Plant diversity is predicted to be positively linked to the diversity of herbivores and predators in a foodweb. Yet, the relationship between plant and animal diversity is explained by a variety of ...competing hypotheses, with mixed empirical results for each hypothesis. We sampled arthropods for over a decade in an experiment that manipulated the number of grassland plant species. We found that herbivore and predator species richness were strongly, positively related to plant species richness, and that these relationships were caused by different mechanisms at herbivore and predator trophic levels. Even more dramatic was the threefold increase, from low- to high-plant species richness, in abundances of predatory and parasitoid arthropods relative to their herbivorous prey. Our results demonstrate that, over the long term, the loss of plant species propagates through food webs, greatly decreasing arthropod species richness, shifting a predator-dominated trophic structure to being herbivore dominated, and likely impacting ecosystem functioning and services.
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Soil microorganisms are critical to ecosystem functioning and the maintenance of soil fertility. However, despite global increases in the inputs of nitrogen (N) and phosphorus (P) to ecosystems due ...to human activities, we lack a predictive understanding of how microbial communities respond to elevated nutrient inputs across environmental gradients. Here we used high-throughput sequencing of marker genes to elucidate the responses of soil fungal, archaeal, and bacterial communities using an N and P addition experiment replicated at 25 globally distributed grassland sites. We also sequenced metagenomes from a subset of the sites to determine how the functional attributes of bacterial communities change in response to elevated nutrients. Despite strong compositional differences across sites, microbial communities shifted in a consistent manner with N or P additions, and the magnitude of these shifts was related to the magnitude of plant community responses to nutrient inputs. Mycorrhizal fungi and methanogenic archaea decreased in relative abundance with nutrient additions, as did the relative abundances of oligotrophic bacterial taxa. The metagenomic data provided additional evidence for this shift in bacterial life history strategies because nutrient additions decreased the average genome sizes of the bacterial community members and elicited changes in the relative abundances of representative functional genes. Our results suggest that elevated N and P inputs lead to predictable shifts in the taxonomic and functional traits of soil microbial communities, including increases in the relative abundances of faster-growing, copiotrophic bacterial taxa, with these shifts likely to impact belowground ecosystems worldwide.
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Human-driven ecosystem simplification has highlighted questions about how the number of species in an ecosystem influences its functioning. Although biodiversity is now known to affect ecosystem ...productivity, its effects on stability are debated. Here we present a long-term experimental field test of the diversity-stability hypothesis. During a decade of data collection in an experiment that directly controlled the number of perennial prairie species, growing-season climate varied considerably, causing year-to-year variation in abundances of plant species and in ecosystem productivity. We found that greater numbers of plant species led to greater temporal stability of ecosystem annual aboveground plant production. In particular, the decadal temporal stability of the ecosystem, whether measured with intervals of two, five or ten years, was significantly greater at higher plant diversity and tended to increase as plots matured. Ecosystem stability was also positively dependent on root mass, which is a measure of perenniating biomass. Temporal stability of the ecosystem increased with diversity, despite a lower temporal stability of individual species, because of both portfolio (statistical averaging) and overyielding effects. However, we found no evidence of a covariance effect. Our results indicate that the reliable, efficient and sustainable supply of some foods (for example, livestock fodder), biofuels and ecosystem services can be enhanced by the use of biodiversity.
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Seed production that varies greatly from year to year, known as "masting" or "mast-fruiting" behavior, is a population-level phenomenon known to exhibit geographic synchrony extending, at least in ...some cases, hundreds of kilometers. The two main nonexclusive hypotheses for the driver of such geographically extensive synchrony are (1) environmental factors (the Moran effect), and (2) the mutual dependence of trees on outcrossed pollen (pollen coupling). We tested 10 predictions relevant to these two hypotheses using 18 years of acorn production data on two species of California oaks. Data were obtained across the entire ranges of the two species at 12 sites (10 for each species) separated by up to 745 km. In general, our results provided strong support for the importance of the Moran effect as a driver of spatial synchrony in and between these two species. Particularly compelling was evidence of close concordance between spatial synchrony in acorn production and key environmental factors extending over the range of both species and significant spatial cross-synchrony between the two species, despite considerable differences in their geographical ecology. Because oaks are monoecious, female flowers are not necessarily related to pollen production, and thus, our tests do not address the role of pollen coupling in bisexual species where pollen and flower production are necessarily correlated. For the oak species considered here, however, the Moran effect is a key driver of large-scale spatial synchrony in acorn production.
