The secret role of elephants : mediators of habitat scale and within-habitat scale predation risk

Abstract: Elephants have long been recognized as important mediators of savannah vegetation structure. With their activities, like feeding on woody plants they may increase visibility of a habitat and produce coarse woody debris within that habitat. These changes in vegetation may influence major interactions between organisms, such as competition or even predation. I focused on the latter and experimentally tested the influence of elephant-induced vegetation changes on habitat and within-habitat scale risk. The experiment was performed in Hluhluwe-iMfolozi Park, South Africa on resource hot spots in savannah grasslands, i.e. grazing lawns. This controlled for resource availability and ensured sufficient herbivore visitation. I created experimental manipulations that should influence perceived predation risk at habitat and within-habitat scale. Firstly, I compared treatment plots with high and low visibility at habitat scale. I expected herbivores to prefer high visibility plots. Secondly, I added coarse woody debris (CWD) to both closed and open plot, at within-habitat scale. CWD may act as escape impediments or may decrease visibility and thus increase perceived predation risk. I expected herbivores to choose plots without CWD. Then I added predator scat to all treatment plots to introduce immediate predation risk which should additionally increase the perceived risk. I expected smaller, more vulnerable herbivores to respond stronger to immediate risk than larger ones. Herbivore response, i.e. visitation, occupancy and herd size, to risk manipulations was monitored with camera traps. Impala and warthog reduced visitation of plots with coarse woody debris, suggesting they perceived risk at within-habitat scale. However, the response of impala was strongly dependent on presence of predator scat and time of day – impala only reduced visitation to plots with CWD at night and when risk was immediate. Warthog avoided lawn edges and impala did as well at night, but increased the visitation of closed plots without CWD during the day. These responses show the variation in perception of habitat scale risk of different prey species. White rhino did not respond clearly to within-habitat or habitat scale risk, which provides unique experimental evidence that this megaherbivore may indeed be relatively invulnerable to predation. My results show that elephant induced vegetation changes influence both fine scale and habitat scale predation risk, however the smaller herbivore species perceive risk differently than larger ones and the perceived risk changes in presence of predator cues.