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When herbivore numbers drop, plants ditch thorny defenses

The risk of predation by African wild dogs like these alters the behavior of their prey, making plant communities less thorny.
[Credit: Dustin Rubenstein]

Plants can persist in landscapes full of hungry plant eaters, or herbivores, either by shielding themselves with special defenses like thorns, or by putting down roots in risky regions where carnivores -- who hunt the herbivores -- roam.

The study revealing these results, published in the 17 October issue of the journal Science, shows how predators and plant defenses interact to shape plant communities.

Even though most ecosystems are full of herbivorous animals, like deer, elephants and giraffes, plants remain on the planet. Just how they thrive when they are a common meal for so many animals has motivated debate among scientists; some say plants persist because large predators attack and kill herbivores, lowering the numbers of these plant--eaters. Others point to special defenses -- like thorns or bitter chemicals -- that plants have evolved.

To better understand if or how these factors interact, the University of British Columbia's Adam Ford and colleagues focused on a plant that African antelopes like: the Acacia tree. It can be full of thorns (A. etbaica) or less thorny (A. brevispica), with antelope preferring the latter, less well--defended plant.

Ford and his team monitored GPS--collared antelope and their main predators, leopards and wild dogs, in the East African savanna.

In one experiment, Ford's team made part of the savanna less dangerous for antelope by removing woody cover that concealed predators. Antelope feeding in these areas greatly increased.

In a separate experiment, the researchers tested whether antelope really do prefer the less thorny Acacia species, A. brevispica. Removing thorns from A. etbaica branches and attaching them to A. brevispica branches greatly boosted the amount of A. etbaica the antelope ate, revealing the importance of thorns as a plant defense mechanism.

A GPS analysis of numbers of the two Acacia species throughout the savanna region showed that Acacia trees kept their thorns in open, non--woody areas where antelope roamed freely. By contrast, the less thorny Acacia species, A. brevispica, was more plentiful in high--risk areas that antelope avoided.

The work of Ford and his team reveals how herbivores' predator avoidance behavior and plants' anti--herbivore defenses work together to shape plant communities.

Human activities that influence populations of large carnivores are already altering these interactions, the researchers note.