Grazing Lawns Research Articles

Grassland albedo as a nature-based climate prospect: the role of growth form and grazing

Abstract Nature-based solutions for mitigating climate change focus largely on land management to reduce carbon emissions and enhance carbon sequestration. Tree planting, commonly advocated for carbon offset, threatens grassland biodiversity and may induce positive radiative forcing (warming) by lowering albedo. Before making decisions about land-use changes in grasslands, an understanding of the fine-scale albedo of grassy versus woody vegetation is needed. Existing satellite-based albedo products offer global coverage with temporally fine, but spatially coarse, resolution, whereas fine-scale in situ grassland albedo data are sparse. We examined the hypotheses that albedo varies seasonally between grass type patches, between shrub and grass patches, and with grazing at the patch scale. Using a tripod-mounted albedometer, we quantified albedo of seven distinct grassland patches in South Africa’s eastern Karoo during early and late dormancy and growing seasons. Patches included intensely-grazed grazing lawn (Cynodon dactylon), grazed and less-grazed red tussock grass (Themeda triandra), grazed and less-grazed white tussock grass (Eragrostis lehmanniana), shrub (Pentzia incana) encroached grass, and bare ground. Season influenced albedo in all patches and, additionally, we found strong differences for the same period between years due to varying rainfall and temperature patterns. For grass-dominated patches, albedo differences were most pronounced during early dormancy, likely due to an effect of grass inflorescences. Albedo of intensely-grazed grazing lawns was consistently higher than other patches, except during early dormancy when white tussock grass albedo was equally high. We found no albedo difference between grazed and less-grazed tussock patches of either red or white grass. Shrub-encroached patches exhibited consistently lower albedo than other patches. Our findings underscore the nuanced relationship between grassland patches and albedo, with shrub encroachment, proposed afforestation, and certain grasses possibly increasing warming potential through reduced albedo. As climate initiatives extend into grasslands, understanding these patterns is essential for climate change mitigation and grassland conservation.

Small‐scale fires interact with herbivore feedbacks to create persistent grazing lawn environments

Abstract Fire‐herbivory feedbacks strongly influence the formation of grazing lawns in savanna ecosystems. Preliminary findings suggest that small‐scale (<25 ha) fires can engineer grazing lawns by concentrating herbivores on the post‐burn green flush; however, the persistence of such grazing lawns over the longer term and without repeated fire is unknown. We used high‐resolution Light Detection and Ranging (LiDAR) to investigate the long‐term effects of fire manipulation on short grass structure (height, cover, volume and spatial continuity) and grazing lawn establishment in Kruger National Park, South Africa. We analysed the effects of fire exclusion and experimental burns applied over a 7‐year period (2013–2019) followed by a 1‐year cessation of burning at varying spatial scales during the early and late dry seasons. Fires contributed a fourfold increase in short grass cover, regardless of fire season or size. The distribution of grass height differed significantly between fire‐induced grazing lawns and recently unburnt parts of the landscape where controlled fires were excluded over the experimental period. The volume (corresponding to bulk density) of short grass on the landscape responded strongly to fires, with grass volume <20 cm in height increasing with both early and late dry season fires. Early dry season fires caused larger and more homogeneous short grass patches. Furthermore, early dry season fires were more influential in increasing the cover of the shortest grass height class (1–5 cm). Synthesis and applications. Our results demonstrate that fire‐induced grazing lawns can persist over the longer term, even when fires are no longer applied, leading to the creation of vertical and horizontal heterogeneity in the grass layer. Small‐scale fires, therefore, represent a feasible management approach to expanding grazing lawn extent, potentially benefiting grazer coexistence and diversity.

Grazing in a megagrazer‐dominated savanna does not reduce soil carbon stocks, even at high intensities

