How do large-mammal communities reassemble after being pushed to the brink of extinction? Few data are available to answer this question, as it is rarely possible to document both the decline and ...recovery of wildlife populations. Here we present the first in-depth quantitative account of war-induced collapse and postwar recovery in a diverse assemblage of large herbivores. In Mozambique's Gorongosa National Park, we assembled data from 15 aerial wildlife counts conducted before (1968-1972) and after (1994-2018) the Mozambican Civil War (1977-1992). Pre-war total biomass density exceeded 9,000 kg km-2, but populations declined by >90% during the war. Since 1994, total biomass has substantially recovered, but species composition has shifted dramatically. Formerly dominant large herbivores-including elephant (Loxodonta africana), hippo (Hippopotamus amphibius), buffalo (Syncerus caffer), zebra (Equus quagga), and wildebeest (Connochaetes taurinus)-are now outnumbered by waterbuck (Kobus ellipsiprymnus) and other small to mid-sized antelopes. Waterbuck abundance has increased by an order of magnitude, with >55,000 individuals accounting for >74% of large-herbivore biomass in 2018. By contrast, elephant, hippo, and buffalo, which totaled 89% of pre-war biomass, now comprise just 23%. These trends mostly reflect natural population growth following the resumption of protection under the Gorongosa Restoration Project; reintroductions (465 animals of 7 species) accounted for a comparatively small fraction of the total numerical increase. Waterbuck are growing logistically, apparently as-yet unchecked by interspecific competition or predation (apex-carnivore abundance has been low throughout the post-war interval), suggesting a community still in flux. Most other herbivore populations have increased post-war, albeit at differing rates. Armed conflict remains a poorly understood driver of ecological change; our results demonstrate the potential for rapid post-war recovery of large-herbivore biomass, given sound protected-area management, but also suggest that restoration of community structure takes longer and may require active intervention.
In savannas, the tree-grass balance is governed by water, nutrients, fire and herbivory, and their interactions. We studied the hypothesis that herbivores indirectly affect vegetation structure by ...changing the availability of soil nutrients, which, in turn, alters the competition between trees and grasses. Nine abandoned livestock holding-pen areas (kraals), enriched by dung and urine, were contrasted with nearby control sites in a semi-arid savanna. About 40 years after abandonment, kraal sites still showed high soil concentrations of inorganic N, extractable P, K, Ca and Mg compared to controls. Kraals also had a high plant production potential and offered high quality forage. The intense grazing and high herbivore dung and urine deposition rates in kraals fit the accelerated nutrient cycling model described for fertile systems elsewhere. Data of a concurrent experiment also showed that bush-cleared patches resulted in an increase in impala dung deposition, probably because impala preferred open sites to avoid predation. Kraal sites had very low tree densities compared to control sites, thus the high impala dung deposition rates here may be in part driven by the open structure of kraal sites, which may explain the persistence of nutrients in kraals. Experiments indicated that tree seedlings were increasingly constrained when competing with grasses under fertile conditions, which might explain the low tree recruitment observed in kraals. In conclusion, large herbivores may indirectly keep existing nutrient hotspots such as abandoned kraals structurally open by maintaining a high local soil fertility, which, in turn, constrains woody recruitment in a negative feedback loop. The maintenance of nutrient hotspots such as abandoned kraals by herbivores contributes to the structural heterogeneity of nutrient-poor savanna vegetation.
Ecology of grazing lawns in Africa Hempson, Gareth P.; Archibald, Sally; Bond, William J. ...
Biological reviews of the Cambridge Philosophical Society,
08/2015, Volume:
90, Issue:
3
Journal Article
Peer reviewed
ABSTRACT
Grazing lawns are a distinct grassland community type, characterised by short‐stature and with their persistence and spread promoted by grazing. In Africa, they reveal a long co‐evolutionary ...history of grasses and large mammal grazers. The attractiveness to grazers of a low‐biomass sward lies in the relatively high quality of forage, largely due to the low proportion of stem material in the sward; this encourages repeat grazing that concomitantly suppresses tall‐grass growth forms that would otherwise outcompete lawn species for light. Regular grazing that prevents shading and maintains sward quality is thus the cornerstone of grazing lawn dynamics. The strong interplay between abiotic conditions and disturbance factors, which are central to grazing lawn existence, can also cause these systems to be highly dynamic. Here we identify differences in growth form among grazing lawn grass species, and assess how compositional differences among lawn types, as well as environmental variables, influence their maintenance requirements (i.e. grazing frequency) and vulnerability to degradation. We also make a clear distinction between the processes of lawn establishment and lawn maintenance. Rainfall, soil nutrient status, grazer community composition and fire regime have strong and interactive influences on both processes. However, factors that concentrate grazing pressure (e.g. nutrient hotspots and sodic sites) have more bearing on where lawns establish. Similarly, we discuss the relevance of enhanced rates of nitrogen cycling and of sodium levels to lawn maintenance. Grazer community composition and density has considerable significance to grazing lawn dynamics; not all grazers are adapted to foraging on short‐grass swards, and differences in body size and relative mouth dimensions determine which species are able to convert tall‐grass swards into grazing lawns under different conditions. Hence, we evaluate the roles of different grazers in lawn dynamics, as well as the benefits that grazer populations derive from having access to grazing lawns. The effects of grazing lawns can extend well beyond their borders, due to their influence on grazer densities, behaviour and movements as well as fire spread, intensity and frequency. Variation in the area and proportion of a landscape that is grazing lawn can thus have a profound impact on system dynamics. We provide a conceptual model that summarises grazing lawn dynamics, and identify a rainfall range where we predict grazing lawns to be most prevalent. We also examine the biodiversity associated with grazing lawn systems, and consider their functional contribution to the conservation of this biodiversity. Finally, we assess the utility of grazing lawns as a resource in a rangeland context.
