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Energy flow and functional compensation in Great Basin small mammals under natural and anthropogenic environmental change

Significance Small mammals play a critical role in the maintenance of desert ecosystems. Using Quaternary fossils from the Great Basin, we show that energy flow through a small mammal community during today’s heightened climate warming differs from that experienced during natural rapid warming in the past. This discrepancy highlights a modern breakdown in energetic compensation among functional groups, and stresses the importance of novel anthropogenic impacts, such as the replacement of shrublands by invasive annual grasses introduced to North American deserts more than a century ago. Use of the fossil record to untangle the effects of climate and anthropogenic habitat change on ecosystem function today is thus critical for understanding how ecosystems will respond to future environmental change. Research on the ecological impacts of environmental change has primarily focused at the species level, leaving the responses of ecosystem-level properties like energy flow poorly understood. This is especially so over millennial timescales inaccessible to direct observation. Here we examine how energy flow within a Great Basin small mammal community responded to climate-driven environmental change during the past 12,800 y, and use this baseline to evaluate responses observed during the past century. Our analyses reveal marked stability in energy flow during rapid climatic warming at the terminal Pleistocene despite dramatic turnover in the distribution of mammalian body sizes and habitat-associated functional groups. Functional group turnover was strongly correlated with climate-driven changes in regional vegetation, with climate and vegetation change preceding energetic shifts in the small mammal community. In contrast, the past century has witnessed a substantial reduction in energy flow caused by an increase in energetic dominance of small-bodied species with an affinity for closed grass habitats. This suggests that modern changes in land cover caused by anthropogenic activities—particularly the spread of nonnative annual grasslands—has led to a breakdown in the compensatory dynamics of energy flow. Human activities are thus modifying the small mammal community in ways that differ from climate-driven expectations, resulting in an energetically novel ecosystem. Our study illustrates the need to integrate across ecological and temporal scales to provide robust insights for long-term conservation and management.