Recent studies suggest that wild animals can promote ecosystem carbon sinks through their impacts on vegetation and soils. However, livestock studies show that intense levels of grazing reduce soil organic carbon (SOC), leading to concerns that rewilding with large grazers may compromise ecosystem carbon storage. Furthermore, wild grazers can both limit and promote woody plant recruitment and survival on savanna grasslands, with both positive and negative impacts on SOC, depending on the rainfall and soil texture contexts. We used grazing lawns in one of the few African protected savannas that are still dominated by megagrazers (> 1000 kg), namely white rhinoceros Ceratotherium simum, as a model to study the impact of prolonged and intense wild grazing on SOC stocks. We contrasted SOC stocks between patches of varying grazing intensity and woody plant encroachment in sites across different rhino habitat types. We found no differences in SOC stocks between the most‐ and least grazed plots in any of the habitats. Intermediately grazed plots, however, had higher SOC stocks in the top 5 cm compared to most and least grazed plots, but only in the closed‐canopy woodland habitat and not in the open habitats. Importantly, we found no evidence to support the hypothesis that wild grazing reduces SOC, even at high grazing intensities by the world's largest megagrazer. Compared to the non‐encroached reference plots, woody encroached plots had higher SOC stocks in soils with low clay content and lower SOC stocks in soils with high clay content, although only in the top 5 cm. Accordingly, our study highlights that wild grazers may influence SOC indirectly through their impact on tree‐grass ratios in grassy ecosystems. Our study thus provides important insights for future natural climate solutions that focus on wild grazer conservation and restoration.Keywords: fire, grazing impact, rewilding, soil carbon, white rhinoceros, woody encroachment

Anthropogenic Disturbance, Not Topographic Dispersal Barriers, Limit Forest and Maintain Madagascar's Derived Savannas

Photo 1. Central Highland grassland (RN7 national road; Isalo district). Levels of grass endemicity in Madagascar are most similar to those of the derived grasslands of the previously forested islands of New Guinea, New Zealand, Japan, and the Mascarenes (Mauritius, Réunion, and Rodrigues). Annual burning of Malagasy treeless grasslands west of the escarpment to support zebu cattle is so intensive that it exposes topsoil and selects a fire-adapted anthropogenic grass assemblage of low palatability, dominated by Loudetia simplex, Schizachyrium sanguineum, and Trachypogon spicatus. This habitat is driven by pastoralism, not topography. Photo credit: Grant Joseph. Photo 2. Critically Endangered Verreaux's sifaka (Propithecus verreauxi) in remnant forest at Zombitse, surrounded by anthropogenic grassland. Zombitse is just one of many areas west of the escarpment harboring closed-canopy forests. Over 90% of vertebrates, being predominately forest-adapted, are limited to such areas across Madagascar, despite the fact that almost 80% of the island not under agriculture is now grassland. This evolutionary anomaly is best explained by treeless grasslands having been historically smaller, or ephemeral. Photo credit: Grant Joseph. Photo 3. Despite sharing common ancestry with African Herpestids that have easily evolved to open habitats [e.g., Cynictis penicillata (a); Suricata suricatta (b)], Malagasy Euplerids [e.g., the Endangered Mungotictis decemlineata (c); the Vulnerable and decreasing Cryptoprocta ferox (d)] are forest-adapted. The same is true of all non-arboreal Malagasy mammal lineages, which have failed to evolve into savanna–grassland specialists, suggesting that open habitats have either not been sufficiently stable or sufficiently vast to promote open-habitat adapted species. Photo credit: Colleen Seymour (a); Grant Joseph (b–d). Photo 4. Aside from the remaining subhumid forest patches on the Central Highlands which share many tree species with eastern lowland forest (thus contradicting the proposal of range-pinning), there are multiple canopied, non-savanna systems west of the escarpment, harboring species dependant on woody vegetation. Examples include deciduous forest, e.g., Kirindy Forest, harboring Coua cristata (a); Zombitse Forest, home to the Endangered Lepilemur hubbardorum (b); Tsingy De Bemaraha Forest sustaining Critically Endangered Propithecus deckenii (c), and Spiny Forest, e.g., Raniala Forest, here providing habitat for the Vulnerable and decreasing Furcifer antimena (d). Photo credit: Grant Joseph. Photo 5. Zebu (Bos taurus indicus) cattle number 7 to 10 million head, and form obligate C4 grazing lawns on Madagascar. There is no evidence to date for any Malagasy obligate grazers, past or present. Treeless grazing lawns now occur where paleoecological evidence supports recent existence of C3 forest and woodland tree species, containing subfossils representing extinct herbivores that included a large component of C3 species in their diets. Transformation of these woodlands and forests to grassland, and extirpation of endemic tree-adapted herbivores occurred coeval with the switch from human gathering and hunting to pastoralism and farming. Photo credit: Grant Joseph. These photographs illustrate the article “Dispersal limitation and fire feedbacks maintain mesic savannas in Madagascar: Comment” by Grant S. Joseph and Colleen L. Seymour published in Ecology. https://doi.org/10.1002/ecy.4045

Reduction in biomass of freshwater arctic vegetation by foraging and nesting hyperabundant herbivores shows recovery

