Human population has increased substantially since the last glacial period, especially across temperate areas with easy access to water sources. Human-mediated landscape transformation can exclude mammal species from their former habitats. Thus, we anticipate a change in the realized niches of mammal species as increased human population forces them to shift within their fundamental niches. In our study, we collected species occurrences from 11,700 ybp to the Present for 46 North American mammal species covering six taxonomic orders (Artiodactyla, Carnivora, Chiroptera, Insectivora, Lagomorpha and Rodentia). We inferred temperature and precipitation for each location using paleoclimate simulations (CCSM3) and modern data. We used these data to calculate niche overlap between multiple past time bins and the Present to identify whether the realized niches of these mammals do indeed shift through time. Time bins include Early Holocene (11,700 to 4,200 ybp), Late Holocene (4,200 ybp to 1500 AD), Post-European (1500 to 1850 AD), Post-Industrial (1850 to 1950 AD) and Present (1950 AD to the Present). We also calculated the environmental niches of modern urban habitat and croplands and calculated their niche overlap with mammal species’ from the Early Holocene and the Present. We then obtained a “human tolerance index” for each species by measuring how overlap with these two habitats has changed over time.
Results/Conclusions
On average, species niche overlap with the Present decreases as you move further back in time. We find that 61% of the species in the study have significantly changed their environmental realized niche at some point across the 5 time intervals, especially after European settlement and the Industrial Revolution in North America.
With European settlements, medium-sized carnivorans and dietary generalists significantly expand their realized niches. Mammals bigger than 10 kg become more likely to change their ecological niche across the Post-Industrial to the Modern transition. Additionally, we find decreased human tolerance with increasing body mass. Have been specially extirpated from habitats where human activities are high. Additionally, dietary generalist mammals display higher human tolerance than dietary specialist species.
Overall, our results suggest that geographic and anthropogenic dispersal limitations together with a low niche plasticity might prevent animals from migrating to new localities as human population increases and climate changes. Geographic models that integrate fossil and modern niche preferences will help elucidate the reasons behind the observed patterns. Moreover, understanding these patterns will help us formulate better conservation plans for the species we wish to protect.