University of Nebraska-Lincoln and the Canadian Museum of Nature researchers say climate change has had no impact on homogenisation of mammals in North America.
Besides climate, the researchers’ study, published on Thursday by Science Daily, shows hunting and farming by humans homogenised, or blending, mammals in North America more than twice as homogeneous noted 10,000 years back.
Homogenisation means the degree to which a mammal species from one ecological community resembles the species composition in its surrounding communities.
Mammals [from the Latin mamma or breast] represent a group of vertebrate animals from Mammalia class. Female animals of this class have breasts which produce milk.
The experts analysed 8,831 fossils representing 365 mammal species from 366 sites across North America. The team assessed homogenisation based on the fossil data.
Nebraska's Kate Lyons, the CMN's Danielle Fraser and international colleagues focused on the past 30,000 years. This timespan includes the absence of humans on the continent, their migration throughout it, and their shift from hunting and gathering to agriculture.
This team of experts infers the high blending in the mammal communities of North America to human settlements. Human activity eroded mammal’s unique character by hastening similarities among many species.
Previous studies that examined North American mammals, from tens of millions to millions of years ago, point to climate as the primary reason for the homogenisation and heterogenisation [or diverse].
However, the researchers found little evidence for effect of climate on blending between 10,000 and 500 years ago. From about 20,000 to 15,000 years ago, a warming North America saw the retreat of glaciers that had enveloped nearly all of modern-day Canada and much of the northern United States.
Warmer climates yield more gradual north-south gradients in temperature and rain. Nebraska University Biological Sciences Assistant Professor Lyons says, “That warming-driven homogeneity in climate breeds homogeneity in mammal communities, too.”
If climate had contributed to mammal communities’ homogenisation, the team would have expected that homogenisation to accelerate before 10,000 years ago. That it did not, shows climate probably had little to do with it, says Lyons.
Other research, that studied the past century to the last few decades, documents recent human influences of land conversion, poaching and territorial encroachment.
So far, no team has succeeded in establishing a baseline of homogenisation, or the real extent of human contributions to it, by examining the phenomenon both before and after humans arrived on the Earth.
Lyons says, “Our conclusion is that this does have to do with early human activities and the arrival of humans into the Americas.”
Their study shows modern-day North American mammal communities are more than twice as homogeneous as they were around 10,000 years ago. By the end of the 21st century this could increase nearly four times as homogeneous.
This change is equivalent to the current difference in homogenisation between the subtropics of central Mexico and the comparatively uniform mammal communities of the Arctic, they say.
This trend emerged earlier. Small mammals, weighing at least one kilogram, showed this trend more. The process of mammal blending hastened around 12,000 years. Around this era, humans hunted mammoths, sabre-toothed cats, dire wolves and other large mammals into extinction, the researchers say.
These findings suggest the spate of large-mammal extinctions contributed to homogenisation. The disappearance of large mammals unique to individual communities would have directly increased their similarity, says Lyons.
Lyons and colleagues showed those extinctions drove smaller species to expand their ranges, filling the geographic voids left by the larger mammals. Expansion would have led to more territorial overlap, Lyons says, further homogenising communities in the process.
Blending of mammals in North America speeded up more over the past 5,000 years. This period marked a rise in human population ten times. Widespread farming increased across places that would later become central and eastern United States.
"It happened much later in North America than on other continents,” says Lyons, “But that's when humans in North America went from being hunter-gatherers to a settled community dependent on agriculture.”
With fewer keystone species, Lyons says, homogenised mammal communities may also boast fewer ways to respond, and possibly survive, the ongoing challenges of climate change and further human encroachment.
[Sudeep Sonawane, an India-based journalist, has worked in five countries in the Middle East and Asia. Email: [sudeep.sonawane@gmail.com]
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