Exclosure Experiment Research Articles

Land use intensification alters the relative contributions of plant functional diversity and soil properties on grassland productivity.

Understanding the mechanisms of grassland productivity variation is critical for global carbon cycling and climate change mitigation. Heretofore, it is unknown how different environmental factors drive small-scale spatial variation in productivity, and whether land use intensification, one of the most important global changes, can regulate the processes that drive productivity change. Here we performed an 18-year exclosure experiment across six sites with high-intensity mowing/grazing history in northern China to examine the effects of land use intensification on plant functional diversity, soil properties, and their relative contributions to above-ground net primary productivity (ANPP). We found that plant functional diversity and soil properties contributed to the variation in ANPP both independently and equally in enclosed grasslands (plant diversity: 20.6%; soil properties: 19.5%). Intensive land use significantly decreased the Rao's quadratic entropy (RaoQ) and community-weighted mean value (CWM) of plant height, and further suppressed the contributions of plant functional diversity to ANPP. In contrast, intensive land use increased soil available N, P, pH, electrical conductivity, and homogeneity of soil available P, and strengthened their contributions to ANPP (31.5%). Our results indicate that high-intensity land use practices in grasslands decrease the role of plant functional diversity, but strengthen the effects of soil properties on productivity. We, therefore, suggest that plant functional diversity can be used effectively to boost productivity in undisturbed grasslands, while soil properties might be a more critical consideration for grassland management in an areas with increased land use.

Wild herbivores enhance resistance to invasion by exotic cacti in an African savanna

Abstract Whether wild herbivores confer biotic resistance to invasion by exotic plants remains a key question in ecology. There is evidence that wild herbivores can impede invasion by exotic plants, but it is unclear whether and how this generalises across ecosystems with varying wild herbivore diversity and functional groups of plants, particularly over long‐term (decadal) time frames. Using data from three long‐term (13‐ to 26‐year) exclosure experiments in central Kenya, we tested the effects of wild herbivores on the density of exotic invasive cacti, Opuntia stricta and O. ficus‐indica (collectively, Opuntia), which are among the worst invasive species globally. We also examined relationships between wild herbivore richness and elephant occurrence probability with the probability of O. stricta presence at the landscape level (6150 km2). Opuntia densities were 74% to 99% lower in almost all plots accessible to wild herbivores compared to exclosure plots. Opuntia densities also increased more rapidly across time in plots excluding wild herbivores. These effects were largely driven by megaherbivores (≥1000 kg), particularly elephants. At the landscape level, modelled Opuntia stricta occurrence probability was negatively correlated with estimated species richness of wild herbivores and elephant occurrence probability. On average, O. stricta occurrence probability fell from ~0.56 to ~0.45 as wild herbivore richness increased from 6 to 10 species and fell from ~0.57 to ~0.40 as elephant occurrence probability increased from ~0.41 to ~0.84. These multi‐scale results suggest that any facilitative effects of Opuntia by wild herbivores (e.g. seed/vegetative dispersal) are overridden by suppression (e.g. consumption, uprooting, trampling). Synthesis. Our experimental and observational findings that wild herbivores confer resistance to invasion by exotic cacti add to evidence that conserving and restoring native herbivore assemblages (particularly megaherbivores) can increase community resistance to plant invasions.

Fire and Herbivory Interactively Suppress the Survival and Growth of Trees in an African Semiarid Savanna

