Title: Bottom-up versus top-down megafauna–vegetation interactions in ancient Beringia
Abstract: During the last glacial period, the Bering Land Bridge was inhabited by a diverse bestiary of large mammals, many now extinct (1), and covered by the steppe-tundra, a widespread, now largely vanished biome (2). Given the twinned losses of big animals and a major ecosystem during the last deglaciation, a central question is to understand whether and how these events were connected. Did bottom-up processes rule, in which changing climates led to changing habitats, which then triggered population declines and, ultimately, extinctions of Arctic megaherbivores? Did top-down processes govern, in which declining populations of megaherbivores, pressured by the regional arrival of humans by at least 32,000 y ago (3), led to altered nutrient cycling and reduced grazing strength, facilitating a widespread replacement of a grass–forb steppe-tundra (4) with a shrub tundra dominated by willow ( Salix ), birch ( Betula ), and alder ( Alnus ) (5)? Or, given that these are complex ecosystems governed by multiple interacting processes, were extinctions and ecosystem transformations all part of one larger positive feedback loop? In PNAS, Monteath et al. (6) shed light on these questions of top-down and bottom-up processes through a careful analysis of the temporal and spatial patterns of late Quaternary megafaunal extinctions and vegetation transformation in eastern Beringia, now known as Alaska. Better answers to these questions have clear implications for contemporary global change ecology and conservation biology. For, if climate was the ultimate driver of Beringian ecosystem transformations and extinctions, then this adds urgency to today’s efforts to conserve habitats and helping species adapt to climate change, for example, via the resist–accept–direct framework (7). Conversely, evidence that top-down trophic processes were decisive would add impetus to efforts to restore ecosystem functioning through rewilding efforts (8), by either reintroducing extirpated species to their former range … [↵][1]1Email: jwwilliams1{at}wisc.edu. [1]: #xref-corresp-1-1