Indirect effects of introduced trout on Cascades frogs (Rana cascadae) via shared aquatic prey

1. The introduction of trout to montane lakes has negatively affected amphibian populations across the western United States. In northern California’s Klamath–Siskiyou Mountains, introduced trout have diminished the distribution and abundance of a native ranid frog, Rana (=Lithobates) cascadae. This is primarily thought to be the result of predation on frog larvae. However, if trout feed on larval aquatic insects that are available to R. cascadae only after emergence, then resource competition may also affect this declining native amphibian. 2. Stomach contents of R. cascadae were compared between lakes that contained trout and those from which introduced trout were removed. Total prey mass in stomach contents relative to frog body mass was not significantly different between lakes with fish and fish‐removal lakes, but in the former R. cascadae consumed a smaller proportion of adult aquatic insects. The stomach contents of fish included larvae of aquatic insects that are, as adults, eaten by R. cascadae. 3. Rana cascadae consumed fewer caddisflies (Trichoptera) and more grasshoppers (Orthoptera) at lakes with higher densities of fish. At lakes with greater aquatic habitat complexity, R. cascadae consumed more water striders (Hemiptera: Gerridae) and terrestrial spiders (Araneae). 4. We suggest that reductions in the availability of emerging aquatic insects cause R. cascadae to consume more terrestrial prey where trout are present. Thus, introduced trout may influence native amphibians directly through predation and, indirectly, through pre‐emptive resource competition.     

Tansley Review No. 110.

S UMMARY 367
I. I NTRODUCTION 367
II. N UMBER 368
III. S IZE 379
IV. A IR SPACE IN THE SEEDS 381
V. F LOATATION AND DISPERSAL 383
1. Air 383
(a) Physical considerations 383
(b) Dispersal 387
(c) Birds 415
2. Water 416
(a) Physical considerations 416
(b) Dispersal 416
VI. C ONCLUSIONS 417
Acknowledgements 417
References 418
Orchid seeds are very small, extremely light and produced in great numbers. Most range in length from c . 0.05 to 6.0 mm, with the difference between the longest and shortest known seeds in the family being 120-fold. The 'widest' seed at 0.9 mm is 90-fold wider than the 'thinnest' one, which measures 0.01 mm (because orchid seeds are tubular or balloon-like, 'wide' and 'thin' actually refer to diameter). Known seed weights extend from 0.31 lg to 24 μg (a 78-fold difference). Recorded numbers of seeds per fruit are as high as 4000000 and as low as 20–50 (80000–200000-fold difference). Testae are usually transparent, with outer cell walls that may be smooth or reticulated. Ultrasonic treatments enhance germination, which suggests that the testae can be restrictive. Embryos are even smaller: their volume is substantially smaller than that of the testa. As a result, orchid seeds have large internal air spaces that render them balloon-like. They can float in the air for long periods, a property that facilitates long-distance dispersal. The difficult-to-wet outer surfaces of the testa and large internal air spaces enable the seeds to float on water for prolonged periods. This facilitates distribution through tree effluates and/or small run-off rivulets that may follow rains. Due to their size and characteristics, orchid seeds may also be transported in and on land animals and birds (in fur, feathers or hair, mud on feet, and perhaps also following ingestion).     

A LuxR homologue of Xanthomonas oryzae pv. oryzae is required for optimal rice virulence

In Gram-negative bacteria a typical quorum sensing (QS) system usually involves the production and response to acylated homoserine lactones (AHLs). An AHL QS system is most commonly mediated by a LuxI family AHL synthase and a LuxR family AHL response regulator. This study reports for the first time the presence of a LuxR family-type regulator in Xanthomonas oryzae pv. oryzae ( Xoo ), which has been designated as OryR. The primary structure of OryR contains the typical signature domains of AHL QS LuxR family response regulators: an AHL-binding and a HTH DNA binding motif. The oryR gene is conserved among 26 Xoo strains and is also present in the genomes of close relatives X. campestris pv. campestris and X. axonopodis pv. citri . Disrupting oryR in three Xoo strains resulted in a significant reduction of rice virulence. The wild-type Xoo strains do not seem to produce AHLs and analysis of the Xoo sequenced genomes did not reveal the presence of a LuxI-family AHL synthase. The OryR protein was shown to be induced by macerated rice and affected the production of two secreted proteins: a cell-wall-degrading cellobiosidase and a 20-kDa protein of unknown function. By expressing and purifying OryR it was then observed that it was solubilized when grown in the presence of rice extract indicating that there could be a molecule(s) in rice which binds OryR. The role of OryR as a possible in planta induced LuxR family regulator is discussed.     