There has been a long-standing interest in understanding how interactions between fire and herbivory influence woody vegetation dynamics in savanna ecosystems. However, controlled, replicated experiments examining how different fire regimes interact with different herbivore groups are rare. We tested the effects of single and repeated burns, crossed with six replicated herbivore treatments, on the mortality and growth of woody vegetation in the Kenya Long-term Exclosure Experiment plots located in a semi-arid savanna system in central Kenya. Burned plots experienced higher tree mortality overall, but differences between burns and non-burns were only significant in plots excluding all wild herbivores and in plots accessible to megaherbivores. Cattle ameliorated the negative effects of repeat burns on tree mortality, perhaps by suppressing fuel load accumulation. Across all herbivore treatments, trees experienced a significant reduction in height within the first two years after fire (top-kill), which was followed by a gradual recovery. Saplings and coppices subjected to repeated burns regrew faster than those that were burned once, except in the presence of megaherbivores. This study highlights strong context-dependent interactions between fire and different herbivore groups, and extends previous approaches to understanding fire–herbivory interactions, which have tended to lump the effects of different herbivore groups, or study them separately.

Trophic cascades in tropical rainforests: Effects of vertebrate predator exclusion on arthropods and plants in Papua New Guinea

AbstractInsect herbivores have the potential to consume large amounts of plant tissue in tropical forests, but insectivorous vertebrates effectively control their abundances, indirectly increasing plant fitness accordingly. Despite several studies already sought understanding of the top‐down effects on arthropod community structure and herbivory, such studies of trophic cascades in old tropics are underrepresented, and little attention was paid to top‐down forces in various habitats. Therefore, we examine how flying insectivorous vertebrates (birds and bats) impact arthropods and, consequently, affect herbivore damage of leaves in forest habitats in Papua New Guinea. In a 3‐month long predator exclosure experiment conducted at four study sites across varying elevation and successional stage, we found that vertebrate predators reduced arthropod density by ∼52%. In addition, vertebrate predators decreased the mean body size of arthropods by 26% in leaf chewers and 47% in non‐herbivorous arthropods but had only a small effect on mesopredators and sap suckers. Overall, the exclusion of vertebrate predators resulted in a ~ 41% increase in leaf damage. Our results, across different types of tropical forests in Papua New Guinea, demonstrate that flying vertebrate insectivores have a crucial impact on plant biomass, create a selective pressure on larger and non‐predatory prey individuals and they prey partition with mesopredators.

Exotic goats do not affect the seed bank but reduce seedling survival in a human-modified landscape of Caatinga dry forest

Livestock browsing and trampling can deplete plant regeneration and thus drive natural ecosystems towards degradation. This phenomenon may be particularly true in tropical dry forests, which support extensive livestock herds feeding on native vegetation, but has rarely been experimentally addressed. Here we examine how browsing and trampling by exotic goats impacts seed bank and seedling survivorship of woody plant species in a Caatinga dry forest by performing a 4-year exclosure experiment. Soil seed bank and transplanted seedlings from four woody plants species were monitored through a year across 14 and 10 paired plots (free goat-access vs exclosure plots), respectively. Regardless of treatment, the soil seed bank was low in density 14 seeds/m2 (±13, standard deviation) and taxonomically impoverished (24 species). Small to intermediate-sized seeds from dry fruits (75%) and abiotically dispersed species (65%) prevailed. The most common attractive structure was an elaiosome (33%), but pulp (14%) and aril (9.5%) were also recorded. 60% of the species presented some type of dormancy and 32% presented a pleurogram. 57% of the seeds were classified as forming a transient seed bank, while seasonally transient seeds accounted for 43% of the total seed bank. Seed bank attributes were not affected by goats, including proportion of viable seeds (25.85 ± 41.3 in exclosure plots vs 31.45 ± 42.49 in free-access plots) and diversity. However, seedling survival one year after transplantation was higher for the plant species in the exclosure plots (68.4%) as compared to the free-access plots (39%). Although there were no significant differences in seed bank, our results suggest that free-ranging goats in densities typical of the Caatinga region can impose elevated seedling mortality to the dominant woody dry forest species even considering a short period of time.

The role of diversity, body size and climate in dung removal: A correlative and experimental approach.

