Ecology Letters (2011) 14: 407-418 ABSTRACT: A general understanding of biological invasions will provide insights into fundamental ecological and evolutionary problems and contribute to more ...efficient and effective prediction, prevention and control of invasions. We review recent papers that have proposed conceptual frameworks for invasion biology. These papers offer important advances and signal a maturation of the field, but a broad synthesis is still lacking. Conceptual frameworks for invasion do not require invocation of unique concepts, but rather should reflect the unifying principles of ecology and evolutionary biology. A conceptual framework should incorporate multicausality, include interactions between causal factors and account for lags between various stages. We emphasize the centrality of demography in invasions, and distinguish between explaining three of the most important characteristics by which we recognize invasions: rapid local population increase, monocultures or community dominance, and range expansion. As a contribution towards developing a conceptual synthesis of invasions based on these criteria, we outline a framework that explicitly incorporates consideration of the fundamental ecological and evolutionary processes involved. The development of a more inclusive and mechanistic conceptual framework for invasion should facilitate quantitative and testable evaluation of causal factors, and can potentially lead to a better understanding of the biology of invasions.
Abstract
We have examined the distribution of the position angle (PA) of the Galactic center filaments with lengths
L
> 66″ and <66″ as well as their length distribution as a function of PA. We find ...bimodal PA distributions of the filaments, and long and short populations of radio filaments. Our PA study shows the evidence for a distinct population of short filaments with PA close to the Galactic plane. Mainly thermal, short-radio filaments (<66″) have PAs concentrated close to the Galactic plane within 60° < PA < 120°. Remarkably, the short filament PAs are radial with respect to the Galactic center at
l
< 0° and extend in the direction toward Sgr A*. On a smaller scale, the prominent Sgr E H
ii
complex G358.7-0.0 provides a vivid example of the nearly radial distribution of short filaments. The bimodal PA distribution suggests a different origin for two distinct filament populations. We argue that the alignment of the short-filament population results from the ram pressure of a degree-scale outflow from Sgr A* that exceeds the internal filament pressure, and aligns them along the Galactic plane. The ram pressure is estimated to be 2 × 10
6
cm
−3
K at a distance of 300 pc, requiring biconical mass outflow rate 10
−4
M
⊙
yr
−1
with an opening angle of ∼40°. This outflow aligns not only the magnetized filaments along the Galactic plane but also accelerates thermal material associated with embedded or partially embedded clouds. This places an estimate of ∼6 Myr as the age of the outflow.
Methods for sampling ecological assemblages strive to be efficient, repeatable, and representative. Unknowingly, common methods may be limited in terms of revealing species function and so of less ...value for comparative studies. The global decline in pollination services has stimulated surveys of flower-visiting invertebrates, using pan traps and net sampling. We explore the relative merits of these two methods in terms of species discovery, quantifying abundance, function, and composition, and responses of species to changing floral resources. Using a spatially-nested design we sampled across a 5000 km(2) area of arid grasslands, including 432 hours of net sampling and 1296 pan trap-days, between June 2010 and July 2011. Net sampling yielded 22% more species and 30% higher abundance than pan traps, and better reflected the spatio-temporal variation of floral resources. Species composition differed significantly between methods; from 436 total species, 25% were sampled by both methods, 50% only by nets, and the remaining 25% only by pans. Apart from being less comprehensive, if pan traps do not sample flower-visitors, the link to pollination is questionable. By contrast, net sampling functionally linked species to pollination through behavioural observations of flower-visitation interaction frequency. Netted specimens are also necessary for evidence of pollen transport. Benefits of net-based sampling outweighed minor differences in overall sampling effort. As pan traps and net sampling methods are not equivalent for sampling invertebrate-flower interactions, we recommend net sampling of invertebrate pollinator assemblages, especially if datasets are intended to document declines in pollination and guide measures to retain this important ecosystem service.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
Magnetized radio filaments are found in abundance in the inner few hundred parsecs of our galaxy. Progress in understanding this population of filaments has been slow over the last few ...decades, in part due to a lack of detection elsewhere in the galaxy or in external galaxies. Recent highly sensitive radio continuum observations of radio galaxies in galaxy clusters have revealed remarkable isolated filamentary structures in the intracluster medium (ICM) that are linked to radio jets, tails, and lobes. The origin of this class of filaments is not understood either. Here, we argue that the underlying physical mechanisms responsible for the creation of the two populations are the same because of their similarities in morphology, spacing between the filaments, aspect ratio, and magnetic energy densities to the thermal pressure of the medium and that both populations have undergone synchrotron aging. These similarities provide an opportunity to investigate the physical processes in the interstellar medium (ISM) and ICM for the first time. We consider that the origin of the filaments in both the Galactic center and ICM is a result of the interaction of a large-scale wind with clouds, or the filaments arise through the stretching and collection of field lines by turbulence in a weakly magnetized medium. We examine these ideas in four radio galaxy filaments associated with four radio galaxies—IC 40B, IC 4496, J1333–3141, ESO 137–006—and argue that much can be understood in the future by comparing these two populations of filaments.
