Search

Many savanna-dependent species in Africa including large herbivores and apex predators are at increasing risk of extinction. Achieving effective management of protected areas (PAs) in Africa where lions live will cost an estimated US$ >1–2 billion/year in new funding. We explore the potential for fire-management-based carbon financing programs to fill this funding gap and benefit degrading savanna ecosystems. We demonstrate how introducing early dry season fire management programs could produce potential carbon revenues (PCRs) from either a single carbon financing method (avoided emissions) or from multiple sequestration methods ranging from US$ 59.6–655.9 million/year (at US$ 5/ton) or US$ 155.0 million/year to US$ 1.7 billion/year (at US$ 13/ton). We highlight variable but significant PCRs for savanna PAs from US$ 1.5–44.4 million/year per PA. We suggest investing in fire management programs to jump-start the United Nations Decade of Ecological Restoration to help restore degraded African savannas and conserve imperiled keystone herbivores and apex predators.

Variation in defense traits likely depends on access to different resources and risk from herbivory. Plant defense theories have predicted both positive and negative associations between defense traits and access to resources, but relatively few studies have explored intraspecific variation in defense traits along multiple resource and mammalian herbivory risk gradients. We assessed relationships between herbivory intensity, multiple resources, and plant defense traits using a widely distributed tropical savanna herb,Solanum incanum. As independent measures of risk from large mammal herbivores are rare, we used a satellite-based vegetation index to predict herbivory intensity at the landscape scale. We found that the satellite-based estimate of herbivory intensity was positively associated with browser abundance and total soil P, but negatively associated with rainfall. Intraspecific defense traits too varied substantially across sites (n=43) but only variation in spine density was associated with herbivory intensity and plant resources, such that spine density was positively associated with both rainfall and soil P, but bimodally associated with herbivory intensity. Taken together, it suggests that defenses maybe favored either where resources for defense are abundant under low but still present risk (i.e, at high rainfall sites) or where resource-expensive plant tissue is at high risk (i.e, at high soil P sites). This hints at the possibility of a shift from a resource-associated (bottom-up) to an herbivory-associated (top-down) control of allocation to defenses along an environmental gradient. Additionally, the independent effect of soil P on a carbon-based defense, spine density, suggests potential for resources that are not components of defenses to also influence allocation to defense traits. Thus, our study provides evidence for the influence of multiple drivers, resources, and herbivory intensity, on anti-herbivore defenses and their shifting relative importance on allocation to defenses along an environmental gradient.

QUESTIONS: Grasslands support ecosystem services, promote diversity, and assist in carbon sequestration. However, grasslands worldwide are diminishing in area, and understanding the drivers shaping the remaining grasslands is critical for their maintenance. The North American tallgrass prairie covers approximately 13% of its historical range and is shaped by fire and herbivory. Fire frequency negatively correlates with plant species richness, while bison (Bos bison) — the historical grazers — offset this effect. However, bison populations have declined, and large browsers are increasing in density. Few studies though have examined the role of large browsers — particularly white‐tailed deer (Odocoileus virginianus) — and their interaction with fire frequency in tallgrass prairies. Here, we addressed two questions: (a) What are the impacts of deer on plant diversity, species identities, and relative abundances; and (b) is there an interactive effect between the pressures exerted by deer and the well‐documented effects of fire in driving plant community responses? LOCATION: This study took place at the Konza Prairie Biological Station in northeastern Kansas, USA. METHODS: Using a 22‐year deer exclosure experiment, we examined differences in plant species richness, evenness, and plant community composition between plots that were either accessible or inaccessible to deer, in areas burned annually or once every four years. RESULTS: We did not find significant effects of deer or interactive effects between deer and burning frequency on any metric of the plant community measured, including plant species richness, evenness, and plant community composition. CONCLUSIONS: Contrary to the impact that deer have in other ecosystems (e.g. forests), our results indicate that deer do not affect the plant community of herbaceous‐dominated tallgrass prairies. These results indicate that while the loss of bison‐grazers has shifted tallgrass prairie plant communities to C₄ grass‐dominated systems, the shift to browsing‐dominated herbivore pressure from deer has a minimal effect on the plant community.