The mechanisms by which climatic changes influence ecosystem functions, that is, by a direct climatic control of ecosystem processes or by modifying richness and trait compositions of species communities, remain unresolved. This study is a contribution to this discourse by elucidating the linkages between climate, land use, biodiversity, body size and ecosystem functions. We disentangled direct climatic from biodiversity-mediated effects by using dung removal by dung beetles as a model system and by combining correlative field data and exclosure experiments along an extensive elevational gradient on Mt. Kilimanjaro, Tanzania. Dung removal declined with increasing elevation, being associated with a strong reduction in the richness and body size traits of dung beetle communities. Climate influenced dung removal rates by modifying biodiversity rather than by direct effects. The biodiversity-ecosystem effect was driven by a change in the mean body size of dung beetles. Dung removal rates were strongly reduced when large dung beetles were experimentally excluded. This study underscores that climate influences ecosystem functions mainly by modifying biodiversity and underpins the important role of body size for dung removal.

Fallow deer foraging alone does not preserve the vegetation of traditionally sheep‐grazed calcareous grasslands

Abstract The goal of this study was to evaluate to what extent wild ungulates (fallow deer) can contribute to the maintenance of semi‐natural calcareous grasslands, which are a threatened habitat type (natura 2000 code *6210). In a 10‐year exclosure experiment, we tested the effects of ungulate foraging using three treatments: (A) control with combined foraging of herded sheep and wild fallow deer, (B) sheep exclosure with only deer foraging and (C) total exclosure with no foraging. Treatments not grazed by sheep (B, C) were characterized by significantly declining species numbers, litter accumulation and shrub encroachment. Despite high population densities, the effect of fallow deer alone (B) was weak: Succession of woody species was only partly inhibited, while annuals, short‐growing and rosette‐building plant species were strongly suppressed by litter accumulation. Only the combination of sheep and fallow deer foraging preserved vegetation structure and species richness and led to a promotion of target species. Synthesis and applications. We conclude that we need to continue the traditional land‐use forms such as sheep grazing in order to maintain calcareous grasslands. However, we should also raise our awareness for wild animals and analyse more in depth their potential contribution to the conservation management of open habitats.

No evidence for the simultaneous induction of structural and chemical defences in spiny southern African savanna trees

AbstractIt is still not well established whether plant chemical and physical defence traits can be simultaneously induced in savanna trees and how this would affect plant nutritional quality. Here, we use a long‐term mammalian herbivore exclosure experiment in a dystrophic semi‐arid African savanna to test how chemical (condensed tannins [CTs] and total polyphenols [TPs]) and structural (spines and thorns) defences are affected by herbivore exclusion in five common savanna woody species. We also tested whether nutritional quality interacted with either physical or chemical defences after protected trees were reintroduced to herbivores. Trees that remained available to herbivores did not increase CTs and TPs, rather these were found to be higher without mammalian herbivory in some species. In contrast, herbivory resulted in a significant induction of structural defences (i.e. longer and thicker spines) and at the same time, improved nutritional quality (i.e. higher available crude protein and lower C:N ratios) for some species. We found that the plant defence traits of the treatment which previously excluded herbivores were mostly similar to the control, which both differed from the exclosure, suggesting that legacies of herbivore exclusion on plant defence traits are mostly short‐lived.

Submerged vegetation colonizes behind artificial wave shelter after a 10-year time-lag and persists under high grazing pressure by waterbirds