Trophic rewilding seeks to rehabilitate degraded ecosystems by repopulating them with large animals, thereby re-establishing strong top-down interactions. Yet there are very few tests of whether such ...initiatives can restore ecosystem structure and functions, and on what timescales. Here we show that war-induced collapse of large-mammal populations in Mozambique's Gorongosa National Park exacerbated woody encroachment by the invasive shrub Mimosa pigra-considered one of the world's 100 worst invasive species-and that one decade of concerted trophic rewilding restored this invasion to pre-war baseline levels. Mimosa occurrence increased between 1972 and 2015, a period encompassing the near extirpation of large herbivores during the Mozambican Civil War. From 2015 to 2019, mimosa abundance declined as ungulate biomass recovered. DNA metabarcoding revealed that ruminant herbivores fed heavily on mimosa, and experimental exclosures confirmed the causal role of mammalian herbivory in containing shrub encroachment. Our results provide mechanistic evidence that trophic rewilding has rapidly revived a key ecosystem function (biotic resistance to a notorious woody invader), underscoring the potential for restoring ecological health in degraded protected areas.
1. Small fenced reserves are perceived to require management interventions to maintain ecosystems in a natural state. Such interventions are typically initiated and assessed on the basis of ...short-term observations, while slow changes are often misunderstood or missed entirely. Long-term monitoring is therefore crucial to understand the effect of reserve management on ecosystem structure and functioning. 2. We analysed a 15-year data set of herbaceous vegetation data for 59 monitoring sites in two nature reserves of different size and with different management regimes in a semi-arid, South African savanna. Community composition and vegetation structure (basal cover) were assessed in response to rainfall and management variables. 3. No directional changes over time were found. Basal cover as measured by two proxies (distance to the nearest tuft and tuft diameter) increased with current and previous year rainfall. The relative abundance of functional groups also responded to rainfall, with perennial grasses increasing in relative abundance following a wet year. Compositional responses, as measured by a dissimilarity index of species relative abundance, showed a 2-year lag and responses were larger following dry years than wet years. 4. The response to rainfall of distance to the nearest tuft was significantly weaker in the smaller reserve, while compositional change during dry years was larger than in the larger reserve. 5. Synthesis and applications. Most of the variation in herbaceous basal cover and community composition was associated with differences between years (time) at each site, rather than with differences between sites (space), indicating that inter-annual variation in rainfall is the most important driver of herbaceous layer dynamics in these systems. However, management did modify the effects of rainfall on herbaceous structure and community composition. The smaller reserve, which had higher grazer and waterhole density, showed greater fluctuations in key herbaceous variables. Such reserves are common in southern Africa and probably require more careful management than larger reserves in the face of global climate change.
Abstract
Experiments have begun demonstrating that the fear (antipredator behavioral responses) large carnivores inspire in ungulates can shape ecosystem structure and function. Most such experiments ...have focused on the impacts of either just one large carnivore, or all as a whole, rather than the different impacts different large carnivores may have in intact multi-predator-prey systems. Experimentally testing the relative fearfulness ungulates demonstrate toward different large carnivores is a necessary first step in addressing these likely differing impacts. We tested the fearfulness ungulates demonstrated to playbacks of lion (Panthera leo), African wild dog (Lycaon pictus), cheetah (Acinonyx jubatus) or non-predator control (bird) vocalizations, in Greater Kruger National Park, South Africa. Ungulates ran most to lions, then wild dogs, and then cheetahs, demonstrating a very clear hierarchy of fear. Those that did not run looked toward the sound more on hearing large carnivores than controls, looking most on hearing lions. Notably, prey species-specific population level kill rates by each predator did not predict the patterns observed. Our results demonstrate that different large carnivores inspire different levels of fear in their ungulate prey, pointing to differing community-level impacts, which we discuss in relation to the ongoing worldwide decline and loss of large carnivores.