AbstractArctic‐nesting geese are specialist herbivores of grasses and sedges (collectively, graminoids). Under moderate grazing pressure, these migratory herbivores can create and maintain arctic grazing lawns with high nutritional content and low aboveground biomass. Nutrient and energy subsidies from southern agricultural landscapes during winter have improved survival among populations of Ross's (Anser rossii) and lesser snow geese (Anser caerulescens caerulescens), leading to marked population growth. Resulting goose hyperabundance has raised conservation concern for resilience of arctic ecosystems to withstand cumulative and intense pressures of herbivory and nest construction. We used both design‐based (experimental herbivore exclosures) and model‐based methods to investigate changes to plant community structure in direct response to foraging and nesting by these species within the Queen Maud Gulf (Ahiak) Migratory Bird Sanctuary, Nunavut, Canada. Annual nest construction and foraging by up to ~1.3 million geese at a large colony at Karrak Lake markedly reduced aboveground biomass of forage (graminoids) and non‐forage (foliose and fruticose lichens) vegetation, with spatial variation in reduction associated with intensity of use by geese. Within vast brood‐rearing regions, foraging reduced above‐ and belowground plant biomass of lowland plant communities by 61% and 29%, respectively, between 1994 (when herbivore exclosures were established) and 2017. In addition to landscape diversity associated with abiotic properties created by geomorphic processes, long‐term herbivory by geese further increased spatial heterogeneity in vegetation at the landscape scale. Although foraging geese nearly completely depleted aboveground plant biomass in some parts of their brood‐rearing areas, belowground biomass was largely conserved, and thus, plant communities had strong potential for aboveground regeneration. We propose that the effects of high‐density nesting and foraging by Ross's and lesser snow geese in the central Canadian Arctic, while extensive, are ephemeral and prone to reversion following cessation of grazing and nesting pressure, such as that afforded by shifts in herbivore distribution or local population decline.

The effects of herbivore aggregations at water sources on savanna plants differ across soil and climate gradients.

Water sources in arid and semiarid ecosystems support humans, wildlife, and domestic animals, forming nodes of activity that sculpt surrounding plant communities and impact critical grazing and soil systems. However, global aridification and changing surface water supply threaten to disrupt these water resources, with strong implications for conservation and management of these ecosystems. To understand how effects of herbivore aggregation at water impact plant communities across contexts, we measured herbivore activity, plant height, cover (trees, grasses, forbs, and bare ground), diversity, and composition at 17 paired water sources and matrix sites across a range of abiotic factors in a semiarid savanna in Kenya. The effects of proximity to surface water and herbivore aggregation on plant communities varied substantially depending on soil and rainfall. In arid areas with nutrient-poor sandy soils, forb and tree cover were 50% lower at water sources compared to neighboring matrix sites, bare ground was 20% higher, species richness was 15% lower, and a single globally important grazing grass (Cynodon dactylon) dominated 60% of transects. However, in mesic areas with nutrient-rich finely textured soils, species richness was 25% higher, despite a 40% increase in bare ground, concurrent with the decline of a dominant tall grass (Themeda triandra) and increase in C.dactylon and other grass species near water sources. Recent rainfall was important for grasses; cover was higher relative to matrix sites only during wet periods, a potential indication of compensatory grazing. These findings suggest that effects of herbivore aggregation on vegetation diversity and composition will vary in magnitude, and in some cases direction, depending on other factors at the site. Where moisture and nutrient resources are high and promote the dominance of few plant species, herbivore aggregations may maintain diversity by promoting grazing lawns and increasing nondominant species cover. However, in arid conditions and sites with low nutrient availability, diversity can be substantially reduced by these aggregations. Our results highlight the importance of considering abiotic conditions when managing for effects of herbivore aggregations near water. This will be particularly important for future managers in light of growing global aridification and surface water changes.

Forage quality in grazing lawns and tall grasslands in the subtropical region of Nepal and implications for wild herbivores

