the northern Atlantic Forest study region showing all surveyed forest patches.

For detailed maps see Fig. S1, S2, S3.

Figure 2. Size distribution of all forest patches surveyed (left, N = 196) or not surveyed (right, N = 613,997 patches >0.25 ha) within the entire study region (∼252,669 km²).

Numerical representation of different-sized patches are expressed in terms of proportions of the total.

Figure 3. Species-area relationships (SARs) for (a) species richness, (b) a measure of total biomass, and (c) a measure of total species vulnerability for 18 mammal species surveyed at 196 forest patches of the Atlantic forest of northeastern Brazil.

Solid circles (and corresponding regression lines and 95% confidence intervals) indicate the five existing strictly protected forest areas in the entire study region, for which intercepts were clearly higher. All other data points (gray circles) represent unprotected forest sites.

Figure 4. Levels of forest patch occupancy for 18 mammal species surveyed throughout the Atlantic Forest study region of northeastern Brazil.

Species are ordered top to bottom according to increasing level of patch occupancy. Dark-gray bars indicate the proportion of all 196 forest patches occupied by each species (see Table 1 for full Latin and English names), and solid circles indicate the aggregate forest area contained within all occupied patches.