Construction of artificial wave shelters is a promising measure to stimulate submerged vegetation in large wind exposed lakes. Here we tested whether the construction of shelter results in the colonization of submerged vegetation and whether grazing by waterbirds hampers vegetation development under those sheltered conditions. We studied the effect of breakwaters that were constructed between 1992 and 1996 in the large (695 km2), wind exposed and turbid Lake Markermeer, The Netherlands. We used monitoring data to evaluate the development of submerged vegetation and its relation with the abundance of herbivorous birds and conducted a field grazer exclosure experiment to determine the effect of grazers on the newly established vegetation. We found that in the sheltered area, a dense charophyte dominated vegetation developed over 16 years, while submerged vegetation remained very sparse outside the breakwaters. The area also attracted many herbivorous birds, especially molting Mute swans during summer. After the breakwaters had been completed, 10 years passed before the development of the charophyte vegetation started. This delay was probably not caused by bird grazing, but more likely due to unfavorable light conditions in the first decade. The exclosure experiment confirmed that while grazing reduced macrophyte biomass by 50% and plant height by 45%, it did not affect vegetation cover, which remained high (90–95%) throughout summer. The water depth in most of the study area exceeded the depth range at which Mute swans prefer to forage and this probably prevented overgrazing. We conclude that building artificial wave shelters is an effective measure to stimulate submerged vegetation in large wind exposed lakes.

Livestock impacts on an iconic Namib Desert plant are mediated by abiotic conditions

Resolving the relative contributions of top-down versus bottom-up drivers of vegetation dynamics is a major challenge in drylands. In the coming decades, growing livestock populations and shifts in water availability will simultaneously impact many arid systems, but a lack of empirical data on plant responses to these pressures limits understanding of how plants will respond. Here, we combine ground and drone observations from an herbivore exclosure experiment to identify ungulate visitation patterns and their impacts on the cover and melon production of !nara (Acanthosicyos horridus), a large, long-lived desert plant in the hyper-arid Namib Desert. !Nara are of key ecological, social, and economic importance to Namib ecosystems and to the local Topnaar people. At our study site, we find that among native and domestic herbivores, free-ranging donkeys have the largest impact on !nara cover and melon production. !Nara cover was negatively affected by herbivores close to the desert-ephemeral river ecotone during a dry period, whereas !nara cover increased on all plants across the landscape during a wetter period, regardless of herbivore access. !Nara near the river channel and those protected from herbivores had more mature melons, particularly during the wetter period. At this site, the potential for conflict between Topnaar !nara melon harvesting and pastoral practices varies with a plant’s distance from the river and prevailing abiotic conditions. This work advances monitoring approaches and adds empirical support to the understanding that top-down and bottom-up regulation of plant dynamics varies with spatiotemporal context, even within landscapes.

Effects of Waterbird Herbivory on Dominant Perennial Herb Carex thunbergii in Shengjin Lake

Abundant food resources in riparian zones provide efficient foraging sites for waterbirds. Herbivory is a key ecosystem process that has widely recognized effects on primary production and vegetation structure and composition. However, there is limited understanding of impacts of waterbird herbivory on riparian zone vegetation. In this study, a bird exclosure experiment with five levels of foraging intensities (no foraging, very little foraging, light foraging, moderate foraging and heavy foraging) was set up in Shengjin Lake to study the effects of waterbird foraging on the community structure of sedge meadows and individual traits of the dominant plant Carex thunbergii. Foraging intensity had little effect on community structure. The dry mass of C. thunbergii decreased with the increasing foraging time. Waterbird foraging reduced leaf dry mass under heavy foraging by 27.7% and root dry mass by 45.6% compared to CK (no foraging). Waterbird foraging increased allocation to shoot growth but had a weak effect on elemental allocation of C. thunbergii. The foraging intensity significantly affected the morphological traits of C. thunbergii. The results of structural equation modeling showed that RSR (root: shoot ratio represents the ratio of dry mass) and RL (root length) are key traits in driving the dry mass decline in the presence of bird foraging. This study may contribute to a better understanding of the adaptability of perennial herb plants to waterbird foraging and maintain the healthy development of wetland ecosystems.

