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Centaurea diffusa is one of the most destructive invasive weeds in the western USA and allelopathy appears to contribute to its invasiveness (Callaway & Aschehoug 2000). Here we identify a chemical from the root exudates of C. diffusa, 8-hydroxyquinoline, not previously reported as a natural product, and find that it varies biogeographically in its natural concentration and its effect as an allelochemical. 8-Hydroxyquinoline is at least three times more concentrated in C. diffusa-invaded North American soils than in this weed's native Eurasian soils and has stronger phytotoxic effects on grass species from North America than on grass species from Eurasia. Furthermore, experimental communities built from North American...
The detrimental effects of invasive plant species on ecosystems are well documented. While much research has focused on discovering ecological influences associated with invasiveness, it remains unclear how these influences interact, causing some introduced exotic species to become invasive threats. Here we develop a framework that incorporates the influences of propagule pressure, frequency independent growth rates, feedback relationships, resource competition and spatial scale of interactions. Our results show that these ecological influences interact in complex ways, resulting in expected outcomes ranging from inability to establish, to naturalization, to conditional invasion dependent on quantity and spatial...
The rapid expansion of road networks has reduced connectivity among populations of flora and fauna. The resulting isolation is assumed to increase population extinction rates, in part because of the loss of genetic diversity. However, there are few cases where loss of genetic diversity has been linked directly to roads or other barriers. We analysed the effects of such barriers on connectivity and genetic diversity of 27 populations of Ovis canadensis nelsoni (desert bighorn sheep). We used partial Mantel tests, multiple linear regression and coalescent simulations to infer changes in gene flow and diversity of nuclear and mitochondrial DNA markers. Our findings link a rapid reduction in genetic diversity (up to...
Although belowground ecosystems have been studied extensively and soil biota play integral roles in biogeochemical processes, surprisingly we have a limited understanding of global patterns in belowground biomass and community structure. To address this critical gap, we conducted a meta-analysis of published data (> 1300 datapoints) to compare belowground plant, microbial and faunal biomass across seven of the major biomes on Earth. We also assembled data to assess biome-level patterns in belowground microbial community composition. Our analysis suggests that variation in microbial biomass is predictable across biomes, with microbial biomass carbon representing 0.6-1.1% of soil organic carbon (r(2) = 0.91) and 1-20%...
Warming-induced release of CO2 from the large carbon (C) stores in arctic soils could accelerate climate change. However, declines in the response of soil respiration to warming in long-term experiments suggest that microbial activity acclimates to temperature, greatly reducing the potential for enhanced C losses. As reduced respiration rates with time could be equally caused by substrate depletion, evidence for thermal acclimation remains controversial. To overcome this problem, we carried out a cooling experiment with soils from arctic Sweden. If acclimation causes the reduction in soil respiration observed after experimental warming, then it should subsequently lead to an increase in respiration rates after cooling....
Plant invasions have dramatic aboveground effects on plant community composition, but their belowground effects remain largely uncharacterized. Soil microorganisms directly interact with plants and mediate many nutrient transformations in soil. We hypothesized that belowground changes to the soil microbial community provide a mechanistic link between exotic plant invasion and changes to ecosystem nutrient cycling. To examine this possible link, monocultures and mixtures of exotic and native species were maintained for 4 years in a California grassland. Gross rates of nitrogen (N) mineralization and nitrification were quantified with 15N pool dilution and soil microbial communities were characterized with DNA-based...
We examined plant community responses to interactions between arbuscular mycorrhizal (AM) fungi and availability of atmospheric CO2 and soil N. Communities of 14 plant species were grown in mesocosms containing living or killed AM fungal inoculum, ambient or elevated atmospheric CO2 and low or enriched soil N. After one growing season, significantly different plant communities existed in the different treatments. Plant species richness was lowest in +N mesocosms and highest in +AM + CO2 mesocosms. At ambient CO2, AM fungi reduced richness but at elevated CO2 they increased it. This was caused by changes in mortality rates of several C3 forbs and may suggest that CO2 enrichment ameliorates the carbon cost of some...
