Nitrogen fixation in a desert stream ecosystem

Few measurements of nitrogen fixation exist for streams. Desertstreams are warm, well lighted, and often supportabundant cyanobacterial populations; thus N₂ fixationmay be significant in these N-poor ecosystems. N₂fixation was measured in situ by acetylene reductionfor two patch types (Anabaena mat and anepilithic assemblage). Patch-specific rates were highcompared with published values (maximum 775 µgN₂ [83 µmol C₂H₄]mg chl a ^-1 h^-1or 51 mg N₂ [5.4 mmol C₂H₄] m_-2 h_-1).Daytime fixation was higher than nighttimefixation, and temperature, light and inorganic Nconcentration explained 52% of variance in hourlyrates over all dates. Diel input-output budgets wereconstructed on five dates when cyanobacteria werepresent in the stream. Diel N₂ fixation rates weremeasured for comparison with reach-scale diel nitrogenretention, to assess the importance of this vector to Neconomy of the stream. Fixation accounted for up to85% of net N flux to the benthos, but its importancevaried seasonally. Finally, we applied biomass-specificfixation rates to 1992 and 1993 biomass data to obtainseasonal and annual N₂ fixation estimates.Cyanobacteria were absent or rare during winter andspring, thus most of the annual N₂ fixation occurredduring summer and autumn. Annual rates of nitrogenfixation for 1992 and 1993 (8.0 g/m₂ and 12.5g/m₂) were very high compared to other streams,and moderately high compared to other ecosystems.Like other phenomena in this disturbance-proneecosystem, nitrogen fixation is strongly influenced bythe number and temporal distribution of flood events.     

Osteology of the South Asian Genus Psilorhynchus McClelland, 1839 (Teleostei: Ostariophysi: Psilorhynchidae), with investigation of its phylogenetic relationships within the order Cypriniformes

Members of the genus Psilorhynchus are small benthic fishes, commonly referred to as torrent minnows, which inhabit the fast to swift flowing water bodies of the Indo‐Burma region and the Western Ghats of Peninsular India. Despite being described scientifically in the mid 18^th century, the morphology of Psilorhynchus remains poorly known and its phylogenetic placement within the order Cypriniformes is a matter of considerable debate. In this paper the osteology of Psilorhynchus sucatio is described and illustrated in detail. Notes and/or illustrations on the osteology of 12 other species of Psilorhynchus are also provided for the first time. A phylogenetic investigation of the position of Psilorhynchus within the order Cypriniformes is also conducted. Analysis of 127 morphological characters scored for 52 ingroup taxa (including 12 species of Psilorhynchus) and four outgroup taxa resulted in 14 equally parsimonious cladograms (287 steps long; consitency index, CI = 0.48; retention index, RI = 0.88). Psilorhynchus is recovered as the sister group to the family Cyprinidae, and is regarded as a member of the superfamily Cyprinoidea, which forms the sister group to the Cobitoidea (including all other cypriniform families). The sistergroup relationship between Psilorhynchus and Cyprinidae is supported by eight derived characters (five of which are homoplastic within the order Cypriniformes). The monophyly of Psilorhynchus is supported by 16 derived characters (eight of which are homoplastic within Cypriniformes). Three species groups of Psilorhynchus are proposed, the Psilorhynchus balitora group (including P. amplicephalus, P. balitora, P. breviminor, P. nepalensis, P. rahmani, P. pavimentatus, and P. brachyrhynchus), the Psilorhynchus gracilis group (including P. gracilis, P. melissa, P. robustus, and P. tenura), and the Psilorhynchus homaloptera group (including P. arunachalensis, P. homaloptera, P. microphthalmus, and P. pseudecheneis). The continued use of the family group name Psilorhynchidae is recommended. Comments on the interrelationships of the Cypriniformes are also provided. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011.     

