Have we underestimated our impact on the natural world?

According to a team of ecologists from the University of Calgary, humans may have a far greater impact on natural ecosystems than we thought.

While humans certainly have an influence on the ecosystems around them, in general, ecologists have held that 'top-down' and 'bottom-up' effects have a large amount of control over the population of plants and animals in an ecosystem (although whether species occupy specific levels ('trophic levels') of the ecosystem is apparently still up for debate).

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Top-down effects are those that are controlled by predators — the top, or 'keystone', predator in the ecosystem controls the health of the other plants and animals in the area. An oft-used example is the sea otter. Kelp forests are preyed upon by sea urchins, which are a favourite food of the sea otter. Sea otters eat the sea urchins, which keeps the kelp forest healthy (which, in turn, supports a healthy population of other organisms). Remove the sea otters and the urchin population will destroy the kelp forest, creating an 'urchin barren'.

Bottom-up effects are those that are controlled by the nutrient supply, plants and other smallest organisms, which determine the health of larger organisms. Change the dominant plant-life in an area and you influence what 'higher' species live there. Leech nitrogen out of the ground and the plants die without a food source, which causes insects and animals to move to another area or die along with them.

Having completed a five-year study of the ecosystems of southwestern Alberta, the research team found that human activity may be having a much greater effect on ecosystems than natural top-down and bottom-up controls.

"We painstakingly monitored wolves, elk, cattle and plant species, as well as humans for five years. We evaluated how these species interacted across the landscape and ultimately found that humans dominated the ecosystem," said Tyler Muhly, a rare species ecologist and adjunct assistant professor with the Faculty of Environmental Design at the University of Calgary, according to a press statement.

The results of the study, which was led by Muhly, showed that it was the effects of humans, specifically in what grazing plants they grew for cattle, that had the largest influence on the other species in the area, dominating over natural effects. Humans clear forest areas to allow grasses to dominate, or plant specific crops for grazing food ('forage'), which keep cattle and elk in those areas to take advantage of the forage. This, in turn, keeps wolves in the area, close to the cattle and elk that they see as prey.

"Our results contrast with research conducted in protected areas that suggested food chains are primarily regulated by predators," Muhly said in the statement. "Rather, we found that humans influenced other species in the food chain in a number of direct and indirect ways, thus overshadowing top-down and bottom-up effects."

The study, published online at PLOS ONE involved the Natural Sciences and Engineering Research Council of Canada (NSERC), Shell Canada, Parks Canada, the Alberta Government, the University of Alberta, the University of Calgary and the University of Montana. The researchers used a combination of high-tech tracking devices and motion-activated cameras to study the distribution of animals, plants and humans in the region of southwestern Alberta, which included those areas to the south and west of Calgary, bounded by the provincial border with British Columbia and the US-Canada border.

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"Understanding the significance of the impact that humans have on ecosystems is a critical component in formulating long-term and effective conservation strategies," said Prof. Marco Musiani, a professor of wildlife management and landscape ecology at the University of Calgary, in the press statement.

"Our results led us to believe that ecologists have underestimated the impact of humans on natural food chains," said Musiani. "The data we collected shows that humans are deliberately or inadvertently engineering ecosystems regardless of whether they would be naturally pre-disposed to top-down or bottom-up effects. Even in protected areas, the influence of humans might be greater than we previously thought."

(Photo courtesy: The Canadian Press/Liam Richards, PLOS ONE/Muhly et al.)

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