A principal challenge in ecology is to integrate physiological function (e.g. photosynthesis) across a collection of individuals (e.g. plants of different species) to understand the functioning of the entire ensemble (e.g. primary productivity). The control that organism size exerts over physiological and ecological function suggests that allometry could be a powerful tool for scaling ecological processes across levels of organization. Here we use individual plant allometries to predict how nutrient content and productivity scale with total plant biomass (phytomass) in whole plant communities. As predicted by our model, net primary productivity as well as whole community nitrogen and phosphorus content all scale...
Organic carbon (C) and nitrogen (N) are essential for heterotrophic soil microorganisms, and their bioavailability strongly influences ecosystem C and N cycling. We show here that the natural (15)N abundance of the soil microbial biomass is affected by both the availability of C and N and ecosystem N processing. Microbial (15)N enrichment correlated negatively with the C : N ratio of the soil soluble fraction and positively with net N mineralization for ecosystems spanning semiarid, temperate and tropical climates, grassland and forests, and over four million years of ecosystem development. In addition, during soil incubation, large increases in microbial (15)N enrichment corresponded to high net N mineralization...
The amount of carbon plants allocate to mycorrhizal symbionts exceeds that emitted by human activity annually. Senescent ectomycorrhizal roots represent a large input of carbon into soils, but their fate remains unknown. Here, we present the surprising result that, despite much higher nitrogen concentrations, roots colonized by ectomycorrhizal (EM) fungi lost only one-third as much carbon as non-mycorrhizal roots after 2 years of decomposition in a piñon pine (Pinus edulis) woodland. Experimentally excluding live mycorrhizal hyphae from litter, we found that live mycorrhizal hyphae may alter nitrogen dynamics, but the afterlife (litter-mediated) effects of EM fungi outweigh the influences of live fungi on root...
The cycles of the key nutrient elements nitrogen (N) and phosphorus (P) have been massively altered by anthropogenic activities. Thus, it is essential to understand how photosynthetic production across diverse ecosystems is, or is not, limited by N and P. Via a large-scale meta-analysis of experimental enrichments, we show that P limitation is equally strong across these major habitats and that N and P limitation are equivalent within both terrestrial and freshwater systems. Furthermore, simultaneous N and P enrichment produces strongly positive synergistic responses in all three environments. Thus, contrary to some prevailing paradigms, freshwater, marine and terrestrial ecosystems are surprisingly similar in terms...
The effects that below-freezing temperature (frost) can have at times of year when it is unusual are an interesting ecological phenomenon that has received little attention. The physiological consequence of formation of ice crystals in plant tissue is often death of the plants, or at least of sensitive parts that can include flower buds, ovaries, and leaves. The loss of potential for sexual reproduction can have long-lasting effects on the demography of annuals and long-lived perennials, because the short-term negative effects of frosts can result in longer-term benefits through lowered populations of seed predators. The loss of host plants can have dramatic consequences for herbivores, even causing local extinctions,...
'C4 photosynthesis' refers to a suite of traits that increase photosynthesis in high light and high temperature environments. Most C4 plants are grasses, which dominate tropical and subtropical grasslands and savannas but are conspicuously absent from cold growing season climates. Physiological attributes of C4 photosynthesis have been invoked to explain C4 grass biogeography; however, the pathway evolved exclusively in grass lineages of tropical origin, suggesting that the prevalence of C4 grasses in warm climates could be due to other traits inherited from their non-C4 ancestors. Here we investigate the relative influences of phylogeny and photosynthetic pathway in determining the ecological distributions of C4...
The ecosystem engineering concept focuses on how organisms physically change the abiotic environment and how this feeds back to the biota. While the concept was formally introduced a little more than 10 years ago, the underpinning of the concept can be traced back to more than a century to the early work of Darwin. The formal application of the idea is yielding new insights into the role of species in ecosystems and many other areas of basic and applied ecology. Here we focus on how temporal, spatial and organizational scales usefully inform the roles played by ecosystem engineers and their incorporation into broader ecological contexts. Two particular, distinguishing features of ecosystem engineers are that they...