Subtropical grasslands interspersed in forests often present mosaics of tall grasslands and grazing lawns with a high variation in structure, biomass and nutrient concentration. However, the impact of such variation on forage quality is still poorly known. We quantified physical and chemical properties of grasses of grazing lawns and tall grasslands, interspersed in the forested region of Bardia National Park, Nepal during the hot-dry season. This area falls within Cwa-climate (Köppen-Geigen climate classification). We found that grasses in grazing lawns had an average bulk density of ~5400 g.m-3 whereas tall grasslands had an average bulk density of ~1000 g.m-3 only. Forage in grazing lawns was comprised of a higher percentage of green leaf (up to 60%) compared to tall grassland (up to 40%). Phosphorus levels in green leaves were below maintenance requirements of wild herbivores (especially for grazers and mixed feeders) on both grazing lawns and tall grasslands. However, average crude protein levels in green leaves from both the grazing lawns and tall grasslands could meet the herbivores maintenance requirement (~7%). Only green leaves on grazing lawns had crude protein levels sufficient enough (9.7%) to meet the requirements of herbivores for maintenance and gestation, though not for lactation. We conclude that, during the hot-dry season, grazing lawns provide forage with a higher quantity and quality than tall grasslands. Consequently, grazing lawns can make a significant contribution to the maintenance or even growth of the grassland dependent wild ungulate population, such as chital (Axis axis), a primary prey species of the endangered tiger (Panthera tigris) in Bardia National Park. The insight of this study will provide a basis for restoring grazing lawns for quality forage, and aid in the conservation and management of wild grazers and mixed feeders.

Tracking nutrients in space and time: Interactions between grazing lawns and drought drive abundances of tallgrass prairie grasshoppers.

We contrast the response of arthropod abundance and composition to bison grazing lawns during a drought and non‐drought year, with an emphasis on acridid grasshoppers, an important grassland herbivore.Grazing lawns are grassland areas where regular grazing by mammalian herbivores creates patches of short‐statured, high nutrient vegetation. Grazing lawns are predictable microsites that modify microclimate, plant structure, community composition, and nutrient availability, with likely repercussions for arthropod communities.One year of our study occurred during an extreme drought. Drought mimics some of the effects of mammalian grazers: decreasing above‐ground plant biomass while increasing plant foliar percentage nitrogen.We sampled arthropods and nutrient availability on and nearby (“off”) 10 bison‐grazed grazing lawns in a tallgrass prairie in NE Kansas. Total grasshopper abundance was higher on grazing lawns and the magnitude of this difference increased in the wetter year of 2019 compared to 2018, when drought led to high grass foliar nitrogen concentrations on and off grazing lawns. Mixed‐feeding grasshopper abundances were consistently higher on grazing lawns while grass‐feeder and forb‐feeder abundances were higher on lawns only in 2019, the wetter year. In contrast, the abundance of other arthropods (e.g., Hemiptera, Hymenoptera, and Araneae) did not differ on and off lawns, but increased overall in 2019, relative to the drought of 2018.Understanding these local scale patterns of abundances and community composition improves predictability of arthropod responses to ongoing habitat change.

The role of browsers in maintaining the openness of savanna grazing lawns

Abstract In savannas, ruminant herbivores can have divergent impacts on tree recruitment and subsequent woody cover. Whereas heavy grazing by cattle results in woody thickening, intensive grazing by wildlife instead tends to be associated with lower woody cover. To disentangle why woody cover is low in areas heavily grazed by wildlife, we investigated (a) whether grazing lawns attract indigenous mammalian browsers, and if a preference for short‐grass habitat decreases with browser body mass as predator susceptibility decreases; and (b) whether browser attraction to grazing lawns translates into the suppression of woody plants, including seedlings and saplings, thus maintaining the openness of heavily grazed short‐grass areas. In Kruger National Park, South Africa, we contrasted browser abundance (using dung counts) on grazing lawns and on low‐herbivory sites characterised by tall grass. Additionally, a herbivore exclosure experiment was set up to investigate the combined impact of browser removal and grass height habitat type on seedling survival and sapling growth of a dominant woody plant species. Finally, in Hluhluwe‐iMfolozi Park (HiP), we examined the long‐term (10 years) impact of browser removal on the growth rates of a range of woody species, monitored across ten sites along a gradient of herbivory ranging from grazing lawn to tall grass. Steenbok and impala selected short‐ over tall grass as preferred browsing sites, while elephant preferred tall grass. Browser abundance on short grass decreased with browser body mass, indicating that predator avoidance might be a key factor driving mesoherbivores to utilise grazing lawns. Seedling survival was lowest on grazing lawns when browsers were present, with mortality occurring in two out of every three seedlings. Similarly, sapling growth was lowest on grazing lawns, although browser removal had no significant effect. Evidence for increased browser impact on grazing lawns was clearest from our long‐term herbivore exclosure experiment in HiP, which demonstrated that browsers strongly modify the growth rates of woody plants in short‐grass habitats. Synthesis. These results provide support for the hypothesis that browsers, particularly browsing mesoherbivores and mixed feeders, are attracted to short‐grass habitats, and that they help maintain grazing lawn openness by suppressing seedling survival and woody plant growth where grass is kept short by grazers.

