Native ungulates of diverse body sizes collectively regulate long-term woody plant demography and structure of a semi-arid savanna

Authors: SANKARAN, MAHESH - The Tata Institute Of Fundamental Research; Augustine, David; RATNAM, JAYASHREE - University Of Leeds

Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/2013
Publication Date: 8/23/2013
Publication URL: http://handle.nal.usda.gov/10113/58412
Citation: Sankaran, M., Augustine, D.J., Ratnam, J. 2013. Native ungulates of diverse body sizes collectively regulate long-term woody plant demography and structure of a semi-arid savanna. Ecological Applications. 101:1389-1399.

Interpretive Summary: Herbivores and fire both affect tree cover and abundance in savannas. However, whether herbivores are capable of reducing trees in the absence of other additional disturbances such as fire is unclear. Here, we report results from a 10 year study in a savanna in East Africa where we experimentally removed large herbivores that eat tree leaves and bark (browsers), which at the study site included elephants, impala, giraffe, eland and dik-dik antelope. Our results indicate that these herbivores influence all aspects of tree population growth and community development at the site. Removal of herbivores resulted in a 7 times greater rate of shrub and tree recruitment, 2.5 times lower mortality, and a 3 times greater woody biomass within a decade. Browsers impacted growth and mortality of all size classes of woody vegetation. Browsing of seedlings by small antelope, particularly dik-dik, created a ‘browsing trap’ that prevented small shrubs from being recruited into size classes taller than 1.5 feet. Browser impacts also varied by tree species, and although browsers affected the recruitment of all dominant species, they had no effect on mortality of Acacia etbaica individuals over 1.5 feet tall. A. etbaica was the only dominant species to increase in both size and biomass outside exclosures over the decade, suggesting a long-term shift in woody species composition towards dominance by this species. In contrast to patterns observed in the absence of browsers, tree biomass remained unchanged in browsed sites over the decade. Our results indicate that browsing herbivores provide an important ecosystem service by suppressing shrub and tree encroachment on rangelands managed for cattle production.

Technical Abstract: Large mammalian herbivores and fire are both well recognized to play important roles in regulating tree cover and biomass in savannas. However, the extent to which browsing ungulates are capable of regulating tree populations in the absence of other synergistic disturbances such as fire is unclear. Here, we report results from a 10 year replicated herbivore exclusion experiment in a semiarid savanna in East Africa where fires have been actively suppressed for decades. Our results indicate that browsing ungulates can exert significant ‘top-down’ controls in semi-arid savannas, influencing all aspects of tree demography and dramatically altering the structure and function of savanna vegetation. Exclusion of herbivores resulted in a 7-fold increase in shrub and tree recruitment, a 2.5 fold decrease in mortality, and a 3-fold increase in woody biomass within a decade. Browsers impacted growth and mortality of all size classes of woody vegetation. Extensive browsing of seedlings by small bodied ungulates, particularly dik-dik, suppressed woody recruitment by generating a ‘browsing trap’ very similar to a ‘fire trap’, while browsing by elephants and medium sized ungulates suppressed growth and survival of individuals in larger size classes. Overall, recruitment of woody vegetation into the smallest size class and growth transitions to larger size classes were lower, and mortality and reversion to smaller size classes higher in browsed compared to unbrowsed sites. Browser impacts also varied by species, and although browsers significantly impacted recruitment of all dominant species, they had no effect on mortality of Acacia etbaica individuals over 0.5m tall. A. etbaica was the only dominant species to show a net increase in both basal area and biomass outside exclosures over the decade, suggesting a long-term, browser-induced directional shift in woody species composition towards dominance by this species. In contrast to patterns observed inside exclosures, woody biomass and basal area remained unchanged in browsed sites over the decade. Our results indicate that native browsing ungulates provide a critical ecosystem service by suppressing shrub encroachment on rangelands managed for cattle production.