Do bacterial and fungal communities assemble differently during primary succession?

High‐throughput sequencing technologies are now allowing us to study patterns of community assembly for diverse microbial assemblages across environmental gradients and during succession. Here we discuss potential explanations for similarities and differences in bacterial and fungal community assembly patterns along a soil chronosequence in the foreland of a receding glacier. Although the data are not entirely conclusive, they do indicate that successional trajectories for bacteria and fungi may be quite different. Recent empirical and theoretical studies indicate that smaller microbes (like most bacteria) are less likely to be dispersal limited than are larger microbes – which could result in a more deterministic community assembly pattern for bacteria during primary succession. Many bacteria are also better adapted (than are fungi) to life in barren, early‐successional sediments in that some can fix nitrogen and carbon from the atmosphere – traits not possessed by any fungi. Other differences between bacteria and fungi are discussed, but it is apparent from this and other recent studies of microbial succession that we are a long way from understanding the mechanistic underpinnings of microbial community assembly during ecosystem succession. We especially need a better understanding of global and regional patterns of microbial dispersal and what environmental factors control the development of microbial communities in complex natural systems.     

Population dynamics along a primary succession gradient: do alpine species fit into demographic succession theory?

Background and Aims

Understanding processes and mechanisms governing changes in plant species along primary successions has been of major importance in ecology. However, to date hardly any studies have focused on the complete life cycle of species along a successional gradient, comparing pioneer, early and late-successional species. In this study it is hypothesized that pioneer species should initially have a population growth rate, λ, greater than one with high fecundity rates, and declining growth rates when they are replaced by late-successional species. Populations of late-successional species should also start, at the mid-successional stage (when pioneer species are declining), with growth rates greater than one and arrive at rates equal to one at the late successional stage, mainly due to higher survival rates that allow these species to persist for a long time.

Methods

The demography of pioneer- (Saxifraga aizoides), early (Artemisia genipi) and late-successional species (Anthyllis vulneraria ssp. alpicola) was investigated together with that of a ubiquitous species (Poa alpina) along the Rotmoos glacier foreland (2300–2400 m a.s.l., Central Alps, Austria) over 3 years. A matrix modelling approach was used to compare the main demographic parameters. Elasticity values were plotted in a demographic triangle using fecundity, individual growth and survival as vital rates contributing to the population growth rates.

Key Results

The results largely confirmed the predictions for population growth rates during succession. However, high survival rates of larger adults characterized all species, regardless of where they were growing along the succession. At the pioneer site, high mortality rates of seedlings, plantlets and young individuals were recorded. Fecundity was found to be of minor relevance everywhere, but it was nevertheless sufficient to increase or maintain the population sizes.

Conclusions

Demographically, all the species over all sites behaved like late-successional or climax species in secondary successions, mainly relying on survival of adult individuals. Survival serves as a buffer against temporal variation right from the beginning of the primary succession, indicating a major difference between primary and secondary succession.Key words: Demography, elasticity, glacier foreland, matrix model, population growth, primary succession, strategy, Saxifraga aizoides, Artemisia genipi, Anthyllis vulneraria ssp. alpicola, Poa alpina     

Assembly of plant communities in coastal wetlands—the role of saltcedar Tamarix chinensis during early succession

Aims The mechanisms of plant community assembly are hypothesized to vary at different stages of succession. Here, we explore the local assemblage structure of a herbaceous plant community at its early stage of succession in a supratidal wetland. Specifically, we assess the role of Chinese saltcedar (Tamarix chinensis), the lone dominant shrub species, in shaping the spatial structure and species composition in the local plant community, after landscape alteration.Methods We used the multivariate trend-surface analysis for analyzing the spatial structure of the community composition. A null model was also used to detect potential biotic interactions between species. Statistical significance was derived from a permutation test by randomizing the presence-absence matrix and functional traits independently. Sensitivity analysis by randomly selecting 50 subplots and repeating the null model tests was also done. Finally, rank correlation analysis was used to study the relationship between effect sizes and distance to nearest T. chinensis individuals.Important findings The herbaceous plant community was highly structured and shaped by the presence of T. chinensis. At local scale, two functional traits, plant height and leaf area, were found to be significantly convergent. Dispersal, environmental stress and interspecific competition played a trivial effect on the local community assembly. The facilitating effect of T. chinensis on the pioneering herbaceous plants, through acting as a wind shelter, was put forward as the dominant community assembly process.     

Host specificity in ectomycorrhizal communities: what do the exceptions tell us?

