Hydrologic sources of carbon cycling uncertainty throughout the terrestrial–aquatic continuum

Linking hydrologic interactions with global carbon cycling will reduce the uncertainty associated with scaling-up empirical studies and facilitate the incorporation of terrestrial–aquatic linkages within global and regional change models. Much of the uncertainty in estimates of carbon fluxes associated with precipitation and hydrologic transport results from the extensive spatial and temporal heterogeneity in both intrinsic functioning and anthropogenic modification of hydrological cycles. To better understand this variation we developed a landscape ecological approach to coupled hydrologic–carbon cycling that merges local mechanisms with multiple-scale spatial heterogeneity. This spatially explicit framework is applied to examine variability in hydrologic influences on carbon cycling along a continental scale water availability gradient with an explicit consideration of human sources of variability. Hydrologic variation is an important component of the uncertainty in carbon cycling; accounting for this variation will improve understanding of current conditions and projections of future ecosystem responses to global change.     

Along the speciation continuum in sticklebacks

Speciation can be viewed as a continuum, potentially divisible into several states: (1) continuous variation within panmictic populations, (2) partially discontinuous variation with minor reproductive isolation, (3) strongly discontinuous variation with strong but reversible reproductive isolation and (4) complete and irreversible reproductive isolation. Research on sticklebacks (Gasterosteidae) reveals factors that influence progress back and forth along this continuum, as well as transitions between the states. Most populations exist in state 1, even though some of these show evidence of disruptive selection and positive assortative mating. Transitions to state 2 seem to usually involve strong divergent selection coupled with at least a bit of geographic separation, such as parapatry (e.g. lake and stream pairs and mud and lava pairs) or allopatry (e.g. different lakes). Transitions to state 3 can occur when allopatric or parapatric populations that evolved under strong divergent selection come into secondary contact (most obviously the sympatric benthic and limnetic pairs), but might also occur between populations that remained in parapatry or allopatry. Transitions to state 4 might be decoupled from these selective processes, because the known situations of complete, or nearly complete, reproductive isolation (Japan Sea and Pacific Ocean pair and the recognized gasterosteid species) are always associated with chromosomal rearrangements and environment‐independent genetic incompatibilities. Research on sticklebacks has thus revealed complex and shifting interactions between selection, adaptation, mutation and geography during the course of speciation.     

Variation in allometry along the weapon-signal continuum

Evolutionary Ecology - Structures used in intrasexual competition span a continuum, with pure weapons that are used exclusively in physical fights at one extreme and pure aggressive signals that...     

Instability of the sexual continuum

Maynard Smith and Szathmary have posed the problem of demonstrating the conjectured instability of a continuum of sexual types with finite interbreeding. Here, I propose a model in which one can analyse exactly when and how the existence of the instability can depend on an Allee effect, and how the growth rate and typical scale of the unstable perturbations depend on the strength and range of competition, mating preference, fecundity and offspring variance due to Mendelian segregation and mutation. Instabilities of various kinds are shown to occur in the majority of parameter regimes. In short, the continuum often breaks up into incipient species.     

Platelets and the immune continuum

Platelets are anucleate cells that are crucial mediators of haemostasis. Most immunologists probably don't think about platelets every day, and may even consider these cells to be 'nuisances' in certain in vitro studies. However, it is becoming increasingly clear that platelets have inflammatory functions and can influence both innate and adaptive immune responses. Here, we discuss the mechanisms by which platelets contribute to immunity: these small cells are more immunologically savvy than we once thought.     

Limitations of the continuum assumption in cancellous bone

Most existing stress analyses of the skeleton which consider cancellous bone assume that it can be modelled as a continuum. In this paper we develop a criterion for the validity of this assumption. The limitations of the continuum assumption appear in two areas: near biologic interfaces, and in areas of large stress gradients. These limitations are explored using a probabilistic line scanning model for density measurement, resulting in an estimate of density accuracy as a function of line length which is experimentally verified. Within three to five trabeculae of an interface, a continuum model is suspect. When results as predicted using continuum analyses vary by more than 20-30% over a distance spanning three to five trabeculae, the results are suspect.     

Aluminium in human sweat

It is of burgeoning importance that the human body burden of aluminium is understood and is measured. There are surprisingly few data to describe human excretion of systemic aluminium and almost no reliable data which relate to aluminium in sweat. We have measured the aluminium content of sweat in 20 healthy volunteers following mild exercise. The concentration of aluminium ranged from 329 to 5329 μg/L. These data equate to a daily excretion of between 234 and 7192 μg aluminium and they strongly suggest that perspiration is the major route of excretion of systemic aluminium in humans.     