We report the discovery of a widespread population of collisionally excited methanol J = 4 sub(-1) to 3 sub(0) E sources at 36.2 GHz from the inner 66' x 18' (160 x 43 pc) of the Galactic center. ...This spectral feature was imaged with a spectral resolution of 16.6 km s super(-1) taken from 41 channels of a Very Large Array continuum survey of the Galactic center region. The revelation of 356 methanol sources, most of which are maser candidates, suggests a large abundance of methanol in the gas phase in the Galactic center region. There is also spatial and kinematic correlation between SiO (2-1) and CH sub(3)OH emission from four Galactic center clouds: the +50 and +20 km s super(-1) clouds and G0.13-0.13 and G0.25 + 0.01. The enhanced abundance of methanol is accounted for in terms of induced photodesorption by cosmic rays as they travel through a molecular core, collide, dissociate, ionize, and excite Lyman Werner transitions of H sub(2). A time-dependent chemical model in which cosmic rays drive the chemistry of the gas predicts CH sub(3)OH abundance of 10 super(-8) to 10 super(-7) on a chemical timescale of 5 x 10 super(4) to 5 x 10 super(5) years. The average methanol abundance produced by the release of methanol from grain surfaces is consistent with the available data.
A revised set of propositions about ecology in arid Australia is presented, based on research literature since publication of Stafford Smith and Morton (1990). Fourteen propositions distil our ...argument that most features of the Australian deserts are explicable in terms of two dominant physical and climatic elements: rainfall variability, leading to extended droughts and occasional flooding rains; and widespread nutrient poverty. Different landscapes within the arid zone show these features to varying degrees, and so it is important to think about different places separately when considering our propositions. Plant life-histories strongly reflect temporal patterns of soil moisture; because Australian deserts receive more variable rainfall than most others, there is a distinctive spectrum of life-histories. Low levels of phosphorus (together with abundant soil moisture on irregular occasions) favour plants producing a relative excess of carbohydrate (C). In turn, C-rich plant products sometimes lead to fire-prone ecosystems, assemblages dominated by consumers of sap and other C-based products, and abundant detritivores (particularly termites). Fluctuations in production due to variable rainfall provide openings for consumers with opportunistic life-histories, including inhabitants of extensive but ephemeral rivers and lakes. Most consumer species exhibit some dietary flexibility or utilise more dependable resources; these strategies give rise to greater stability in species dynamics and composition of assemblages than might first be imagined under the variable rainfall regime. Aboriginal people have had long-standing ecological influence as they accessed resources. For each proposition we suggest the extent to which it is ‘different’, ‘accentuated’ or ‘universal’ in comparison with other deserts of the world, recognising that this categorisation is in need of critical testing. Further tests of each proposition are also suggested to fill the many gaps that still exist in our knowledge of the structure and functioning of Australia’s deserts.
► Revised propositions about arid Australia are presented based on research since Stafford Smith and Morton (1990, Journal of Arid Environments 18, 255-278. ► Australian plant life-histories distinctively reflect variable temporal patterns of soil moisture. ► Low phosphorus levels favour plants producing a relative excess of carbohydrate, leading to fire-proneness and abundant detritivores. ► Irregular fluctuations in production due to variable rainfall encourage consumers with opportunistic life-histories.
Aims.
In this study we determine the morphology and mass-loss rate of jets emanating from the companion in post-asymptotic giant branch (post-AGB) binary stars with a circumbinary disc. In doing so ...we also determine the mass-accretion rates onto the companion, and investigate the source feeding the circum-companion accretion disc.
Methods.
We perform a spatio-kinematic modelling of the jet of two well-sampled post-AGB binaries, BD+46°442 and IRAS 19135+3937, by fitting the orbital phased time series of H
α
spectra. Once the jet geometry, velocity, and scaled density structure are computed, we carry out radiative transfer modelling of the jet for the first four Balmer lines to determine the jet densities, thus allowing us to compute the jet mass-loss rates and mass-accretion rates. We distinguish the origin of the accretion by comparing the computed mass-accretion rates with theoretically estimated mass-loss rates, both from the post-AGB star and from the circumbinary disc.