Droughts are predicted to increase in frequency and intensity for large parts of Africa. It is unclear how this may impact native grazers in protected areas, and how these outcomes may differ under ...contrasting management approaches. Reducing artificial water sources and increasing the size of protected areas have been proposed as management responses that could possibly increase system resilience and buffer herbivores against large-scale mortality during droughts. Long-term spatially explicit buffalo, elephant and hippo census data from the Kruger National Park and adjacent private protected areas during three droughts (1982/1983, 1991/1992 and 2015/2016) provided a unique opportunity to explore how a range of management actions influenced megaherbivore responses across six regions. Drought responses varied significantly at a regional scale for all three megaherbivores, supporting the notion that size, and the associated emergent property of spatio-temporal heterogeneity, provides a buffer during droughts. Furthermore, regional buffalo population declines (as a function of mortality, movement and birth rates) were highest in areas with high waterhole density and concomitant low levels of herbaceous biomass, compared with areas that experienced similar rainfall, but had lower waterhole density and higher herbaceous biomass. Elephant mortalities were negligible, but elephants displayed regional movement patterns, seemingly attracted to higher water provision densities during the drought. Our results highlight that drought interacts with spatio-temporal environmental gradients and management actions and illustrates how the Greater Kruger National Park is increasingly functioning as an integrated ecosystem for megaherbivores.
Protected areas (PAs) adjacent to the Kruger National Park (KNP) are divided into fenced and unfenced properties. We discuss land-use development to provide context of how these savannas have evolved ...over the past century. Although the PAs have embraced the basic philosophies of the KNP management approach and have similar high-level objectives, they function at different spatial, and consequently temporal scales, largely because of fencing and the subsequent provision of water in these areas. We examine bottom up (environmental, rainfall) conditions using rainfall as a percentage of the mean and the Standard Precipitation Index (SPI) leading into five drought periods dating back to 1982/1983 and top-down drivers focusing on animal number and type (geo-referenced data from annual helicopter counts) on fenced versus areas open to the larger KNP system. We then examine the response variables, grass species composition, cover and standing crop (all using the Multiple Indicator Method) and the capacity of these to attain a variety of potential land use objectives. Using a dataset spanning some 30 years, we provide scenarios that offer an early warning system to managers irrespective of land use objectives and management approach (laissez-faire or active).
1. Elucidation of the mechanism determining the spatial scale of patch selection by herbivores has been complicated by the way in which resource availability at a specific scale is measured and by ...vigilance behaviour of the herbivores themselves. To reduce these complications, we studied patch selection by an animal with negligible predation risk, the African elephant. 2. We introduce the concept of nutrient load as the product of patch size, number of patches and local patch nutrient concentration. Nutrient load provides a novel spatially explicit expression of the total available nutrients a herbivore can select from. 3. We hypothesized that elephant would select nutrient-rich patches, based on the nutrient load per 2500 m 2 down to the individual plant scale, and that this selection will depend on the nitrogen and phosphorous contents of plants. 4. We predicted that elephant would cause more adverse impact to trees of lower value to them in order to reach plant parts with higher nutrient concentrations such as bark and root. However, elephant should maintain nutrient-rich trees by inducing coppicing of trees through re-utilization of leaves. 5. Elephant patch selection was measured in a homogenous tree species stand by manipulating the spatial distribution of soil nutrients in a large field experiment using NPK fertilizer. 6. Elephant were able to select nutrient-rich patches and utilized Colophospermum mopane trees inside these patches more than outside, at scales ranging from 2500 down to 100 m 2 . 7. Although both nitrogen and phosphorus contents of leaves from C. mopane trees were higher in fertilized and selected patches, patch choice correlated most strongly with nitrogen content. As predicted, stripping of leaves occurred more in nutrient-rich patches, while adverse impact such as uprooting of trees occurred more in nutrient-poor areas. 8. Our results emphasize the necessity of including scale-dependent selectivity in foraging studies and how elephant foraging behaviour can be used as indicators of change in the availability of nutrients.
There is a growing concern that the feeding habits of the African elephant, which include pushing over, uprooting and snapping trees, may have a negative impact on other herbivores. Browsed trees are ...known to respond by either increasing production (shoots and leaves) or defence (secondary compounds). It is not clear, however, what proportion of the browsed biomass can be made available at lower feeding heights after a tree is pushed over or snapped; thus, it is also unclear how the forage quality is affected. In a field survey in Kruger National Park, South Africa, 708 Mopane trees were measured over four elephant utilization categories: snapped trees, pushed-over trees, uprooted trees and control trees. The elephants' impact on the leaf biomass distribution was quantified, and the forage quality (Ca, P, K and Mg, N, digestibility and condensed tannin CT concentrations) were analyzed. Pushed-over and uprooted trees had the maximum leaf biomass at lower heights (< 1 m), snapped trees at medium heights (1-2 m) and control trees at higher heights (> 2 m). In all three utilization categories, the minimum leaf biomass was seven times higher than it was for control trees at a height of below 1 m. Leaf nitrogen content increased in all three categories and was significantly higher in snapped trees. CT concentrations increased slightly in all trees that were utilized by elephants, especially on granitic soils in the dry season. The results provide the insight that elephants facilitate the redistribution and availability of browse and improve the quality, which may positively affect small browsing herbivores.
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