Large herbivore impact on plant biomass along multiple resource gradients in the Serengeti

Abstract Herbivores form an important link in the transfer of energy within a food web and are strongly influenced by bottom‐up trophic cascades. Current hypotheses suggest that herbivore consumption and impact on plants should scale positively with plant resource availability. However, depending on the effect of resources on plant quantity and quality, herbivore impact may vary with different types of resources. We test four alternative hypotheses for the relationship between plant biomass, herbivore impact on plant biomass and plant resource gradients, each based on how resources might affect plant abundance and quality to herbivores. We measured plant biomass for four non‐consecutive years in a long‐term grazing exclosure experiment in the Serengeti National Park that includes seven sites that vary substantially in rainfall and soil and plant nitrogen (N) and phosphorus (P). Our data supported the hypothesis that herbivore impact is controlled by plant quality, in this case driven by plant P, as herbivore effects on biomass decreased with higher rainfall but increased with greater plant P, but not N content. To our knowledge, this is the first experimental study to indicate that wild mammalian herbivory is associated with P availability rather than N. Synthesis. Our results suggest that P, in addition to water and N, may play a more important role in driving trophic interactions in terrestrial systems than previously realized. Given the uncertainties in rainfall due to climate change and increasing anthropogenic manipulations of global N and P cycles, our findings emphasize the need to consider multiple resources for understanding how trophic interactions might be influenced by environmental variables.

The effects of livestock grazing on vegetation in a semiarid grassland: a test of three hypotheses

AbstractQuestionsLivestock grazing is an important driver of plant diversity in grasslands. Different mechanisms summarized in three main hypotheses have been proposed to explain its varying effects depending on productivity. The insufficient compensation hypothesis (ICH) states that livestock causes the extinction of species that cannot compensate for biomass removal in unproductive sites, reducing species richness. The competitive release hypothesis (CRH) states that livestock increases plant diversity by attenuating competition, especially in productive environments. In the convergent evolution hypothesis (CEH), livestock has minor effects on diversity in unproductive sites because plants that endure stress are pre‐adapted to grazing. These hypotheses have rarely been compared formally, and have mostly been tested on large spatial scales. Here we simultaneously test these hypotheses on a local scale.LocationNatural semi‐arid grassland, Oaxaca, Mexico.MethodsWe performed a long‐term exclosure experiment, recording plant diversity annually over 17 years. In 2017 we measured species richness over a grazing intensity gradient. We used linear mixed‐effects models, nestedness and beta‐diversity analyses to evaluate diversity and composition in relation to grazing and productivity.ResultsPlant diversity generally increased with grazing intensity. In line with CRH, species richness increased as grazing became more intensive due to a progressive incorporation of increasingly poor competitors to the community, with a seemingly stronger effect under high productivity. This suggests that herbivores limit competitive species and prevent competitive exclusion. ICH may play a secondary role because the effect was reversed in low productivity inside exclosures.ConclusionsOur finding implies that in productive sites, the exclusion of livestock can be detrimental to plant diversity as a lack of grazing accelerates competitive exclusion. By contrast, in limiting environments, grazing can preserve diversity if livestock are kept at low stocking densities. Our results show that small‐scale variations in productivity need to be considered when designing livestock management.

Waterfowl grazing on winter wheat: Quantifying yield loss and compensatory growth

Herbivorous waterfowl populations have increasingly adapted to forage on agricultural land and triggered a parallel rise in damage to agricultural crops. Winter wheat (Triticum aestivum) is the most commonly reported crop damaged in northern Europe, and farmers increasingly demand management actions to mitigate economic impacts. Here, we apply two parallel approaches (exclosure experiments in undisturbed fields and yield assessments in fields subject to scaring) to quantify the impact of waterfowl grazing on winter wheat yield, crop height and nutrient content. We demonstrate that waterfowl grazing led to a substantial reduction in crop height during winter and early spring, but also that compensatory growth led to no significant effect on crop height at the time of harvest. In terms of grain yield, the effect of waterfowl grazing varied from non-significant to a 6% loss, and on fields subject to scaring, the negative effect of grazing was only significant when grazing continued into spring (the case for 7.6% of the area sampled). In addition, the exclosure experiments indicated that plots grazed both winter and spring had a protein content 9% lower than ungrazed plots. While these figures express an economic loss to the affected farmers, our findings also indicate that substantial economic impacts from grazing waterfowl on winter wheat was rare in our study area. This may also be the case elsewhere in the temperate region, when most waterfowl abandon crop foraging in due time to allow for compensatory growth in late spring, and as long as the early developmental stages survive the early impacts from grazing. A substantial scaring effort had only a limited effect on grain yield loss when fields were grazed during spring, but seemed to prevent damage on fields that were grazed only during winter. Future decision-making in relation to the waterfowl-agriculture conflict may benefit from studies looking in-depth at the costs and benefits associated with scaring efforts.