Results.
The spatio-kinematic model of the jet reproduces the observed absorption feature in the H
α
lines. The jets have an inner region with extremely low density in both objects. The jet model for BD+46°442 is tilted by 15° with respect to the orbital axis of the binary system. IRAS 19135+3937 has a smaller tilt of 6°. Using our radiative transfer model, we find the full 3D density structure of both jets. By combining these results, we can compute the mass-loss rates of the jets, which are of the order of 10
−7
− 10
−5
M
⊙
yr
−1
. From this we estimate mass-accretion rates onto the companion of 10
−6
− 10
−4
M
⊙
yr
−1
.
Conclusions.
Based on the mass-accretion rates found for these two objects, we conclude that the circumbinary disc is most likely the source feeding the circum-companion accretion disc. This is in agreement with the observed depletion patterns in post-AGB binaries, which is caused by re-accretion of gas from the circumbinary disc that is under-abundant in refractory elements. The high accretion rates from the circumbinary disc imply that the lifetime of the disc will be short. Mass transfer from the post-AGB star cannot be excluded in these systems, but it is unlikely to provide a sufficient mass-transfer rate to sustain the observed jet mass-loss rates.
ABSTRACT
Ecosystem monitoring is fundamental to our understanding of how ecosystem change is impacting our natural resources and is vital for developing evidence‐based policy and management. However, ...the different types of ecosystem monitoring, along with their recommended applications, are often poorly understood and contentious. Varying definitions and strict adherence to a specific monitoring type can inhibit effective ecosystem monitoring, leading to poor program development, implementation and outcomes. In an effort to develop a more consistent and clear understanding of ecosystem monitoring programs, we here review the main types of monitoring and recommend the widespread adoption of three classifications of monitoring, namely, targeted, surveillance and landscape monitoring. Landscape monitoring is conducted over large areas, provides spatial data, and enables questions relating to where and when ecosystem change is occurring to be addressed. Surveillance monitoring uses standardised field methods to inform on what is changing in our environments and the direction and magnitude of that change, whilst targeted monitoring is designed around testable hypotheses over defined areas and is the best approach for determining the causes of ecosystem change. The classification system is flexible and can incorporate different interests, objectives, targets and characteristics as well as different spatial scales and temporal frequencies, while also providing valuable structure and consistency across distinct ecosystem monitoring programs. To support our argument, we examine the ability of each monitoring type to inform on six key types of questions that are routinely posed for ecosystem monitoring programs, such as where and when change is occurring, what is the magnitude of change, and how can the change be managed? As we demonstrate, each type of ecosystem monitoring has its own strengths and weaknesses, which should be carefully considered relative to the desired results. Using this scheme, scientists and land managers can design programs best suited to their needs. Finally, we assert that for our most serious environmental challenges, it is essential that we include information from each of these monitoring scales to inform on all facets of ecosystem change, and this is best achieved through close collaboration between the scales. With a renewed understanding of the importance of each monitoring type, along with greater commitment to monitor cooperatively, we will be well placed to address some of our greatest environmental challenges.
ABSTRACT
We study the environment of Sgr A* using spectral and continuum observations with the Atacama Large Millimeter/submillimeter Array and Very Large Array. Our analysis of subarcsecond H30 α, ...H39 α, H52 α , and H56 α line emission towards Sgr A* confirms the recently published broad-peak ∼500 km s−1 spectrum towards Sgr A*. We also detect emission at more extreme radial velocities peaking near −2500 and 4000 km s−1 within 0.2 arcsec. We then present broad-band radio continuum images at multiple frequencies on scales from arcseconds to arcminutes. A number of elongated continuum structures lie parallel to the Galactic plane, extending from ∼0.4 arcsec to ∼10 arcmin. We note a non-thermal elongated structure on an arcminute scale emanating from Sgr A* at low frequencies between 1 and 1.4 GHz where thermal emission from the minispiral is depressed by optical depth effects. The position angle of this elongated structure and the sense of motion of ionized features with respect to Sgr A* suggest a symmetric, collimated jet emerging from Sgr A* with an opening angle of ∼30○ and a position angle of ∼60○ punching through the medium before accelerating a significant fraction of the orbiting ionized gas to high velocities. The jet with an estimated mass flow rate of ∼1.4 × 10−5 M⊙ yr−1 emerges perpendicular to the equatorial plane of the accretion flow near the event horizon of Sgr A* and runs along the Galactic plane. To explain a number of east–west features near Sgr A*, we also consider the possibility of an outflow component with a wider angle launched from the accretion flow at larger radii.
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