Ecological legacies of civil war: 35‐year increase in savanna tree cover following wholesale large‐mammal declines

Abstract

Summary Large mammalian herbivores (LMH) exert strong effects on plants in tropical savannas, and many wild LMH populations are declining. However, predicting the impacts of these declines on vegetation structure remains challenging. Experiments suggest that tree cover can increase rapidly following LMH exclusion. Yet it is unclear whether these results scale up to predict ecosystem‐level impacts of LMH declines, which often alter fire regimes, trigger compensatory responses of other herbivores and accompany anthropogenic land‐use changes. Moreover, theory predicts that grazers and browsers should have opposing effects on tree cover, further complicating efforts to forecast the outcomes of community‐wide declines. We used the near‐extirpation of grazing and browsing LMH from Gorongosa National Park during the Mozambican Civil War (1977–1992) as a natural experiment to test whether megafaunal collapse increased tree cover. We classified herbaceous and tree cover in satellite images taken (a) at the onset of war in 1977 and (b) in 2012, two decades after hostilities ceased. Throughout the 3620‐km2 park, proportional tree cover increased by 34% (from 0.29 to 0.39) – an addition of 362 km2. Four of the park's five major habitat zones (including miombo woodland, Acacia–Combretum–palm savanna, and floodplain grassland) showed even greater increases in tree cover (51–134%), with an average increase of 94% in ecologically critical Rift Valley habitats. Only in the eastern Cheringoma Plateau, which had historically low wildlife densities, did tree cover decrease (by 5%). The most parsimonious explanation for these results is that reduced browsing pressure enhanced tree growth, survival and/or recruitment; we found no directional trends in rainfall or fire that could explain increased tree cover. Synthesis. Catastrophic large‐herbivore die‐offs in Mozambique's flagship national park were followed by 35 years of woodland expansion, most severely in areas where pre‐war wildlife biomass was greatest. These findings suggest that browsing release supersedes grazer–grass–fire feedbacks in governing ecosystem‐level tree cover, consistent with smaller‐scale experimental results, although the potentially complementary effect of CO2 fertilization cannot be definitively ruled out. Future work in Gorongosa will reveal whether recovering LMH populations reverse this trend, or alternatively whether woody encroachment hinders ongoing restoration efforts.

Scrubbing Up: Multi-Scale Investigation of Woody Encroachment in a Southern African Savannah

“…Savannahs are characterised by the co-occurrence of a grass layer with an overstorey tree layer of variable density [2][3][4], and savannah tree cover levels strongly affect ecosystem function [4,5]. The tree-grass mix of savannahs exists in a dynamic state, with changes in the balance of these vegetation types potentially resulting in switches between alternate states of forest or grassland [6][7][8].…”

“…Savannahs with mean annual precipitation levels of around 650mm or below are reasonably stable, but tend to change towards forest above this level (unless other disturbance processes allow a mix of grass and trees to be maintained) [1,14]. Other potential drivers include fire [16][17][18][19][20][21][22][23], disturbance by megaherbivores, particularly elephant (Loxodonta africana) [4,19,[24][25][26][27], grazing by domestic herbivores [28], scale-dependent interactions between climate, fire and herbivory [17,[29][30][31][32], and increasing CO 2 fertilization [5,9,10,17,[33][34][35][36][37][38][39][40][41][42][43]. The vulnerability of a region to changes in the tree-grass balance of savannas will likely depend on the prevailing land use history, the environmental setting, and the functional traits of woody plants that govern their responsiveness to both local and global drivers [12].…”

“…Within savannah environments, the relative dominance of herbs and grasses versus woody plants is important for biodiversity [4]. This impacts on the abundance and distribution of animal populations [44,45], including trophic interactions, by modulating the degree of concealment versus visibility for predators and prey [4,46,47], spatial patterns of soil fertility [32] and soil moisture via shading and evapotranspiration [48].…”

“…This impacts on the abundance and distribution of animal populations [44,45], including trophic interactions, by modulating the degree of concealment versus visibility for predators and prey [4,46,47], spatial patterns of soil fertility [32] and soil moisture via shading and evapotranspiration [48]. The tree-grass mix also has consequences for ecosystem services including carbon sequestration [49], along with regional-scale feedbacks to the Earth system due to changes in gas exchange and albedo [50].…”

“…Woody encroachment within savannahs is recognised as a global phenomenon [5,9,50], and is of increasing concern because of its potential impacts on biodiversity and ecosystem processes [4]. Such changes have been extensive in both farmed and protected savannahs globally over the last century [50], and have knock-on effects for conservation efforts.…”