Herbivory is a major energy transfer within ecosystems; an open question is under what circumstances it can stimulate aboveground seasonal primary production. Despite multiple field demonstrations, past theory considered herbivory as a continuous process and found stimulation of seasonal production to be unlikely. Here, we report a new theoretical model that explores the consequences of discrete herbivory events, or episodes, separated in time. We discovered that negative density (biomass) dependence of plant growth, such as might be expected from resource limitation of plant growth, favors stimulation of seasonal production by infrequent herbivory events under a wide range of herbivory intensities and maximum plant relative growth rates. Results converge to those of previous models under repeated, short‐interval herbivory, which generally reduces seasonal production. Model parameters were estimated with new and previous data from the Serengeti ecosystem. Patterns of observed frequent and large magnitude stimulated production in these data agreed generally with those predicted by the episodic herbivory model. The model thus may provide a new framework for evaluating the sustainability and impact of herbivory., The issue of whether herbivores can stimulate plant production remains controversial. Here we develop a theoretical framework for understanding how discrete or episodic herbivory events can stimulate plant production under a wide variety of conditions without increasing nutrient availability. The model predictions, based on new data for relative growth rates of grassland plants, agree reasonably well with previously published data for grazing impacts on above‐ground production in the Serengeti.

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. 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.

Declining biodiversity represents one of the most dramatic and irreversible aspects of anthropogenic global change, yet the ecological implications of this change are poorly understood. Recent studies have shown that biodiversity loss of basal species, such as autotrophs or plants, affects fundamental ecosystem processes such as nutrient dynamics and autotrophic production. Ecological theory predicts that changes induced by the loss of biodiversity at the base of an ecosystem should impact the entire system. Here we show that experimental reductions in grassland plant richness increase ecosystem vulnerability to invasions by plant species, enhance the spread of plant fungal diseases, and alter the richness and structure of insect communities. These results suggest that the loss of basal species may have profound effects on the integrity and functioning of ecosystems.

SUMMARYLions (Panthera leo) in Africa have lost nearly half their population in just the last two decades, and effective management of the protected areas (PAs) where lions live will cost an estimated USD >$1 B/year in new funding. We explore the potential for launching a fire management and habitat restoration carbon-financing program to help fill this PA management funding gap. We demonstrate how introducing early dry season fire management programs could produce potential carbon revenues (PCR) from either a single carbon-financing method (avoided emissions) or from multiple sequestration methods of USD $59.6-$655.9 M/year (at USD $5/ton) or USD $155.0 M–$1.7 B/year (at USD $13/ton). We highlight variable but significant PCR for PAs with the greatest potential for restoring lion numbers between USD $1.5–$44.4 M per PA. We suggest investing in lion-centric fire management programs to jump-start the United Nations Decade of Ecological Restoration and help preserve African lions across their range.SCIENCE FOR SOCIETYThe United Nation’s recently launched the Decade of Ecological Restoration in response to planet-wide land degradation. This study analyses the potential for savanna fire management programs to restore fire regimes that can generate new sources of revenue from carbon financing for chronically under-funded protected areas in Africa with lions, as lions are a key indicator of savanna ecosystem health. We estimated the amount of carbon saved by shifting fires that normally burn in the late dry season (and emit more carbon) to the early dry season (that accrue more carbon in the soil and woody vegetation). Based on current carbon market values we found substantial potential to eliminate or significantly reduce the $>1B annual funding gap needed to save the lion from extinction. Given additional benefits for nature and people from new savanna fire programs, we recommend integrated conservation and development projects direct more funding to some of the least developed countries with high biodiversity and support fire management programs in Africa.

Substantial evidence suggests plants and herbivores are limited by multiple resources but their role in driving plant-herbivore interactions is still poorly understood. Here we model multiple resource limitation of plants and herbivores and derive analytical solutions for steady-state plant biomass, herbivore biomass and herbivore impact. The model predicts “apparent” limitation of herbivore biomass by resources that otherwise do not limit herbivore growth. Consequently, higher supply of plant-growth limiting resources allows herbivores to persist at lower supplies of herbivore growth-limiting resources. Likewise, increased supply of these non-limiting resources to herbivores can dramatically increase herbivore impacts on plants. Additionally, the outcomes of herbivore exclusion experiments should differ along different resource gradients, depending on herbivore response to plant resource concentrations. Analysis of existing and new data from marine, freshwater and terrestrial systems supports several of these predictions. Our analysis expands ecologists’ understanding of plant-herbivore dynamics to accommodate multiple limiting resources.

