#02 2021

Are our nature reserves evolving into a Jurassic World?

By Rion E. Lerm, Technician, SAEON Ndlovu Node

Elephants are significant ecosystem engineers, moderating the type and quality of habitat for other fauna. Although much of their impact on the landscape is evident during a visit to a nature reserve or protected area where these magnificent pachyderms roam freely, some effects may not be immediately noticeable as they develop over time.

Strategies that enable other species to co‑exist with the “engineers” include different life‑history traits such as tree‑living vs. ground‑living. The impact of elephants on lower-order animals such as invertebrates or lesser‑known vertebrates is not yet fully understood, despite the ecosystem processes these animals may provide.

A common dwarf gecko (Lygodactylus capensis). This species’ love for corkwood trees led to it disappearing from areas where elephants were introduced decades ago (Photo: G. Masterson; available from http://vmus.adu.org.za/?vm=ReptileMAP-219)

The gecko’s favourite hangout: a velvet corkwood (Commiphora mollis) (Photo: O. M. van der Bank; available from http://vmus.adu.org.za/?vm=TreeMAP-51)

How does elephant rewilding affect lizard species? 

A recent study investigated whether the effects of elephant rewilding/reintroduction were significantly impacting lizard species across a savanna landscape.

The north‑eastern low‑lying region of South Africa is mostly made up of protected areas, nature reserves, game farms and tribal land, all with varied management histories. For example, one study site in the paper (Klaserie Private Nature Reserve that adjoins Kruger National Park) was unprotected prior to 1972 and elephants were a rare occurrence. The African savanna elephant population in the reserve rapidly increased after the fences between the reserve and Kruger were dropped around 1993/1994.

The second study site, Balule Nature Reserve to the west, was established in 1993 from former pastoral land. In Balule, elephants were only detected in 2003 when the last of the fences were dropped between Balule and Klaserie.

Study site 3 was a disconnected zone, part of Balule but without any elephant presence. Here, typical mammalian residents included cattle and wildlife in the form of mesofauna compared with a range of wild meso‑ and megafauna (including elephants) that roamed the other two sites in more recent times.

Within each of the sites, large quadrats were placed along comparable ecological settings where the abundance of three lizard species was recorded. These were the common dwarf gecko, common variable skink and the striped skink (collectively dubbed lizards).

Woody plant species taller than one metre were also recorded, but the red bushwillow and velvet corkwood stood out as favourites among these scaly ectotherms. Hence, the remainder of the woody plants were grouped together as “other” in subsequent statistical analyses.

The predictions of this study were simple: 1) the density of woody plants and ground wood items utilised by the lizards would increase with elephant reintroduction time; 2) the abundance of the tree‑living common dwarf gecko would decrease with increasing elephant reintroduction time due to its reliance on trees; and 3) the abundance of the other two lizards (the skinks) would not be affected as they are generalist species able to make use of various habitat attributes.

After the data was collected and some fancy statistical analyses were performed (stuff that make the layman go “Yikes!”), the results were as follows:

  • A total of 53 lizards were observed across the three study sites after actively searching the quadrats for hours. Of these, 58% were common dwarf gecko, 36% the common variable skink and only 6% were striped skink.
  • The tree‑living gecko abundance was significantly higher in the zone that had no elephants and younger reintroduction times, compared with older elephant introduction times.
  • The geckoes were observed on woody plants 87% of the time, half of which were on the corkwood trees and a third on the bushwillows.
  • The skinks were observed on a variety of habitat attributes including woody plants, trees, ground wood and terra firma.
  • In line with expectations, tree density in the zone with no elephants was significantly higher than in the other sites where these animals roamed freely. Specifically, the velvet corkwood was densest where elephants were absent. The bushwillow was densest at the intermediate elephant reintroduction time (Balule), suggesting the corkwood is a favourite item on the elephants’ menu.
  • Tree damage and ground wood abundance were significantly higher where elephants were present.
  • The results point to a significant relationship between the gecko and the corkwood trees and a positive one at that. By using the statistical models, a similar relationship was identified between the skinks and other woody plants.

African savanna elephant (Loxodonta africana) are ecosystem engineers, moderating the type and quality of habitat for other fauna (Photo: Shutterstock)

Modified landscapes 

Habitat modifications beyond a certain point (time coupled with rising population sizes) brought about by these ecosystem engineers significantly and negatively impacted another species indirectly.

It is not often that our perceptions of patterns and processes are backed by science when we observe these driving through South Africa’s Big Five reserves. More importantly, trophic rewilding is a novel conservation tool that will depend on studies like this that attempt to predict how “engineers” will impact biodiversity and what types of landscapes they will produce.

From these results it seems that to prevent a Jurassic World where giants rule, there needs to be some cap on the population size of megafauna. The question remains: how many are too many before we will start noticing more species losses?