Michael G. Bowler

Coexistence and relative abundance in forest trees

Contemporary acceleration of biodiversity loss makes increasingly urgent the need to understand the controls of species coexistence. Tree diversity in particular plays a pivotal role in determining terrestrial biodiversity, through maintaining diversity of its dependent species and with them, their predators and parasites. Most theories of coexistence based on the principle of limiting similarity suggest that coexistence of competing species is inherently unstable; coexistence of competitors must be maintained by external forces such as disturbance, immigration or ‘patchiness’ of resources in space and time. In contrast, storage theory postulates stable coexistence of competing species through temporal alternation of conditions favouring recruitment of one species over the other. Here we use storage theory to develop explicit predictions for relative differences between competitors that allow us to discriminate between coexistence models. Data on tree species from a primary forest on the Mexican Pacific coast support a general dynamic of storage processes determining coexistence of similar tree species in this community, and allow us to reject all other theories of coexistence.

PHYLOGENY, NICHES, AND RELATIVE ABUNDANCE IN NATURAL COMMUNITIES

Community structure refers to the number of species in a community and the pattern of distribution of individuals among those species. We use a novel way of representing community structure to show that abundance within closely related pairs of co-occurring tree species in a highly diverse Mexican forest is more equitable than is abundance within more distantly related pairs. This observation is at odds with the fundamental assumption of neutral models of community structure, i.e., that species are interchangeable. The observed patterns suggest niche apportionment, in which interaction is focused pairwise between congeners but falls away from the phylogenetic structure above the genus level. Thus niche processes may significantly affect community structure through regulating relative abundance in a substantial proportion of species, which in turn potentially enhances community stability. One such mechanism of stable coexistence has already been shown to be active in this forest.

Symmetry in the changing jets of SS 433 and its true distance from US

We present the deepest yet radio image of the Galactic jet source SS 433, which reveals over two full precession cycles (>2 × 163 days) of the jet axis. Systematic and identifiable deviations from the traditional kinematical model for the jets are found: variations in jet speed, lasting for as long as tens of days, are necessary to match the detailed structure of each jet. It is remarkable that these variations are equal and opposite, matching the two jets simultaneously. This explains certain features of the correlated redshift residuals found in fits to the kinematic model of SS 433 reported in the literature. Asymmetries in the image caused by light-travel time enabled us to measure the jet speeds of particular points to be within a range from 0.24c to 0.28c, consistent with, yet determined independently from, the speeds derived from the famous moving optical emission lines. Taken together with the angular periodicity of the zigzag/corkscrew structure projected on the plane of the sky (produced by the precession of the jet axis), these measurements determine beyond all reasonable doubt the distance to SS 433 to be 5.5 ± 0.2 kpc, significantly different from the distance most recently inferred using neutral hydrogen measurements together with the current rotation model for the Galaxy.

A NEW APPLICATION OF STORAGE DYNAMICS: DIFFERENTIAL SENSITIVITY, DIFFUSE COMPETITION, AND TEMPORAL NICHES

We have developed our recent demonstration that a temporal niche axis may support the stable coexistence of tree species. We previously applied a two-species lottery model assuming differences between species in environmental sensitivity to generate the predictions that the better competitor will show greater fluctuation in recruitment and be the less common species, predictions supported unanimously by data from a Mexican deciduous forest. Here, we expand on that model and explore the implication from our earlier study that competition among tree species may be focused, rather than the more commonly assumed diffuse, by applying a version of the model dealing with diffuse competition. We found focused competition to be the better explanation of the available data. Our results constitute a serious challenge to the entrenched opinion that competition among tree species is diffuse and to the proposition that competition may be unimportant in structuring tree communities. Another component of the earlier work was the comparison of closely related species. Although species may be ecologically similar for reasons other than phylogenetic relatedness, the additional underlying genetic similarity and the shared evolutionary history of close relatives makes them good candidates for the processes driving storage dynamics. We draw on the Gentry data set to show that congeneric species co-occur in plant communities in all habitat types worldwide, indicating that the dynamics we describe may take place in almost any community containing woody species, in any part of the world.

SS 433: Observation of the Circumbinary Disk and Extraction of the System Mass

The so-called stationary Hα line of SS 433 is shown to consist of three components. A broad component is identified as emitted in that wind from the accretion disk that grows in speed with elevation above the plane of the disk. There are two narrow components, one permanently redshifted and the other permanently shifted to the blue. These are remarkably steady in wavelength and must be emitted from a circumbinary ring, orbiting the center of mass of the system rather than orbiting either the compact object or its companion: perhaps the inner rim of an excretion disk. The orbiting speed (approximately 200 km s−1) of this ring material strongly favors a large mass for the enclosed system (around 40 M☉), a large mass ratio for SS 433, a mass for the compact object plus accretion disk of ~16 M☉, and hence the identity of the compact object as a rather massive stellar black hole.

