Cattle, wild herbivores, and drought interact to drive vegetation change over 17 years in an East African savanna

Background/Question/Methods

Rainfall and herbivory are two of the main factors driving grassland dynamics. Heavy livestock grazing in combination with drought, for example, can cause dramatic and lasting changes in community composition, structure, and function. In grasslands around the world, native large mammalian herbivores have typically been replaced by domestic livestock. This is particularly dramatic in African savannas, where cattle (grazers) have replaced a diverse suite of herbivores that span a wide range of body sizes and foraging habits. However, the consequences of such changes, particularly in combination with other factors such as rainfall, are not well understood. The Kenya Long-term Exclusion Experiment (KLEE) was established in 1995 to examine the separate and combined effects of cattle and native wild herbivores on savanna dynamics. This experiment is situated in the Laikipia region of northern Kenya and consists of 18 large-scale (4-ha) plots assigned to six treatments: no large herbivores, cattle only, meso-wildlife only (excludes elephants and giraffes), meso-wildlife plus cattle, all wildlife, and all wildlife plus cattle. Herbaceous species composition and cover have been surveyed twice yearly since the start of the experiment. Rainfall is measured daily and other changes (e.g. tree cover) are monitored periodically.

Results/Conclusions

Over 17 years, herbaceous cover has consistently been reduced in plots to which cattle have access, relative to other treatments. This impact of cattle appears to have caused a divergence in plant community composition between cattle-accessible and cattle-absent treatments. Most notably, cover of one perennial grass, Brachiaria lachnantha, has increased dramatically in cattle-absent treatments and appears to be competitively excluding other species of grasses and forbs. The combination of cattle absence and droughts in 2001, 2006, and 2009 facilitated spread of this grass. Conversely, cattle, particularly in combination with native wildlife, have enabled greater species richness and evenness to persist, particularly after droughts. Cattle and wildlife appear to jointly contribute to resilience in the herbaceous community through at least three mechanisms: first, by grazing B. lachnantha and preventing its dominance; second, by creating small patches of bare ground in which less common species can establish; and third, via elephant herbivory, which has reduced tree cover and indirectly benefited some understory species. Our results illustrate that cattle (at moderate stocking rates) and native wildlife may together play a vital role in maintaining savanna structure and diversity, particularly where wildlife abundances are no longer high enough to maintain natural disturbance regimes.