Intraspecific variation in the strength of density dependence in aphid populations

Abstract.  1. Experimental evidence is presented for positive, negative, and no density dependence from 32 independent density manipulations of milkweed aphids ( Aphis nerii ) in laboratory and field experiments. This substantial variation in intraspecific density dependence is associated with temperature and host-plant species.
2. It is reported that as population growth rate increases, density dependence becomes more strongly negative, suggesting that the monotonic definition of density dependence used in many common population models is appropriate for these aphids, and that population growth rate and carrying capacity are not directly proportional.
3. For populations that conform to these assumptions, population growth rate may be widely applicable as a predictor of the strength of density dependence.     

Density-dependent egg dispersion in flowers of Silene vulgaris by the seed predator Hadena confusa (Noctuidae)

Abstract.

• 1 Hadena moths drink nectar from, pollinate, and oviposit into the flowers of the genus Silene. In the gynodioecious Silene vulgaris (Moench) Garcke (Caryophyllaceae), eggs were laid by Hadena confusa (Hufn.) females both in male phase flowers, which could not be pollinated by the moth at the time of oviposition, and in female phase and female flowers that allowed pollination.
• 2 Flowers in which an egg had been laid had a lowered probability of receiving a second egg laid by another female during the same night.
• 3 The degree of flower discrimination by the moth with respect to the presence of eggs was positively correlated with the proportion of flowers containing eggs.
• 4 The oviposition behaviour of H.confusa is moulded by four factors: (1) a high probability of chosen male phase flowers later becoming pollinated by other moths, (2) the presence of conspecific eggs and risk for larval competition, (3) probability of presence of conspecific eggs also in neighbouring flowers, (4) capability of larvae to move to adjacent flowers.

     

Heterogeneity and density dependence in a field study of a tephritid-parasitoid interaction

Abstract.

• 1 The spatial distributions of two tephritid flies (Urophora stylata (Fabricius) and Terellia serratulae L.) attacking thistle flower heads and the levels of parasitism from their associated parasitoid guilds were studied over a 7-year period.
• 2 Using these data it is possible to seek both temporal, density dependent relationships between average levels of parasitism and host density per generation, and also any spatial patterns of parasitism contributing to stability that may be operating within the same field system.
• 3 Parasitism by the two most important generalist parasitoids of T.serratulae is a direct function of average T.serratulae density per year. There is little evidence of any stabilizing heterogeneity arising from the spatial distribution of parasitism within generations.
• 4 Temporal density dependence of Urophora stylata cannot be confirmed from the 7 years of study but there is evidence of spatial heterogeneity which may have an important effect on the dynamics of the host population.

     

Photosynthetic glow peaks and their relationship with the free energy changes

This paper is concerned with relating thermoluminescence to the total free-energy change, G, involved in detrapping a particular electron-hole pair as a photosynthetic sample is warmed from an initial low temperature. It extends a mathematical discussion of four possible mechanisms introduced in an earlier paper [DeVault, Govindjee and Arnold, Proc Nat'l Acad Sci USA 80: 983–987 (1983)]; here, particular attention is paid to the dependence of the absolute temperature of the maximum of a glow-peak, T _m , on the total free-energy change, G. The conclusion from the cases studied is that T _m =G/(k _B W) where G is evaluated at T _m , W is a complicated function of temperature and of thermodynamic parameters in the steps of the mechanism, and k _B is the Boltzmann constant. If the rate limiting step in the mechanism of detrapping is not preceded by any step in which G is appreciably negative, W is likely to have a value of about 33 and T _m is approximately proportional to G. Otherwise W can become much smaller and more strongly dependent on temperature and T _m is no longer proportional to G. These conclusions are of significance in lending theoretical support to the practice of inferring redox midpoint potential changes from shifts in T _m .     

Crown profile of foliage area characterized with the Weibull distribution in a hinoki (Chamaecyparis obtusa) stand

Summary Thirten sample trees of various sizes in a 29-year-old hinoki [Chamaecyparis obtusa (Sieb, et Zucc.) Endl.] plantation were felled and subjected to the stratified clip technique. Crown profile of foliage area fitted well with the Weibull distribution. The crown profile tended to be more skewed toward the top of crowns in smaller trees than in larger trees. This tendency was reflected in the value of the shape parameter of the Weibull distribution. The shape parameter ranged from 1.73 to 3.23 and gradually increased up to an asymptotic value with an increase of stem diameter at breast height. The scale parameter of the distribution ranged from 1.0 to 4.2 and tended to increase in proportion to stem diameter at breast height. Foliage area of a tree correlated well with stem diameter at breast height through an ordinary allometric equation. Tree height could be approximated fairly well by a generalized allometric equation as a function of stem diameter at breast height. On the basis of the census of stem diameter at breast height, canopy profile could be constructed synthesizing crown profiles of foliage area for individual trees in the stand. Leaf area index was estimated to be 6.6 ha ha^–1.     

Temperature and enzymic control of malate metabolism in berries of Vitis vinifera

The specific activities of grape enzymes concerned with malic acid metabolism were determined at various stages of berry development under two temperature regimes. It was found that the lower acidity levels occurring in high temperature grapes at maturity are due to a changed pattern in acid breakdown rather than to reduced malic acid accumulation. The differences in acid disappearance cannot be explained by a shift in relative enzyme activities towards malate consumption with rising temperature.     

Temperature and the regulation of activity of some mitochondrial enzyme systems in ecto- and endo-thermic animals

1. 1.|No obvious correlation was found between the temperature dependence of the apparent K_m values for either mitochondrial succinoxidase or NADH oxidase systems prepared from rat or rainbow trout liver tissue and the previously observed pattern of susceptibility of these same enzymes to thermal inactivation.

2. 2.|Apparent activation energies (E_A) have been calculated from the data and, although these are equivocal when considered in the above context, it is concluded that the possibility is not excluded that entropy may be a more important variable than enthalpy in the action of these enzymes in ectothermic animals.

Ca2+-activated K+ permeability in human erythrocytes: Modulation of single-channel events

Elevated levels of intracellular Ca²⁺ activate a K⁺-selective permeability in the membrane of human erythrocytes. Currents through single channels were analysed in excised inside-out membrane patches. The effects of several ions that are known to inhibit K⁺ fluxes are described with respect to the single-channel events. The results suggest that the blocking ions can partly move into the channels (but cannot penetrate) and interact with other ions inside the pore. The reduction of single-channel conductance by Cs⁺, tetraethylammonium and Ba²⁺ and of single-channel activity by quinine and Ba²⁺ is referred to different rates of access to the channel. The concentration- and voltage-dependent inhibition by ions with measurable permeability (Na⁺ and Rb⁺) can be explained by their lower permeability, with single-file movement and ionic interactions inside the pore.