Probabilistic Mapping and Spatial Pattern Analysis of Grazing Lawns in Southern African Savannahs Using WorldView-3 Imagery and Machine Learning Techniques

Savannah grazing lawns are a key food resource for large herbivores such as blue wildebeest (Connochaetes taurinus), hippopotamus (Hippopotamus amphibius) and white rhino (Ceratotherium simum), and impact herbivore densities, movement and recruitment rates. They also exert a strong influence on fire behaviour including frequency, intensity and spread. Thus, variation in grazing lawn cover can have a profound impact on broader savannah ecosystem dynamics. However, knowledge of their present cover and distribution is limited. Importantly, we lack a robust, broad-scale approach for detecting and monitoring grazing lawns, which is critical to enhancing understanding of the ecology of these vital grassland systems. We selected two sites in the Lower Sabie and Satara regions of Kruger National Park, South Africa with mesic and semiarid conditions, respectively. Using spectral and texture features derived from WorldView-3 imagery, we (i) parameterised and assessed the quality of Random Forest (RF), Support Vector Machines (SVM), Classification and Regression Trees (CART) and Multilayer Perceptron (MLP) models for general discrimination of plant functional types (PFTs) within a sub-area of the Lower Sabie landscape, and (ii) compared model performance for probabilistic mapping of grazing lawns in the broader Lower Sabie and Satara landscapes. Further, we used spatial metrics to analyse spatial patterns in grazing lawn distribution in both landscapes along a gradient of distance from waterbodies. All machine learning models achieved high F-scores (F1) and overall accuracy (OA) scores in general savannah PFTs classification, with RF (F1 = 95.73±0.004%, OA = 94.16±0.004%), SVM (F1 = 95.64±0.002%, OA = 94.02±0.002%) and MLP (F1 = 95.71±0.003%, OA = 94.27±0.003%) forming a cluster of the better performing models and marginally outperforming CART (F1 = 92.74±0.006%, OA = 90.93±0.003%). Grazing lawn detection accuracy followed a similar trend within the Lower Sabie landscape, with RF, SVM, MLP and CART achieving F-scores of 0.89, 0.93, 0.94 and 0.81, respectively. Transferring models to the Satara landscape however resulted in relatively lower but high grazing lawn detection accuracies across models (RF = 0.87, SVM = 0.88, MLP = 0.85 and CART = 0.75). Results from spatial pattern analysis revealed a relatively higher proportion of grazing lawn cover under semiarid savannah conditions (Satara) compared to the mesic savannah landscape (Lower Sabie). Additionally, the results show strong negative correlation between grazing lawn spatial structure (fractional cover, patch size and connectivity) and distance from waterbodies, with larger and contiguous grazing lawn patches occurring in close proximity to waterbodies in both landscapes. The proposed machine learning approach provides a novel and robust workflow for accurate and consistent landscape-scale monitoring of grazing lawns, while our findings and research outputs provide timely information critical for understanding habitat heterogeneity in southern African savannahs.

Vegetation dynamics in the face of a major land-use change: a 30-year case study from semi-arid South Africa

Although wildlife production is widely considered beneficial for semi-arid environments, few studies have reported on the long-term environmental effects of converting from livestock production to game ranching. Asante Sana Game Reserve in South Africa was stocked with domestic livestock for centuries. However, after 1996 game ranching was adopted in the reserve. We investigated the long-term (1987–2017) spatial and temporal change in vegetation productivity and type on the reserve using Landsat multispectral data and soil adjusted vegetation index (SAVI), and explored the relationship between changes in vegetation productivity and the potential drivers. The results show that while Thicket has decreased overall, it has expanded into parts of Grassland and Shrubland, and Bare-ground has expanded into parts of Shrubland and Thicket. Overall vegetation productivity increased over time, with the greatest increases in Thicket and Grazing lawn and the lowest in Shrubland and Bare-ground. Changes in rainfall, fire and stocking rates were all significant predictors of the observed changes in vegetation productivity, but rainfall had the strongest effect. This research highlights some of the complex responses that arise in semi-arid vegetation when domestic livestock production is replaced by game ranching, and points towards key issues that can be addressed by management.

