Habitat disturbance effects on the physiological stress response in resident white-bearded wildebeest

Background/Question/Methods

Regarded as a keystone species, white-bearded wildebeest (Connochaetes taurinus) are found across the grassland savannahs of eastern and southern Africa.  Over the past 40 years, however, many local populations have become threatened with extinction.  Directly related to these population declines is the widespread and pervasive expansion of agriculture and large-scale ranching that have occurred across the region.  These processes fragment the landscape, forming habitat discontinuities and imposing barriers to daily and seasonal animal movement.  Unknown, however, is the effect that these landscape disturbances have on the internal state of the animal.  That is, do increased levels of anthropogenic habitat fragmentation and disturbance result in elevated levels of stress?  Using a validated laboratory technique, we quantified fecal glucocorticoid metabolites (i.e., stress hormones) extracted from wildebeest fecal samples collected over a 3-month study period and across three study areas in Kenya with varying degrees of disturbance.  We used an information-theoretic approach to rank models and investigated the effect that landscape factors of disturbance have on observed stress levels.

Results/Conclusions

Our highest ranking model included an interaction between locally collected plant biomass and disturbance, the number of calves in a group, and ΔNDVI (change in Normalized Difference Vegetation Index).  A strong positive effect related to biomass and disturbance suggested that tall/standing biomass and high levels of disturbance contribute to elevated levels of stress in wildebeest.  A strong negative effect related to ΔNDVI was also observed, suggestive that new growth lowers average stress levels.  Our research suggests that increased levels of habitat disturbance may have an adverse effect on wildebeest populations across the region when habitat conditions deteriorate.  We did not identify statistically significant differences in fecal glucocorticoid metabolite levels between study areas.  Wildebeest, however, likely avoid areas of high anthropogenic disturbance, which may indirectly lead to lowered fitness.  Used together with ongoing radio-collaring efforts throughout the region, these results provide a greater understanding of the effects of natural and anthropogenic habitat disturbance on a dominant herbivore that has experienced recent and pervasive population declines.