Immediate and long-term nitrogen oxide emissions from tropical forest soils exposed to elevated nitrogen input

Tropical nitrogen (N) deposition is projected to increase substantially within the coming decades. Increases in soil emissions of the climate‐relevant trace gases NO and N₂O are expected, but few studies address this possibility. We used N addition experiments to achieve N‐enriched conditions in contrasting montane and lowland forests and assessed changes in the timing and magnitude of soil N‐oxide emissions. We evaluated transitory effects, which occurred immediately after N addition, and long‐term effects measured at least 6 weeks after N addition. In the montane forest where stem growth was N limited, the first‐time N additions caused rapid increases in soil N‐oxide emissions. During the first 2 years of N addition, annual N‐oxide emissions were five times (transitory effect) and two times (long‐term effect) larger than controls. This contradicts the current assumption that N‐limited tropical montane forests will respond to N additions with only small and delayed increases in soil N‐oxide emissions. We attribute this fast and large response of soil N‐oxide emissions to the presence of an organic layer (a characteristic feature of this forest type) in which nitrification increased substantially following N addition. In the lowland forest where stem growth was neither N nor phosphorus (P) limited, the first‐time N additions caused only gradual and minimal increases in soil N‐oxide emissions. These first N additions were completed at the beginning of the wet season, and low soil water content may have limited nitrification. In contrast, the 9‐ and 10‐year N‐addition plots displayed instantaneous and large soil N‐oxide emissions. Annual N‐oxide emissions under chronic N addition were seven times (transitory effect) and four times (long‐term effect) larger than controls. Seasonal changes in soil water content also caused seasonal changes in soil N‐oxide emissions from the 9‐ and 10‐year N‐addition plots. This suggests that climate change scenarios, where rainfall quantity and seasonality change, will alter the relative importance of soil NO and N₂O emissions from tropical forests exposed to elevated N deposition.     

Unravelling causal components of the Ordovician Radiation: the Builth Inlier (central Wales) as a case study

Hypotheses about the causes of biodiversification during the Ordovician have been focused in three main areas: tectonic activity and nutrient supply, palaeogeography, and ecological escalation. There is as yet no consensus on mechanisms, and it is unclear whether it is better to study the patterns at local or regional scales. By applying ecological knowledge to the available palaeontological information, it can be shown that neither tectonic nor palaeogeographic effects could account for the permanence of the diversity rise, in the absence of elements of ecological escalation. However, it may be possible to identify trigger mechanisms resulting in enhanced speciation or reduced extinction. Areas of local diversity increase should be distinguished from speciation centres. An ongoing study of the Middle Ordovician Builth-Llandrindod Inlier of central Wales, conducted over 10 years, has identified elements of all three of the above categories of causal mechanisms affecting local diversity. This implies that the patterns of causal relationship and diversification are complex even at very local scales, and at this stage we should not anticipate a clear correlation of global diversity with any single factor. More data are needed from small-scale but intensive studies before we can generalize about the causal mechanisms of the Ordovician Radiation.     

Refuge availability: a key to understanding the summer disappearance of Daphnia

SUMMARY. 1. The mid-summer declines of Daphnia species in three small lakes were investigated to examine the relative roles of reduced natality and increased mortality. Reduced natality (assessed by quantifying clutch size, lipid index, and available food) could not account for the decline in daphnid abundance in any of the populations examined.
2. The role of increased mortality imposed by zooplanktivorous fish was assessed by estimating the sizes of the mid-water refuge areas where daphnids could escape fish predation. The boundaries of the refuge areas were estimated from field measurements and literature values and were based on gradients of temperature, dissolved oxygen and light. Observed decreases in refuge thicknesses correlated well with the mid-summer declines of large-bodied Daphnia species in all three lakes. Intermediatesized daphnids were less affected as the refuges thinned and small-bodied species increased in abundance.
3. The importance of refuge thickness in modifying zooplanktivore-induced mortality was further tested in large enclosures where refuge thickness was experimentally modified. In the presence of zooplanktivorous fish, large-bodied Daphnia , which used the refuge, persisted when the refuge was thick but disappeared when it thinned. Daphnia galeata mendotae , which did not occupy the refuge zone, was rapidly eliminated regardless of refuge thickness. We conclude that refuge availability plays a major role in Daphnia population dynamics.     

