Mass sterilization of a common palm species by elephants in Kruger National Park, South Africa

Elephant herbivory in KNP presently prevents a widespread woody palm Hyphaene petersiana from reaching reproductive size. Out of 65 individual palms sampled inside the Nwaxitshumbe elephant exclosure, 60 (32 females, 28 males) were mature (92%). The mean maximum height of individuals within the enclosure was 7.0 m (range 1.5–11 m). This palm reaches maturity between 4–5.3 m in height as evidenced by the mean height of the tallest immature stems per individual as 5.3 m and the mean height of the shortest mature stems as 4 m (n = 20). Outside the exclosure the mean height of the 75 surveyed individuals was only 1.6 m (max 3.2 m, only 30% > 2 m). Not one of these were reproductive, with most being several (2.5+) m short of being reproductive (Figs. 1, 2, 3). Signs of elephant herbivory of the palm outside the exclosure were widespread, as has been found elsewhere in Africa¹⁹. We found no seedlings inside or outside the exclosure (Fig. 3). Outside the exclosure this is due to a widespread lack of reproduction. The absence of elephants and their role in dispersal and germination^7,8,9 explain the lack of recruitment inside the exclosure, despite the production of many thousands of fruits annually over several decades.

This simple result of mass sterilization by elephants is important for biodiversity conservation for at least three reasons. Firstly, and critically, sterile plants cannot evolve new adaptations, such as to the looming threat of global change, nor can they disperse seeds to move with moving climate zones. Secondly, without seedling recruitment populations will eventually go extinct, although in the case of this highly persistent resprouting palm, this is only likely with sudden or significant environmental change because this species can live for about a century²⁰. Thirdly, because sterile plants do not produce flowers, fruits and large stems this too has biodiversity implications. We observed ad hoc that the outer layer of the fruits of this palm (Fig. 1) is eaten by vervet monkeys (Chlorocebus pygerythrus), porcupines (Hystrix africaeaustralis) and squirrels (Paraxerus cepapi). Elephants also consume Hyphaene fruits^7,8,9. We observed the palm flowers to be heavily visited by pompilid wasps, that African palm swifts (Cypsiurus parvus) were only nesting in tall palms inside the exclosure and that woodpeckers used the tall soft stems for nest sites. Sterilization therefore has diverse biodiversity consequences. These negative impacts are based on data from one location and for only one plant species, but these impacts are likely geographically widespread and to occur on other common woody KNP species. As minimum size to maturity in plant species is well known to scale with their maximum height^17,18 and therefore broken, but potentially tall trees are likely sterile, as was the case for H. petersiana. For example, the geographically widely-distributed important savanna tree Colophospermum mopane (“mopane”) can reach 10–25 m tall but is most often a short (< 2 m), broken tree described as being “planed”, “hedged”, “dwarfed” or “bonsaied” by elephants in KNP and elsewhere^3,21,22. An extensive (> 60 km transect) google earth survey of H. petersiana showed an almost total absence of mature individuals outside of elephant exclosures and a survey of a population of 40 individuals of the congener H. coriacea, indicated that 75% of individuals were sterile.

Since there are only a few antelope (approximately 8 ha per animal during the period 2000–2017 according to SanParks records) within the exclosure, grass biomass is much higher inside than outside. The exclosure is actively burned to maintain the grazing for these rare antelope and although many of the palms inside the exclosure had been burned recently, their canopies had escaped damage because they are several metres above the high grass-biomass fueled fire zone. Many fruits on the ground below mature individuals were damaged by the fire. Outside the exclosure elephant herbivory keeps plants short and therefore when fires take place, fire damages fronds and this exacerbates the lack of plants becoming tall enough to become reproductive. The achievement of reproductive size inside the exclosure is due to the absence of elephants rather than an absence of fire.

The impact of elephant herbivory on reducing the size of this palm outside compared to inside the Nwaxitshumbe exclosure was previously noted by Levick and Rogers¹². However, they interpreted elephant herbivory as having a positive impact on this palm, because of a greater relative stem density outside the exclosure. Also, they suggested that tall vegetation in the exclosure “would be less permeable to vectors such as wind and water”¹². They missed the dramatic and negative impact on reproductive status despite H. petersiana fruits being conspicuously large (up to 10 cm in length) and individual fruit-loads often exceed 100 fruits (Fig. 1). We suggest this was missed because assessing reproductive condition is not a routine conservation assessment of the impacts of herbivory. The debatable positive impacts of elephant herbivory on this palm suggested by Levick and Rogers¹² should be weighed up against more definitely negative impacts on the reproductive status of plants and the additional negative impacts this has, for example on frugivores and pollinators. We suggest that managers consider the conservation impacts of elephants, both positive and negative, on the sexual reproduction of resprouting plants. Although fruiting is less obvious for most plant species than for H. petersiana, given its large fruits, it would nevertheless be relatively easy to assess the minimum size a species needs to be, to be sexually reproductive. Species with tall maximum heights may be a priority. Also, if the present high level of elephant herbivory in KNP is reduced, fruiting by well-established resprouts of this palm could occur within two decades, because they are capable of rapid growth²⁰. However, there is no plan¹⁴ to directly control the presently steadily increasing population¹, although there are plans to reduce access to artificial waterpoints¹⁴ Finally, we emphasize the general conservation problem that resprouting plant species such as H. petersiana present¹⁵. Although they are able to increase stem density despite chronic elephant herbivory or persist in situ in the absence of elephants, their loss of reproduction or loss of their dispersal mutualists, means that they are nevertheless presently “the living dead”²³ in KNP.

Source: Ecology - nature.com