J Insect Conserv (2008) 12:493–498 DOI 10.1007/s10841-007-9089-2
ORIGINAL PAPER
Elephant impact on dragonflies Michael J. Samways Æ Paul B. C. Grant
Received: 22 February 2007 / Accepted: 3 May 2007 / Published online: 30 May 2007 Springer Science+Business Media B.V. 2007
Abstract African elephants and other indigenous megaherbivores have a major impact on local vegetation structure, including aquatic communities, as their big feet and large mass pound the fringes of water bodies. This disturbance is likely to have a profound influence on the structure and composition of insect assemblages in these habitats. We investigated which dragonfly (Odonata) species were tolerant of trampling by elephants and other game. Assemblage composition differed according to extremely high, very high or high disturbance levels. Dragonfly abundance was greatest where impact was high, and decreasing when disturbance became very high or extremely high. Several odonate species are well-adapted to fairly high levels of disturbance, although too much is impoverishing. Medium and low impact sites were geographically separated, and this, combined with much lower disturbance levels, had a considerable influence on promoting regional dragonfly diversity. Several regional specialist species only occurred in the geographically separated, low-impact sites. The full complement of dragonflies is present only when there is a combination of various disturbance levels combined with spatial variation. Elephant impact is similar to that of humans, with too much of either or both, leading to a species-poor, habitatgeneralist dragonfly assemblage. Keywords Elephant trampling Dragonfly biodiversity Parallels with human impact
M. J. Samways (&) P. B. C. Grant Department of Conservation Ecology and Entomology, Centre for Agricultural Biodiversity, University of Stellenbosch, Post Bag X1, Matieland 7602, South Africa e-mail:
[email protected]
Introduction Anthropogenic habitat loss is the primary reason for current loss of biodiversity. This loss comes about from fragmentation of natural ecosystems and the reduction of remnant patch quality (Fahrig 2003). Prior to the massive human impact of the last 6,000 years, the European landscape was disturbed by vertebrates (Vera 2000). Similarly, on the African continent, there always has been, and in places still is, a major impact from megaherbivores (Samways and Kreuzinger 2001; Gibson 2004). These animals include elephant (Loxodonta africana), which can influence plant assemblage composition and pattern (Dublin et al. 1990; Van de Vijver et al. 1999; Lombard et al. 2001), and dung beetle assemblages (Botes et al. 2006). Dragonflies are considered as valuable indicators of human disturbance of African riverine systems (Samways and Steytler 1996). Yet some of these systems are naturally disturbed by elephants and other large megaherbivores, characteristic of the savanna. This has probably been going on for many millennia, long before human agricultural influence. It can be seen today where human disturbance is still relatively minimal, as in parts of southern Africa. Dragonflies and other physically small components of these ecosystems have adapted to these impacts, which include substantial trampling and harvesting of plant material. Dragonflies are particularly prone, being aquatic insects in a dry landscape, living precisely where the large animals congregate to drink. This presents an interesting ecological situation, because it is in these highly disturbed, focal aquatic areas that some dragonfly species are remarkably common. Yet not all local areas are affected by elephants in this way, with these animals preferring some specific watering locations over others. The local megaherbivore
123
494
pressure puts enormous demands on the dragonfly assemblage and would seem to be a major natural factor affecting dragonfly assemblage composition and pattern. The aim here was to determine the extent to which megaherbivores, especially elephants, influence the relative abundance and species richness of dragonfly species. Additionally, the objective was to establish which dragonfly species are tolerant of such extreme natural disturbance and which are not. A final aim was to determine whether the various elephant population levels are positive or negative drivers of dragonfly diversity.
Sites, materials and methods The primary research area was the Linyanti River, Botswana, between Kings Pool (1826¢ S, 2342¢ E; 954 m a.s.l.) and Duma Tau (1831¢ S, 2334¢ E; 940 m a.s.l.). It is a slow-moving, meandering, perennial river with fringing reeds, papyrus and grass, and with many marginal pools and some lagoons. The fringing muddy bottom is heavily affected by elephants, which trample pools to such an extent that the river margins become heavily puddled and in places devoid of vegetation (Fig. 1). Other megaherbivores in the area have a much more localized and lesser affect, and these include hippopotamus (Hippopotamus amphibius), Cape buffalo (Syncerus caffer), zebra (Equus burchelli), and red lechwe (Kobus leche). In addition, a large chacma baboon (Papio ursinus) population removed water lilies on a regular basis. The extent of impact on the grass banks is also great, to the extent that the grass is grazed down to lawn length, to mostly only a few centimetres tall. Fringing the bank, mostly about 30–40 m from the river’s edge is a gallery forest of bushes and trees of various sizes (Fig. 1). The various levels of megaherbivore
Fig. 1 A site at Duma Tau, Botswana, highly impacted by elephants and other large vertebrates
123
J Insect Conserv (2008) 12:493–498
impact are listed in Table 1, which are also the dependent variables used in the analyses. A further site, Chitabe (1931¢ S, 2322¢ E; 940 m a.s.l.), received medium impact from elephants, and had more extensive water bodies and localized pools (pans), leaving the flowing system lightly grazed and grass reaching an average height of about 50 cm (Table 1). As there was no low-impact site on the Linyanti River system, the nearest possibility was to include a site on the Okavango River system, but it is recognized here that this also implies wide geographical separation, and thus not providing totally comparable data with the other sites. This site was at Vumbura (1859¢ S, 2250¢ ; 940 m a.s.l.), where the slow-moving Okavango system is mostly composed of an abundance of reeds, papyrus and tall (1 m) grass. The large mammals are less abundant and make use of regular paths, leaving most of the vegetation intact, and having little stirring effect on the muddy bottom (Table 1). During April 2002, at each of the five sites (representing extremely high, very high, high, medium and low impact) ten transects 100 m long were selected. These transects (50 in total) were each along one side of the water’s edge, 3 m wide, and included moving (