Droughts Decouple African Savanna Grazers from Their Preferred Forage with Consequences for Grassland Productivity

Grazing lawn and flammable-tussock grass communities are contrasting resource pools for mammalian grazers in terms of forage quantity and quality. Drought events fundamentally alter forage availability within these communities and therefore should alter herbivore use with repercussions for the recovery and functioning of ecosystems after drought. During and after an intense El Nino drought (2014–2017) in Kruger National Park, South Africa, we addressed two questions: (1) how does herbivore use of different grass types change during a drought and (2) how do these changes affect grass productivity post-drought? We monitored grazer use of three different grass communities (lawn, tussock and burned-tussock) at a landscape scale and measured primary productivity monthly during and post-drought. For the first drought year, grazer numbers were highest on grazing lawn communities. This pattern continued into the second dry growing season, until herbivores finally left the study area. Both lawns and tussock grasslands recovered rapidly after the first good rainfall (productivity > 150 g m−2 per month). However, grazers did not return to feed on the same patches they had frequented pre-drought resulting in grazing lawn grasses self-shading and senescing. Longer droughts have the potential to decouple grazers and grazing lawns with negative impacts on lawn productivity and persistence that could drive the loss of lawns in savanna landscapes and impact mesoherbivore populations. It is clear from our results that grazer effects need to be incorporated into drought frameworks to understand the consequences of droughts for grassland function.

Geese mediate vegetation state changes with parallel effects on N cycling that leave nutritional legacies for offspring

AbstractAlong the coastal fringe of the Yukon–Kuskokwim River Delta in southwestern Alaska, geese maintain grazing lawns dominated by a rhizomatous sedge that, when ungrazed, transitions to a taller, less palatable growth form that is taxonomically described as a different species. Nutrients recycled in goose feces, in conjunction with grazing, are critical to the rapid, nutritious growth of grazing lawns, and selective foraging on lawns has positive life‐history consequences for goslings. To examine whether bidirectional vegetation shifts were accompanied by parallel changes in N cycling, we studied how 15N‐urea and 13C15N‐glycine were processed through soils and plants of native and recently reverted vegetation states. Biomass and plant 15N uptake from plots reverted to the tall growth form using exclosures and from those shifted to grazing lawns by experimental clipping and then goose grazing were identical to their native counterparts. Total recovery of 15N within the tall vegetation types was significantly greater than within grazing lawns, although when expressed on a per‐gram biomass basis, percentage of 15N recovery was significantly higher in grazing lawns compared with the tall vegetation state. Patterns of 13C enrichment in CO2 soil efflux showed rapid use of 13C‐glycine as a respiratory substrate within the first hour following injection, with both the timing and magnitude of efflux occurring at similar time points for all four vegetation types. However, higher soil respiration rates and a shorter half‐life for 13C‐glycine in soils from tall meadows resulted in a greater proportional loss of 13CO2 compared with grazing lawns. Despite daily‐to‐weekly tidal inundation, all of 15N from labeled substrates could be accounted for within 1 m of the injection grid from soils of both states after 30 d, with significant levels of 15N in soils and vegetation after one year. Geese have remarkably high fidelity to brood‐rearing areas, returning as adults to the same grazing lawns where they were raised as goslings. Our data suggest that the role fecal‐derived nutrients play in the positive feedback loop between geese and their food resources can provide a long‐term legacy that spans generations.

Increased litter decomposition rates of exotic invasive species Hieracium pilosella (Asteraceae) in Southern Patagonia, Argentina

Plant invasions can affect key ecosystem processes regulating carbon and nutrient balances, i.e., litter decomposition. Exotic herb Hieracium pilosella (hawkweed, Asteraceae) recently invaded steppes of Tierra del Fuego island, Southern Patagonia, Argentina, where plant communities exhibit different vulnerabilities to invasion. To investigate H. pilosella invasion’s effects on litter decomposition, we conducted a field experiment in two plant communities, a tussock grassland and a grazing lawn, having contrasting environmental conditions and different vulnerabilities of being invaded by H. pilosella. At each community, we decomposed litter of dominant native species and of the exotic herb during 24 months. In addition, we characterized the litter qualities of the different species, and the microclimates of the communities. Exotic litter decomposition was remarkably faster and showed a higher association to microenvironmental variability compared to native litter decomposition. H. pilosella decomposed three-times faster than native litter in the moister, and more susceptible to invasion by H. pilosella plant community, the tussock grassland. By contrast, decomposition of native litter was not associated to the contrasting environmental conditions of the studied communities. Results suggest that introduced species may exhibit different ecosystem process rates and controls compared to resident species, affecting ecosystem nutrient and carbon cycles.

