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Mom, I want to be an ecosystem engineer …
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Our natural world is shrinking, and with it the parcels of land we set aside for conservation assume greater importance. Even areas as large as the Kruger National Park do not function at the spatial scale which they used to. But ecological theory suggests that it is essential to maintain ecological processes within these reduced systems if biodiversity is to persist.
Elephants contribute to a number of fundamental ecological processes in African savannas, and should therefore be a desired component. The impact of elephants on savanna vegetation is, however, widely recognised by images of woodlands reduced to shrubs.
Are impacts of this apparently extreme nature merely part of an ‘elephant-vegetation’ cyclical relationship, or are they unsustainable for maintaining biodiversity? To what extent should a system be allowed to run its course, or should elephants be closely managed? “How many elephants can a specific system carry?”
These perplexing issues have been the subject of much heated debate over the past few decades. Their importance has once again surfaced in southern Africa where elephant populations continue to grow in reserves where the boundaries are now fixed. In response to increasing concern in South Africa, the Minister of Water and Environmental Affairs has sought the input of a wide range of experts for defining elephant policy.
We need long-term observation
Effective policy depends on scientific information of high quality. Despite an impressive bibliography for elephant ecology, policy formulation based on the information at hand is not as convincing as it might be. A primary reason is that much of our information about the impact of elephants is based on short-term observation of systems which change slowly, often in response to infrequent events. We need to understand the long-term behaviour of these systems, as is the mandate of SAEON.
In the case of elephants, SAEON has contributed to the maintenance of a study on their impacts in the Venetia-Limpopo Nature Reserve founded by De Beers in the early 1990’s. The reserve is well suited for a study of this nature. There is detailed information available on the reserve’s woody vegetation from before and commensurate with the reintroduction of elephants – about 9 000 individual plants are monitored.
The elephant population is at low density. Forty elephants were introduced into 350 square kilometres between 1991 and 1994. The population grew to 100 or so by 2010. A density of one elephant per 3.5 square kilometres is substantially lower than those recorded for many other large African reserves of comparable climate. Severe impacts on woody plants were therefore not expected, although this expectation was tempered by the fact that little grass suitable for elephants was available as a consequence of the degraded state of the reserve owing to a history of imprudent livestock ranching practices (elephants are preferentially grazers if green grasses are available). Indeed, it was hoped that elephants might reduce abundance of the ubiquitous mopane tree, well known to be a staple of their diet, and thereby promote grass production.
These ecosystem engineers had not read the script. Indeed mopane proved to be the staple woody diet species, but it was supplemented through pronounced selection for a number of uncommon woody species.
The severity of impact of elephants’ feeding relates not so much to the quantity they consume, but to the manner in which they gain access to plant material. They may uproot a tree or pollard its stem by pushing it over in order to access its crown, excavate a tree or shrub in order to eat carbohydrate-rich roots, or eventually ringbark a tree by stripping bark for its sugar-rich phloem material.
Their use of woody material is heightened during periods of dire nutritional need such as at the height of the dry season, especially during drought years. Impact on an individual tree of any of these methods of feeding depends on the ability of that individual to resprout, which varies depending on its species and size. In many cases pollarding, partial or complete uprooting, or ringbarking may result in the death of an adult tree or shrub; in others the plant may continue to live, albeit in an altered state, through coppicing from roots or the stem base.
Ecosystem engineering impacts
Conspicuous, severe impacts were first recorded for Acacia woodlands that had invaded seasonal wetlands. Most of these have been transformed to the open grassland state they were before bush encroachment occurred after the 1950’s.
Next, the name ‘kanniedood’ (“cannot die”) proved ironic for the Commiphora species whose light wood was no match for the attention of elephants. Vegetation types bearing this name that were mapped before the reintroduction of elephants are no longer recognisable as such.
The population of the enigmatic Adenia spinosa is in freefall, with only 20 remaining from a population of 120 recorded in the mid-1990’s. The bark of the charismatic maroela and wild sering trees appears to be particularly delectable and has resulted in more than 50% of the adult population having been lost within only 13 years. Some species such as the False Marula and some shrub kanniedood species appear to be more resilient. Although commonly uprooted, they produce vigorous coppice growth from the roots. Similarly, the ubiquitous mopane usually coppices prolifically following utilisation.
The value of long-term study lies not only in quantifying levels of adult mortality. It was expected that elephants would kill trees and that the impact might be conspicuous in a system from which elephants had been absent for over a century. If adults are being killed then a population may still be secure if there is an adequate rate of seedling regeneration and growth rates are sufficient for recruitment of adults from smaller size classes.
Detailed (read tedious, laborious) measures of these processes have revealed that many of the impacted species recruit very infrequently, if at all, and some are slow-growing. I have not yet seen any other than an adult Marula after 29 years of working there. Growth is invariably slow in a drought-prone environment such as this (half the mean annual rainfall of Gauteng), but simple population models have revealed that even the fastest growing Commiphora species cannot recruit adults quickly enough to replace losses to mortality. One outcome is that a species may be locally extirpated from the reserve.
Of the 32 tree species which were once well represented on this reserve, populations of 18 of these are in decline and their future does not look promising if current patterns persist. The arborescent Aloe species may already have become extirpated, but this example underscores the complexities to be addressed because its demise is ostensibly as much the result of the attentions of eland, kudu, and other browsers as it is those of elephants.
Pachyderm paradox
This case study has highlighted the paradox of keeping elephants in medium-sized reserves. The elephant population of the reserve appears sustainable as evidenced by its continued growth, attributed to the abundance of its staple food item Mopane. But current levels of woody diversity are not. The size of the elephant population is apparently well within its “carrying capacity” if elephants were the only concern, but it may have well exceeded its bounds if all woody species are to persist.
Changes in the woody vegetation of the Venetia-Limpopo Nature Reserve have been recorded for 30 years, with specific attention paid to the impact of elephants since 1997. Thirty years approximates the working lifespan of most of us, but the personal lesson is that it constitutes a small fraction of the lifespan of the woody denizens of this semi-arid savanna.
Photographs reveal little or no pronounced change in some woody individuals after all this time. Improved insights of the dynamics and functioning of these systems can be gained, but it requires commitment to maintaining data collection when the work returns are not initially evident.
A corporate body ensured that this endeavour endured; the same can be expected of government institutions. For those who work in organisations tasked with the management of our natural renewable resources, be critical of decision-makers who target monitoring activities for budget cuts.
But be equally critical of whether effective use is being made of the information being collected.
