External disturbances are powerful drivers of community and ecosystem reorganization over most of the Earth’s terrestrial surface. As such, they cause losses of plant biomass, reset succession, modify stocks and fluxes of nutrients, and change trophic dynamics. Furthermore, most ecosystems are subjected to multiple disturbance agents, and these disturbances do not operate in isolation from one another. Despite this, most studies have focused on how communities and ecosystems respond to a single type of disturbance, and few have considered the consequences of interactive effects among disturbance agents (Harvey et al. 2014, Buma, 2015). Additionally, few studies have addressed how the effects of disturbances, and in particular the interactive effects among different disturbances, varies across contrasting communities or is determined by environmental context (Sankaran and McNaughton 1988).
In savanna ecosystems, the two most important disturbance agents are usually fire (as an abiotic agent) and herbivory (as a biotic agent). These agents operate not just by altering ecosystem processes, but also through governing the transition between grass- and tree-dominated vegetation. By the virtue of both agents consuming plant biomass, there are some distinct parallels through which they operate, to the extent that fire has been described as a ‘global herbivore’ (Bond and Keeley, 2005). But these agents also have important differences, because fire is driven by the nature of the fuel load and occurs intermittently, whereas herbivory is driven by foliar quality and quantity and can occur more continually. Therefore we could expect both similarities and differences in how these two disturbance agents alter ecosystems. This should, in turn, have potential consequences for the interactive effects among them.
The Editor’s Choice article for this issue of Journal of Ecology, ‘Woody plant biomass and carbon exchange depend on elephant-fire interactions across a productivity gradient in African savanna’ focuses on interactive effects of fire and large herbivores. The authors utilized a replicated field experiment set up in each of four sites of South Africa’s Kruger National Park (KNP) that vary in productivity due to differences in precipitation and soil fertility. In each site, treatments consisted of three different fire regimes (none, annual and triennial) that have been maintained for up to 56 years. In KNP, culling of a megaherbivore, i.e., elephants, was performed until 1994 (37-40 years after the experiment started), after which their densities greatly increased. This ‘natural experiment’ enabled analysis of the impacts of periods with and without high elephant densities across contrasting fire regimes and among contrasting sites.
Their study revealed that the two primary disturbance agents in KNP, fire and elephants, exerted contrasting effects. Fire removed comparable amounts of vegetation across the four sites despite large differences in productivity. In contrast, elephants removed a higher proportion of woody plant biomass in the more productive sites. Furthermore, there were interactive effects of fire and elephants because increased fire frequency had negative effects on woody plant biomass during the low-elephant but not the high-elephant periods. It also highlighted the importance of elephants in reversing increases in woody plant productivity in a manner that fire could not, thereby leading to the ecosystem switching from a carbon sink to a carbon source, and with this effect being greatest at the most productive site. In combination, these results point to the capacity of elephants to alter the impact of fire and to propel a transition from dense woodland to open savanna that is independent of site conditions.
This study should interest a large number of Journal of Ecology readers, not just because it is focused on a charismatic animal in an iconic ecosystem, but because it highlights one of the clearest examples yet of the long-term interactive effects between two major but contrasting disturbance agents. As such it provides compelling evidence that the understanding of the impacts of one of these disturbance agents is incomplete without explicit consideration of the other. It also highlights what can be achieved through combining a natural experiment with a formal (i.e., manipulative) experiment. While a handful of ecologists unfortunately remain resistant to natural experiments on the basis of alleged confounding factors, studies such as this highlight the power of combining both types of approaches within a single study (cf., Fukami and Wardle 2005), together with robust long-term data, to disentangle the role of interacting drivers of ecosystem processes in a manner that is otherwise difficult or impossible to achieve.
David A. Wardle
Associate Editor, Journal of Ecology
Photo credits: Adam Pellegrini