Grazing by large savanna herbivores indirectly alters ant diversity and promotes resource monopolisation.

In savannas, grazing is an important disturbance that modifies the grass layer structure and composition. Habitat structural complexity influences species diversity and assemblage functioning. By using a combination of natural sites and manipulated experiments, we explored how habitat structure (grazing lawns and adjacent bunch grass) affects ant diversity and foraging behaviour, specifically the efficiency of resource acquisition, resource monopolisation and ant body size. We found that in the natural sites there was no difference in the amount of time ants took to locate resources, but in the manipulated experiments, ants were faster at locating resources and were more abundant in the simple treatments than in the more complex treatments. Ant body size was only affected by the manipulated experiments, with smaller ants found in the more complex treatments. In both the grazing lawn and bunch grass habitats there were differences in assemblage patterns of ants discovering resources and those dominating them. Seasonality, which was predicted to affect the speed at which ants discovered resources and the intensity of resource monopolisation, also played a role. We show that ants in winter monopolised more baits and discovered resources at a slower rate, but only at certain times within the experiment. Grazing in conjunction with season thus had a significant effect on ant diversity and foraging behaviour, with dominant ants promoted where habitat complexity was simplified when temperatures were low. Our results indicate that structural complexity plays a major role in determining ant assemblage structure and function in African savannas.

Ecological engineering through fire‐herbivory feedbacks drives the formation of savanna grazing lawns

AbstractVariation in grass height is beneficial to biodiversity conservation in savanna landscapes. Theory predicts that small fires can promote short‐grass areas within savannas. We experimentally assessed the influence of fire season and size on grass height and the resultant response of wild grazer communities and tested three hypotheses: (1) repeated small fires in tall‐grass savannas increase short‐grass grazer densities in the post‐burn environment; (2) increased grazer densities maintain grass height in a short, palatable state and drive feedbacks that exclude fire; and (3) late‐dry season burns concentrate grazers more effectively than early dry season burns.We repeatedly applied annual treatments (unburned, early‐ and late‐burns) in 0.25‐, 5‐ and 25‐ha plots over a period of 3 years in a tall‐grass savanna system in Kruger National Park, South Africa. Dung counts for grazer density and grass height data were collected along 50‐m transects. Grass height was measured in paired 1‐m2herbivore exclosures on plots before and after applied fires.Dung data indicate that wildebeest occurred most frequently in grass heights below 5 cm. Their preference for plots regardless of fire size or season increased over time with each repeated burn. Zebra and buffalo favoured burns immediately post‐fire, but buffalo did not actively select for burnt areas over longer time periods.By the second year of treatment, herbivory maintained 28% and 91% of the grass height below 10 cm in the early‐ and late‐season burns respectively. In contrast, herbivory on the unburned treatments had no effect on grass height.Synthesis and applications. Fires less than 25 ha in size attracted sufficient grazing herbivores to shorten grass height. Repetition of the fire treatments resulted in the active selection of these areas in the longer term by wildebeest, impala and, to a lesser degree, zebra. Grazing pressure was high enough to initiate positive feedbacks and maintain lawns after only two seasons of burning and, depending on the season of burn, reduced grass height to a level that excluded repeat fires. Our study demonstrated that theory on grazer use of the post‐fire environment can be implemented practically by applying small repeated burns to promote the formation of short‐grass areas within savannas.

Restoring lost ecological function: ecological surrogates facilitate maintenance of coastal turf communities

ABSTRACTLost or extinct ecosystem function presents challenges for managers of natural ecosystems for conservation. One solution is to use ecological surrogates to replace these processes but the settings under which this often controversial approach succeeds are unclear. We tested whether mammalian grazing could be used as a surrogate for grazing by an extinct avian herbivore guild in two threatened coastal turf communities that are structured by an exposure and salt gradient, and vulnerable to non-native encroachment and invasion. Prostrate species had higher cover and species density relative to erect taxa when grazed, with the effect greater for native than non-native taxa. Prostrate species, dominated by native taxa, were most abundant closest to the ocean, with a secondary peak in abundance 25–35 m inland. A positive nutrient feedback mechanism consistent with a grazing lawn was not observed suggesting a primary role for the salinity gradient, augmented by opportunistic grazing of taller species, in structuring these communities. Synthesis and application: mammalian grazers can replace the likely functional role of extinct avian herbivores in turf communities under certain environmental settings.

Grassland structural heterogeneity in a savanna is driven more by productivity differences than by consumption differences between lawn and bunch grasses.

