Population dynamic consequences of competition within and between age classes

We investigate the population dynamics of a semivoltine species whose juvenile development takes two years to complete, and is followed by a very short reproductive adult stage. Reproduction is synchronized so at any given time the juvenile population consists of two cohorts. Coexistence of the two cohorts requires that the strength of intea-cohort competition exceeds that of inter-cohort competition, an extension of the competitive exclusion principle. The population may exhibit population cycles with low integer period. We identify two mechanisms for two-year cycles, and note that four-year cycles are a natural consequence of overcompensation. Three year cycles can occur and we discuss the mechanisms involved.     

A cross-diffusion model of forest boundary dynamics

A simple mathematical model of mono-species forest with two age classes which takes into account seed production and dispersal is presented in the paper. This reaction — diffusion type model is then reduced by means of an asymptotic procedure to a lower dimensional reaction — cross-diffusion model. The existence of standing and travelling wave front solutions corresponding to the forest boundary is shown for the later model. On the basis of the analysis, possible changes in forest boundary dynamics caused by antropogenic impacts are discussed.     

Population dynamics of Potamothrix alatus paravanicus Poddubnaya & Pataridze (Tubificidae) in different areas of Lake Sevan

Potamothrix alatus paravanicus Poddubnaya & Pataridze (Tubificidae) is the most abundant species of benthic invertebrate in Lake Sevan and the only species in the deep profundal. Differences in annual temperature, length of the stratification period (low oxygen content) and bottom sediment characteristics influence the population of P. a. paravanicus in different depth zones of Lake Sevan. From shallow to deep water a regular increase of the following indices was observed in 1984–1987: annual biomass from 1.2–2.0 to 8.2–17.0 g m^–2 wet weight; mean individual wet weight of clitellate specimens from 2.0–3.8 to 8.3–16.6 mg; total life span from one and a half to more than 20 years; the length of breeding season from 2–3 to 12 months.An important ecological distinction between P. a. paravanicus and allied Potamothrix hammoniensis (Michaelsen) is the much lower fecundity of the former species.     

Phytoplankton strategies and time scales in a eutrophic reservoir

Phytoplankton species groups were studied in a eutrophic reservoir at different time scales (daily, weekly, monthly and yearly). Four strategic groups along the r-K continuum were defined and their seasonal time courses were followed. Their temporal distribution of relative biomass reflected resource partitioning because each strategic group dominated the phytoplankton community at different times in the year. However, the relationships between strategic groups changed with the time scale involved. At the daily scale an inverse relationship between r- and K-groups occurred whereas at supradaily scales such relationships did not hold. Species groups reflected strategic groups. No relationship between population growth and losses was found, suggesting that both were not counterbalanced. In the long term (supraannual) the phytoplankton community changed very much in this reservoir, its ecological memory being small as compared with that of lakes.     

Dynamic properties of water at phosphatidylcholine lipid-bilayer surfaces as seen by deuterium and pulsed field gradient proton NMR

The dynamic properties of water in phosphatidylcholine lipid/water dispersions have been studied, applying a combination of ²H-NMR techniques (quadrupole splitting and spin-lattice relaxation time) and self-diffusion measurements using pulsed field gradient (PFG) ¹H-NMR. The hydration properties of POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine) were compared with those of DOPC (1,2-dioleoyl-sn-glycero-3-phosphatidylcholine) and EYL (egg yolk phosphatidylcholine (lecithin)). A model is presented that assumes an exponentially decaying influence of the bilayer surface on water dynamics as well as on water orientation with increasing hydration. This assumption is based on an exponentially decaying hydration potential which results from direct lipid-water and water-water interactions. The model describes successfully the experimental data for a large water concentration range, especially at low hydration, where other models failed. With the exception of a small fraction of water which is significantly influenced by the surface in slowing down the mobility, the interbilayer water has isotropic, free water characteristics in terms of correlation times and molecular order. Hydration properties of POPC are comparable with those of EYL but differ from DOPC. At very low water content the correlation times of headgroup segmental reorientation and water are similar, indicating a strong coupling of this water to the lipid lattice. The hydration properties of the three lipids studied are explained in terms of slightly different headgroup conformations due to different lateral packing of the molecules by their fatty-acid chain composition.     

