Peter J.H. Sharpe

Reaction kinetics of poikilotherm development.

A stochastic thermodynamic model of poikilotherm development has been derived from the Eyring equation assuming multiple activity states of the underlying developmental control enzymes. This analysis brings together into a general model the day-degrees concept and the Arrhenius hypothesis as interpreted by Eyring. The compensating effect of enzyme inactivation at high and low temperatures incorporated into the model has the following consequences. 1. (i) It demonstrates the validity of the linear approximation (day-degree concept) in the mid-temperature region for some organisms. 2. (ii) It effectively establishes a low-temperature threshold for development. 3. (iii) It reduces the rate of development at higher temperatures, thereby establishing both an optimum and upper threshold for development. The resulting equation has been found applicable to a wide range of organisms.

Distribution model of organism development times.

Abstract It is shown in this analysis that the distribution of organism development times for constant and variable temperatures can be described based upon one simple assumption. This assumption is that the concentration of enzymes which are rate controlling for development are symmetrically distributed about some genetically determined mean concentration. It then follows mathematically that the skew in the distribution in development times, observed by Stinner, Butler, Bacheler & Tuttle (1975) and others, results naturally from the transformation from development rates to emergence times. The distribution model is shown to agree with observed data for (i) boll weevil, Anthonomus grandis Boheman, and (ii) cotton fleahopper, Pseudatomoscelis seriatus Reuter, reared under both constant and variable temperature regimes. The resulting model enables predictions of the distribution of emergence times for organisms reared under any set of variable temperature field conditions.

Ozone alters carbon allocation in loblolly pine: assessment with carbon-11 labeling.

Loblolly pine (Pinus taeda L.) seedlings were exposed to 0.120 micromol mol(-1) (ppm) ozone for 7 h per day, 5 days per week for 12 weeks. No visible damage resulted from this regime. A short-lived radioisotope of carbon ((11)C) was used to characterize changes in plant physiology caused by ozone, the first time this technique has been used for ozone exposure studies. In comparison to plants kept in charcoal-filtered air, pines exposed to ozone exhibited reductions in photosynthesis (16%), speed of phloem transport (11%), phloem photosynthate concentration (40%) and total carbon transport toward roots (45%). Photosynthate not transported to the roots appeared to accumulate in the stems. Primary branches of pines exposed to ozone were some 50-60% heavier than those of control pines. Ozone was thus shown to have a significant short-term impact on phloem transport processes that results in a shift in allocation of photosynthate favoring stems.

An analysis of the mechanics of guard cell motion

Abstract This paper presents a mechanical analysis of the cellular deformations which occur during the opening and closing of stomata. The aperture of the stomatal pore is shown to be a result of opposing pressures of the guard and adjacent epidermal cells. The analysis indicates that the epidermal cells have a mechanical advantage over the guard cells. With no mechanical advantage, an equal reduction in the turgor pressure of both guard and epidermal cells would have a neglible effect upon stomatal aperture. However, due to the mechanical advantage of the surrounding cells, the stomatal aperture increases with equal reductions in turgor, until the adjacent epidermal cells become flaccid. The minimum diffusion resistance of the pore occurs at this point. Further reductions in guard cell turgor lead to closure of the pore. The analysis further demonstrates how the shape, size, wall thickness and material properties of the guard cell walls influence their behavior.

Ecological Field Theory: the concept and field tests

Ecological field theory (EFT) quantifies plant spatial influences as pulsating geometric zones about individual plants. It provides the basis for a methodology to include spatial interactions between plants of different size, function and growth-form in models of plant community dynamics. The key components of EFT are: 1. the influence domain of individuals (D), 2. the field intensity within the domains (I), 3. the influence surface (ID) and 4. the intensity of interactions (II). The means to calculate these key components are outlined and several tests of the methodology as applied to a semi-arid eucalypt woodland are presented. The tests include a comparison of measured shrub growth with a computer implementation of EFT (the RESCOMP model) and spatial growth data from the eucalypt woodland to support the postulates included in EFT. Practical uses expected for the method are in agroforestry, landscape rehabilitation, simulations of disturbance effects and in determining invasibility of plant communities.