Fatty acid cycling in the fasting rat

Adipose tissue lipolysis and fatty acid reesterification by liver and adipose tissue were investigated in rats fasted for 15 h under basal and calorigenic conditions. The fatty acid flux initiated by adipose fat lipolysis in the fasted rat is mostly futile and is characterized by reesterification of 57% of lipolyzed free fatty acid (FFA) back into adipose triglycerides (TG). About two-thirds of FFA reesterification are carried out before FFA release into plasma, whereas the rest consists of plasma FFA extracted by adipose tissue. Thirty-six percent of the fasting lipolytic flux is accounted for by oxidation of plasma FFA, whereas only a minor fraction is channeled into hepatic very low density lipoprotein-triglycerides (VLDL-TG). Total body calorigenesis induced by thyroid hormone treatment and liver-specific calorigenesis induced by treatment with beta, beta'-tetramethylhexadecanedioic acid (Medica 16) are characterized by a 1.7- and 1.3-fold increase in FFA oxidation, respectively, maintained by a 1.5-fold increase in adipose fat lipolysis. Hepatic reesterification of plasma FFA into VLDL-TG is negligible under both calorigenic conditions. Hence, total body fatty acid metabolism is regulated by adipose tissue as both source and sink. The futile nature of fatty acid cycling allows for its fine tuning in response to metabolic demands.     

Identifying the structure of cycling in ecosystems

Given a steady-state network of flows within an ecosystem, it becomes possible to systematically and automatically enumerate all distinct simple cycles of flows. Cycles may be grouped according to shared smallest arcs. The original network may then be decomposed into a web of purely cycled flow and a residual acyclic graph consisting of once-through pathways. The aggregation of elementary cycles according to shared vulnerable arcs seems particularly effective in locating those transfers which stress is most likely to disturb.     

Sulfur cycling in forests

Sulfur is essential for the production of certain amino acids in plants. As amino acid sulfur is the major form of sulfur in trees, there is a strong relationship between organic S and organic N in tree tissue. Sulfur deficiencies occur in parts of southeastern Australia and northwestern North America, remote from pollutant inputs. Since bilogical S requirements of forests are modest (< 5 kg · ha^–1 yr^–1 for net vegetative increment), however, atmospheric S inputs in polluted regions (10–80 kg · ha^–1 yr^–1 ) often exceed not only the forest ecosystem S requirement but also its ability to biologically accumulate S. There is some increase in the SO^2– ₄–S content of forest vegetation in response to elevated atmospheric S inputs, but this capacity is apparently easily saturated. Soil SO^2–2 ₄adsorption is often the dominant feature of S cycling in polluted ecosystems and often accounts for net ecosytem S accumulations.Contribution from a symposium on the role of sulfur in ecosystem processes held August 10, 1983, at the annual meeting of the A.I.B.S., Grand Forks, ND; Myron Mitchell, convenor.     

The continuum and the individualistic association

Summary A geometrical model is described, within whose framework classes and continua of vegetation can both be accommodated; it is postulated that each point of view may be appropriate in different cases.It is shewn the three antitheses — between classification and a continuum; between the individualistic and the integrated view of the plant community; and between continuous intergradation in the field of different vegatation types, and the existence of distinct stands with well-marked boundaries —have no necessary connexion with one another, and that any combination of these views can be held without inconsistency. All three antitheses stand in need of extensive objective tests — not with a view to accepting one alternative and rejecting the other, but to determine how far each view is applicable.The a priori and arguments in favour of the existence of integrated plant communities are detailed, and a plea is made for the extension of work designed to ascertain whether, and how widely, such integrated plant communities do in fact occur.

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Résumé Un modèle géométrique est formulé dans le cadre duquel les classements et les continua de la végétation peuvent être englobés. On admet que chaque point de vue puisse être juste dans les cas divers.Les trois antithèses — entre la classification et les continua, entre les associations individualisées et intégrées, et entre les communautés végétales aux limites définies et les transitions continues — ne se rapportent pas nécessairement, et n'importe quelle combinaison de ces idées peut être soutenue sans desaccord. Toutes les trois ont besoin d'épreuves objectives et amples, non pour accepter l'une en rejetant l'autre, mais pour savoir jusqu'à quel point chacune peut se trouver valable.Les thèses a priori en faveur de l'existence des communautés végétales intégrées sont détaillées et on fait appel à l'extension des travaux destinés à constater l'existence réelle et l'importance de tels communautés.

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Die Redaktion von Vegetatio hält es für angebracht, neben den ihrigen auch Ansichten, die sie nicht oder doch nicht durchgehends teilt, hervortreten zu lassen. Wir sind überzeugt, dass die Pflanzendecke der Erde in ihrem ganzen Ausmass der Unterteilung in pflanzensoziologische Gruppierungen höheren oder niedrigeren Ranges zugänglich ist; ihre Begrenzung kann scharf oder aber weniger scharf und gleitend sein.     

Hybrid zones and the speciation continuum in Heliconius butterflies

Tropical butterflies in the genus Heliconius have long been models in the study of the stages of speciation. Heliconius are unpalatable to predators, and many species are notable for multiple geographic populations with striking warning colour pattern differences associated with Müllerian mimicry. A speciation continuum is evident in Heliconius hybrid zones. Examples range from hybrid zones across which (a) there is little genetic differentiation other than at mimicry loci, but where hybrids are common, (b) to ‘bimodal‘ hybrid zones with strong genetic divergence and few hybrids, (c) through to ‘good’ sympatric species, with hybridization extremely rare or absent. Now, in this issue of Molecular Ecology, Arias et al. ( 2012 ) have found an intermediate case in Colombian Heliconius cydno showing evidence for assortative mating and molecular differences, but where hybrids are abundant.