Megafaunal impact on ecosystems

 

How animals change vegetation

Herbivores eat, damage and trample on plants. Across the world they have an enormous influence on the types of vegetation found in a landscape.

When herbivores disappear from a place, the vegetation changes. Plants that were previously eaten, have the chance to spread. Anything that was trampled has the opportunity to grow tall.

In northern Eurasia and Beringia, where mammoths once roamed with bison and horses, their heavy grazing suppressed woody growth and encouraged grazing-resistant grasses. The grasses dried out the upper soils, reducing waterlogging. With the megafauna extinction, the waterlogging returned.

Carnivores shape landscapes too by controlling the movements of herbivores. They prevent population explosions by eating herbivores and also create a “climate of fear.” After wolves were reintroduced to Yellowstone in 1995 the park’s elks had to stay on the move and couldn’t graze so intensively. This helped the park’s Willow trees to recover, which then benefited the local beavers. (For more about this story visit Yellowstone Park’s website.)

The absence or presence of large animals in a landscape changes it. Imagine how great the changes to vegetation are when the animals that disappear are the size of mammoths, giant ground sloths and sabretooth tigers!

How megafauna extinction changed vegetation

In 2016 an article was published in PNAS about the impact of megafauna extinction on vegetation. The authors (Mahlia, Doughty, Galetti, Smith, Svenning and Terborgh) observed that we live in a world shaped by the absence of megafauna.

the loss of megafauna may have an enduring but little-recognized legacy on the functioning of the contemporary biosphere. Much of our current understanding of ecosystem ecology and biogeochemistry has been developed in a world artificially depleted of giants. Article title “Megafauna and ecosystem function from the Pleistocene to the Anthropocene”

The authors looked at the disappearance of megaherbivores, plant eating animals weighing 1,000 kg or more, and also megacarnivores, meat eating animals weighing 100 kg or more.

The authors explain that there are three categories of ecosystem function.

  • Green world: vegetation growth controlled by nutrient and water availability
  • Black world: vegetation growth controlled by fires
  • Brown world: vegetation growth controlled by vegetation

When megaherbivores disappear, a brown world can become either a green one or a black one. (This is not a set in concrete rule, and other environmental factors may cause exceptions to it.)

In a black world, a fire-controlled system, the plants that succeed are those that have adapted to make fire a part of their life cycle.

Fires sharply increased at Lynch’s Crater in northeast Australia after a reduction in megafauna around 40,000 to 42,000 years ago. At the same time the landscape changed from mixed open woodland to a dense forest “dominated by fire-adapted sclerophyll trees.”

The authors say it’s likely also that megafauna decline in eastern North America “caused an enhanced fire regime and the replacement of open and patchy woodlands by forest.”

In Europe the loss of megafauna led to transitions from a brown world to a green one. In northwest Europe the megafauna helped to create a mixed woodland environment with areas of dense trees and areas of open parkland. After the megafauna extinctions the land became dominated by “relatively dense tree cover.”

Most of the information and quotes on this webpage come from the article “Megafauna and ecosystem function from the Pleistocene to the Anthropocene.” There is a lot more to the article than I’ve mentioned here, so please do read it if you can.