Arbuscular mycorrhizal fungi (AM fungi) and soil organic matter (SOM) can be important factors in soil fertility, cycling of nutrients, and plant productivity. It is still unclear whether greater AM fungi abundance is advantageous for plant productivity under nutrient-poor tropical soils despite the relatively common lack of phosphorus (P) and the purported benefit of AM fungi in obtaining and exchanging P with plants for carbon. We explored whether AM fungi and/or SOM augmented plant productivity in different field soils to test the hypotheses that AM fungi were important contributors to plant productivity and that the contribution by AM fungi is higher on soils with lower organic matter and presumably lower nutrient availability compared to soils with higher organic matter. We conducted a factorial experiment in the greenhouse with potted soils of either high or low organic matter (SOM) collected from each of three different land uses, grazed by wildlife in a protected area (Serengeti National Park, Tanzania), grazed by livestock, and cropland. Half the soils were sterilized to remove soil microbes, including AM fungi. Two grass species, Zea mays and Themeda triandra, were grown for 12 weeks in 8 replicates of each soil type and sterilization treatment. About 52.4% and 62.6% of Z. mays roots grown in non-sterilized soils were colonized by AM fungi in low and high SOM, respectively, and 38.1% and 46.7% of T. triandra roots grown in non-sterilized soils were colonized by AM fungi in low and high SOM respectively. Overall, the production of both plant species was significantly higher on control soils than sterilized soils, indicating that AM fungi likely contributed to productivity, and on soils with higher SOM. However, the separate contribution to the productivity of SOM and soil microbes varied significantly among plant species and soils from different land uses. Zea mays productivity increased most strongly to higher SOM, and declined with sterilization in agricultural, but not livestock or wildlife grazed soils. In contrast, T. triandra production was largely insensitive to SOM or sterilization except on wildlife-grazed soils, where it increased most strongly in unsterilized soils. Soil microbe impacts on productivity, therefore, may be driven more by host plant species than by lower nutrient supply, as associated with lower SOM. Furthermore, the results suggest that efforts to enhance productivity in uncultivated lands should perhaps focus on altering plant species composition, while efforts to enhance productivity in agriculture soils might not depend on beneficial soil microbes or additional fertilizer but instead on effective crop rotations to reduce soil pathogens.

Ubiquitous declines in biochemical reaction rates above optimal temperatures (Topt) are normally attributed to enzyme state changes, but such mechanisms appear inadequate to explain pervasive Topt well below enzyme deactivation temperatures (Tden). Here, a meta-analysis of 92 experimental studies shows that product formation responds twice as strongly to increased temperature than diffusion or transport. This response difference has multiple consequences for biochemical reactions, such as potential shifts in the factors limiting reactions as temperature increases and reaction-diffusion dynamics that predict potential product inhibition and limitation of the reaction by entropy production at temperatures below Tden. Maximizing entropy production by the reaction predicts Topt that depend on enzyme concentration and efficiency as well as reaction favorability, which are patterns not predicted by mechanisms of enzyme state change. However, these predictions are strongly supported by patterns in a meta-analysis of 121 enzyme kinetic studies. Consequently, reaction-diffusion thermodynamics and entropy production may constrain organism performance at higher temperatures, yielding temperature optima of life that may depend on reaction characteristics and environmental features rather than just enzyme state changes.

Understanding the diversity and community structure of arbuscular mycorrhizal fungi (AMF) is important for potentially optimizing their role in the functioning of terrestrial ecosystems. Land use can have profound effects on AMF abundance, productivity, and species composition, but such effects have been explored less in tropical landscapes where agriculture is often less intensive and wildlife conservation can be predominant. Here, we conducted a landscape-scale investigation to quantify the effects of land uses on the AMF diversity in the Greater Serengeti Ecosystem, Tanzania. Abundance, productivity, and diversity of the spores of AM fungal species were determined from field soils by morphological techniques. A total of 315 AM fungal spores belonging 9 species and four families were recorded. Land use had a significant (P AM fungal families exhibited significant differences in their abundance among land use types. While Glomeraceae had high abundance under all land uses, Gigasporaceae and Scutellosporaceae exhibited the highest abundance in wildlife grazed sites and Acaulosporaceae had high abundance in livestock-grazed and crop agriculture sites. Species composition within families was mostly similar across land uses, with the exception of Scutellospora sinuosum and Acaulospora denticulata, which exhibited much greater abundance in wildlife grazed sites than in livestock-grazed or crop agriculture sites. These results suggest that protected area grassland sites subject to wildlife grazing held a more abundant, diverse, and compositionally distinct AM fungal community than both livestock-grazed and crop agriculture sites, even after accounting for the influence of land use on soil N, Organic carbon, P and pH that affect AM fungi. From the perspective of sustainable production, it is very important to ascertain tillage practices and moderate grazing that can promote mycorrhizal diversity and, therefore, contribute to the long-term sustainability of agro-pastoral ecosystems under tropical conditions.

