Los Angeles Zoo & Botanical Gardens
Los Angeles, CA
All of you probably have herbivorous animals in your zoos, and in your notes somewhere you have recorded the plants they prefer to eat. What is special about that relationship? What adaptations have the plants and animals developed to be dependent on each other? Let’s look at some examples from different habitats and various points of view.
Let’s first consider the large browsers on the African savanna such as giraffe, elephant, and various antelopes. They enjoy the leaves and flowers of African acacia trees such as the whistling thorn tree, despite the trees’ several defenses and attempts to keep them away. One common defense is spines and thorns. The giraffe, in particular, can slide its long 18” tongue around the tough thorns to find the more nutritious leaves and flowers.
Another defense of the tree is the combative ants that make their nests in the dried thorn sites. Besides keeping the tree clean of ‘boring’ beetles, the bruchid, as well as mildew, pollen and scale, the ants march out to sting and bite the muzzle of any browser that dares disturb their work. Young giraffes move on quickly, but older ones are indifferent to the sting and stop browsing only when they are ‘fed up.’ An elephant, on the other hand, is strong enough to destroy the whole tree, but its brute size cannot protect it from a mass attack of angry ants.
The acacia tree has one more powerful adaptation to deter its relationship with browsers. As soon as a browser begins to nibble, the tree counters by making a nasty bitter taste in the leaves by producing an alkaloid chemical compound. At the same time, the tree also sends out an alarm to other acacia trees as far as 50 yards away. It emits an ethylene gas into the air warning them to start production of their bitter treat. The message travels downwind. Smart browsers have learned to eat upwind or suffer indigestion. For all its defenses, the acacia tree needs the herbivores to disperse its seeds to maintain a healthy savannah ecosystem.
Turning to an example in North America, in 1997 researchers from Oregon State University noticed that aspens, cottonwoods and willow trees were on the decline in Yellowstone National Park. Over 15,000 elk as well as deer and smaller herbivores were feasting on them, but the tallest aspen was less than 18 inches high in 1998. With fewer and smaller willows available to slow the water in creeks and streambeds, beavers in the Park were unable to build dams and literally disappeared. A complex web had unraveled. Why?
When Yellowstone Park was created in 1872, gray wolves were present, but they were seen as a threat to ranching livestock nearby. The wolves were controlled by poisoning and killing raids. Between 1914 and 1926, park managers killed at least 136 wolves in the park. The park had no wolves for 70 years.
In 1995, wolves were reintroduced to Yellowstone. They began to prey on the elk and deer populations. Reducing the number of browsers wasn’t enough for the willows, cottonwoods and aspens to return, however. The beavers were still missing from the ecosystem. The willows and beavers needed each other to slow the waterways in Yellowstone Park in order to survive. In 2001, 129 beavers were reintroduced into the Park. Eventually, creeks and streams were slowed as vegetation returned, providing better habitats for beavers and fish. The elk and deer have become stronger as they have had to run farther and faster to stay ahead of the wolves. In doing so, their hooves have aerated the soil allowing trees and grasses to grow taller with more food for birds and bears.
When I started as a docent at the Los Angeles Zoo, I learned about agoutis and pacas. Curious about these rodents and what they were ‘good for,’ I researched their usefulness in the South American rainforest. Agoutis and pacas have long sharp incisors and bulging jaw muscles, the only animals strong enough to crack open the hard outer shell of Brazil nut seed pods. The pod is the size of a baseball and drops from the Brazil nut tree when it is ripe. The mid-size rodents find 15-20 individual nuts inside the pod. They eat some and hide others in the ground for another meal or for when times get tough. In the next rainy season the seed/nuts germinate, guaranteeing a future of nuts.
The nutty connection continues with the empty seed pods, useless to the agouti but very helpful to other forest animals, especially the Brazil nut poison frog (Dendrobates casteneoticus). The very tiny amphibian reproduces in the empty seed pods and nowhere else. In fact, the pods provide such a desirable nursery that insects compete with the frog for hatching rights to the pods. If the insects hatch first, their larvae eat off of other larvae and juvenile frogs. If the frogs hatch first, they have an ‘all you can eat’ buffet from the pod and insects.
It gets nuttier. The only insect adapted to pollinate the small yellow blossoms of the Brazil nut tree is the beautiful little orchid bee from the Family Euglossinni. The female bee has the ability to penetrate the defenses of the small yellow flower with her long, strong tongue and collects the nectar that provides the bee with energy. The very tiny pollen collected along the way sticks to the bee to pollinate another blossom. The smaller male orchid bee, meanwhile, is perfuming himself in the fragrances of nearby orchid blossoms to attract the females and pollinating the orchids at the same time.
The Brazil nut tree depends on these animals for its survival. Human propagation of the tree has been only modestly successful, yet the Brazil nut is a staple food source and livelihood for more than 400,000 Amazonian people and a $65 million industry, the last product coming from the natural forest.
In contrast to the agouti and Brazil nut tree, the koala and the eucalyptus tree have a very different relationship. A drowsy koala snuggled in the elbow of a eucalyptus tree is a charming sight. Yet the koala needs to eat about 1,000 eucalyptus leaves a day! The leaves, however, are tough and fibrous often toxic making them unappetizing to other Australian herbivores. Koalas are picky in their leaf menu and look for leaves with higher protein and lower fiber content. They have adapted flat cheek teeth to grind the leaves that mix with bacteria in their saliva to begin digestion.
While all of this is good for the koala, the eucalyptus tree doesn’t care for the ‘strip search’ very much. It eventually suffers from diseases and struggles with photosynthesis when its leaves are eaten. Fortunately the tree doesn’t die from this experience and has a healthier relationship with kookaburras, cockatoos, Hedana spiders, bush tail possums and sugar gliders.
One more intriguing relationship between plants and animals comes from (the) Mauritius Islands in the Indian Ocean. The islands used to be home to large birds and giant reptiles including two species of tortoises (Cylindraspis) that dispersed the seeds of native plants on the islands. Early explorers and sailors decimated the animal populations, leaving the plants, including the ebony tree, vulnerable to extinction without its giant animal partners. The ebony tree provides Mauritius with a profitable economic resource, its wood exported at a price of $17,000 per cubic meter to make furniture, piano keys, clarinets, and bagpipes.
An untested and controversial experiment was tried in the year 2000 to replace the extinct giant tortoises using Aldabra tortoises to disperse ebony tree seeds. When a substitute species is introduced into an ecological niche in order to continue the role of its extinct predecessor, the idea is called rewilding. Until then rewilding was considered controversial and little scientific data supported its value.
A biologist from the University of Bristol along with the Mauritian Wildlife Foundation introduced a few Aldabra tortoises to a small Mauritius island where ebony trees were being chopped down for charcoal and rats were eating the few remaining seedlings. The Aldabra tortoises quickly set to work eating the ebony fruit and dispersing seeds across the island. The trees are slow growing, but new seedling colonies are widespread on the island; the number of Aldabra tortoises is growing, too. An amazing web is being restored as well as part of Mauritius’ economy.
We’ve only scratched the surface of many interesting examples of how animals and plants depend on each other. When you go back to your zoos, look again at the marvelous browsers and grazers and learn of one connection they have to a plant. I promise it will only lead to more connections, enriching our understanding that you can’t have one without the other.