At high stocking rates, cattle do not functionally replace wild herbivores in shaping understory community composition.

Over a quarter of the world's land surface is grazed by cattle and other livestock, which are replacing wild herbivores, potentially impairing ecosystem structure, and functions. Previous research suggests that cattle at moderate stocking rates can functionally replace wild herbivores in shaping understory communities. However, it is uncertain whether this is also true under high stocking rates and the effects of wild herbivore on plant communities are moderate, enhanced, or simply additive to the effects of cattle at high stocking rates. To evaluate the influence of cattle stocking rates on the ability of cattle to functionally replace wild herbivores and test for interactive effects between cattle and wild herbivores in shaping understory vegetation, we assessed herbaceous vegetation in a long-term exclosure experiment in a semi-arid savanna in central Kenya that selectively excludes wild mesoherbivores (50-1000 kg) and megaherbivores (elephant and giraffe). We tested the effects of cattle stocking rate (zero/moderate/high) on herbaceous vegetation (diversity, composition, leafiness). We also tested how those effects depend on the presence of wild mesoherbivores and megaherbivores. We found that herbaceous community composition (primary ordination axis) was better explained by the presence/absence of herbivore types than by total herbivory, suggesting that herbivore identity is a more important determinant of community composition than total herbivory at high cattle stocking rates. The combination of wild mesoherbivores and cattle stocked at high rates led to increased bare ground and annual grass cover, reduced perennial grass cover and understory leafiness, and enhanced understory diversity. These shifts were weaker or absent when cattle were stocked at high stocking rates in the absence of wild mesoherbivores. Megaherbivores tempered the effects of cattle stocked at high rates on herbaceous community composition but amplified the effects of high cattle stocking rate on bare ground and understory diversity. Our results show that cattle at high stocking rates do not functionally replace wild herbivores in shaping savanna herbaceous communities contrary to previous findings at moderate stocking rates. In mixed-use rangelands, interactions between cattle stocking rate and wild herbivore presence can lead to non-additive vegetation responses with important implications for both wildlife conservation and livestock production.

Less Is More: Lowering Cattle Stocking Rates Enhances Wild Herbivore Habitat Use and Cattle Foraging Efficiency

Over a quarter of the world’s land surface is grazed by cattle and other livestock, which are replacing wild herbivores and widely regarded as drivers of global biodiversity declines. The effects of livestock presence versus absence on wild herbivores are well documented. However, the environmental context-specific effects of cattle stocking rate on biodiversity and livestock production are poorly understood, precluding nuanced rangeland management recommendations. To address this, we used a long term exclosure experiment in a semi-arid savanna ecosystem in central Kenya that selectively excludes cattle (at different stocking rates), wild mesoherbivores, and megaherbivores. We investigated the individual and interactive effects of cattle stocking rate (zero/moderate/high) and megaherbivore (>1,000 kg) accessibility on habitat use (measured as dung density) by two dominant wild mesoherbivores (50–1,000 kg; zebraEquus quaggaand elandTaurotragus oryx) across the “wet” and “dry” seasons. To explore potential tradeoffs or co-benefits between cattle production and wildlife conservation, we tested for individual and interactive effects of cattle stocking rate and accessibility by wild mesoherbivores and megaherbivores (collectively, large wild herbivores) on the foraging efficiency of cattle across both seasons. Eland habitat use was reduced by cattle at moderate and high stocking rates across both dry and wet seasons and regardless of megaherbivore accessibility. We observed a positive effect of megaherbivores on zebra habitat use at moderate, but not high, stocking rates. Cattle foraging efficiency (g dry matter step–1min–1) was lower in the high compared to moderate stocking rate treatments during the dry season, and was non-additively reduced by wild mesoherbivores and high cattle stocking rates during the wet season. These results show that high stocking rates are detrimental to wild mesoherbivore habitat use and cattle foraging efficiency, while reducing to moderate stocking rates can benefit zebra habitat use and cattle foraging efficiency. Our findings demonstrate that ecosystem management and restoration efforts across African rangelands that involve reducing cattle stocking rates may represent a win-win for wild herbivore conservation and individual performance of livestock.