Increasing pressure for communities to conserve wildlife in mixtures with livestock faces scepticism about whether such management is sustainable. The study by Sitters et al. shows that wildlife–livestock coexistence may be sustainable, but only if megaherbivores are included.

Grass roots can harbor abundant endophytic N 2 -fixing microbes (diazotrophs), but their abundance and activity compared to those on legumes and in soil crusts is still unknown. Here, in a natural ecosystem, the Serengeti of East Africa, we explored whether herbivores and soil nutrients limited grass root endophyte diazotroph abundance and their root mass-specific and area-specific N 2 -fixation, as they often do for diazotrophs symbiotic with legumes and those free-living in soil. N 2 -fixation and copy number of the nitrogenase gene nifH was measured with stable isotope and molecular methods, respectively, for the dominant grass Themeda triandra , and legume, Indigofera volkensii , and in the top 5 cm of soil in a 16-year herbivore exclosure experiment across four sites that varied in mean annual rainfall and soil N, P, and moisture. T. triandra nifH gene copy number was highly variable across sites and individuals but often approached or exceeded that of I. volkensii roots and soils. T. triandra roots generally exhibited lower root mass-specific N 2 -fixation (activity), which was not reduced by herbivores and increased in drier soils. In contrast, I. volkensii activity was only reduced by herbivores and soil diazotrophs were mostly inactive. T. triandra exhibited greater area-specific N 2 -fixation than I. volkensii , due to its much greater root biomass, but this difference was reduced by herbivores. Grass-associated endophytic diazotrophs may fix far more N 2 in natural systems than previously realized, and may be limited by different factors those affecting symbiotic legume and free-living soil diazotrophs.

Arbuscular mycorrhizal fungi (AM fungi) and soil organic matter (SOM) can be important factors in soil fertility, cycling of nutrients, and plant productivity. It is still unclear whether greater AM fungi abundance is advantageous for plant productivity under nutrient-poor tropical soils despite the relatively common lack of phosphorus (P) and the purported benefit of AM fungi in obtaining and exchanging P with plants for carbon. We explored whether AM fungi and/or SOM augmented plant productivity in different field soils to test the hypotheses that AM fungi were important contributors to plant productivity and that the contribution by AM fungi is higher on soils with lower organic matter and presumably lower nutrient availability compared to soils with higher organic matter. We conducted a factorial experiment in the greenhouse with potted soils of either high or low organic matter (SOM) collected from each of three different land uses; grazed by wildlife in a protected area (Serengeti National Park, Tanzania), grazed by livestock, and cropland. Half the soils were sterilized to remove soil microbes, including AM fungi. Two grass species, Zea mays and Themeda triandra , were grown for 12 weeks in 8 replicates of each soil type and sterilization treatment. About 52.4% and 62.6% of Z. mays roots grown in non-sterilized soils were colonized by AM fungi in low and high SOM, respectively, and 38.1% and 46.7% of T. triandra roots grown in non-sterilized soils were colonized by AM fungi in low and high SOM respectively. Overall, plant biomass of both plant species was significantly higher on control soils than sterilized soils, indicating that AM fungi likely contributed to productivity, and on soils with higher SOM. However, the separate contribution to productivity of SOM and soil microbes varied significantly among plant species and soils from different land uses . Z. mays productivity increased most strongly to higher SOM, and declined with sterilization in agricultural, but not livestock or wildlife grazed soils. In contrast, T triandra production was largely insensitive to SOM or sterilization except on wildlife-grazed soils, where it increased most strongly in unsterilized soils. AM fungi impacts on productivity therefore may be driven more by host plant species than by lower nutrient supply, as associated with lower SOM. Furthermore, efforts to enhance productivity in uncultivated lands should perhaps focus on altering plant species composition, while efforts to enhance productivity in agriculture soils might not depend on beneficial soil microbes or additional fertilizer but instead on effective crop rotations.

