Quantifying the Extent of North American Mammal Extinction Relative to the Pre-Anthropogenic Baseline Marc A. Carrasco1,2*, Anthony D. Barnosky1,2,3, Russell W. Graham4,5 1 Department of Integrative Biology, University of California, Berkeley, California, United States of America, 2 Museum of Paleontology, University of California, Berkeley, California, United States of America, 3 Museum of Vertebrate Zoology, University of California, Berkeley, California, United States of America, 4 Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania, United States of America, 5 Earth and Mineral Sciences Museum, The Pennsylvania State University, University Park, Pennsylvania, United States of America Abstract Earth has experienced five major extinction events in the past 450 million years. Many scientists suggest we are now witnessing a sixth, driven by human impacts. However, it has been difficult to quantify the real extent of the current extinction episode, either for a given taxonomic group at the continental scale or for the worldwide biota, largely because comparisons of pre-anthropogenic and anthropogenic biodiversity baselines have been unavailable. Here, we compute those baselines for mammals of temperate North America, using a sampling-standardized rich fossil record to reconstruct species-area relationships for a series of time slices ranging from 30 million to 500 years ago. We show that shortly after humans first arrived in North America, mammalian diversity dropped to become at least 15%–42% too low compared to the ‘‘normal’’ diversity baseline that had existed for millions of years. While the Holocene reduction in North American mammal diversity has long been recognized qualitatively, our results provide a quantitative measure that clarifies how significant the diversity reduction actually was. 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. Data currently are not available to make similar quantitative estimates for other continents, but qualitative declines in Holocene mammal diversity are also widely recognized in South America, Eurasia, and Australia. Extending our methodology to mammals in these areas, as well as to other taxa where possible, would provide a reasonable way to assess the magnitude of global extinction, the biodiversity impact of extinctions of currently threatened species, and the efficacy of conservation efforts into the future. Citation: Carrasco MA, Barnosky AD, Graham RW (2009) Quantifying the Extent of North American Mammal Extinction Relative to the Pre-Anthropogenic Baseline. PLoS ONE 4(12): e8331. doi:10.1371/journal.pone.0008331 Editor: Anna Stepanova, Paleontological Institute, Russian Federation Received October 1, 2009; Accepted November 19, 2009; Published December 16, 2009 Copyright: ß 2009 Carrasco et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: Funding was provided by the National Science Foundation Grants EAR-0310221 and DEB-0543641 (Barnosky) and EAR-9005144 and EAR-9807499 (Graham). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: [email protected] Introduction the power function. While alternatives to the power law have been proposed [21–25], it remains the most frequently used in assessing Species diversity in any region varies within some limits through species-area relationships. SARs have proven useful in comparing time. Therefore, in order to measure the extent to which humans diversity among different regions and groups, in estimating are causing a biodiversity crisis [1,2], perhaps even a sixth mass potential extinctions given changes in area suitable for particular extinction [3–5], it is necessary to know how biodiversity naturally groups of species (though those estimations are not without fluctuates in the absence of humans and, if the diversity today falls controversy [4,11,20]), in determining baseline targets for below this pre-anthropogenic baseline, by how much. Those conservation, and in island biogeography theory [10,12–19,26,27]. numbers have been difficult to estimate because of complexities in We assessed SARs for terrestrial, non-volant mammals at 19 assembling the requisite data [6–8]. Here we provide that different intervals of time (Table 1) in 10 different biogeographic estimation, by utilizing 28,019 occurrences of fossil mammal provinces (Figure 1). Pre-anthropogenic time intervals were those species from across temperate North America to compute species- falling between 30 million to approximately 11,500 years ago. The area relationships (SARs) for many pre-anthropogenic time slices anthropogenic time slice was the Holocene, 11,500 to 500 calendar and post-anthropogenic time. years ago, beginning with the first evidence for widespread humans The species-area