Population Interactions and the Determinants of Population Size

Abstract The flux of individuals within populations is dependent upon six controlling processes: the intrinsic rate of increase of the plant, intraspecific competition for resources, interspecific competition, natural enemies, mutualisms and refuge effects such as the immigration of seeds from other populations. Although population interactions are generally believed to play a major role in determining the flux of individuals within populations, little attempt has been made to quantify the strength of these interactions and their role in the dynamics of populations. In this paper we examine the role of competition, herbivory and mutualistic interactions in determining the dynamics of a range of annual plant species. Firstly, it is shown that the dynamics of three weed species ( Bromus sterilis, Galium aparine, Papaver rhoeas ) in an experimental community in an arable cropping system of winter wheat are determined primarily by the rapid population growth of B. sterilis . Interactions between the species play a minor part in the dynamics of the system. Secondly, it is shown that current levels of grazing by overwintering populations of brent geese have a minor impact on the abundance of Salicornia europaea , but that increased grazing has the potential to reduce abundance and increase the instability of S. europaea populations. This is a consequence of the aggregative response of the geese, which results in an increasing proportion of the seeds of S. europaea being eaten as plant density increases. Thirdly, it is shown that there is a complex interaction between root pathogenic and arbuscular mycorrhizal fungi in natural field populations of Vulpia ciliata and that the benefit of mycorrhizal fungi to the plant is in providing protection against pathogens.     

The trackmaker of Apatopus (Late Triassic, North America): implications for the evolution of archosaur stance and gait

Abstract:  For some decades, a major focus of research has been on how locomotor modes changed in some archosaurian reptiles from a more or less 'sprawling' to an 'erect' posture, whether there were discrete intermediate stages, and how many times 'erect' posture evolved. The classic paradigm for the evolution of stance and gait in archosaurs, a three-stage transition from sprawling to 'semi-erect' to erect posture, has been replaced by a subtler understanding of a continuum of changing limb joint angles. We suggest a further separation of terminology related to stance vs. gait so as not to entail different processes: 'sprawling' and 'erect' should refer to continua of stance; 'rotatory' and 'parasagittal' are more appropriate ends of a continuum that describes the motions of gait. We show that the Triassic trackway Apatopus best fits the anatomy and proportions of phytosaurs, based on a new reconstruction of their foot skeleton; it is less likely to have been made by another pseudosuchian or non-archosaurian archosauromorph. Moreover, the trackmaker was performing the high walk. A phytosaurian trackmaker would imply that the common ancestor of pseudosuchians, and therefore archosaurs could approximate the high walk (depending on phylogeny), and if so, erect stance and parasagittal gait did not evolve independently in pseudosuchians and ornithosuchians, although the kinematic mechanisms differed in the two groups. It remains to be seen how far outside Archosauria, if at all, more or less erect posture and parasagittal gait may have evolved.     

Instream habitat use by blue duck (Hymenolaimus malacorhynchos) in a New Zealand river

1. The feeding habitat of a river specialist, blue duck (Hymenolaimus malacorhynchos (Gmelin 1789): Anatidae), was characterized in terms of water depth and velocity on eight occasions over a 13-month period in a river in the central North Island of New Zealand using video to record activity and relocate feeding sites. 2. Of the five feeding activities identified (‘pecking’, ‘grazing’, ‘head-dipping’, up-ending’ and ‘diving’), adult blue duck used mostly head-dipping (> 60% of feeding events on all dates), although diving or grazing from submerged surfaces of exposed boulders comprised major proportions of feeding behaviour (up to 33%) on occasions. Variations in feeding behaviour between dates partly reflected changes in antecedent flow conditions and the annual cycle of the birds. 3. Grazing and diving occurred in significantly faster water (mostly 0.3–0.45 m s^–1) and at significantly different depths (mean = 0.10 and 0.55 m, respectively) than head-dipping (0.20 m depth and 0.28 m s^–1 velocity). Adult feeding depths and velocities at four sites on different dates averaged 0.20 m and 0.31 m s^–1, respectively. Most feeding by 3–4-week-old ducklings occurred over a similar distribution of water velocities to adults but over a wider range of depths. 4. Adult birds fed in significantly shallower and lower velocity water than was available on the two dates that comparisons could be made. Ducklings also fed over a slower range of water velocities but were not selective in terms of water depth. 5. Energetically more expensive search methods were employed at times of high apparent energy demand to access flow microhabitats where larger bodied prey were more likely to be encountered. 6. These data indicate that, like other aquatic organisms, river birds can be influenced by basic hydraulic elements of river flow, but show at the same time that adult blue duck can accommodate variable lotic environments efficiently.     

