Biophysical Ecology

David M. Gates

• Publisher: Dover Publications
• Year: 2012
• ISBN:
  9780486140797
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

This classic and highly influential text presents a uniquely comprehensive view of the field of biophysical ecology. In its analytical interpretation of the ecological responses of plants and animals to their environments, it draws upon studies of energy exchange, gas exchange, and chemical kinetics.
The first four chapters offer a preliminary treatment of the applications of biophysical ecology, discussing energy and energy budgets and their applications to plants and animals, and defining radiation laws and units. Succeeding chapters concern the physical environment, covering the topics of radiation, convection, conduction, and evaporation. The spectral properties of radiation and matter are reviewed, along with the geometrical, instantaneous, daily, and annual amounts of both shortwave and longwave radiation. The book concludes with more elaborate analytical methods for the study of photosynthesis in plants and energy budgets in animals, in addition to animal and plant temperature responses.
This text will prove of value to students and environmental researchers from a variety of fields, particularly ecology, agronomy, forestry, botany, and zoology.

Chapter 1

Introduction

Ecology is the study of the relationship of plants and animals to their environment and to one another and of the influence of man on ecosystems. The word ecology is derived from the Greek words oikos, meaning house or place to live, and logos, meaning science or study. The German zoologist Ernest Haeckel was an early user of the word Ökologie in 1866 and described it as a separate field of scientific knowledge, “the relation of the animal to its organic as well as its inorganic environment, particularly its friendly or hostile relations to those animals or plants with which it comes in contact.” A book with the word Ökologie in the title was published by Hans Reiter in 1885, but it is difficult to say precisely when the science of ecology began to take form as a discipline since it has always been inextricably interwoven with natural history. In America, the field of ecology became active about the turn of the century. In 1899, Henry Cowles, of the University of Chicago, published his classic ecological study of the sand dunes of Lake Michigan. Soon after that, ecology was recognized as a distinct professional discipline. In 1907, Victor Shelford, of the University of Illinois, reported on succession among communities of tiger beetles in direct association with plant succession. An excellent summary of the history of ecology is given by Kendeigh (1974) and of plant ecology by McIntosh (1974).

The definition of ecology makes it clear that it is a science which necessitates understanding of the physical environment, involving the fields of physics, meteorology, geology, chemistry, and so forth, combined with an understanding of biology, including systematics, community dynamics, anatomy, physiology, genetics, and other subjects. The science of ecology, by its very nature, is among the most complex of all the sciences and, because of this inherent complexity, must draw upon knowledge from the other sciences. Ecology is done poorly if either the biotic or abiotic aspects of the subject are not treated in a fully correct and rigorous scientific manner. Each ecological process or event must be studied in its full complement of physical and biological components. This requires that the physical principles of ecology be dealt with by the ecologist as thoroughly and correctly as the physicist deals with physics and the chemist with chemistry. At the same time, the ecologist must have a competent understanding of physiology, genetics, systematics, and other branches of biological science. This is a difficult order, yet a necessary one. Mathematical skills are also needed. Ecology, to be done well, must involve all the techniques of modern science. Fortunately, the modern computer is a very sophisticated instrument, capable of enormous data storage and complex mathematical manipulations.

All of life involves energy flow and material flow. Not a single animal or plant lives or breathes without the transformation of energy. The most microscopic change within an organism involves utilization of energy. Energy is involved whether it is the coursing of blood through the veins and arteries of an animal, the transfer of electrons in the photosynthetic process of plants, the division or expansion of cells, the beating of a heart, the flying of a bird, or simply the bending of a branch in the wind. Fundamental to the study of ecology is an understanding of energy flow and of energy transfer from one form to another within the biological and physical systems. Also fundamental to ecology is an understanding of mass transport within the environment. Life is not a static process within the organism; every cell, tissue, and organ is at all times chemically and physically active. An ecologist cannot remove him- or herself from understanding these factors, for they are often important in determining how an organism will respond to the forces and factors of the environment. Biophysical ecology is basically, therefore, an approach to ecology founded upon a thorough understanding of the sciences of energy and fluid flow, gas exchange, chemical kinetics, and other processes. This understanding is enhanced by using mathematical formulations of physical processes and relating them to the unique properties of organisms.