Population dynamics of the potato tuber moth on processing tomatoes in Israel

The potato tuber moth (PTM),Phthorimaea operculella (Zeller) (Lepidoptera: Gelechiidae), is a major pest of processing tomatoes,Lycopersicon esculentum Mill. (Solanaceae), in Israel. The larvae penetrate the tomato fruit through the stem end and present a serious threat to crop quality. Foliage and fruit samples were taken in nine commercial tomato fields located in Israel's three main tomato growing areas, two of which are potato growing areas as well. PTM was not found where potatoes were absent. Potato harvest in nearby fields was found to be the most significant factor affecting seasonal trends in PTM population density in tomatoes. All four larval instars were found in foliage on all sampling dates. Significantly higher proportions of first instars were found during the population density increase which followed potato harvest. Damaged fruits did not contain first instar larvae, indicating that PTM never undergoes complete development within tomato fruit. Fruit damage levels at harvest were positively correlated to the peak mean population densities on foliage and the date they were observed. In tomato fields not adjacent to potatoes, infestation was first observed at the edge of the field. Both before and after the potato harvest in nearby fields, population density at the edge of the field was significantly higher than at the center. In tomato fields adjacent to potatoes, no significant differences were found between population densities at the edge and center before the potatoes were harvested. After the potato harvest, population density at the center of tomato fields was higher than at the edge. Deceased, October 1988     

Biogeochemical cycling of sulfur and iron in sediments of a south-east Asian mangrove,Phuket Island,Thailand

Benthic sulfate reduction and sediment pools of sulfur and iron were examined during January 1992 at 3 stations in the Ao Nam Bor mangrove, Phuket, Thailand. Patterns of sulfate reduction rates (0–53 cm) reflected differences in physical and biological conditions at the 3 stations, and highest rates were found at the vegetated site within the mangrove (Rhizophora apiculata) forest. Due to extended oxidation of mangrove sediments, a large portion of the added³⁵S-label was recovered in the chromium reducible pools (FeS₂ and S⁰) (41–91% of the reduced sulfur). Pyrite was the most important inorganic sulfur component, attaining pool sizes 50–100 times higher than acid volatile pools (FeS). HCl-extractable (0.5 M HCl) iron pools, including Fe(II)_HCl and Fe(III)_HCl, were generally low and Fe(III)_HCl was only present in the upper surface layers (0–5 cm). Maximum concentrations of dissolved Fe²⁺ (35–285 M) occurred just about the depth where dissolved H₂S accumulated. Furthermore Fe²⁺ and H₂S coexisted only where concentrations of both were low. There was an accumulation of organic sulfur in the deep sediment at 2 stations in the inner part of the mangrove. The reoxidation of reduced sulfides was rapid, and storage of sulfur was minor in the upper sediment layers, where factors like bioturbation, the presence of roots, or tidal mixing enhance oxidation processes.Author of correspondence.     

Prerequisite for His4 in deltorphin A for highδ opioid receptor selectivity

Summary Analysis of deltorphin A position 4 analogues included: backbone constrained N MeHis, spinacine (Spi), N MePhe and the tetrahydroisoquinoline-3-carboxylic acid (Tic); spatially confined side-chain (Phg); and imidazole alkylation ofl- andd-His⁴ enantiomers. High selectivity was lost with the following replacements: N MeHis⁴, N MePhe⁴ and Phg⁴ reduced binding and the constrained residues also increasedµ binding; ring closure between the side-chain and amino group to yield Spi⁴ or Tic⁴ increasedµ affinity. Imidazole methylation of His⁴ marginally affected opioid binding and doubled selectivity; alkylatedd-His⁴-derivatives generally maintained selectivity in spite of decreased affinities. Thus, His⁴ imidazole preserves selectivity by facilitating high binding and by repulsion at theµ receptor. Several low energy conformers of deltorphin A indicated that the His⁴ imidazole preferred a spatial orientation parallel to the phenolic side-chain of Tyr¹ suggestive that this conformation might contribute to high affinity and selectivity.     

Host-plant influence on the population ecology of the jack pine budworm, Choristoneura pinus (Lepidoptera: Tortricidae)

Abstract.

• 1 Newly-emerged, second-instar jack pine budworm (Choristoneura pinus Freeman) establish spring feeding sites preferentially in the pollen cones of their host tree, Pinus banksiana Lamb.
• 2 Laboratory studies showed that the rate of establishment and survival of jack pine budworm on pollen cones was high throughout the entire spring emergence period of the insect.
• 3 In contrast, the rate of establishment and survival of jack pine budworm on vegetative buds was very poor early in the spring. Vegetative buds were only acceptable as feeding sites to the jack pine budworm for a relatively brief period in late spring.
• 4 Field studies showed that the change in population density of jack pine budworm during the spring emergence stage, as expressed by k-values, was a function of the abundance of pollen cones in the stand. Population reduction was greatest in those stands with the fewest pollen cones.
• 5 Direct measurement of spring dispersal by jack pine budworm showed that dispersal and consequent losses to the budworm population were greatest in stands with the fewest pollen cones.
• 6 We conclude that changes in the density of jack pine budworm are strongly influenced by production of pollen cones in the host stand. Because pollen cone production is related to previous years of defoliation by the jack pine budworm, we propose that pollen cones act as a density-dependent factor governing the density of early-stage jack pine budworm.
• 7 The resulting dynamics are compared to those of other budworm species and used to explain observed regional and temporal patterns of jack pine budworm outbreaks.