Mammalian herbivores prefer burned areas and this attraction has largely been attributed to the increased nutrient content of the postfire green flush and more recently to the avoidance of predators. However, alternative reasons for this attraction could be: (i) to avoid disease carrying and behaviour changing invertebrates; (ii) because burned areas are warmer microclimates; or (iii) to obtain minerals from the ash. This study tests for differences in tick and fly (Diptera) numbers between burned and unburned areas in Serengeti National Park, Tanzania. It also tests for differences in ground and air column temperatures between burned and unburned areas and for differences in the mineral content of ash in burned areas compared to the mineral content of green leaves in unburned areas. We found no difference in the abundance of either type of invertebrate between burned and unburned areas. Only ground temperature was higher in burned areas and this was only during the middle of the day, when increases in temperature would be less important than at night. Ash was higher in Al, Ca, Cu, Mg, Mn and P than nearby green leaves from unburned vegetation. Thus, obtaining minerals from ash is the only alternative reason for attraction to burned areas that maybe supported by this study. Resume Les mammiferes herbivores preferent les zones brulees, et cet attrait a ete en grande partie attribue a l'augmentation du contenu en nutriments de la repousse verte qui suit les feux et, plus recemment, a la possibilite d'y detecter les predateurs. Pourtant, il pourrait exister d'autres raisons a cet attrait: (i) pour eviter les invertebres qui transportent des maladies et dont le comportement change; (ii) parce que les zones brulees ont des microclimats plus chauds; ou (iii) pour absorber les mineraux des cendres. Cette etude verifie les differences du nombre de tiques et de mouches (dipteres) entre des zones brulees et non brulees du Parc National de Serengeti, en Tanzanie. Elle etudie aussi les differences de temperature au sol et dans les colonnes d'air entre ces zones et celles du contenu en mineraux dans les cendres des zones brulees et dans les feuilles vertes des zones non brulees. Nous n'avons pas trouve de difference dans l'abondance de ces deux types d'invertebres entre zones brulees ou pas. Seule la temperature du sol etait plus elevee dans les zones brulees, et cela seulement en milieu de journee, quand cette augmentation de temperature a moins d'importance que pendant la nuit. Les cendres contenaient plus d'Al, Ca, Cu, Mg, Mn et P que les feuilles vertes de la vegetation proche non brulee. Donc, la seule raison alternative de frequenter les zones non brulees que peut soutenir cette etude est l'attraction pour les mineraux des cendres.

Arbuscular mycorrhizal fungi (AMF) can be important mutualists to plant hosts in acquiring soil nutrients. Past work has not explored whether previous land-cover history influences current AMF abundance in croplands and whether different land-cover histories in grazed but not cultivated areas influence AMF. This study was conducted to assess the effects of land-cover history in and near Serengeti National Park on AMF abundance in areas with three different land uses. The results showed that land-cover history influenced a number of soil physicochemical properties following conversion of grassland to cropland or woodland to cropland during the past 27 years. Different original land cover generally did not significantly influence current AMF abundance in croplands or livestock-grazed soils. However, livestock-grazed current grasslands that were formerly woodlands had lower AMF abundance than sites that had been grasslands since 1984. These results suggest that lower AMF abundance in livestock-grazed and cropland areas as compared to protected wildlife-grazed areas may reflect reduced total carbon inputs and higher disturbance and are not strongly influenced by the legacy of previous land cover. Given that recent studies have detected legacy effects on AMF, such effects may reflect more the impact on the taxonomic composition of AMF rather than their total abundance.

Environmental changes are expected to shift the distribution of functional trait values in plant communities through a combination of species turnover and intraspecific variation. The strength of these shifts may depend on the availability of individuals with trait values adapted to new environmental conditions, represented by the functional diversity (FD) of existing community residents or dispersal from the regional species pool. We conducted a 3-year nutrient- and seed-addition experiment in old-field plant communities to examine the contributions of species turnover and intraspecific variation to community trait shifts, focusing on four key plant functional traits: vegetative height, leaf area, specific leaf area (SLA), and leaf dry matter content (LDMC). We further examined the influence of initial FD and seed availability on the strength of these shifts. Community mean height, leaf area, and SLA increased in response to fertilization, and these shifts were driven almost entirely by intraspecific variation. The strength of intraspecific shifts in height and leaf area was positively related to initial intraspecific FD in these traits. Intraspecific trait responses to fertilization varied among species, with species of short stature displaying stronger shifts in SLA and LDMC but weaker shifts in leaf area. Trait shifts due to species turnover were generally weak and opposed intraspecific responses. Seed addition altered community taxonomic composition but had little effect on community trait shifts. These results highlight the importance of intraspecific variation for short-term community functional responses and demonstrate that the strength of these responses may be mediated by community FD.