Classic ectomycorrhizal symbioses are mutualisms that involvethe exchange of fixed carbon for mineral nutrients between plantroots and fungi. They are unique in the way they contain featuresof both intimate and diffuse symbioses. The degree of host specificityvaries, particularly among the fungi. Here we examine two exceptionalcases of specificity to see what they tell us about the advantagesof specificity, how it is initiated, and the potential rolethat it plays in complex ecosystems. The first case involvesnon-photosynthetic epiparasitic plants, which contrary to virtuallyall other plants, exhibit high levels of specificity towardtheir fungal hosts. The second case involves suilloid fungi;this is the largest monophyletic group of ectomycorrhizal fungithat is essentially restricted to associations with a singleplant family. In both cases, new symbioses are initiated bydormant propagules that are stimulated to germinate by chemicalcues from the host. This reduces the cost of wasting propaguleson non-hosts. The advantages of specificity remain unclear inboth cases, but we argue that increased benefit to the specialistmay result from specialized physiological adaptations. We reexaminethe idea that specialist fungi may help their hosts competein complex ecosystems by reducing facultative epiparasitismby other plants, and suggest an alternative hypothesis for theobserved pattern.     

Resource discovery in ant communities: do food type and quantity matter?

1. Omnivorous woodland ant species trade off between the ability to find and behaviourally control food resources. Dominant species can limit the ability of subordinates to harvest certain food items. However, subordinate species, by being faster discoverers, could gain access to such food items by arriving at them first. 2. In this study, we tested the hypothesis that resource‐directed discovery occurs in ant communities and that good discoverers preferentially discover high value resources. We did this by measuring time to discovery and the number of discoveries of high and low levels of two resource types, crickets and honey, for species occurring in Texas and Arizona woodland ant communities. 3. Ants discovered resources roughly 10 times faster in Texas than in Arizona. They discovered crickets more rapidly than honey in both communities, but there was no difference in the discovery of different resource levels. We also found that species were not biased in their discovery of different resource types or levels. 4. These results provide indirect evidence that discovery is directed by resource stimuli but that such directedness does not impact interspecific exploitative competition.     

Spatial dynamics in model plant communities: what do we really know?

A variety of models have shown that spatial dynamics and small-scale endogenous heterogeneity (e.g., forest gaps or local resource depletion zones) can change the rate and outcome of competition in communities of plants or other sessile organisms. However, the theory appears complicated and hard to connect to real systems. We synthesize results from three different kinds of models: interacting particle systems, moment equations for spatial point processes, and metapopulation or patch models. Studies using all three frameworks agree that spatial dynamics need not enhance coexistence nor slow down dynamics; their effects depend on the underlying competitive interactions in the community. When similar species would coexist in a nonspatial habitat, endogenous spatial structure inhibits coexistence and slows dynamics. When a dominant species disperses poorly and the weaker species has higher fecundity or better dispersal, competition-colonization trade-offs enhance coexistence. Even when species have equal dispersal and per-generation fecundity, spatial successional niches where the weaker and faster-growing species can rapidly exploit ephemeral local resources can enhance coexistence. When interspecific competition is strong, spatial dynamics reduce founder control at large scales and short dispersal becomes advantageous. We describe a series of empirical tests to detect and distinguish among the suggested scenarios.     

Does the energy equivalence rule apply to intertidal macrobenthic communities?

Energy equivalence assumes equal contribution of large and small species to production and energy flow in communities. As in a double logarithmic plot, physiological rates decline with body weight by –0.25, log biomass should increase by 0.25 and log abundance decline by –0.75 with log species weight, when this concept is valid. This was tested with annual data sets of the macrobenthos of 4 intertidal sites in the German Wadden Sea (Königshafen) and 3 sites in a south Portuguese lagoon (Ria Formosa). Only abundance data from two of these sites displayed significantly negative slopes with mean body size of the species. Biomass and secondary production data were significantly positively correlated with mean body size for all Ria Formosa sites and also for the biomass of a mussel bed in Königshafen. However, high variation in body size of the individuals of a species limits interpretation of these plots.It is preferable to test this concept by body weight classes regardless of its species composition. At Königshafen, biomass and production displayed two distinct peaks. One peak at small body size was caused by browsing species. The other peak at larger body size was caused by animals which potentially extract their food from the water column. This bimodality was only vaguely reflected at one station in the Ria Formosa, possibly because of a dominance of detritus feeding species. In a normalized form (log biomass or production / width of size classvs. log size class), these spectra imply a dominance of small individuals in biomass and production at all sites (except for a mussel bank at Königshafen). This is interpreted as a consequence of permanent disturbances.     

Bacterial succession during 500?years of soil development under agricultural use

Soil bacterial succession under intensive anthropogenic disturbances is not well known. Using terminal restriction fragment length polymorphisms and 454 pyrosequencing of 16S rRNA genes, this study investigated how soil bacterial diversity and community structure changed under two agricultural land uses (paddy rice and upland cropping) in relation to soil development along a 500-year chronosequence created by intermittent reclamation of estuarine salt marshes. Multivariate analysis revealed orderly changes in soil physicochemical properties and bacterial community structure with time, confirming the occurrence of soil development and bacterial succession. Patterns of soil development and bacterial succession resembled each other, with recent land uses affecting their trajectories but not the overall direction. Succession of bacterial community structure was mainly associated with changes in ??-Proteobacteria and Verrucomicrobia. Two stages of bacterial succession were observed, a dramatic-succession stage during the first several decades when bacterial diversity increased evidently and bacterial community structure changed rapidly, and a long gradual-succession stage that lasted for centuries. Canonical correspondence analysis identified soil Na⁺, potentially mineralizable nitrogen, total phosphorous, and crystallinity of iron oxyhydrates as potential environmental drivers of bacterial succession. To conclude, orderly succession of soil bacterial communities occurred along with the long-term development of agroecosystems, which in turn was associated with soil physicochemical changes over time.     