Foundations of stochastic development

Abstract A consistent mathematical theory of stochastic poikilotherm development has been derived based upon a minimum set of biological assumptions obtained from the literature. In the subsequent analysis, the resulting developmental rate can be justifiably represented as a random variable. Three cases are considered: (1) developmental rates dependent only on temperature, (2) rates dependent on both temperature and age, and (3) rates dependent on a general function of temperature and time. The analysis provides a mathematical foundation for the current practice of superimposing a probability distribution function on a biological time scale to describe the development of individuals from a population.

Pheromone dispersion in forests

A simple generic model for the dispersion of pheromones in a forestedecosystem is presented. Methods for the calculation of various concentration related parameters are described. The influence of different micrometeorological conditions on concentration profiles is discussed. Two separate studies from two different geographical locations lend support to the predictions of the dispersion model. For two different species of Dendroctonus beetles, aggregation behavior was correlated with meteorological condition resulting from inversion profiles.

Object-oriented simulation: plant growth and discrete organ to organ interactions

Abstract This paper reviews and applies new hierarchical approaches to ecological modelling. These new approaches are made possible by the development of the object-oriented paradigm. This paradigm draws upon the notion of ‘universal’ or classes dating back to early Greek philosophy. It is an intriguing approach to simulation because it is based upon the concepts of hierarchy and taxonomy, two of the basic organizing principles in ecology. Adopting an object-oriented approach to simulation can result in a reduction of mathematical and statistical abstraction. The object-oriented approach lends itself directly to incorporation of mechanisms within appropriate hierarchies. A case study is presented outlining the design steps for simulating plant growth objects (roots, stem, leaves, fruit, whole plant, etc.). The design steps are shown in graphical form to illustrate the differences between object-oriented and traditional procedural approaches. Cotton growth and development has been selected for the case study because of the large knowledge base available for the explicit representation of age and size for each organ. Inclusion of mechanisms at the level of the individual organ provides additional information for crop management. Computing fruit growth at discrete branch locations results in the ability to manage for optimum fiber yield and reduced pest vulnerabilities for individual bolls. Variability in light interception, leaf age, and resulting carbohydrate supply for leaves at specific positions leads naturally to variability in individual boll fiber yield. The goal of the model to capture the behavior of individual organs as a function of their interaction with other organs was achieved. The object-oriented paradigm facilitates the formulation of a simulation procedure in which an individual organ interacted with other organs and the crop microclimate. This led us one step closer to answering the question, ‘How do individual characteristics and behaviors result in given population patterns?’.

Disturbance propagation by bark beetles as an episodic landscape phenomenon

Landscapes are the resultant of ecological processes and events operating on many different space-time scales. Large scale disturbance is recognized as a major influence on landscape patterns, but the impact of small scale events is often overlooked. We develop an hierarchical framework to relate lightning and bark beetle population dynamics to the southern pine forest landscape using the concepts of disturbance propagation and amplification. The low level lightning disturbance can be propagated to the landscape level when weather and forest stand structure facilitate bark beetle epidemics. We identify epidemics as biotically-driven episodes that alter landscape structure. The concept of the landscape as the spatial dimension of these episodes is represented in a conceptual model linking insect-host and landscape mosaic interactions.

A biophysical model of southern pine beetle, Dendroctonus frontalis Zimmermann (Coleoptera: Scolytidae), development☆

Abstract Predicting the population dynamics of the southern pine beetle, Dendroctonus frontalis Zimmermann, in forests of the U.S.A. requires predictive models of cohort development. The cryptic nature of the insect, however, prevents direct observations of its development, and undefined host requirements of early stage larvae prevent rearing it in the laboratory. Consequently, obtaining the information needed to formulate predictive models is difficult. This study describes experimental, analytical, and modeling techniques used to obtain information on beetle development times, the distribution of those times, and percent mortality over a full range of constant temperatures. Our data indicate that D. frontalis is highly sensitive to temperature. Cohort development occurs over extended intervals of time with the durations of development influenced by temperature in a nonlinear fashion. Information on development rates and the distributions of development times were used to formulate biophysical models of life stage development and development from eggs to adult emergence. Tests of the latter model here and elsewhere (Feldman et al., 1981a) showed good agreement with data from several field populations. These tests provided a basis for incorporating the model into a larger population dynamics model for this insect (Feldman et al., 1981b).