The Utah prairie dog (Cynomys parvidens), listed as threatened under the United States Endangered Species Act, was the subject of an extensive eradication program throughout its range during the 20th century. Eradication campaigns, habitat destruction/fragmentation/conversion, and epizootic outbreaks (e.g., sylvatic plague) have reduced prairie dog numbers from an estimated 95,000 individuals in the 1920s to approximately 14,000 (estimated adult spring count) today. As a result of these anthropogenic actions, the species is now found in small isolated sets of subpopulations. We characterized the levels of genetic diversity and population genetic structure using 10 neutral nuclear microsatellite loci for twelve populations (native and transplanted) representative of the three management designated “recovery units,” found in three distinct biogeographic regions, sampled across the species' range. The results indicate (1) low levels of genetic diversity within colonies (H e = 0.109–0.357; H o = 0.106‐ 0.313), (2) high levels of genetic differentiation among colonies (global F ST = 0.296), (3) very small genetic effective population sizes, and (4) evidence of genetic bottlenecks. The genetic data reveal additional subdivision such that colonies within recovery units do not form single genotype clusters consistent with recovery unit boundaries. Genotype cluster membership support historical gene flow among colonies in the easternmost West Desert Recovery Unit with the westernmost Pausaugunt colonies and among the eastern Pausaugunt colonies and the Awapa Recovery unit to the north. In order to maintain the long‐term viability of the species, there needs to be an increased focus on maintaining suitable habitat between groups of existing populations that can act as connective corridors. The location of future translocation sites should be located in areas that will maximize connectivity, leading to maintenance of genetic variation and evolutionary potential.

Mycorrhizas influence plant productivity, community composition, nutrient dynamics and soil structure, yet the factors determining the abundance and diversity of these symbioses in natural ecosystems are poorly understood. We studied arbuscular mycorrhizal (AM) fungi (Glomeromycota) in long-term grazed and un-grazed plots at eight sites forming natural gradients of soil properties and rainfall in the Serengeti National Park, Tanzania. Four metrics of AM fungal abundance and diversity were examined: (1) abundance and species composition of spores, (2) standing crop of AM hyphae in the soil, (3) production of new AM hyphae in buried mesh bags and (4) a bioassay of AM fungal inoculum potential. Our first goal was to test the predictions of the Functional Equilibrium Model that AM fungi should be most abundant in grazed plots (because of increased nutrient demands caused by overcompensation) and at sites with the lowest soil phosphorus (P) and rainfall. Our second goal was to use multi-group structural equation models (SEMs) to investigate the interrelated influences of grazing, soil properties, rainfall and root biomass on the abundance and species composition of AM fungal communities. Two multi-group SEMs were generated: one to examine abundance patterns of AM fungi and another to examine patterns in the spores or auxiliary cells of different families of AM fungi. We observed an extremely high abundance and diversity of AM fungal spores. Only one prediction of the Functional Equilibrium Model was supported; production of new hyphae was highest in the lowest P soils. The abundance and species composition of AM fungi varied across sites but the only variable that responded to grazing was the abundance of Glomeraceae spores, with higher abundance in fenced (un-grazed) plots. Multi-group SEMs revealed that rainfall, total soil P, soil organic matter and soil texture were all important predictors, but the relative importance of each factor varied depending upon the AM metric or taxon observed. Our findings suggest niche differentiation among AM fungal taxa and demonstrate that climate and soil properties are important predictors of the abundance and community composition of these important, hidden components in the Serengeti ecosystem.

Arbuscular mycorrhizal fungi (AMF) are important microbial symbionts for plants especially when soil phosphorus (P) and nitrogen (N) are limited. Little is known about the distribution of AM hyphae in natural systems of tropical soils across landscapes and their association with different land uses. We studied mycorrhizal hyphal abundance in a wildlife grazed system, a livestock grazed system and under cultivated soils in and near Serengeti National Park, Tanzania. Samples of the upper 15 cm of soil beneath locally dominant plant species were collected. Hyphae were preserved on permanent slides and the length of hyphae per cubic centimeter of soil was calculated. Significant differences (p

We argue for expanding the role of theory in ecology to accelerate scientific progress, enhance the ability to address environmental challenges, foster the development of synthesis and unification, and improve the design of experiments and large-scale environmental-monitoring programs. To achieve these goals, it is essential to foster the development of what we call efficient theories, which have several key attributes. Efficient theories are grounded in first principles, are usually expressed in the language of mathematics, make few assumptions and generate a large number of predictions per free parameter, are approximate, and entail predictions that provide well-understood standards for comparison with empirical data. We contend that the development and successive refinement of efficient theories provide a solid foundation for advancing environmental science in the era of big data.

