Connecting the Fifth Ape to the Sixth Mass Extinction

Recent research conducted out of UC Berkeley and Penn State University has quantitatively demonstrated that mammal diversity in North America has plummeted since the arrival of humans about 13,000 years ago. The study (Available at PLoS One) examines changes in the number and variety of mammal species from several different biogeographic provinces within the United States and over a time period ranging from the Oligocene up to about 500 years ago.

To conduct the study, researchers established a ‘diversity baseline’ by calculating species-area relationship values from counts of fossilized mammals. Species-area relationship values are basically ratios of species occurrence to geographic area. Once this base line was determined, time intervals without human presence were compared to periods since the arrival of humans into North America about 13,000 years ago.

After finding a correlation between humans and declining mammal diversity, the scientists concluded that if “mass extinctions are defined as loss of at least 75% of species on a global scale, our data suggest that North American mammals had already progressed one-fifth to more than halfway (depending on biogeographic province) towards that benchmark, even before industrialized society began to affect them.”

The study is unique in that it utilized fossil evidence to establish a diversity base line covering a thirty million year period. The quantification of this paleontological data aids in giving greater empirical support to qualitative assessments of declining diversity worldwide. As stated in the paper’s conclusion, “[w]orldwide, about 60 mammal species have gone extinct in the past 400 years, and some 25% of remaining species are considered under threat of extinction, observations which contribute to notions we are experiencing a sixth mass extinction.”

Carrasco, M., Barnosky, A., & Graham, R. (2009). Quantifying the Extent of North American Mammal Extinction Relative to the Pre-Anthropogenic Baseline PLoS ONE, 4 (12) DOI: 10.1371/journal.pone.0008331