Science & Environment

Carnivores help trees thrive without thorns, study says

Silhouette of Acacia etbaica (Image: A. T. Ford) Image copyright A.T.Ford
Image caption Thorns are among the defences available to plants to stop them being eaten by herbivores

The presence of carnivores helps plants without thorny defences thrive, a study of life on the savannah reveals.

Researchers found that species without thorns thrived in areas favoured by carnivores because plant-eating animals deemed it too risky to graze at these sites.

The team added that declining carnivore numbers was likely to have an impact on the links that connect carnivores, herbivores, plants and people.

The study appears in Science magazine.

"Our observations indicate that carnivores, like leopards and wild dogs, shape where herbivores eat," explained co-author Adam Ford from the University of British Columbia, Canada.

"Plant defences, such as thorns, shape what herbivores eat."

"Plants have two pathways to success. You either protect yourself from herbivores by growing large thorns, or thrive in areas that are risky to your predators; plant eaters."

He added that the delicate ecological equilibrium between the animal and plant kingdoms was likely to be disrupted in some regions.

Dr Ford observed: "As human activities continue to reduce populations of predators, herbivores like impala become willing to feed in areas that used to be risky, consuming more preferred vegetation and - ironically - allowing less-preferred thorny plant species to take over."

A study published earlier this year suggested that three-quarters of the planet's large carnivores were experiencing declines in their populations.

It added that the majority now only occupied less than half of their historic range, and this contraction could have a wide-reaching and long-lasting impact on ecosystems.

Image copyright A.T.Ford
Image caption The study showed that impala preferred grazing in areas where not favoured by carnivores as hunting grounds

In their paper, Dr Ford and his colleagues wrote: "The observation that most ecosystems support abundant plant life, despite the existence of herbivores that eat plants, has motivated a great deal of research and debate in ecology."

Image copyright D.Rubenstein
Image caption The presence of predators did influence where the impala chose to eat, the study showed

The team added that this debate had led to the development of two broad ideas.

The first being the "green world hypothesis", which argues the plants indirectly benefit from carnivores because the animals not only control herbivore populations, they also "trigger risk-avoidance behaviour" that reduces vegetation consumption in areas favoured by potential predators.

The other idea, the "plant defence hypothesis", suggests plants have evolved structural and chemical defences to deter herbivores.

Dr Ford and the team said that although the ideas were once viewed as separate arguments, they were no longer considered to be mutually exclusive and it was now necessary to understand how plant defence mechanisms and predation interacted.

In order to do this, the team fitted GPS collars to 20 adult impala, an African antelope, and four leopards and five wild dogs.

The researchers then combined the data gathered by the tracking devices with a high-resolution satellite image of tree cover in the study area - savannah 100km north of the equator in Kenya.

The satellite image also enabled the team to determine where impala were choosing to gather and feed, and where the predators were located.

As part of the study, the team also conducted experiments to see how effective there thorns were in protecting acacia trees from impala.

They removed thorns from a thorny species (Acacia etbacia), and added thorns to a less-thorny species (A. brevispica), and compared how many leaves were eaten by impala from these branches.

They found that impala avoided eating trees with thorns, irrespective of species of tree.

The team also found that there was a three-fold increase in the number of stems of the less-thorny acacia species in areas where impala were prevented from browsing.

Dr Ford explained that the results showed that neither carnivores or plant defences held absolute sovereignty over the composition of the landscape.

The presence of carnivores influenced where the impala fed, while the tree defence mechanism influenced what the herbivores ate.

He observed: "We're only beginning to understand the linkages between carnivores, their prey, plants and people."

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