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"Quinn, J. F."
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Neuroprotective natural antibodies to assemblies of amyloidogenic peptides decrease with normal aging and advancing Alzheimer's disease
A number of distinct β-amyloid (Aβ) variants or multimers have been implicated in Alzheimer's disease (AD), and antibodies recognizing such peptides are in clinical trials. Humans have natural Aβ-specific antibodies, but their diversity, abundance, and function in the general population remain largely unknown. Here, we demonstrate with peptide microarrays the presence of natural antibodies against known toxic Aβ and amyloidogenic non-Aβ species in plasma samples and cerebrospinal fluid of AD patients and healthy controls aged 21-89 years. Antibody reactivity was most prominent against oligomeric assemblies of Aβ and pyroglutamate or oxidized residues, and IgGs specific for oligomeric preparations of Aβ1-42 in particular declined with age and advancing AD. Most individuals showed unexpected antibody reactivities against peptides unique to autosomal dominant forms of dementia (mutant Aβ, ABri, ADan) and IgGs isolated from plasma of AD patients or healthy controls protected primary neurons from Aβ toxicity. Aged vervets showed similar patterns of plasma IgG antibodies against amyloid peptides, and after immunization with Aβ the monkeys developed high titers not only against Aβ peptides but also against ABri and ADan peptides. Our findings support the concept of conformation-specific, cross-reactive antibodies that may protect against amyloidogenic toxic peptides. If a therapeutic benefit of Aβ antibodies can be confirmed in AD patients, stimulating the production of such neuroprotective antibodies or passively administering them to the elderly population may provide a preventive measure toward AD.
Journal Article
Bird Metacommunities in Temperate South American Forest: Vegetation Structure, Area, and Climate Effects
Spatial structure in metacommunities and their relationships to environmental gradients have been linked to opposing theories of community assembly. In particular, while the species sorting hypothesis predicts strong environmental influences, the neutral theory, the mass effect, and the patch dynamics frameworks all predict differing degrees of spatial structure resulting from dispersal and competition limitations. Here we study the relative influence of environmental gradients and spatial structure in bird assemblages of the Chilean temperate forest. We carried out bird and vegetation surveys in South American temperate forests at 147 points located in nine different protected areas in central Chile, and collected meteorological and productivity data for these localities. Species composition dissimilarities between sites were calculated, as well as three indices of bird local diversity: observed species richness, Chao estimate of richness, and Shannon diversity. A stepwise multiple regression and partial regression analyses were used to select a small number of environmental factors that predicted bird species diversity. Although diversity indices were spatially autocorrelated, environmental factors were sufficient to account for this autocorrelation. Moreover, community dissimilarities were not significantly related to distance between sites. We then tested a multivariate hypothesis about climate, vegetation, and avian diversity interactions using a structural equation modeling (SEM) approach. The SEM showed that climate and area of fragments have important indirect effects on avian diversity, mediated through changes in vegetation structure. Given the scale of this study, the metacommunity framework provides useful insights into the mechanisms driving bird assemblages in this region. Taken together, the weak spatial structure of community composition and diversity, as well as the strong environmental effects on bird diversity, support the interpretation that species sorting has a predominant role in structuring avian assemblages in the region.
Journal Article
Invasibility of experimental habitat islands in a California winter annual grassland
In an experimental test of plant community invasibility, we introduced seeds of a native ruderal, California poppy (Eschscholzia californica), at fixed density into experimental plots in a California winter annual grassland. Each of the 42 plots, which ranged in size from 2 m^2 to 32 m^2, had been studied for 4 yr previous to the introduction, with the common observation that a subset of plots of each size consistently held more species than others. It was primarily in these more species-rich plots that establishment and reproduction by the experimental invader occurred. Success of the invader per plot, measured as the total number of plants germinating, producing seeds, or perennating, varied with plot size, but the statistical contribution of plot size was secondary to that of local species number. Contributing variables were the extent of small mammal disturbance (positive) and the degree to which a single resident plant species (in particular, Bromus diandrus) dominated a plot (negative). In contrast to theories of competitive exclusion via niche partitioning, species-rich plots were more invasible.