If we look about the world we live in, it is obvious that there are fairly distinct communities of organisms, such as those comprising a forest, prairie, pond, or stream. Not only are there a variety of communities in the world, but each community has a distinct set of edaphic environmental features. The term ecosystem is a convenient concept first proposed by Professor A. G. Tansley in 1935 to describe the collective sum of biotic and abiotic components of a segment of the landscape. The definition of the term used here is that proposed by J. W. Marr (1961): “An ecosystem is an ecological unit, a subdivision of the landscape, a geographic area that is relatively homogeneous and reasonably distinct from adjacent areas. It is made up of three groups of components—organisms, environmental factors and ecological processes.” The term ecosystem may be applied to a meadow, forest, lake, sand dune, or another readily recognized unit of the landscape. The ecosystem includes interactions between the plants and animals in an area with the climate and physiography of the region. In order to understand the response of a particular organism to its environment, however, knowledge of the climate and physiography of a region is not sufficient. The microclimate and physiography in the immediate vicinity of the organism must be known as well. Traditionally, ecologists have preferred to study ecosystems from a macroscopic standpoint by attempting to describe the community structure, identify the species present, and understand the distribution and association of plants and animals, population dynamics, and the general interaction of climate with the plant and animal community. Other ecologists have been concerned with understanding the trophic levels within ecosystems and the flow of energy and nutrients among the various trophic levels. Each of these approaches is very necessary and worthwhile in its contribution to our understanding of ecosystems. Another approach is also required, however, in order that a better understanding of the detailed processes underlying the major events occurring within ecosystems is achieved. This is a reductionist approach to ecology; it involves understanding the detailed processes going on within ecosystems at the level of the individual organism, as well as organism-to-organism interactions. Much of this falls within the subdiscipline known as physiological ecology. Very often in physiological ecology, however, the physical aspects of ecology are not as rigorously treated as they might be. For this reason, and in order to give additional emphasis to the physics and biophysics of the subject matter, I have used the term biophysical ecology to describe the subject of this book. The term biophysical ecology is necessarily redundant because the study of ecology, by definition, includes the physics of the environment and the biophysics of physiological processes. Nevertheless, this is the term that best describes the subject matter to which this book is devoted.

A Reductionist Approach

The events occurring within an organism, and between an organism and the immediate environment, are fundamental to our understanding of ecological processes. The reductionist approach is predicated on the fact that the ecosystem is the sum and product of its parts, when all interactions are taken into account. Whether a particular approach to the study of ecology is considered reductionistic or holistic does not really matter. The important thing is to study ecology with full use of the skills, tools, analyses, and insights of modern science.

A detailed approach to ecosystem study naturally involves knowledge gained from cellular biology, molecular biology, physiology, biochemistry, anatomy, systematics, physics, geology, meteorology, climatology, and so on. The ecologist is the synthesizer of all this information as applied to description and understanding of the ecosystem. This does not mean the ecologist must be a physiologist, cellular biologist, or biochemist. In fact, the ecologist must be quite careful to avoid being diverted into another specialty since every student of ecology will find one or more of these other disciplines especially intriguing. In order to operate effectively as an ecologist, one must learn these other disciplines well yet avoid being diverted by them, all the while applying the principles of these disciplines to the problems of ecology. Since the ecologist is a synthesizer, and because understanding an ecosystem requires understanding biological as well as physical events, it is necessary that the ecologist also be well grounded in the subject matter of physics, chemistry, and mathematics. As a general rule, it is easier and more useful if the student is trained in these hard-core, analytical subjects early and then increases the amount of coursework in biology. The reverse procedure is generally less satisfactory, but this is a matter of personal choice.

Because of the complexity of the subject matter, our ability to understand cause and effect within ecosystems is severely limited and requires the application of various tools of modern science. The single most important new tool available to the ecologist today is the computer. By this means, we extend our capacity, not for logic, but for data storage and processing. Just as sensory perceptions can be extended through the use of electronic detectors, photographic film, recorders, and other means, so also can we extend our capacity for storage and handling of large amounts of data by the use of computers.

