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Acacia Self-Defense: NO PLACE TO RUN, NO PLACE TO HIDE KIRK ANDERSON, CURATOR OF GARDENS


Prey species, those animals that are the predatory targets of other wildlife, often rely upon speed or camoufl age to elude their predators. T ese are eff ective defense mechanisms they have developed to give them a fi ghting chance at survival. But what happens if a prey species is incapable of motion? Has absolutely no ability to avoid the approach or reach or grasp of its predator? Is rooted to the ground, not just fi guratively, but literally? T is is exactly the predicament faced by the thorny acacia trees throughout Africa’s savannas and woodlands. T ey are the favorite prey of the giraff e, which savors the tree’s nutritious leaves and seedpods.


T e familiar appearance of the acacia, with a bare trunk forking up to equally bare branches that support an elevated canopy full of leaves, is not how the tree actually grows, but rather the result of browsing by herbivores. T ese predators strip the leaves from the lower branches, while the upper canopy beyond reach remains covered in greenery. T e height of the “browse line,” that horizontal fl at bottom of the leafy canopy that hovers above bare branches, corresponds directly to the tallest predator foraging on the tree. In Africa, the tallest herbivore is, of course, the giraff e. Since the giraff e can reach nearly twenty feet in height and may consume up to 140 pounds a day, the acacia is an easy target and just ripe for the picking.


14 www.LivingDesert.org


So what’s a ‘defenseless’ plant to do? Acacias rely on multiple defense mechanisms to deter herbivores such as giraff es, some more eff ective than others.


T e most readily apparent line of defense is the acacia’s sharp thorns. T ese hide-piercing fl esh-grabbing spikes, arrayed among the acacia’s branches, can be up to four inches long. Some thorns are straight like a stiletto, others curved like a claw. Straight thorns tend to be paired while curved ones can either be paired or arrayed singly. While these thorns act as a deterrent against many herbivore predators, the giraff e in particular is well equipped to get past them. Tender new growth on the branch tips is eaten thorns and all, before the spikes have a chance to harden. T e giraff e uses its twenty-inch long desensitized tongue to strip leaves from the thorny branches, and its strong molars to crush any thorns that may end up in its mouth. Even though the acacia’s thorns do not fully protect the tree from tissue and foliage loss, they do lessen damage by restricting bite size and retarding bite rates, thereby slowing the rate of consumption.


A second line of acacia defense is provided by ants; biting, stinging, swarming, ready to off er up their lives in defense of the colony, turning a mouthful of foliage into a painful experience. Many plants draw ants into their canopies with the lure of a sugary reward from special glands on the leaf stems called nectaries. But certain species of acacia, like the whistling thorn (Acacia drepanolobium), go one step further; they supply room and board within the swollen bases of the thorns. T ese domatia (literally, “small houses”) are hollowed out by


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