Journal Article
Settlement and Transport of Benthic Invertebrates in an Intermittent Upwelling Region
We monitored settlement of sea urchins (Strongylocentrotus spp.) and crabs (primarily Cancer spp.) and concurrent physical variables at Bodega Marine Laboratory (BML) and Salt Point (northern California) from April through August 1992. Upwelling favorable winds led to lower temperatures, higher salinities, and lower subsurface pressures at BML, while periods of relaxation from upwelling typically caused a lagged reversal of each of these trends. This response to relaxation was due to a combination of alongshore and onshore flow of near-surface water at this site. Crab settlement was significantly correlated with the rate of temperature increase, corresponding to relaxation events. In contrast, sea urchins settled primarily during an unusual relaxation event which possibly involved remote forcing. Sea urchin and crab settlement were negatively correlated. Possible reasons include residence in different water masses, different biomechanical characteristics, and predation by crab larvae on sea urchin larvae. The time-course of settlement at Salt Point was similar to that at BML for both species groups.
Journal Article
The Influence of Colonization in Nested Species Subsets
Biotic communities inhabiting collections of insular habitat patches often exhibit compositional patterns described as \"nested subsets\". In nested biotas, the assemblages of species in relatively depauperate sites comprise successive subsets of species in relatively richer sites. In theory, nestedness may result from selective extinction, selective colonization, or other mechanisms, such as nested habitats. Allopatric speciation is expected to reduce nestedness. Previous studies, based largely on comparisons between land-bridge and oceanic archipelagos, have emphasized the role of selective extinction. However, colonization could also be important in generating strong patterns of nestedness. We apply a recently published index of nestedness to more than 50 island biogeographic data sets, and examine the roles of colonization, extinction, endemism, and, to a limited extent, habitat variability on the degree on nestedness. Most data sets exhibit a significant degree of nestedness, although there is no general tendency for land-bridge biotas to appear more nested than oceanic ones. Endemic species are shown to generally reduce nestedness. Comparisons between groups of non-endemic species differing in over-water or inter-patch dispersal ability indicate that superior dispersers generally exhibit a greater degree of nestedness than poorer dispersers, a result opposite that expected if colonization were a less predictable process than extinction. These results suggest that frequent colonization is likely to enhance nestedness, thereby increasing the compositional overlap among insular biotas. The prevalence of selective extinction in natural communities remains in question. The importance of colonization in generating and maintaining nested subsets suggests that (1) minimum critical areas will be difficult to determine from patterns of species distributions on islands; (2) multiple conservation sites are likely to be required to preserve communities in subdivided landscapes; and (3) management of dispersal processes may be as important to preserving species and communities as is minimizing extinctions.
Journal Article
Correlated Environments and the Persistence of Metapopulations
Many species are distributed into \"metapopulations\", consisting of many ephemeral local populations in partially isolated patches of habitat. Metapopulation models have generally ignored correlation among environmental conditions in constituent populations. The effects of environmental correlation in the probability of extinction on the persistence times of finite metapopulations are examined, using a computer simulation. When the means and variances of the subpopulation extinction probabilities are both high (e.g. 0.25), temporal environmental correlation among populations may greatly reduce the average time to metapopulation extinction, especially for large metapopulations. However, when the extinction rate and/or its variance take on low to moderate values, even complete correlation in probabilities of extinction among populations may lead to relatively little diminution in global persistence time.
