Biotic responses of canids to the terminal Pleistocene megafauna extinction

Trophic downgrading is a major concern for conservation scientists. The largest consumers in many ecosystems have become either rare or extirpated, leading to worry over the loss of their ecosystem function. However, trophic downgrading is not a uniquely modern phenomenon. The extinction of 34 genera of megafauna from North America ～13 000 yr ago must have led to widespread changes in terrestrial ecosystem function. Studies that have examined the event address impacts on vegetative structure, small mammal communities, nutrient cycling, and fire regimes. Relatively little attention has been paid to community changes at the top of the food chain. Here, we examine the response of carnivores in North America to the Pleistocene extinction. We employ fossil data to model the climatic niche of endemic canids, including the extinct dire wolf Canis dirus, over the last 20 000 yr. Quantifying the abiotic niche allows us to account for expected changes due to climate fluctuations over the Late Quaternary; deviations from expected responses likely reveal influences of competition and/or resource availability. We quantify the degree of niche conservatism and interspecific overlap to assess species and community responses among canids. We also include in our analyses a novel introduced predator, the domestic dog Canis lupus familiaris, which accompanied humans into the New World. We find that endemic canid species display low fidelity to their climatic niche through time, We find that survivors increasingly partition their climatic niche throughout the Holocene and, surprisingly, do not expand into niche space presumably vacated by the extinction of very large carnivores. These results suggest that loss of megaherbivores and competition with humans likely outweighed advantages conferred from the loss of very large predators. We also find that wolves and dogs decrease their niche overlap throughout the Holocene, suggesting a distinctive relationship between dogs and man.