Early footsteps in the Americas Despite a plethora of archaeological research over the past century, the timing of human migration into the Americas is still far from resolved. In a study of exposed outcrops of Lake Otero in White Sands National Park in New Mexico, Bennett et al . reveal numerous human footprints dating to about 23,000 to 21,000 years ago. These finds indicate the presence of humans in North America for approximately two millennia during the Last Glacial Maximum south of the migratory barrier created by the ice sheets to the north. This timing coincided with a Northern Hemispheric abrupt warming event, Dansgaard-Oeschger event 2, which drew down lake levels and allowed humans and megafauna to walk on newly exposed surfaces, creating tracks that became preserved in the geologic record. —AMS

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Intraguild predation (IGP) is an extreme form of competition that involves a dominant predator (IG predator), a subordinate predator that is also a superior exploitative competitor (IG prey), and their shared prey. Theory predicts three possible equilibria, which parallel increasing resource enrichment: exclusion of the IG predator, stable coexistence, and exclusion of the IG prey. Here, we report on the existence of two concurrent states in a model carnivore system dominated by IGP. Using species occurrence data obtained from randomly distributed remote cameras, we modeled the probabilities of detection and occupancy of the kit fox (Vulpes macrotis) and its IG predator, the coyote (Canis latrans), at White Sands National Monument, New Mexico, USA, using a recently developed conditional two-species occupancy model. Kit foxes persisted in habitats of low resource abundance, which could not support coyotes. Coexistence occurred in adjacent habitats of higher resource enrichment, with coyote occupancy strongly correlated with small mammal abundance. The odds of a coyote being present vs. absent in prey-rich habitats was 332:1 (6 0.006; SE) whereas the odds of a coyote being present vs. absent in prey-poor habitats was 1:4 (6 0.11); thus, coyotes were much more likely to be present in prey-rich habitats. Kit foxes were more frequently photographed in prey-poor habitats avoided by coyotes. The odds of kit foxes being present vs. absent at a site where coyotes were absent was 7.9:1 (6 0.082), which was greater than the odds of kit foxes being present vs. absent at a site where coyotes were present (2.2:1 6 0.081). These findings indicate that kit foxes avoid coyotes, but that their adaptations to arid conditions enable them to exploit habitats unsuitable for coyotes. Consequently, the primary driver of this spatial separation is the inability of coyotes to use prey-poor habitats, rather than kit fox avoidance of coyotes. Our results reveal the existence of concurrent IGP states within the same ecological community, which is consistent with theoretical predictions, and highlight the power of the conditional two-species occupancy model to illuminate how the abundance of shared prey shapes the competitive dynamic in ecological systems dominated by IGP.

Contemporaneous sloth and human footprints from the terminal Pleistocene at White Sands National Monument suggest stalking.

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