ALBERT

Description: Edaphic variation in plant community composition is widespread, yet its underlying mechanisms are rarely understood and often assumed to be physiological. In East African savannas, Acacia tree species segregate sharply across soils of differing parent material: the ant-defended whistling thorn, A. drepanolobium (ACDR), is monodominant on clay vertisols that are nutrient rich but physically stressful, whereas poorly defended species such as A. brevispica (ACBR) dominate on nutrient-poor but otherwise less-stressful sandy loams. Using a series of field experiments, we show that large-mammal herbivory interacts with soil properties to maintain this pattern. In the absence of large herbivores, transplanted saplings of both species established on both soil types. Browsers strongly suppressed survival and growth of ACDR saplings on sandy soil, where resource limitation constrained defensive investment. On clay soil, ACBR saplings established regardless of herbivory regime, but elephants prevented recruitment to maturity, apparently because trees could not tolerate the combination of biotic and abiotic stressors. Hence, each tree species was filtered out of one habitat by browsing in conjunction with different edaphic factors and at different ontogenetic stages. Browser abundance was greater on sandy soil, where trees were less defended, consistent with predicted feedbacks between plant community assembly and herbivore distributions. By exploring two inversely related axes of soil “quality” (abiotic stress and nutrient content), our study extends the range of mechanisms by which herbivores are known to promote edaphic specialization, illustrates how the high cost of a protection mutualism can constrain the realized niche of host trees, and shows that large-scale properties of savanna ecosystems are shaped by species interactions in cryptic ways that mimic simple abiotic determinism. These results suggest that ongoing declines in large-herbivore populations may relax spatial heterogeneity in plant assemblages and reduce the beta diversity of communities. This article is protected by copyright. All rights reserved.

Description: Short-rotation woody biomass crops (SRWC) have been proposed as a major feedstock source for bioenergy generation in the Northeastern US. To quantify the environmental effects and greenhouse gas (GHG) balance of crops including SRWC, investigators need spatially explicit data which encompass entire plantation cycles. A knowledge gap exists for the establishment period which makes current GHG calculations incomplete. In this study, we investigated the effects of converting pasture and hayfields to willow ( Salix spp.) and hybrid-poplar ( Populus spp.) SRWC plantations on soil nitrogen (N) cycling, nitrous oxide (N 2 O) emissions, and nitrate ( NO 3 − ) leaching at six sites of varying soil and climate conditions across northern Michigan and Wisconsin, following these plantations from pre conversion through their first 2 years. All six sites responded to establishment with increased N 2 O emissions, available inorganic N, and, where it was measured, NO 3 − leaching; however, the magnitude of these impacts varied dramatically among sites. Soil NO 3 − levels varied threefold among sites, with peak extractable NO 3 − concentrations ranging from 15 to 49 g N kg −1 soil. Leaching losses were significant and persisted through the second year, with 44–112 kg N ha −1 leached in SRWC plots. N 2 O emissions in the first growing season varied 30-fold among sites, from 0.5 to 17.0 Mg- CO 2 eq  ha −1 (carbon dioxide equivalents). N 2 O emissions over 2 years resulted in N 2 O emissions due to plantation establishment that ranged from 0.60 to 22.14 Mg- CO 2 eq  ha −1 above baseline control levels across sites. The large N losses we document herein demonstrate the importance of including direct effects of land conversion in life-cycle analysis (LCA) studies of SRWC GHG balance. Our results also demonstrate the need for better estimation of spatial variability of N cycling processes to quantify the full environmental impacts of SRWC plantations.

Description: Protein disulfide isomerase (PDI) is a chaperone protein in the endoplasmic reticulum that is up-regulated in mouse models of, and brains of patients with, neurodegenerative diseases involving protein misfolding. PDI’s role in these diseases, however, is not fully understood. Here, we report the discovery of a reversible, neuroprotective lead optimized...

Description: Uncertainty surrounding future climate makes it difficult to have confidence that current nutrient management strategies will remain effective. This study used monitoring and modelling to assess current effectiveness (% phosphorus reduction), and resilience (defined as continued effectiveness under a changing climate) of best management practices (BMPs) within 5 catchments of the Lake Simcoe watershed, Ontario. The model INCA-P was used, and monitoring data used to calibrate and validate a series of management scenarios. To assess current BMP effectiveness, models were run over a baseline period 1985-2014 with and without management scenarios. Climate simulations were run (2070-2099), and BMP resilience calculated as the % change in effectiveness between the baseline and future period. Results demonstrated that livestock removal from water courses was the most effective BMP, while manure storage adjustments were the least. Effectiveness varied between catchments, influenced by the dominant hydrological and nutrient transport pathways. Resilience of individual BMPs was associated with catchment sensitivity to climate change. BMPs were most resilient in catchments with high soil water storage capacity, and small projected changes in frozen-water availability and in soil moisture deficits. Conversely, BMPs were less resilient in catchments with larger changes in spring melt magnitude and in overland flow proportions. Results indicated that BMPs implemented are not always those most suited to catchment flow pathways, and a more site-specific approach would enhance prospects for maintaining P reduction targets. Furthermore, BMP resilience to climate change can be predicted from catchment physical properties and present day hydrochemical sensitivity to climate forcing.

