Question

Passage:

The debate surrounding the late Pleistocene megafaunal extinctions—the relatively sudden disappearance of large-bodied mammals such as mammoths, mastodons, and saber-toothed cats across the Americas approximately 11,000 years ago—has long polarized paleontologists, ecologists, and archaeologists. Broadly, the explanatory models can be categorized into anthropogenic (human-driven) and climatic paradigms, though recent multidisciplinary scholarship has introduced novel biological vectors into the discourse.

For decades, the dominant paradigm was the "Pleistocene Overkill" or "Blitzkrieg" hypothesis, most robustly championed by paleoecologist Paul S. Martin. Martin posited that the arrival of specialized, Clovis-point wielding human hunters in the Americas resulted in the rapid, systemic slaughter of immunologically and behaviorally naive megafauna. According to this model, the extinction event was swift and correlated directly with the southward progression of human migration, leaving these massive animals with insufficient evolutionary time to adapt to this novel, hyper-efficient apex predator.

However, detractors of the Blitzkrieg hypothesis argue that the archaeological record severely lacks the requisite volume of kill sites to substantiate such a continent-wide slaughter. Instead, scholars such as R. Dale Guthrie advocate for a structurally driven climatic model. Guthrie contends that the transition from the Pleistocene to the Holocene was marked by a dramatic shift in global weather patterns, replacing the highly diverse, nutrient-rich "mosaic" environments of the Pleistocene with the strictly zoned, less nutritious biomes seen today. This environmental homogenization deprived megafaunal species of the dietary diversity necessary to sustain their immense body masses and complex life histories, leading to severe nutritional stress, lowered reproductive rates, and eventual population collapse, entirely independent of human hunting pressures.

More recently, mammalogist Ross MacPhee and virologist Preston Marx have proposed a third alternative: the Hyperdisease Hypothesis. They argue that neither human hunting nor climatic shifts are sufficient to explain the sudden, phylogenetically broad, and synchronized extinction across vastly diverse habitats. Instead, they suggest that early human migrants, or their accompanying domesticated animals (such as dogs), served as vectors for highly virulent, interspecies pathogens. The immunologically naive endemic fauna possessed no evolutionary resistance to these novel infectious agents. This model relies on epidemiological dynamics, mirroring modern instances of catastrophic population declines following the introduction of novel diseases into isolated island ecosystems.

Finally, ecologists such as Norman Owen-Smith have advanced the Keystone Herbivore Hypothesis, a synthetic model that bridges the gap between anthropogenic and environmental factors. Owen-Smith suggests that humans did not need to hunt all megafauna to extinction to cause a catastrophic die-off. Instead, the selective removal of only the absolute largest "megaherbivores" (like mammoths) by human hunters initiated a cascading ecological collapse. Because these colossal animals functioned as landscape architects—maintaining open, mosaic environments by trampling saplings and consuming dominant vegetation—their initial demise allowed dense, homogenous forests to encroach rapidly. This secondary habitat alteration subsequently destroyed the ecological niches upon which numerous smaller megafaunal species relied, leading to their indirect extinction.

Question:
Based on the passage, match each researcher or group of researchers to the primary mechanism they propose as the cause of the late Pleistocene megafaunal extinctions.