The relationship between geographic area and the latitudinal gradient in species richness in New World birds

One hypothesis for the latitudinal gradient in species richness observed in most animal taxa is that the richness of a region is determined by its geographic area. However, the relationship between geographic area and species richness across regions is generally weak. It has been suggested that this is because species from the tropics spill out of this region of high richness, artificially inflating the richness of other regions. This generates the interesting prediction that the area and richness of extra-tropical regions should be more strongly correlated if tropical species are excluded. We test this prediction using the avifauna of the New World. We find that there is indeed a relationship between the land area and species richness of a region once tropical species are excluded. This relationship is independent of the latitude and productivity of regions. Both latitude and productivity can explain variance in richness unexplained by land area. There is no relationship between land area and species richness if tropical species are not excluded from the analysis, suggesting that tropical species do indeed mask the relationship between richness and area. We conclude that our results generally support the geographic area hypothesis, although tests of its other predictions and on other land masses are required.     

Exploring links between physiology and ecology at macro-scales: the role of respiratory metabolism in insects

The relationships between macro-ecological patterns and physiological investigations in insects, especially those dealing with respiratory metabolism, are assessed in an attempt to encourage the development of the interaction between macroecology and physiological ecology. First, we demonstrate that although physiological ecology has been explicitly concerned with a number of issues relating to species boundaries, many questions remain unanswered. We argue that there are essentially two ways in which the relationship between physiological tolerances and species range boundaries have been investigated. The correlational approach involves physiological inference, physiological prediction, isocline analyses and climatic matching, and has often been criticized for a lack of rigour, while the experimental approach seeks to examine experimentally the relationships between physiological variables and range edges. Second, we use the recent debate on processes underlying latitudinal patterns in body size to caution against the conflation of patterns and processes operating at intraspecific and interspecific levels, the dangers inherent in invoking single explanatory variables, and an undue focus on adaptationist (e.g. optimization) rather than nonadaptationist explanations or some combination of the two. We show that both positive and negative relationships between body size and latitude have been found at the intraspecific level and suggest that interactions between temperature-induced heterochrony, and the relationship between habitat durational stability, growing season length, and generation time can be used to explain these differences. Similar variation in documented patterns is demonstrated at the interspecific level, and the mechanisms usually proffered to explain such clines (especially the starvation/desiccation-resistance hypothesis) are discussed. Interactions between various environmental factors, such as host-plant quality, and their effects on size clines are also discussed. Third, we argue that respiratory metabolism, as a measure of ATP cost, and its spatio-temporal variation are critical to many explanations of macroecological patterns. Adaptive changes in metabolism reputedly involve both depression (stress resistance) and elevation of metabolic rate, although recent studies are increasingly calling these ideas into question. In particular, flow-through respirometry is revolutionizing results by allowing careful separation of resting (or standard) and active metabolic rates. These techniques have rarely been applied to studies of metabolic cold adaptation in insects, one of the most polemical adaptations ascribed to high-latitude and high-altitude species. We conclude by arguing that physiological investigations of species tolerances are important in the context of macroecology, especially species distributional patterns and the possible impact of climate change thereon. However, we caution that relationships between abiotic variables, species tolerances, and distributional ranges may be non-linear and subject to considerable modification by the presence of other species, and that many of the pressing questions posed by macroecology have not been addressed by insect physiologists. Nonetheless, we suggest that because an understanding of the dynamics of species distributions is of considerable importance, especially in the context of current conservation problems, insect physiological ecology has much future scope.