Description: Positive indirect effects of consumers on their resources can stabilize food webs by preventing overexploitation, but the coupling of trophic and non-trophic interactions remains poorly integrated into our understanding of community dynamics. Elephants engineer African savanna ecosystems by toppling trees and breaking branches, and although their negative effects on trees are well documented, their effects on small-statured plants remain poorly understood. Using data on 117 understory plant taxa collected over seven years within 36 one-hectare experimental plots in a semi-arid Kenyan savanna, we measured the strength and direction of elephant impacts on understory vegetation. We found that elephants had neutral effects on most (83-89%) species, with a similar frequency of positive and negative responses among the remainder. Overall, understory biomass was 5-14% greater in the presence of elephants across a range of rainfall levels. Whereas direct consumption presumably accounts for the negative effects, positive effects are likely indirect. We hypothesized that elephants create associational refuges for understory plants by damaging tree canopies in ways that physically inhibit feeding by other large herbivores. Indeed, understory biomass and species richness beneath elephant-damaged trees were 55% and 21% greater, respectively, than under undamaged trees. Experimentally simulated elephant damage increased understory biomass by 37% and species richness by 49% after one year. Conversely, experimentally removing elephant damaged branches decreased understory biomass by 39% and richness by 30% relative to sham-manipulated trees. Camera-trap surveys revealed that elephant damage reduced the frequency of herbivory by 71%, whereas we detected no effect of damage on temperature, light, or soil moisture. We conclude that elephants locally facilitate understory plants by creating refuges from herbivory, which countervails the direct negative effects of consumption and enhances larger-scale biomass and diversity by promoting the persistence of rare and palatable species. Our results offer a counterpoint to concerns about the deleterious impacts of elephant “overpopulation” that should be considered in debates over wildlife management in African protected areas: understory species comprise the bulk of savanna plant biodiversity, and their responses to elephants are buffered by the interplay of opposing consumptive and non-consumptive interactions. This article is protected by copyright. All rights reserved.

Description: The oft-repeated claim that Earth’s biota is entering a sixth "mass extinction" depends on clearly demonstrating that current extinction rates are far above the "background" rates prevailing in the five previous mass extinctions. Earlier estimates of extinction rates have been criticized for using assumptions that might overestimate the severity of the extinction crisis. We assess, using extremely conservative assumptions, whether human activities are causing a mass extinction. First, we use a recent estimate of a background rate of 2 mammal extinctions per 10,000 species per 100 years (that is, 2 E/MSY), which is twice as high as widely used previous estimates. We then compare this rate with the current rate of mammal and vertebrate extinctions. The latter is conservatively low because listing a species as extinct requires meeting stringent criteria. Even under our assumptions, which would tend to minimize evidence of an incipient mass extinction, the average rate of vertebrate species loss over the last century is up to 114 times higher than the background rate. Under the 2 E/MSY background rate, the number of species that have gone extinct in the last century would have taken, depending on the vertebrate taxon, between 800 and 10,000 years to disappear. These estimates reveal an exceptionally rapid loss of biodiversity over the last few centuries, indicating that a sixth mass extinction is already under way. Averting a dramatic decay of biodiversity and the subsequent loss of ecosystem services is still possible through intensified conservation efforts, but that window of opportunity is rapidly closing.

Description: Ecology, Volume 0, Issue 0, Ahead of Print. Determining how competing species coexist is essential to understanding patterns of biodiversity. Indirect facilitation, in which a competitively dominant species exerts a positive effect on one competitor by more strongly suppressing a third, shared competitor, is a potentially potent yet understudied mechanism for competitive coexistence. Here we provide evidence for indirect facilitation in a guild of four African Acacia ant species that compete for nesting space on the host plant Acacia drepanolobium, showing that a competitively dominant acacia ant species indirectly creates establishment opportunities for the most subordinate species that may help to maintain diversity. Using long-term observational data and field experiments, we demonstrate that the competitively dominant ant species outcompetes two competitively intermediate species, while tolerating colonies of the subordinate competitor; this creates opportunities for local colonization and establishment of colonies of the subordinate species within the dominant species' territories. Host plants occupied by this subordinate species are then more likely to be colonized by the intermediate species, which in turn are more likely to be displaced by the dominant species. This process has the potential to generate a cyclical succession of ant species on host trees, contributing to stable coexistence within this highly competitive community.

Description: Neither the number of HIV-1 proviruses within individual infected cells in HIV-1–infected patients nor their genetic relatedness within individual infected cells and between cells and plasma virus are well defined. To address these issues we developed a technique to quantify and genetically characterize HIV-1 DNA from single infected cells in vivo. Analysis of peripheral blood CD4+ T cells from nine patients revealed that the majority of infected cells contain only one copy of HIV-1 DNA, implying a limited potential for recombination in virus produced by these cells. The genetic similarity between HIV populations in CD4+ T cells and plasma implies ongoing exchange between these compartments both early and late after infection.