Marc A. Carrasco

The Impact of the Species–Area Relationship on Estimates of Paleodiversity

Estimates of paleodiversity patterns through time have relied on datasets that lump taxonomic occurrences from geographic areas of varying size per interval of time. In essence, such estimates assume that the species–area effect, whereby more species are recorded from larger geographic areas, is negligible for fossil data. We tested this assumption by using the newly developed Miocene Mammal Mapping Project database of western North American fossil mammals and its associated analysis tools to empirically determine the geographic area that contributed to species diversity counts in successive temporal bins. The results indicate that a species–area effect markedly influences counts of fossil species, just as variable spatial sampling influences diversity counts on the modern landscape. Removing this bias suggests some traditionally recognized peaks in paleodiversity are just artifacts of the species–area effect while others stand out as meriting further attention. This discovery means that there is great potential for refining existing time-series estimates of paleodiversity, and for using species–area relationships to more reliably understand the magnitude and timing of such biotically important events as extinction, lineage diversification, and long-term trends in ecological structure.

Quantifying the extent of North American mammal extinction relative to the pre-anthropogenic baseline.

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.

Variation and its implications in a population of Cupidinimus (Heteromyidae) from Hepburn's Mesa, Montana

ABSTRACT New specimens of the heteromyid genus Cupidinimus have been found at the Barstovian (middle Miocene) Hepburns Mesa site in the Yellowstone Valley, southwestern Montana. The population is not placed within a new or existing species because the high coefficients of variation of several dental measurements indicate the presence of more than one species. However, lack of any clear character differentiation (qualitative or quantitative) within the population makes it impossible to distinguish separate species. Possible explanations proposed for this high variation and lack of specific diagnosibility are: morphologically similar species (including the products of a recent speciation event or introgression); sexual dimorphism; geographic variation; and temporal mixing. The age, location, and size of the surrounding Cupidinimus taxa, the lack of any size differentiation of the population into two species, and the geological history of the area suggest that the variation in the Hepburns Mesa Cupidinimus...

Collateral mammal diversity loss associated with late Quaternary megafaunal extinctions and implications for the future

Abstract Using data from two palaeontological databases, MIOMAP and FAUNMAP (now linked as NEOMAP), we explore how late Quaternary species loss compared in large and small mammals by determining palaeospecies-area relationships (PSARs) at 19 temporal intervals ranging from c. 30 million to 500 years ago in 10 different biogeographical provinces in the USA. We found that mammalian diversity of both large and small mammals remained relatively stable from 30 million years ago up until both crashed near the Pleistocene–Holocene transition. The diversity crash had two components: the well-known megafaunal extinction that amounted to c. 21% of the pre-crash species, and collateral biodiversity loss due to biogeographical range reductions. Collateral loss resulted in large mammal diversity regionally falling an additional 6–31% above extinction loss, and small mammal diversity falling 16–51%, even though very few small mammals suffered extinction. These results imply that collateral losses due to biogeographical range adjustments may effectively double the regional diversity loss during an extinction event, substantially magnifying the ecological ramifications of the extinctions themselves. This is of interest in forecasting future ecological impacts of mammal extinctions, given that c. 8% of USA mammal species, and 22% of mammal species worldwide, are now considered ‘Threatened’ by the IUCN.

The Miocene MammaL Mapping Project (Miomap): An Online Database of Arikareean Through Hemphillian Fossil Mammals

Abstract The Miocene Mammal Mapping Project (MIOMAP), a relational database of all published mammalian vertebrate localities between 30 and 5 million years old from the western United States, is now online for use by the paleontological community. The database is housed at the University of California at Berkeley, served through the Berkeley Natural History Museums, and accessible via the University of California Museum of Paleontology website. Here we outline the salient features of the database to facilitate its use and provide the information needed for users to adapt the data to their own needs. Online queries of the database can be accessed via http://www.ucmp.berkeley.edu/miomap and made through HTML forms or an interactive map created using open source MapServer 4.0 software and Google Earth™. We also highlight past work done using the database and some of its potential applications.