Ecosystem Shift 6,000 Years Ago Caused By Humans

For the past 6,000 years, human beings have been having an extreme effect on the distribution of animals and plants across the globe, some of which had been in place for up to 300 million years, to the point that some species are more segregated from others than they have ever been. A research team from the Evolution of Terrestrial Ecosystems (ETE) program, led by S. Kathleen Lyons, performed a study that looked deeper into plants' and animals' distribution across the landscapes of the planet through fossil records.

The shift has been so dramatic that the team discovered that the consequences of a shift like this could usher in a new stage in global evolution equivalent to that of the evolution of complex organisms from single-cell microbes, according to Smithsonian Magazine.

The team decided to focus on locating patterns among plants and animals, filtering out the relationships that could be classified merely as chance. They looked at two types of relationships - pairs and segregations - to find these patterns. Pairs would include animals that occur together, like cheetahs and giraffes, which are often found in the same areas, while segregations would include animals that are not present when another is. 

Lyons and her researchers found that over the course of the previous 300 million years, it was much more common for species of animals to occur in species pairs than it was for them to segregate. The world was dominated by patterns of aggregation, and this was true for both plants and animals. That is, until 6,000 years ago on the North American continent, brought on by the arrival of human beings dependent on agriculture. 

"It's striking that there's a community structure that is changing in ways it hasn't changed before and that appears to be associated with humans," Erle Ellis, from the University of Maryland, said according to Smithsonian. "I would say it's one of the most interesting indicators I've ever seen of a shift in the biosphere associated with humans."

The ETE team examined almost 360,000 organism pairs from 80 different communities on different continents, extensively searching for ways to rule out connections other than the arrival of human beings. They found that despite climate condition shifts varying drastically over those 300 million years, the average of 64 percent of species pairs being aggregated stayed consistent. After the shift and the arrival of humans, the average of aggregated species pairs dropped by almost half, to 37 percent. Now, species relationships are marked more by segregation than aggregation than ever before, according to Smithsonian.

"We're living in a lot of areas where species used to overlap their distributions," Lyons said. "They don't overlap anymore because they can't get through the areas where we're living now." Humans broke up animal and plant populations with our arrival through the fragmentation and destruction of their habitats, making their ranges smaller and cutting off overlap that previously existed.

Gregory Dietl, the Curator of Cenozoic Invertebrates at the Paleontological Research Institution and a paleontologist, wrote a review for the study in which he claimed that he believed that this can make predictions of the future with past examples much more difficult with the breaking of that 300-million-year-old pattern. Dietl also said it would be reasonable to think that increased species segregation can make those species more vulnerable to environmental changes. 

"It probably means species are more vulnerable to extinction because there are fewer connections between them," Lyons said. "And because their geographic ranges are smaller, their abundances are almost certainly smaller."

While scientists are uncertain about future extinctions or animals' ability to adapt due to the shift in the global ecosystem, they are certain that it has been so immense that it needs to be officially recognized. This has led not only the team, but researchers as far back as 2000 - with its coining by Paul Crutzen and Eugene Stoermer - to refer to the human influence on the environment and ecosystem as ushering in a new age: the Anthropocene.

"There's a tendency to think humans did not become a transformative force until fairly recently," says Ellis. "But this effect can be placed at the very beginnings of agriculture. So it's a very early indicator. The process of humans becoming distinct from other species and the way they transformed the Earth is really the cause of the Anthropocene. So this [study] is interesting in terms of asking where and when did this train leave the station?"

An official start date for the Anthropocene has not been set just yet, but the Anthropocene Working Group will be making that designation in 2016. The group's chair, Jan Zalasiewicz, believes that 1952 is the most likely year that the line will be drawn at, due to the beginning of the nuclear age and its effects really taking hold around then.

If the results found by Lyons and her team can be found to have occurred in other fossil records around the world, it will prove the global influence mankind has had on the evolution of life on Earth unequivocally. Ellis believes that this discovery has opened "a door to a whole new way of looking at changes in the Earth system, changes in the biosphere, changes induced by humans."

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