Arbuscular mycorrhizal fungi (AMF) are more widely distributed and can associate with a wide range of plant species. AMF are keystone organisms that form an interface between soils and plant roots. They are also sensitive to environmental changes. AMF are important microbial symbioses for plants under conditions of P-limitation. The AMF are crucial for the functioning of terrestrial ecosystems as they form symbiotic interactions with plants. Mycorrhizal fungi are known to influence plant diversity patterns in a variety of ecosystems globally. AMF hyphae form an extensive network in the soil. The length is a common parameter used to quantifying fungal hyphae. The mycelial network of AM fungi extends into the soil volume and greatly increases the surface area for the uptake of immobile nutrients. Also, AM symbioses improve plants tolerance to drought and enhance plants’ tolerance of or resistance to root pathogens. Also, the networks of AM hyphae play a crucial role in the formation of stable soil aggregates and in the building up of a macroporous structure of soil that allows penetration of water and air and thereby prevents erosion. The functioning of AMF symbiosis is mediated by direct and indirect effects of biotic and abiotic factors of the surrounding rhizosphere, the community, and the ecosystem. AMF have great potential in the restoration of disturbed land and low fertility soil. However, despite the importance of AMF to terrestrial ecosystems, little is known about the effects of environmental changes on AMF abundance, activity and the impact of these changes on the ecosystem services. Therefore, it is important to gain a clearer understanding of the effects of environmental changes on the AM fungal species to guide conservation and restoration efforts.

Differences in body sizes may create a trade-off between foraging efficiency (foraging gains/costs) and access to resources. Such a trade-off provides a potential mechanism for ecologically similar species to coexist on one resource. We explored this hypothesis for tundra (Cygnus columbianus) and trumpeter swans (Cygnus buccinator), a federally protected species, feeding solely on sago pondweed (Stuckenia pectinata) tubers during fall staging and wintering in northern Utah. Foraging efficiency was higher for tundra swans because this species experienced lower foraging and metabolic costs relative to foraging gains; however, trumpeter swans (a) had longer necks and therefore had access to exclusive resources buried deep in wetland sediments and (b) were more aggressive and could therefore displace tundra swans from lucrative foraging locations. We conclude that body size differentiation is an important feature of coexistence among ecologically similar species feeding on one resource. In situations where resources are limiting and competition for resources is strong, conservation managers will need to consider the trade-off between foraging efficiency and access to resources to ensure ecologically similar species can coexist on a shared resource.

Threats to the Serengeti Protected areas are an important tool for conserving biodiversity and ecosystem functioning. But how well do these areas withstand pressure from human activity in surrounding landscapes? Veldhuis et al. studied long-term data from the Serengeti-Mara ecosystem in East Africa. Human activities at boundary regions cause animals to concentrate in the core of the protected area, which eventually reduces soil carbon storage and nitrogen fixation rates and increases vulnerability to extreme droughts. Similar patterns are likely for many, if not all, large protected areas. Science , this issue p. 1424

Carnivores play a central role in ecosystem processes by exerting top-down control, while fire exerts bottom-up control in ecosystems throughout the world, yet, little is known about how fire affects short-term carnivore distributions across the landscape. Through the use of a long-term data set we investigated the distribution of lions, during the daytime, in relation to burned areas in Serengeti National Park, Tanzania. We found that lions avoid burned areas despite the fact that herbivores, their prey, are attracted to burned areas. Prey attraction, however, likely results from the reduction in cover caused by burning, that may thereby decrease lion hunting success. Lions also do not preferentially utilize the edges of burned areas over unburned areas despite the possibility that edges would combine the benefit of cover with proximity to abundant prey. Despite the fact that lions avoid burned areas, lion territory size and reproductive success were not affected by the proportion of the territory burned each year. Therefore, burning does not seem to reduce lion fitness perhaps because of the heterogeneity of burned areas across the landscape or because it is possible that when hunting at night lions visit burned areas despite their daytime avoidance of these areas.

