The Impala Lilies (Adenium multiflorum) in my parents' garden flowered this year. This little happening led me down a train of thought that ended in a shattering conclusion: the greatest threat to plant diversity is how man has homogenized water across the landscape, or rather created a uniform waterscape.
Oh wow, so how did I get there? A little bit of background is required. Firstly, my parents “garden” is no garden at all — their backdoor opens onto the African savanna of the lowveld. {I remember returning home on one occasion to find the back door out of use as a pride of 15 lions had killed a waterbuck on the path from the car to the house.} The plants growing there are those that have always been there and are subject to all the nibbles and chomping of all the animals that nibbled and chomped them for millennia past. Except….
Except for three Impala Lilies, which are indigenous to the Lowveld, but had never been seen on the farm. My mother has a fondness for this particular plant species, hence a break in tradition of keeping the garden wild, she planted these few plants. And they have suffered ever since. The impala and kudu share an inordinate fondness for these three plants — enjoying the leaves as fast as they were produced.
In the more than fifteen years that these poor little plants eked an existence, and I never seen them flower. Not until this year. This year, I found happy Impala Lilies full of leaves and a display that lit up the winter bush drabness. What had changed? Well, after years of worrying after her little plants, and coaxing them back to life after each mauling by a hungry antelope, she finally decided to keep the animals out for good, so she caged them in (the plants that is). Free from hungry mouths, these plants thrived in their captivity.
A simple observation. No animals meant happy plants. But hang on, Impala Lilies are indigenous to the Lowveld. They’ve evolved in a landscape full up with many different plant eaters. So they should be able to easily withstand herbivory. But. But, I’d never seem them on the farm. Certainly not because the environment was wrong — my mother’s plants certainly attest to how much they enjoy the soil and the rain. But, I’m certain that it is because there are too many hungry mouths to feed, and any other Impala Lilies that may have been on the farm never had a human mother-hen to protect them. I think that this plant species, and many others, have been eaten out existence — what scientists term “local extinction”.
Oh, but I hear your thoughts: “that is the obvious result of overstocking, putting too many game animals onto the land, you silly people”. No, the reserve follows strict scientifically-based and monitored stocking densities, and the plant community, or the biomass of greenery, is healthily intact. There is enough food, barring natural droughts, for all the animals and then some. This is not a management or science problem. Or at least, not directly.
What those Impala Lilies showed me, was that they needed to be away from herbivores. Far away. But where is this in the large expanse of the reserve? Nowhere. Because what used to be refugia — places where plants are protected from plant-eaters — no longer exist.
Almost all of the large herbivores in the African savanna need two things: food and water. Plants grow across the landscape, but in prehistory water was restricted to river courses, often in ephemeral rivers, or small seasonal pans. Large grazers and browsers, especially those that occur at a high density (and thus consume a lot of plants) can on!y do so within foraging distance of water. In prehistory, the closest water source to my mothers Impala Lilies would have been a small river over 15 km away. But now there are at least five dams and water holes within 3 km. And I can think of areas that may have been tens of kilometres, maybe even hundreds, away from water. These areas could only support few animals. But the plants! The plants don’t need to be near a river or any other surface water. They tap into underground water or water stored in the soil.
Thus, there is probably an entire guild of savanna plant species that only ever experienced the mildest of the herbivore munchies. In scientific terms, we could say that the waterscape was highly heterogenous — there were many areas with surface water, but large areas with none, or almost none. These low points in the waterscape would have been home to a few animals like roan, sable, eland and tsessebe with the rare movement of the animals that make larger herds through their midst. {And interestingly, the increase in waterpoints has been blamed for the decline in the antelope mentioned above — read "The Riddle Of The Rare Antelope"}
By putting in boreholes, and creating a new surface water point, humans have encouraged the expansion savanna herbivore populations into the thirsty plant refugia. And although the herbivore-resistant plants do just that, resist, survive and flourish, those like the Impala Lilies start to fade a disappear under onslaught of continuous bites.
This idea of the role of waterscape refugia in protecting plant biodiversity also explains something that I found decidedly odd in another African system, the subtropical thicket of the Eastern Cape. Thicket is home to elephants, where they are the primary agent of disturbance (unlike savannas, where fire could be said to be the dominant reset button in the system). And Thicket can support an incredibly high biomass of elephants — the tree species are especially robust to mega-herbivore hunger, far more so than their savanna counterparts who lose large limbs and whole trunks to elephant ministrations. But there are also a host of plant species that seem to get systematically removed from the landscape once elephants are introduced. Aloe africana is an example. An aloe that is endemic to small part of the subtropical thicket and yet it is very susceptible to elephants and other large herbivores (like the Greater Kudu). So, how can this be? Well, water used to be really scarce in this basin. There are major rivers, running from the escarpment across the narrow strip of coastal lowlands and into the sea. But these are far apart. In prehistory, there would have been vast areas of Thicket that would have been far from water, and thus rarely visited by elephants. (Elephants are a keystone species in Thicket as they open paths that other animals can traverse — No elephants, no paths. No paths, no other herbivores, or at least very few). Ask the Addo ecologists and they’ll tell you that their biggest way to control elephant densities is to open and close water holes. Their management strategy is largely based on controlling water availability.
Nonetheless, there remains far more surface water for roving herbivores in Addo, and across the rest of the country, than there ever has been. In the Karoos (both Succulent K. and Nama K.), I predict that scores of plant species have been lost as the thirsty plant refugia was quenched by boreholes and watering troughs. As I said above, the herbivore resistant community remains, but we lose species that relied on being in that part of the landscape where animals rarely travelled.
The levelling out of the waterscape is likely the greatest threat to plant biodiversity. Creating herbivore-free fenced areas — much larger versions of my mother’s caged Impala Lilies may be what we need to preserve those plant species who relied on the thirst refugia to survive herbivory.
A parting thought… All of those nature reserves where we imagine the ecosystem and biodiversity to be in some sort of prehistory balance — think again. The waterscape has been homogenized in these reserves, and we may be unaware of those plant species that are being lost from the communities as the munching mouths of herbivores move in.
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| Finally in Flower: My mother's Impala Lily (Adenium multiflorum) in flower after being protected from herbivory. This species can withstand a level of browsing, but then only grows as a small stunted shrub. When free from browsing, it can form a tree up to 3 m tall (often seen in Kruger Park camps). It grows from a rootstock that enables it to often resprout after damage. But continuous browsing can kill a plant. |