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What drives masting? The phenological synchrony hypothesis Koenig, Walter D; Knops, Johannes M. H; Carmen, William J ...
Ecology (Durham),
2015-January, 2015, 20150101, January 2015, 2015-Jan, 2015-01-00, Volume:
96, Issue:
1
Journal Article
Peer reviewed
Open access
Annually variable and synchronous seed production, or masting behavior, is a widespread phenomenon with dramatic effects on wildlife populations and their associated communities. Proximally, masting ...is often correlated with environmental factors and most likely involves differential pollination success and resource allocation, but little is known about how these factors interact or how they influence seed production. We studied masting in the valley oak (
Quercus lobata
Née), a California endemic tree, and report evidence that phenological synchrony in flowering driven by microclimatic variability determines the size of the acorn crop through its effects on pollen availability and fertilization success. These findings integrate two of the major factors believed to influence seed production in wind-pollinated species-environmental conditions and pollen limitation-by means of a coherent mechanistic hypothesis for how highly variable and synchronized annual seed production is accomplished. We illustrate how, by means of a simulation based on the mechanism proposed here, climate change may influence masting patterns through its effects on environmental variability.
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Masting, the intermittent and synchronized production of seeds, is a common and important phenomenon throughout the plant kingdom. Surprisingly, the proximate mechanisms by which populations of ...masting plants synchronize their seed sets have been relatively unexplored. We examined how temperature influences the acorn crop of the valley oak Quercus lobata, a masting species common in California, USA, over 33 years in order to assess whether temperature acts directly on acorn crop as a cue or whether it acts instead through intermediate steps indicative of a direct mechanistic connection to acorn production. Compared to several alternatives, the difference in temperature during the spring flowering period over the prior two years (Δt) was a good predictor of annual acorn crop in valley oak, as proposed recently by Kelly et al. Significantly, Δt correlates positively with temperatures the previous April, a likely driver of pollination success in valley oak, and negatively with the previous year's acorn crop, which is in turn negatively correlated with the current year's acorn crop, presumably due to resource limitation. Thus, the success of Δt is not as a cue but rather explained by its close relationship to the proximate drivers that have a direct, mechanistic relationship with acorn crop size.
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In grassland ecosystems, N and P fertilization often increase plant productivity, but there is no concensus if fertilization affects soil C fractions. We tested effects of N, P and N+P fertilization ...at 5, 10, 15 g m-2 yr-1 (N5, N10, N15, P5, P10, P15, N5P5, N10P10, and N15P15) compared to unfertilized control on soil C, soil microbial biomass and functional diversity at the 0-20 cm and 20-40 cm depth in an alpine meadow after 5 years of continuous fertilization. Fertilization increased total aboveground biomass of community and grass but decreased legume and forb biomass compared to no fertilization. All fertilization treatments decreased the C:N ratios of legumes and roots compared to control, however fertilization at rates of 5 and 15 g m-2 yr-1 decreased the C:N ratios of the grasses. Compared to the control, soil microbial biomass C increased in N5, N10, P5, and P10 in 0-20 cm, and increased in N10 and P5 while decreased in other treatments in 20-40 cm. Most of the fertilization treatments decreased the respiratory quotient (qCO2) in 0-20 cm but increased qCO2 in 20-40 cm. Fertilization increased soil microbial functional diversity (except N15) but decreased cumulative C mineralization (except in N15 in 0-20 cm and N5 in 20-40 cm). Soil organic C (SOC) decreased in P5 and P15 in 0-20 cm and for most of the fertilization treatments (except N15P15) in 20-40 cm. Overall, these results suggested that soils will not be a C sink (except N15P15). Nitrogen and phosphorus fertilization may lower the SOC pool by altering the plant biomass composition, especially the C:N ratios of different plant functional groups, and modifying C substrate utilization patterns of soil microbial communities. The N+P fertilization at 15 g m-2 yr-1 may be used in increasing plant aboveground biomass and soil C accumulation under these meadows.