Alterations to habitats that increase the distance at which prey can detect predators can decrease the risk of predation. In grasslands, burning may have this effect for mammalian herbivores by decreasing vegetation height, potentially making burned areas safer habitats. However, few studies have tested the impacts of burning on the perceived predation risk of herbivores. Vigilance can be used as a measure of habitat safety and therefore should be lower in burned areas than unburned areas. We tested the impacts of prescribed burning on vigilance by Thomson's gazelles in Serengeti National Park, Tanzania. Both individual vigilance, proportion of time an individual spends looking, and group vigilance, proportion of a group looking, of Thomson's gazelles was observed in burned and unburned areas before and after presenting a model cheetah to each group. Surprisingly, both individual and group vigilance was the same in both habitats pre- and post-cheetah addition and removal. This is despite the fact that both vegetation height and biomass were lower in burned areas than unburned areas. Thus, it does not appear that Thomson's gazelles perceive burned areas to be safer habitats than unburned areas. Resume Des modifications de l'habitat qui augmentent la distance a laquelle les proies peuvent deceler leurs predateurs peuvent aussi diminuer le risque de predation. Dans les prairies, les feux peuvent avoir cet effet pour les mammiferes herbivores en diminuant la hauteur de la vegetation, ce qui fait eventuellement des zones brulees des habitats plus surs. Cependant, peu d'etudes ont teste les impacts des feux sur la perception du risque de predation chez les herbivores. L'etat de vigilance peut servir de mesure de la securite de l'habitat et il devrait donc etre moindre dans les zones brulees que dans celles qui ne le sont pas. Nous avons teste les impacts des feux prescrits sur la vigilance de gazelles de Thomson dans le Parc National de Serengeti, en Tanzanie. Nous avons observe la vigilance individuelle, la proportion de temps qu'un individu passe a surveiller, et la vigilance du groupe, la proportion des membres du groupe qui sont en train de regarder, dans des aires brulees et non brulees, avant et apres avoir presente a chaque groupe une silhouette de guepard. Curieusement, la vigilance individuelle et la vigilance de groupe etaient les memes dans les deux habitats avant et apres avoir introduit puis enleve le guepard factice alors que et la hauteur de la vegetation et la biomasse etaient plus faibles dans les zones brulees que dans les zones non brulees. Il semble donc que les gazelles de Thomson ne percoivent pas les zones brulees comme des habitats plus surs que les zones non brulees.

A mechanistic understanding of the highly variable effects of herbivores on plant production in different ecosystems remains a major challenge. To explain these patterns, the compensatory continuum hypothesis (CCH) predicts plants to compensate for defoliation when resources are abundant, whereas the growth rate hypothesis (GRH) makes the opposite claim of high herbivory tolerance under resource-poor conditions. The limiting resource model (LRM) tries to reconcile this dichotomy by incorporating the indirect effects of herbivores on plant resources and predicts that the potential for plant compensation is dependent upon whether, and how, herbivory influences limiting resources. Although extensively evaluated in laboratory monocultures, it remains uncertain whether these models can also explain the response of heterogeneous and multi-species natural plant communities to defoliation. Here we investigate community-wide plant response to defoliation and report data from a field experiment in the arid and primarily water-limited Trans-Himalayan grazing ecosystem in northern India involving clipping, irrigation and nutrient-feedback with herbivore dung. Without nutrient-feedback, plants compensated for defoliation in absence of irrigation but failed to compensate under irrigation. Whereas, in the presence of nutrient-feedback plants compensated for defoliation when irrigated. This divergent pattern is not consistent with the CCH and GRH, and is only partially explained by the LRM. Instead, these pluralistic results are consistent with the hypothesis that herbivory may alter the relative strengths of water and nutrient limitation since irrigation increased root:shoot ratio in absence of fertilization in unclipped plots, but not in the corresponding clipped plots. So, herbivory appears to increase relative strength of nutrient-limitation for plants that otherwise seem to be primarily water-limited. Extending the LRM framework to include herbivore-mediated transitions between water and nutrient-limitation may clarify the underlying mechanisms that modulate herbivory-tolerance under different environmental conditions.

Species coexistence involving trophic interactions has been investigated under two theoretical frameworks—partitioning shared resources and accessing exclusive resources. The influence of body size on coexistence is well studied under the exclusive resources framework, but has received less attention under the shared-resources framework. We investigate body-size-dependent allometric extensions of a classical MacArthur-type model where two consumers compete for two shared resources. The equilibrium coexistence criteria are compared against the general predictions of the alternative framework over exclusive resources. From the asymmetry in body size allometry of resource encounter versus demand our model shows, counterintuitively, and contrary to the exclusive resource framework, that a smaller consumer should be competitively superior across a wide range of supplies of the two resource types. Experimental studies are reviewed to resolve this difference among the two frameworks that arise from their respective assumptions over resource distribution. Another prediction is that the smaller consumer may have relatively stronger control over equilibrium resource abundance, and the loss of smaller consumers from a community may induce relatively stronger trophic cascades. Finally, from satiating consumers’ functional response, our model predicts that greater difference among resource sizes can allow a broader range of consumer body sizes to coexist, and this is consistent with the predictions of the alternative framework over exclusive resources. Overall, this analysis provides an objective comparison of the two alternative approaches to understand species coexistence that have heretofore developed in relative isolation. It advances classical consumer–resource theory to show how body size can be an important factor in resource competition and coexistence.