Jet Velocity in SS 433: Its Anticorrelation with Precession-Cone Angle and Dependence on Orbital Phase

We present a reanalysis of the optical spectroscopic data on SS 433 from the last quarter-century and demonstrate that these data alone contain systematic and identifiable deviations from the traditional kinematic model for the jets: variations in speed, which agree with our analysis of recent radio data, in precession-cone angle, and in phase. We present a simple technique for separating out the jet speed from the angular properties of the jet axis, assuming only that the jets are symmetric. With this technique, the archival optical data reveal that the variations in jet speed and in precession-cone angle are anticorrelated in the sense that when faster jet bolides are ejected the cone opening angle is smaller. We also find speed oscillations as a function of orbital phase.

Measurement of the equilibrium relative humidity for common precipitant concentrations: facilitating controlled dehydration experiments

The dehydration of crystals of macromolecules has long been known to have the potential to increase their diffraction quality. A number of methods exist to change the relative humidity that surrounds crystals, but for reproducible results, with complete characterization of the changes induced, a precise humidity-control device coupled with an X-ray source is required. The first step in these experiments is to define the relative humidity in equilibrium with the mother liquor of the system under study; this can often be quite time-consuming. In order to reduce the time spent on this stage of the experiment, the equilibrium relative humidity for a range of concentrations of the most commonly used precipitants has been measured. The relationship between the precipitant solution and equilibrium relative humidity is explained by Raoults law for the equilibrium vapour pressure of water above a solution. The results also have implications for the choice of cryoprotectant and solutions used to dehydrate crystals. For the most commonly used precipitants (10-30% PEG 2000-8000), the starting point will be a relative humidity of 99.5%.

An analytical model assessing the potential threat to natural habitats from insect resistance transgenes

We examine the role of ecological interactions on effective gene flow from genetically manipulated plants to their wild relatives. We do so by constructing and applying to oilseed rape (OSR) an analytical model for interaction between plants with and without an insect resistance (IR) allele in natural communities, incorporating documented levels of herbivore variability. We find that with reasonable values of advantage to the IR allele, little concomitant disadvantage (physiological costs of the allele) restricts it to low proportions of the natural population for large numbers of generations. We conclude that OSR IR transgenes are unlikely to pose an immediate threat to natural communities. Our model identifies those factors best able to regulate particular transgenes at the population level, the most effective being impaired viability of seeds in the period between production and the following growing season, although other possibilities exist. Because solutions rely on ratios, limiting values of regulating factors are testable under controlled conditions, minimizing risk of release into the environment and offering significant advancement on existing testing programmes. Our model addresses folivory but is easily modified for herbivory damaging the seed or directly affecting seed production by infested plants, or for pathogens altering seed survival in the seedbank.

Measurement of the intrinsic variability within protein crystals: implications for sample-evaluation and data-collection strategies

The advent of micro-focused X-ray beams has led to the development of a number of advanced methods of sample evaluation and data collection. In particular, multiple-position data-collection and helical oscillation strategies are now becoming commonplace in order to alleviate the problems associated with radiation damage. However, intra-crystal and inter-crystal variation means that it is not always obvious on which crystals or on which region or regions of a crystal these protocols should be performed. For the automation of this process for large-scale screening, and to provide an indication of the best strategy for data collection, a metric of crystal variability could be useful. Here, measures of the intrinsic variability within protein crystals are presented and their implications for optimal data-collection strategies are discussed.

Temporal niche dynamics, relative abundance and phylogenetic signal of coexisting species

The coexistence of similar species accounts for some 30% of diversity within communities, yet the coexistence and relative abundance of similar species is a continuing ecological conundrum. Using close phylogenetic relatedness as a measure of similarity, we previously demonstrated that neither classic niche theory nor neutral theory can explain the relative abundances of co-occurring pairs of similar tree species in a diverse tropical forest. Here, we show that the stable, focused competition of a temporal niche dynamic fits the distribution of observed fractional abundances (pairwise relative abundances). Previously published, independent evidence of temporal dynamics in this community supports our results; our model identifies additional criteria for field tests of differential sensitivity (DS) temporal dynamics. The success of temporal dynamics at explaining the observed distribution—and the failure of alternative hypotheses to do so—indicates that current diagnostics of community structure and assembly needs general re-examination.