Aquatic invertebrate communities in tank bromeliads: how well do classic ecological patterns apply?

Tank bromeliads (Bromeliaceae) often occur in high densities in the Neotropics and represent a key freshwater habitat in montane forests, housing quite complex invertebrate communities. We tested the extent to which there are species richness–altitude, richness–environment, richness–size, richness–habitat complexity and richness–isolation relationships for the aquatic invertebrate communities from 157 bromeliads in Cusuco National Park, Honduras. We found that invertebrate species richness and abundance correlated most strongly, and positively, with habitat size, which accounted for about a third of the variance in both. Apart from bromeliad size (equivalent of the species–area relationship), we found remarkably little evidence of classic biogeographic and ecological relationships with species richness in this system. Community composition correlated with altitude, bromeliad size and position, though less than 20% of the variation was accounted for by the tested variables. The turnover component of dissimilarity between the communities correlated with altitude, while the nestedness-resultant component was related to bromeliad size. The unexplained variance could reflect a large stochastic component in the system, associated with the ephemerality of the habitat patches (both the plants themselves and the fluctuations in their water content) and stochasticity due to the dispersal dynamics in the system. We conclude that there is a small contribution of classic biogeographic factors to the diversity and community composition of aquatic invertebrates communities in bromeliads. This may be due to the highly dynamic nature of this system, with small patch sizes and high emigration rates. The patterns may mostly be driven by factors affecting colonisation success.     

Why do chimpanzee males attack the females of neighboring communities?

Our closest nonhuman primate relatives, chimpanzees, engage in potentially lethal between‐group conflict; this collective aggressive behavior shows parallels with human warfare. In some communities, chimpanzee males also severely attack and even kill females of the neighboring groups. This is surprising given their system of resource defense polygyny, where males are expected to acquire potential mates. We develop a simple mathematical model based on reproductive skew among primate males to solve this puzzle. The model predicts that it is advantageous for high‐ranking males but not for low‐ranking males to attack females. It also predicts that more males gain a benefit from attacking females as the community's reproductive skew decreases, i.e., as mating success is more evenly distributed. Thus, fatal attacks on females should be concentrated in communities with low reproductive skew. These attacks should also concur with between‐community infanticide. A review of the chimpanzee literature provides enough preliminary support for this prediction to warrant more detailed testing. Am J Phys Anthropol 155:430–435, 2014. © 2014 Wiley Periodicals, Inc.     

Trading places: how do DNA polymerases switch during translesion DNA synthesis?

The replicative bypass of base damage in DNA (translesion DNA synthesis [TLS]) is a ubiquitous mechanism for relieving arrested DNA replication. The process requires multiple polymerase switching events during which the high-fidelity DNA polymerase in the replication machinery arrested at the primer terminus is replaced by one or more polymerases that are specialized for TLS. When replicative bypass is fully completed, the primer terminus is once again occupied by high-fidelity polymerases in the replicative machinery. This review addresses recent advances in our understanding of DNA polymerase switching during TLS in bacteria such as E. coli and in lower and higher eukaryotes.     

The energy cost of flight: do small bats fly more cheaply than birds?

Flapping flight is one of the most expensive activities in terms of metabolic cost and this cost has previously been considered equal for the two extant vertebrate groups which evolved flapping flight. Owing to the difficulty of obtaining accurate measurements without disturbing flight performance, current estimates of flight cost within the group of small birds and bats differ by more than a factor of five for given body masses. To minimize the potential problem that flight behaviour may be affected by the measurements, we developed an indirect method of measuring flight energy expenditure based on time budget analysis in which small nectar-feeding bats (Glossophaginae) could continue their natural rhythm of flying and resting entirely undisturbed. Estimates of metabolic flight power based on 172 24-h time and energy budget measurements were obtained for nine individual bats from six species (mass 7–28 g). Metabolic flight power (P_F) of small bats was found to increase with body mass following the relation P_F = 50.2 M^0.771 (r² = 0.96, n = 13, P_F in W, M in kg). This is about 20–25% below the majority of current predictions of metabolic flight cost for small birds. Thus, either the flight cost of small birds is significantly lower than has previously been thought or, contrary to current opinion, small bats require less energy to fly than birds. Accepted: 29 September 1997     

Plant biology: do DELLAs do defence?

DELLA proteins repress plant growth and developmental processes. Recent data suggest that DELLAs improve survival by imposing growth restraint during plant stress, enabling limited resources to be diverted to pathogen defence.