On temporal partitioning of a community of ectomycorrhizal fungi

Several mechanisms may contribute to the high species richness often reported in ectomycorrhizal (ECM) fungal communities, including spatial and temporal partitioning. Here, we focus on temporal partitioning. Using molecular methods, we determined the frequencies of occurrence of ECM fungal species detected as hyphae and ECM roots in the forest floor of a Pinus resinosa plantation during a 13-month period. We then used a novel statistical procedure to place the most frequently occurring ECM fungal species into groups distinguished by their patterns of relative frequency over time. Three groups with contrasting temporal patterns were distinguishable for fungal species detected as hyphae. Two groups were distinguishable for species detected as ECM roots. Our results support the hypothesis that temporal partitioning occurs among the species of ECM fungi in this community, but we did not address its causes, which may have involved interactions among species' physiological tolerances, temporal environmental variability, temporal patterns of root production, and variation in fungal genet lifespan. These interactions should be the subjects of future research.     

Interannual variation in rainfall,drought stress and seedling mortality may mediate monodominance in tropical flooded forests

Flood tolerance is commonly regarded as the main factor explaining low diversity and monodominance in tropical swamps. In this study we examined seedling mortality in relation to seasonality, i.e., flooding versus drought, of the dominant tree species (Prioria copaifera), and three associated species (Pterocarpus officinalis, Carapa guianensis and Pentaclethra macroloba), in seasonally flooded forests (SFF) in Darien, Panama. Seedling mortality differed among species, years and seasons. Prioria seedlings experienced the lowest overall mortality, and after 3 years many more Prioria seedlings remained alive than those of any of the associated species. In general, within species, larger seedlings had greater survival. Seed size, which can vary by close to 2 orders of magnitude in Prioria, had a confounding effect with that of topography. Large-seeded Prioria seedlings experienced 1.5 times greater mortality than small-seeded seedlings, as large-seeded Prioria seedlings were more likely to be located in depressions. This finding suggests that seed size, plant size and topography are important in understanding SFF regeneration. For all species, seedling mortality was consistently greater during the dry season than during flooding. For Prioria, dry season seedling mortality was correlated with drought stress, that is, high mortality during the long El Niño dry season of 1998 and the normal dry season of 2000, but very low dry season mortality during the mild dry season of 1999. Prioria’s ability to dominate in seasonally flooded forest of Central America is partly explained by its low drought-related mortality in comparison to associated species.     

HOW DID LIFE BECOME SO DIVERSE? THE DYNAMICS OF DIVERSIFICATION ACCORDING TO THE FOSSIL RECORD AND MOLECULAR PHYLOGENETICS

Abstract: The long‐term diversification of life probably cannot be modelled as a simple equilibrial process: the time scales are too long, the potential for exploring new ecospace is too large and it is unlikely that ecological controls can act at global scales. The sum of many clade expansions and reductions, each of which happens according to its own dynamic, probably approximates more a damped exponential curve when translated into a global‐scale species diversification curve. Unfortunately, it is not possible to plot such a meaningful global‐scale species diversification curve through time, but curves at higher taxonomic levels have been produced. These curves are subject to the vagaries of the fossil record, but it is unlikely that the sources of error entirely overwhelm the biological signal. Clades radiate when the external and internal conditions are right: a new territory or ecospace becomes available, and the lineage has acquired a number of characters that open up a new diet or mode of life. Modern high levels of diversity in certain speciose clades may depend on such ancient opportunities taken. Dramatic climatic changes through the Quaternary must have driven extinctions and originations, but many species responded simply by moving to more favourable locations. Ecological communities appear to be no more than merely chance associations of species, but there may be real interactions among species. Ironically, high species diversity may lead to more speciation, not, as had been assumed, less: more species create more opportunities and selective pressures for other species to respond to, rather than capping diversity at a fixed equilibrium level. Studies from the scale of modern ecosystems to global long‐term patterns in the fossil record support a model for the exponential diversification of life, and one explanation for a pattern of exponential diversification is that as diversity increases, new forms become ever more refinements of existing forms. In a sense the world becomes increasingly divided into finer niche space. Organisms have a propensity to speciate freely, species richness within ecosystems appears to generate opportunities for more speciation, clades show all kinds of patterns from sluggish speciation rates and constant diversity through time to apparently explosive speciation, and there is no evidence that rapidly speciating clades have reached a limit, nor that they are driving other clades to extinction. A corollary of this view is that current biodiversity must be higher than it has ever been. Limits to infinite growth are clearly local, regional, and global turnover and extinction events, when climate change and physical catastrophes knock out species and whole clades, and push the rising exponential curve down a notch or two.     