SECOND MESSENGERS AND THE REGULATION OF CA 2+ FLUXES BY CA 2+ -MOBILIZING AGONISTS IN RAT LIVER

Knowledge of the mechanism of action of Ca²⁺-mobilizing agonists in liver has progressed considerably following the discovery that their interaction with specific receptors on the plasma membrane is accompanied by the hydrolysis of PIP₂ and the generation of the second messengers diacylglycerol and IP₃, for the activation of protein kinase C and the mobilization of intracellular Ca²⁺, respectively. Although the second messenger functions of diacylglycerol and IP₃ in these actions seem well established, it is not yet clear how the agonists are able to regulate Ca²⁺ influx across the plasma membrane, an event which is crucial for those actions of the agonists which are dependent on the maintenance of an elevated level of cytosolic Ca²⁺, Whilst there is evidence for the existence of more than one pathway for Ca²⁺ influx in liver, it appears that in each instance the Ca²⁺ influx process is regulated differently to the Ca²⁺ influx through the volage-sensitive Ca²⁺ channels that is known to occur in excitable tissues. At present it is not clear whether any of the Ca²⁺ influx pathways in liver is regulated by direct coupling to the agonist receptor mechanism on the outer surface of the plasma membrane, or whether the regulation involves the production of some second messenger(s). However, indirect evidence from a number of tissues appears to favour the involvement of both IP₃ and IP₄ in the regulation of Ca²⁺ influx. The mechanism by which IP₃ and IP₄ may regulate Ca²⁺ influx remains to be established, but it has been proposed that Ca²⁺ entry into the cell occurs through a pathway connecting the plasma membrane and the endoplasmic reticulum, following the release of intracellular Ca²⁺ from the lumen of the endoplasmic reticulum. Although it is not yet known whether glucagon (or cyclic AMP) activates the same pathway for Ca²⁺ influx as Ca²⁺-mobilizing agonists, the marked potentiation by cyclic AMP of the Ca²⁺ influx induced by Ca²⁺-mobilizing agonists has provided a powerful system with which to study the regulation of Ca²⁺ influx in liver. Whether this Ca²⁺ influx process occurs through some ion exchange mechanism (such as Ca²⁺/Na⁺ exchange) remains to be determined. Results from this study suggests that the Ca²⁺ influx is inhibited by neomycin, acidic pH, and a depolarization of the plasma membrane. The observation that cyclic AMP synergistically potentiates the influx of Ca²⁺ induced by Ca²⁺-mobilizing agonists, that this influx appears to correlate with the reported ability of these agonists to induce PIP₂ hydrolysis and accumulation of IP₃, and that cyclic AMP synergistically potentiates the production of IP₄ by vasopressin, are all consistent with the notion that IP₃ and IP₄ are involved in regulating Ca²⁺ influx. Whilst little is known about the Ca²⁺ transport process itself, these studies coupled with the recent finding that Ca²⁺ influx into the liver cell can occur through different pathways, seem set to lead to a better understanding of this important process in the near future.     

Pigment Cell Refugia in Homeotherms—The Unique Evolutionary Position of the Iris

Homeotherms are generally considered to lack classical active dermal pigment cells (chromatophores) in their integument, attributable to the development of an outer covering coat of hair or feathers. However, bright colored dermal pigment cells, comparable to chromatophores of lower vertebrates, are found in the irides of many birds. We propose that, because of its exposed location, the iris is an area in which color from pigment cells has sustained a selective advantage and appears to have evolved independently of the general integument. In birds, the iris appears to have retained the potential for the complete expression of all dermal chromatophore types. Differences in cell morphology and the presence of unusual pigments in birds are suggested to be the result of evolutionary changes that followed the divergence of birds from reptiles. By comparison, mammals appear to have lost the potential for producing iridophores, xanthophores, or erythrophores comparable to those of lower vertebrates, even though some species possess brightly colored irides. It is proposed that at least one species of mammal (the domestic cat) has recruited a novel iridial reflecting pigment organelle originally developed in the choroidal tapetum lucidum. The potential presence of classical chromatophores in mammals remains open, as few species with bright irides have been examined.