Journal Article
Estimating Species Richness: The Michaelis-Menten Model Revisited
The Michaelis-Menten model has been widely used to estimate the richness (S) of species pools, but is largely untested. We tested whether (1) species accumulation curves follow the form predicted by the model, (2) the model gives unbiased estimates (Ŝ and B̂, respectively) of S and of the sample size, B, needed to detect S/2 species, and (3) performance is robust to community structure. Performance varied with community structure. For model communities with species-abundance distributions based on MacArthur's broken-stick model with 100 or 1000 species, deviations from predicted accumulation curves were slight, and Ŝ and B̂ were unbiased (P ≥ 0.18). For broken-stick communities with 10 species, Ŝ and B̂ overestimated S and B by an average of 17% and 63%, respectively (P < 0.001). For model communities with species-abundance distributions based on Tokeshi's (1990) random-fraction model with 10, 100, or 1000 species, deviations from predicted accumulation curves were large; on average, Ŝ underestimated S by 7-37% (P < 0.001), and (for S = 100 or 1000) B̂ underestimated B by 67-80% (P < 0.001). Vascular plant inventories (S = 42 to 99 species) also showed large deviations from predicted curves; on average, Ŝ underestimated S by 35% (P < 0.001) and B̂ underestimated B by 72% (P < 0.001). Because most natural communities are better described by the random-fraction than the broken-stick model, we suggest the Michaelis-Menten model will typically yield poor estimates of S. Moreover, we argue that accepted criteria for evaluating estimators of S are inadequate.
Journal Article
An Evaluation of Randomization Models for Nested Species Subsets Analysis
Randomization models, often termed \"null\" models, have been widely used since the 1970s in studies of species community and biogeographic patterns. More recently they have been used to test for nested species subset patterns (or nestedness) among assemblages of species occupying spatially subdivided habitats, such as island archipelagoes and terrestrial habitat patches. Nestedness occurs when the species occupying small or species-poor sites have a strong tendency to form proper subsets of richer assemblages. In this paper, we examine the ability of several published simulation models to detect, in an unbiased way, nested subset patterns from a simple matrix of site-by-species presence-absence data. Each approach attempts to build in biological realism by following the assumption that the ecological processes that generated the patterns observed in nature would, if they could be repeated many times over using the same species and landscape configuration, produce islands with the same number of species and species present on the same number of islands as observed. In mathematical terms, the mean marginal totals (column and row sums) of many simulated matrices would match those of the observed matrix. Results of model simulations suggest that the true probability of a species occupying any given site cannot be estimated unambiguously. Nearly all of the models tested were shown to bias simulation matrices toward low levels of nestedness, increasing the probability of a Type I statistical error. Further, desired marginal totals could be obtained only through ad-hoc manipulation of the calculated probabilities. Paradoxically, when such results are achieved, the model is shown to have little statistical power to detect nestedness. This is because nestedness is determined largely by the marginal totals of the matrix themselves, as suggested earlier by Wright and Reeves. We conclude that at the present time, the best null model for nested subset patterns may be one based on equal probabilities of occurrence for all species. Examples of such models are readily available in the literature.
Journal Article
Harvest refugia in marine invertebrate fisheries: models and applications to the red sea urchin, Strongylocentrotus franciscanus
Benthic marine invertebrates with planktonic larvae may exhibit Allee effects in reproduction or recruitment. Hydrodynamic considerations and experimental evidence suggest that species broadcasting gametes suffer greatly reduced fertilization efficiencies as densities decline. There is also evidence for some species, including the red sea urchin, that post-dispersal recruitment success declines at low densities of adults, if adults provide shelter from predators or other sources of mortality. Species displaying strong Allee effects may be susceptible to catastrophic population collapses with slight increases in mortality due to harvesting or natural causes. A simple two patch logistic model suggests that the establishment of a harvest refuge is necessary to prevent collapses and maintain sustainable catches at high levels of harvesting effort. A more detailed, age structured model based on the biology of the red sea urchin, Strongylocentrotus franciscanus produces similar results. Effects of harvesting strategies are sensitive to Allee effects caused by both fertilization efficiency and the disruption of adult spine canopies. Theoretical requirements for the size and spacing of marine reserves depends upon the dispersal abilities of the target species. Multiple reserves, spaced more closely than the average larval dispersal distance, appear to be an effective and conservative strategy for maintaining healthy populations and sustainable levels of harvest.
Journal Article