Keystone Species and the Value of an Ecosystem
Nola, a Northern White Rhinoceros and resident at the San Diego Zoo Safari Park, died this November. On the 22nd of that month, the Safari Park announced her passing and immediately, news outlets around the world picked up the story. It’s always sad when an animal as magnificent and fascinating as a rhinoceros dies, but why did this particular death generate interest on a global scale? Mainly it’s because of the kind of rhino Nola was. She was a Northern White Rhinoceros; one of the last. Now that she’s gone, there are only three left on the planet.
Caring people the world over rally around calls to save the whales, and save the pandas, and save the frogs. “Save the (insert species here)” is a cry heard across our planet. While this may seem a very species-specific approach, these very vulnerable flagship animals are working to represent something beyond themselves. “Save the rhinos” means so much more than working to preserve that species alone. There is tremendous value in focusing our attention on one species. There is also tremendous value in understanding that any one species is part of a much bigger picture. Nola was a beautiful animal and a member of a beautiful species that we are losing. She was also part of something bigger; an interconnected and delicate ecosystem. When we talk about endangered rhinos, we are also talking about the world they are a part of; the other animals, the plants, the fungus, the bacteria. In short, all the other ingredients that make that system what it is.
Imagine making a cake. There are many ingredients that go into a cake; eggs, milk, flour and so on. If you are missing an ingredient when you start, what you end up with will not be the cake you envisioned. Sometimes the change is minimal. For instance, if you make a cake but don’t have any frosting, you wind up with a disappointing but edible cake. Other times the missing ingredient can prove catastrophic. Try making a cake without any baking powder, or flour, or milk. What you wind up with is an inedible mess.
An ecosystem can be a bit like a cake. Remove one ingredient, maybe a particular species of dragonfly or pine tree or bacteria, and the entire ecosystem changes. Sometimes the species that has been removed is the frosting and the ecosystem remains functional but diminished in some way. Other times, however, the species is the baking powder and the whole ecosystem turns into something else.
There is evidence to indicate that Nola and her kin are a baking powder kind of species. Baking powder species are more commonly referred to as “Keystone species” (in architecture, the keystone is the stone that keeps an archway together. Remove it and your whole doorway falls in). Northern white rhinos used to range across the part of central Africa where Uganda, South Sudan, the Democratic Republic of the Congo, and the Central African Republic meet. Rampant poaching devastated their numbers and in 2008 the northern white rhinoceros was declared “extinct in the wild”. At that point, all remaining members of Nola’s species lived in captivity. With the rhinos gone, that ecosystem is now fundamentally changed.
White rhinos are grazers, meaning they eat grasses rather than the leaves of shrubs and trees. It’s easy to think of them as enormous gray lawnmowers but more accurate to think of them as very large gardeners. When gardeners weed the garden, they don’t pull up every plant, just specific ones. The plants left untouched thrive in the absence of competition. Rhinos work the same way. They selectively eat certain grass species, which allows other species to flourish. This leads to greater biodiversity among plants which then attract different types of herbivores (think antelope and zebras). This, in turn, brings in different types of predators (lions, jackals, etc).
With the rhinos gone from that northern range, the opposite will likely occur. Like dominos falling, other populations will shift in response to the missing rhinos. For instance, the grasses the rhinos had been eating will likely grow in faster and thicker, choking out some competing plants. As these plants become harder to find, the species that rely on them will begin to struggle and their populations may diminish. This, in turn, would lead to a decline in the predator species who either starve or move out of the area in search of food. This can lead them to move into human populated areas (think about the mountain lions we now see with some regularity in Orange County suburbs).
The now famous saga of the wolves of Yellowstone National Park illustrates this concept beautifully. Gray wolf populations were already in decline when Yellowstone was established in 1872. The new park offered no protection to the predators and they were aggressively targeted as a hazard and pest. By 1926 the gray wolf had been completely wiped out. In their absence the elk populations spiked, causing massive destruction to stands of aspen and cottonwood trees. In response, the park service had to institute a capture and relocate the program to try and control the elk population. Coyote populations also surged and impacted the pronghorn antelope herds. Populations of many species were thrown into chaos by the removal of one.
In 1995, a controlled reintroduction of gray wolves to Yellowstone began. Since their return, something startling has happened. The ecosystem has begun again to stabilize. The elk population declined, leading to a corresponding resurgence in willows, cottonwoods, and aspens. The overblown coyote population, once the densest in North America, dropped and correspondingly the fox population (which was suppressed by coyotes) has begun to recover. The resurgent foxes are preying upon rodents and ground-nesting birds that eat seeds. Seeds which are now free to germinate and fill in areas over-grazed by the formerly rampant elk population.
A similar situation has occurred in Africa. Nola’s cousin, the southern white rhinoceros, has been systematically reintroduced in areas of Namibia, Botswana, Zimbabwe, and Mozambique; areas they’d been extirpated from in decades past. A recent study conducted by Joris P. G. M. Cromsigt and Mariska te Beest (see below) indicates that returning the rhinos to their former range has had effects similar to what has been seen in Yellowstone.
These two examples provide substantial evidence to indicate that returning keystone species to their former range goes a long way toward stabilizing that particular ecosystem. The problem with the northern white rhinoceros is that there aren’t enough left to establish a new population. So then is that where the whole cake recipe comes apart? Does this mean we must resign ourselves to the destabilization of that ecosystem? Not necessarily. The wolves that once roamed through Yellowstone were not only driven out of the park, they were driven to extinction. When the reintroduction began in the 90’s, the wolves brought in were a Canadian subspecies, similar but taxonomically distinct from the original wolves.
This is a contentious issue in the saga of the Yellowstone wolves but it does open the door to possibility. In the absence of a northern white rhino population to recolonize the northern range with, might it be possible to substitute southern white rhinos instead? Though recognized as distinct sub-species with clear genetic differences, the argument could be made that the southern whites are sufficiently similar to the northern whites to take on their role in that ecosystem. To take it back to the cake analogy, in the absence of whole milk, can a baker can get away with 2%?
The struggle to save the rhinos is ongoing and complicated. There have been major victories and, as demonstrated by Nola, definite setbacks. Globally many populations of rhinos are on the verge of reversing decades long trends. At the same time, the demand for rhino horn continues to drive up poaching numbers. Birth rates are climbing but poaching rates are climbing faster. This is an ongoing crisis and threatens to undo all the gains that have been made over the last century. Those of us who care about these magnificent animals, and beyond that, about the glorious, global network they are a part of owe it to them and to ourselves to stay focused on this goal.
Safari West is lucky to be the home to three beautiful southern white rhinos. Every day we focus our attention on teaching our guests about this species’ place in the world and the challenges the world’s rhinos are facing. We are hoping to see our adult rhinos, Mufasa and Eesha, breed successfully and add another member to that carefully monitored population count. While these goals are Safari-West-specific, we have much loftier hopes as well. We hope our conservation message coupled with the awe-inspiring sight of our three gorgeous rhinos informs and inspires an ever-widening pool of people. People who hopefully become conservation advocates themselves. People who will join us in pursuing these critical conservation aims. Every time we add a voice, the call to save the rhinos gets louder. Every additional rhino in the wild helps to keep the ecosystem in balance. By taking action to save the species, we’re taking action to save the ecosystem and by extension, the whole of our big, beautiful, interconnected world.