Savanna grasslands are characterized by high spatial heterogeneity in vegetation structure, aboveground biomass and nutritional quality, with high quality short-grass grazing lawns forming mosaics with patches of tall bunch grasses of lower quality. This heterogeneity can arise because of local differences in consumption, because of differences in productivity, or because both processes enforce each other (more production and consumption). However, the relative importance of both processes in maintaining mosaics of lawn and bunch grassland types has not been measured. Also their interplay been not been assessed across landscape gradients. In a South African savanna, we, therefore, measured the seasonal changes in primary production, nutritional quality and herbivore consumption (amount and percentage) of grazing lawns and adjacent bunch grass patches across a rainfall gradient. We found both higher amounts of primary production and, to a smaller extent, consumption for bunch grass patches. In addition, for bunch grasses primary production increased towards higher rainfall while foliar nitrogen concentrations decreased. Foliar nitrogen concentrations of lawn grasses decreased much less with increasing rainfall. Consequently, large herbivores targeted the biomass produced on grazing lawns with on average 75 % of the produced biomass consumed. We conclude that heterogeneity in vegetation structure in this savanna ecosystem is better explained by small-scale differences in productivity between lawn and bunch grass vegetation types than by local differences in consumption rates. Nevertheless, the high nutritional quality of grazing lawns is highly attractive and, therefore, important for the maintenance of the heterogeneity in species composition (i.e. grazing lawn maintenance).Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-016-3698-y) contains supplementary material, which is available to authorized users.

The Importance of Coprophagous Macrodetritivores for the Maintenance of Vegetation Heterogeneity in an African Savannah

Grazing ecosystems are often characterized by dynamic vegetation structure mosaics of short grazing lawns and tall grass vegetation that are important for the biodiversity and functioning of these ecosystems. Grazing-induced trampling, causing soil compaction and reduced water infiltration, has been shown to be an important mechanism for lawn grass formation. However, insights in reverse bioturbation mechanisms were mostly lacking, especially how tall vegetation persists under continuous grazing by herbivores. In this study, we explore if defecation by large herbivores in combination with different groups of coprophagous macrodetritivores can locally convert compacted grazing lawn patches back to tall bunch grasslands with a more loose soil. Across a rainfall gradient in an African savannah, we separated the potential roles in this process between dung beetles versus earthworms and termites. We placed different mesh sizes under dung piles and studied the consequences for soil, vegetation, and hydrological properties. We found that soil water infiltration rate, soil organic matter content, electrical conductivity, bunch grass cover, and bunch grass biomass were significantly promoted by dung addition, irrespective of position along the rainfall gradient. In addition, the presence of tunneling dung beetles significantly increased water infiltration rate and biomass of bunch grasses, pointing at a new mechanism whereby macrodetritivores affect the structure and diversity of plant communities. We conclude that coprophagous macrodetritivores interact with large herbivores in contributing to the maintenance of structural heterogeneity in the vegetation of grazing ecosystems, with a special role played by soil-tunneling dung beetles.

Clipping frequency but not nutrients affect the architecture and non-structural carbohydrates of a browsing lawn

“Grazing lawns” are well-known effects of large mammalian herbivores on a short grass sward that is maintained through a positive feedback effect; plants are re-grazed due to their increased palatability. However, there are few studies that have addressed the notion of “browsing lawns.” I simulated browsing and added nutrients to Acacia nigrescens, a palatable and dominant African tree, to examine the effects on the architecture of these trees in a greenhouse. I recorded investments in non-structural carbohydrates and crude protein between roots and stems as a possible mechanism behind such architectural changes. There was a significant decrease in mean branch internode length and an increase in the branching ratio with clipping; the last mentioned indicated that there was more extensive tree branching. There were no significant effects on plant architecture of nutrient addition. There were changes in non-structural carbohydrates and crude protein between the roots and stems due to clipping but not to nutrient addition. Non-structural carbohydrate concentrations decreased with clipping in the roots and increased in the stems. Crude protein decreased in the stems with clipping frequency but not in the roots. There was an increase in root:shoot ratio of crude protein with increasing clipping frequency. A. nigrescens performs like a browsing lawn because of the changes in tree architecture (mean branch internode length and branching ratio) that are consistent with clipping. Clipping but not nutrient addition was important for these trees. Clipping frequency was important for non-structural carbohydrate redistribution from roots to stems. A positive feedback could have long-term effects on the palatability of these trees which should cause large mammalian herbivores to consistently favor A. nigrescens.