Native and domestic browsers and grazers reduce fuels, fire temperatures, and acacia ant mortality in an African savanna

Abstract

Despite the importance of fire and herbivory in structuring savanna systems, few replicated experiments have examined the interactive effects of herbivory and fire on plant dynamics. In addition, the effects of fire on associated ant-tree mutualisms have been largely unexplored. We carried out small controlled burns in each of 18 herbivore treatment plots of the Kenya Long-term Exclosure Experiment (KLEE), where experimentally excluding elephants has resulted in 42% greater tree densities. The KLEE design includes six different herbivore treatments that allowed us to examine how different combinations of megaherbivore wildlife, mesoherbivore wildlife, and cattle affect fire temperatures and subsequent loss of ant symbionts from Acacia trees. Before burning, we quantified herbaceous fuel loads and plant community composition. We tagged all trees, measured their height and basal diameter, and identified the resident ant species on each. We recorded weather conditions during the burns and used ceramic tiles painted with fire-sensitive paints to estimate fire temperatures at different heights and in different microsites (under vs. between trees). Across all treatments, fire temperatures were highest at 0-50 cm off the ground and hotter in the grass under trees than in the grassy areas between trees. Plots with more trees burned hotter than plots with fewer trees, perhaps because of greater fine woody debris. Plots grazed by wildlife and by cattle prior to burning had lower herbaceous fuel loads and experienced lower burn temperatures than ungrazed plots. Many trees lost their ant colonies during the burns. Ant survivorship differed by ant species and at the plot level was positively associated with previous herbivory (and lower fire temperatures). Across all treatments, ant colonies on taller trees were more likely to survive, but even some of the tallest trees lost their ant colonies. Our study marks a significant step in understanding the mechanisms that underlie the interactions between fire and herbivory in savanna ecosystems.

Combining paleo-data and modern exclosure experiments to assess the impact of megafauna extinctions on woody vegetation

“…Experiments with size-selective exclosures on savannas showed that elephants accounted for more than 80% of all woody plant loss across all plant height classes (22) whereas exclusion of elephants resulted in 42% more trees (23). Furthermore, elephants and all large herbivores debark trees and also feed on saplings and adult Fig.…”

Ecosystem structure, function and composition in rangelands are negatively affected by livestock grazing

“…The economies of many people, particularly from rural areas, depend on the provision of goods and services such as milk, meat, wool, and hide derived from livestock. The economic importance of grazing and the reports of positive or neutral effects of grazing on species richness in specific studies (e.g., Socher et al 2013, Fensham et al 2014, Kimuyu et al 2014 has led some to suggest that introducing or reintroducing commercial herds of livestock to intermittently grazed or ungrazed areas (e.g., alpine high country of Australia; Williams et al 2006) might have benefits for biodiversity and ecosystem functioning (Lunt et al 2007, Williamson et al 2014). …”

“…Grazinginduced habitat modification alters species composition by reducing the diversity of plants and terrestrial invertebrates, small mammals, birds, reptiles, and soil crusts (e.g., Williams et al 2008, van Klink et al 2014. Grazing also alters community structure by influencing, for example, the return interval of wildfires and the accumulation of flammable fuel (Kimuyu et al 2014) or plant community composition (Lunt et al 2012, Fensham et al 2014. These direct structural and compositional shifts have often large, indirect effects on ecosystem functions.…”

“…The effects of grazing are largely driven by four main factors: (1) the type of herbivore (e.g., Kimuyu et al 2014), (2) the intensity of grazing pressure (Lunt et al 2007, Eldridge et al 2011), (3) the level of plant productivity (e.g., Senft et al 1987, Proulx andMazumder 1998), and (4) the evolutionary history of grazing (Milchunas and Lauenroth 1993). First, grazing effects are known to vary between native and domestic herbivores (Riginos and Grace 2008) and among different breeds of livestock, which have different foraging behaviors and patch preferences (Squires 1981).…”

Ecological Effects of Grazing in the Northern Tianshan Mountains

“…Grazing has traditionally been viewed as detrimental to plant growth [24,25], but some studies have shown that overcompensation effects (NPP increase) occurred in moderately grazed systems by changing root/shoot allocation [26,27]. Additionally, previous studies have shown that the effects of grazing on ET vary under different environmental conditions [8,[28][29][30][31], but most grazing has typically been shown to decrease ET. Based on the above information, we expect to observe the following in the northern Tianshan Mountains: (1) grazing may generally decrease NPP, but it may increase NPP in some moderately grazed grassland systems; and (2) grazing may generally decrease ET, but WUE does not necessarily follow the trends of NPP and ET under grazing scenarios because it is determined by both NPP and ET.…”