Soil bacterial and fungal communities are linked with plant functional types and soil properties under different grazing intensities

AbstractAs a grassland management strategy in the typical steppe, livestock grazing can affect soil microbes and ecosystem functions via vegetation removal, trampling and manure deposition. However, the effects of grazing and its main consequences (e.g. the shifts in plant functional types and alterations in soil properties) on soil bacterial and fungal community composition remain unresolved. Taking advantage of a 5‐year grazing exclosure experiment that tested different grazing intensities [control: no grazing (G0); light: 0.75 sheep ha−1 (G1); moderate: 1.50 sheep ha−1 (G2); heavy: 2.25 sheep ha−1 (G3); and overgrazing: 3.00 sheep ha−1 (G4)], we examined grazing‐related changes in plant communities, soil chemistry and soil microbial communities. Grazing intensity affected the diversity and composition of soil bacterial and fungal communities via changes in plant species and soil nutrient contents. Increased grazing intensity reduced soil organic carbon (OC), suppressed the dominant bacterial phyla Proteobacteria and Bacteriodetes, and increased the abundance of oligotrophic bacteria in subordinate phyla (e.g. Verrucomicroba and Chloroflexi). Increased grazing intensity also promoted the dominant fungal phylum Ascomycetes because of the poor‐nutrient soil environment. The decreased soil carbon‐to‐nitrogen (C/N) ratio in the G2 treatment was associated with an effect on the fungal community composition. Structural equation modelling demonstrated that responses of the bacterial community to grazing intensity were related to changes in soil OC and soil pH, whereas the combination of C4 plant community biomass and soil pH drove changes in the fungal community. Our study provides novel evidence that grazing affects plant functional group and soil chemistry through the soil pH and C/N ratio, which in turn affects bacterial and fungal communities.Highlights Heavy and overgrazing suppressed bacterial phylum Proteobacteria and increased fungal phylum Ascomycete. Soil OC was the predominant driver of changes in bacterial richness and community composition. C4 plant community biomass modulated fungal richness and community composition.

Large herbivores facilitate an insect herbivore by modifying plant community composition in a temperate grassland.

Large herbivores often co‐occur and share plant resources with herbivorous insects in grassland ecosystems; yet, how they interact with each other remains poorly understood. We conducted a series of field experiments to investigate whether and how large domestic herbivores (sheep; Ovis aries) may affect the abundance of a common herbivorous insect (aphid; Hyalopterus pruni) in a temperate grassland of northeast China. Our exclosure experiment showed that 3 years (2010–2012) of sheep grazing had led to 86% higher aphid abundance compared with ungrazed sites. Mechanistically, this facilitative effect was driven by grazing altering the plant community, rather than by changes in food availability and predator abundance for aphids. Sheep significantly altered plant community by reducing the abundance of unpalatable forbs for the aphids. Our small‐scale forb removal experiment revealed an “associational plant defense” by forbs which protect the grass Phragmites australis from being attacked by the aphids. However, selective grazing on forbs by sheep indirectly disrupted such associational plant defense, making P. australis more susceptible to aphids, consequentially increasing the density of aphids. These findings provide a novel mechanistic explanation for the effects of large herbivores on herbivorous insects by linking selective grazing to plant community composition and the responses of insect populations in grassland ecosystems.