relationship is one of ecology’s few widely- in our study area and steepening of the human population growth recognized ‘‘laws’’ [9–13], which simply states that as sampling curve worldwide [28]. Data were obtained from the MIOMAP area increases, the number of species sampled increases at a database of fossil mammals for time intervals older than 5 million regular rate. SARs are widely used in assessing and comparing years and the FAUNMAP I & II databases for time intervals species diversity [10,12–20] and are generally expressed as between approximately 5 million and 500 years ago [29,30]. S=cAz, where S = number of species, A = area sampled, and z We used only fossil data to avoid the sampling and analytical and c are empirically derived constants that express the slope of complexities of comparing fossil with modern samples. That PLoS ONE | www.plosone.org 1 December 2009 | Volume 4 | Issue 12 | e8331 Quantifying Mammal Extinction Table 1. Temporal Bins into Which Species Occurrences Were terrestrial non-volant mammals are known to have gone extinct in Sorted*. our study area in the past 500 years, there have been severe range reductions of many species and at least nine subspecies have gone extinct [31]. Worldwide, about 60 mammal species have gone Time Interval Age Boundaries Interval Duration extinct in the past 400 years [8], and some 25% of remaining species are considered under threat of extinction [32], observations Holocene ,11,500-500 ,11,000 which contribute to notions we are experiencing a sixth mass Rancholabrean 0.15 Ma,11,500 ,140,000 extinction [3,5,8]. Irvingtonian 1.8-0.15 Ma 1.65 Ma We used standard, accepted techniques to adjust for the well- Blancan 4.7-1.8 Ma 2.9 Ma recognized sampling problems inherent in using fossil data to Late Late Hemphillian 5.9-4.7 Ma 1.2 Ma assess diversity (see Reference [17] and Materials and Methods). We computed species richness per time slice and per geographic Early Late Hemphillian 6.7-5.9 Ma 0.8 Ma area by rarefying the occurrence data using a richness value of 75 Late Early Hemphillian 7.5-6.7 Ma 0.8 Ma occurrences, and plotted these standardized richness counts Early Early Hemphillian 9-7.5 Ma 1.5 Ma against sampled area to determine paleospecies-area relationships Late Clarendonian 10-9 Ma 1.0 Ma [17]. We examined Type IV curves (unnested analyses), which Middle Clarendonian 12-10 Ma 2.0 Ma simply plot species richness against geographic area for each area Early Clarendonian 12.5-12 Ma 0.5 Ma sampled, as well as Type I curves (nested analyses), in which smaller geographic areas are nested inside larger ones. The Late Barstovian 14.8-12.5 Ma 2.3 Ma resulting curves provide the pre-anthropogenic baseline to which Early Barstovian 15.9-14.8 Ma 1.1 Ma the Holocene data points can be compared. Late Hemingfordian 17.5-15.9 Ma 1.6 Ma Details of our methods are summarized below (see Reference Early Hemingfordian 18.8-17.5 Ma 1.3 Ma [17] and Materials and Methods), but two points are worth special Late Late Arikareean 19.5-18.8 Ma 0.7 Ma mention. First, many of the Holocene (anthropogenic time Early Late Arikareean 23.8-19.5 Ma 4.3 Ma interval) samples include more species occurrences than the pre- anthropogenic samples (Table S1), which would be expected to Late Early Arikareean 27.9-23.8 Ma 4.1 Ma bias the results by producing Holocene SARs with higher diversity Early Early Arikareean 30-27.9 Ma 2.1 Ma values (see Materials and Methods). The fact that we find just the *Age bins prior to 1.8 Ma are subdivisions of the North American Land Mammal opposite—lower Holocene diversity with respect to the pre- Ages (NALMAs) [34] while post-Blancan time interval boundaries are those used Holocene time slices—suggests that our conclusions are particu- in FAUNMAP I [30]. larly robust and conservative. doi:10.1371/journal.pone.0008331.t001 Second, the Holocene time interval is the shortest one by far (Table 1). This might be expected to cause comparatively low limited our study to assessing anthropogenic influence only up to species richness values for that time interval, if species richness is 500 years ago, thus yielding a conservative assessment of diversity influenced by accumulating more species in longer time intervals decline in respect to pre-anthropogenic times. Even though no through evolution and immigration, i.e., faunal turnover through Figure 1. Boundaries of the ten biogeographic provinces used. Blue lines and abbreviations demarcate and label the following biogeographic provinces: CC, Central California; MJ, Mojave; CP, Columbia Plateau; GB, Great Basin; SGB, Southern Great Basin; NR, Northern Rockies; CRP, Colorado Plateau; NGP, Northern Great Plains; SGP, Southern Great Plains; and GC, Gulf Coast.