Diversity biases in terrestrial mammalian assemblages and quantifying the differences between museum collections and published accounts: A case study from the Miocene of Nevada

Understanding diversity through time in the fossil record has primarily relied on the raw count of species within a given time interval, or species richness. These estimates are often derived from published fossil data, and standardized for sample size or geographic area. However, most methods that standardize richness by sample size are sensitive to changes in evenness, which introduces a potential problem with relying on published records: published accounts could be more even than the museum collections from which they are drawn. We address this bias in the context of mammalian paleodiversity, comparing published and museum collections of the Hemphillian Thousand Creek fauna to those of the Barstovian Virgin Valley fauna. We rarified specimen data, both number of identified specimens (NISP) and minimum number of individuals (MNI), and presence/absence data to compare published and museum data within and between faunas. Within faunas, published numbers of specimens are more even than museum samples, but the difference for localities in Virgin Valley is not significant. Neither published nor museum numbers of specimens indicate a significant difference between faunas, but the diversity pattern is reversed between the two data sets. Presence/absence rarefactions show no differences between sources; here, published data adequately sample the underlying museum records. Specimen-based evenness is not accurate in the published sample, and therefore we suggest that future studies of diversity in terrestrial mammalian assemblages must assess unpublished collections. Additionally, NISP data for Thousand Creek are more even than MNI data, suggesting that relying solely on NISP for assessing species diversity can also be misleading. Because publication bias alters richness and evenness, diversity estimates using published data must be circumspect about data sources.     

The effects of heavy metal pollution on genetic diversity in zinc/cadmium hyperaccumulator Sedum alfredii populations

The genetic diversity and population structure of seven populations of Sedum alfredii growing in lead/zinc (Pb/Zn) mine spoils or in uncontaminated soils from eastern and southern China were investigated using random amplified polymorphic DNA (RAPD) technology. Four of the sampled sites were heavily contaminated with heavy metals (Zn, Cd, Pb), and extremely high concentrations of Zn, Cd, and Pb were found among these corresponding populations. A significant reduction of genetic diversity was detected in the mining populations. The reduction of genetic diversity could be derived from a bottleneck effect and might also be attributed to the prevalence of vegetative reproduction of the mining populations. Analysis of molecular variance (AMOVA) and the unweighted pair group method with arithmetic mean (UPGMA) tree derived from genetic distances further corroborated that the genetic differentiation between mine populations and uncontaminated populations was significant. Polymorphism with the heavy metal accumulation capability of S. alfredii probably due to the genetic variation among populations and heavy metal contamination could have more impact on the genetic diversity and population structure of S. alfredii populations than geographic distance.     

On the length distribution of external branches in coalescence trees: genetic diversity within species

Let Z(n) denote the length of an external branch, chosen at random from a Kingman n-coalescent. Based on a recursion for the distribution of Z(n), we show that nZ(n) converges in distribution, as n tends to infinity, to a non-negative random variable Z with density x--> 8/(2+x)(3), x>or=0. This result facilitates the study of the time to the most recent common ancestor of a randomly chosen individual and its closest relative in a given population. This time span also reflects the maximum relatedness between a single individual and the rest of the population. Therefore, it measures the uniqueness of a random individual, a central characteristic of the genetic diversity of a population.     

Phylogeography of halophytes from European coastal and inland habitats

The repeated advance and retreat of glaciers during the Pleistocene ice ages have played a major role in shaping the present patterns of genetic variation within and among plant and animal populations of the temperate zone. In Europe, the geographic ranges of many species were confined to a few, mostly southern refugia during periods of full glaciation. Distribution ranges then reexpanded, and uninhabited northern areas were recolonized during the interglacials. These contraction–expansion cycles were repeated at least four times. Paleontological and molecular phylogeographic studies during the last decade have greatly increased our knowledge of refugial areas and postglacial recolonization patterns of European trees, shrubs and Alpine plants since the last glacial maximum about 20,000 years ago. Much less is known yet about non-Alpine herbaceous plants. In the present review, we summarize recent phylogeographic work on halophytic (salt-adapted) plants from coastal and inland habitats in Europe. Major refugial areas for these plants have been identified along the Mediterranean coasts, but some species could also have survived in saline inland localities. In general, recolonization of N and NW Europe occurred in a stepwise fashion along the Atlantic coastline. For a number of species, molecular studies revealed concordant genetic discontinuities on the background of an essentially continuous geographic distribution. Such congruency could be explained by the preferential seed dispersal through sea currents. However, phylogeographic patterns of halophytes also proved to be influenced by other factors like sea-level fluctuations during the Pleistocene, secondary contact between divergent lineages, long-distance dispersal, clonal growth, and special habitat and temperature requirements.     

Consequences of Melaleuca quinquenervia Invasion on Soil Nematodes in the Florida Everglades

The tree Melaleuca quinquenervia invades all types of habitats of South Florida leading to up to 80% loss of aboveground diversity. To examine impacts on the belowground ecosystem, we investigated the composition and diversity of nematodes from soils dominated by the invasive tree and compared them with soils supporting native plant communities at six locations across the Florida Everglades over three years. Despite the significant differences in soil type, hydrology, and native plant composition of the sites, there were consistent differences in nematode communities between soil environments under the native and invaded plant communities. The total abundance and diversity of nematodes in soils dominated by M. quinquenervia was 60% and 80% of adjacent soils under native plants. Fungal-feeding and plant-parasitic nematodes were twice as abundant under native plants as under M. quinquenervia. Nematode communities under M. quinquenervia were bacterivore-dominated, while under native vegetation plant-parasite dominated. The overall diversity of nematodes was 20% lower under the exotic than under native plants, with plant parasites being 36% and fungivores being 30% less diverse. Soil moisture, % of Ca, Mg, and clay particles and total soil C and N were greater in M. quinquenervia soils, but plant-available concentrations of P, K, Ca, and Mg as well as CEC were reduced. Overall, data suggests that the invasion process may modify soil biotic and abiotic conditions that in turn promote the advancement of the exotic M. quinquenervia and displacement of the native plants.