Experimental evidence for opposing effects of high deer density on tick-borne pathogen prevalence and hazard

BackgroundIdentifying the mechanisms driving disease risk is challenging for multi-host pathogens, such as Borrelia burgdorferi sensu lato (s.l.), the tick-borne bacteria causing Lyme disease. Deer are tick reproduction hosts but do not transmit B. burgdorferi s.l., whereas rodents and birds are competent transmission hosts. Here, we use a long-term deer exclosure experiment to test three mechanisms for how high deer density might shape B. burgdorferi s.l. prevalence in ticks: increased prevalence due to higher larval tick densities facilitating high transmission on rodents (M1); alternatively, reduced B. burgdorferi s.l. prevalence because more larval ticks feed on deer rather than transmission-competent rodents (dilution effect) (M2), potentially due to ecological cascades, whereby higher deer grazing pressure shortens vegetation which decreases rodent abundance thus reducing transmission (M3).MethodsIn a large enclosure where red deer stags were kept at high density (35.5 deer km−2), we used an experimental design consisting of eight plots of 0.23 ha, four of which were fenced to simulate the absence of deer and four that were accessible to deer. In each plot we measured the density of questing nymphs and nymphal infection prevalence in spring, summer and autumn, and quantified vegetation height and density, and small mammal abundance.ResultsPrevalence tended to be lower, though not conclusively so, in high deer density plots compared to exclosures (predicted prevalence of 1.0% vs 2.2%), suggesting that the dilution and cascade mechanisms might outweigh the increased opportunities for transmission mechanism. Presence of deer at high density led to shorter vegetation and fewer rodents, consistent with an ecological cascade. However, Lyme disease hazard (density of infected I. ricinus nymphs) was five times higher in high deer density plots due to tick density being 18 times higher.ConclusionsHigh densities of tick reproduction hosts such as deer can drive up vector-borne disease hazard, despite the potential to simultaneously reduce pathogen prevalence. This has implications for environmental pathogen management and for deer management, although the impact of intermediate deer densities now needs testing.Graphical abstract

Large herbivores suppress liana infestation in an African savanna

African savannas are the last stronghold of diverse large-mammal communities, and a major focus of savanna ecology is to understand how these animals affect the relative abundance of trees and grasses. However, savannas support diverse plant life-forms, and human-induced changes in large-herbivore assemblages-declining wildlife populations and their displacement by livestock-may cause unexpected shifts in plant community composition. We investigated how herbivory affects the prevalence of lianas (woody vines) and their impact on trees in an East African savanna. Although scarce (<2% of tree canopy area) and defended by toxic latex, the dominant liana, Cynanchum viminale (Apocynaceae), was eaten by 15 wild large-herbivore species and was consumed in bulk by native browsers during experimental cafeteria trials. In contrast, domesticated ungulates rarely ate lianas. When we experimentally excluded all large herbivores for periods of 8 to 17 y (simulating extirpation), liana abundance increased dramatically, with up to 75% of trees infested. Piecewise exclusion of different-sized herbivores revealed functional complementarity among size classes in suppressing lianas. Liana infestation reduced tree growth and reproduction, but herbivores quickly cleared lianas from trees after the removal of 18-y-old exclosure fences (simulating rewilding). A simple model of liana contagion showed that, without herbivores, the long-term equilibrium could be either endemic (liana-tree coexistence) or an all-liana alternative stable state. We conclude that ongoing declines of wild large-herbivore populations will disrupt the structure and functioning of many African savannas in ways that have received little attention and that may not be mitigated by replacing wildlife with livestock.