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Mammalian herbivores can have pronounced effects on plant diversity but are currently declining in many productive ecosystems through direct extirpation, habitat loss and fragmentation, while being ...simultaneously introduced as livestock in other, often unproductive, ecosystems that lacked such species during recent evolutionary times. The biodiversity consequences of these changes are still poorly understood. We experimentally separated the effects of primary productivity and herbivores of different body size on plant species richness across a 10-fold productivity gradient using a 7-year field experiment at seven grassland sites in North America and Europe. We show that assemblages including large herbivores increased plant diversity at higher productivity but decreased diversity at low productivity, while small herbivores did not have consistent effects along the productivity gradient. The recognition of these large-scale, cross-site patterns in herbivore effects is important for the development of appropriate biodiversity conservation strategies.
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Soil carbon (C) sequestration rates vary widely in abandoned agricultural lands, and factors determining this variation, beyond climate, soil type, and productivity, are poorly understood. One such ...factor is soil disturbance by burrowing mammals. Despite being ubiquitous in all grasslands, the impact of burrowing mammals on soil C dynamics is not well understood. We quantified the major ecosystem processes that are influenced by one such burrowing mammal, plains pocket gophers (Geomys bursarius), in old field ecosystems located in east‐central Minnesota, USA. We found that pocket gopher abundance varied among old fields and that newly formed gopher mounds covered up to 6% of the soil surface annually. We first measured short‐term C pool and flux changes induced by gopher activities. Soil N mineralization did not differ between the soil in gopher mounds and undisturbed soil. However, for the soil under gopher mounds, N mineralization was 30% lower compared with the undisturbed soil. We developed a process model to simulate the long‐term gopher disturbance impact on old field soil C accumulation. This simulation showed that pocket gophers reduced both the rate of soil C accumulation and the total C pool. This reduction is primarily driven by reduced plant C input due to the time it requires for the vegetation to recolonize gopher mounds. Soil organic matter (SOM) decomposition changes had only a minor impact. The process model showed that the depth from which burrowing mammals redistribute soil to the surface is a key factor in determining the overall impact on SOM. In total, our study indicated that soil disturbance by burrowing animals could significantly reduce C storage in old field ecosystems when the mammals are mostly active at the surface soil and can be a significant factor in decreasing overall C sequestration after land abandonment. However, at our study site, gopher abundance decreased with abandonment age, which was likely to have been cause by successional vegetation changes, therefore the gopher disturbance‐induced reduction in soil C is transient and decreases with abandonment age.
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Aboveground–belowground interactions exert critical controls on the composition and function of terrestrial ecosystems, yet the fundamental relationships between plant diversity and soil microbial ...diversity remain elusive. Theory predicts predominantly positive associations but tests within single sites have shown variable relationships, and associations between plant and microbial diversity across broad spatial scales remain largely unexplored. We compared the diversity of plant, bacterial, archaeal and fungal communities in one hundred and forty‐five 1 m2 plots across 25 temperate grassland sites from four continents. Across sites, the plant alpha diversity patterns were poorly related to those observed for any soil microbial group. However, plant beta diversity (compositional dissimilarity between sites) was significantly correlated with the beta diversity of bacterial and fungal communities, even after controlling for environmental factors. Thus, across a global range of temperate grasslands, plant diversity can predict patterns in the composition of soil microbial communities, but not patterns in alpha diversity.
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