In order to achieve an understanding of ecosystems, the ecologist asks a variety of questions. If all life on earth is supported by primary productivity in the form of photosynthesis, how do plants of various sizes, shapes, and structures capture sunlight and grow in a variety of environments? Why are plants and animals distributed the way they are, and what regulates their distribution? If an ecosystem undergoes great disturbance from wind, fire, insects, or pathogens, there follows a series of successional stages in recovery toward the original state. What are the forces and factors that affect this plant and animal succession? How do plants and animals compete for essential materials and factors? For example, how does an aspen compete with a pine for sunshine, water, carbon dioxide, or nutrients? What regulates and determines which species occupy a given habitat at a specific time? What climatic and edaphic factors influence the behavior of an animal, and how do they do so? What determines the niche or habitat that a given plant or animal may occupy? These are each extremely complex questions and lead to even more specific and detailed problems.

It is possible to approach these questions in a purely phenomenological manner. It is also possible to take a much more reductionistic approach, considering as self-evident that any organism is coupled to its environment through an exchange of energy and matter and, furthermore, that such exchanges must obey the basic laws of physics and chemistry as expounded by modern science. Biophysical ecology is based on this premise and the conviction that many of the questions posed above can only be answered in detail by a reductionist approach. Other methods do, of course, add much useful information to the body of ecological knowledge, and for many aspects of ecology, descriptive and phenomenological approaches continue to be very useful.

Physical Factors

This book deals with the physical factors that characterize the environments of plants and animals and the way in which these physical factors interact with the large variety of organisms in the world. It does not, however, concern itself with every kind of biophysical interaction, only with some of the primary factors that characterize the world in which we live. Some of the obvious physical factors in our environment include gravitational, electric, and magnetic fields, electromagnetic radiation, fluids and solids, chemical elements and compounds, sonic fields, temperature, and fluid motion. Clearly, to deal with each of these in detail is too great an undertaking for one volume. I have, therefore, elected to treat energy and gas exchange as subjects of primary importance to the response of plants and animals in their habitats.

Some physical factors, such as the gravitational, electric, or magnetic fields, are omitted from this treatise not because they are insignificant, but because their effects are not of first-order magnitude with respect to the energy status or gas-exchange rates of organisms. We live in a gravitational field which varies extremely slowly with time or changing position on the earth. The gravitational field enters into some considerations but is not a dominant factor. Gravitation becomes an important environmental factor if an animal falls off a precipice and, in addition, affects the direction of growth for the seed hypocotyl and apical meristem, but these are not subjects with which we will be dealing. The ground surface and atmosphere are filled with massive electric fields and enormous surges of electric currents, but again, these phenomena are not of primary concern here. The sounds and noises that fill our environment, from the random noises of Brownian motion to the massive thunderclaps of lightning bolts is another topic that will not be discussed. Nor will topics purely internal to an organism be discussed unless they are a direct part of the process of energy and gas exchange between the organism and the environment.

A Multiplicity of Variables

The process of photosynthesis is fundamental to the growth and response of plants to environmental conditions. Many questions about adaptation, competition, succession, productivity, and other activities concerning plants are directly related to the process of photosynthesis. Plant photosynthesis and respiration respond directly to energy and gas exchange, which are in turn affected by certain environmental variables. Likewise, the metabolic activity of animals responds directly to various environmental factors. Radiation, air temperature, substrate temperature, wind speed, and humidity are all environmental factors that affect the exchange of energy. Carbon dioxide concentration, oxygen concentration, and humidity affect the exchange of gases. A plant or animal has many properties that allow it to respond to environmental factors with sensitivity or insensitivity. For example, the absorptance of a leaf or animal surface to solar radiation determines the degree to which it is warmed by sunshine and, for plants, the extent to which photosynthesis is carried on. The size of an organism directly influences the rate of exchange of energy and gases through the depth of the boundary layer of air adhering to the organism’s surface. The size, shape, and orientation of an organism determine the degree to which the wind affects the temperature of the organism. The rates at which gases (water vapor, carbon dioxide, and oxygen) are exchanged depend upon the permeability or resistance of the plant or animal surface. Without going into more detail, it is easy to see that the ecology of a plant or animal may be directly involved with eight or more independent environmental variables and a half-dozen or more organism parameters. Important dependent variables are leaf temperature and transpiration photosynthetic, and respiration rates for plants and body temperature and metabolic rate for animals.

The problem faced by the ecologist in understanding the interaction or organisms with their environment is the problem of too many variables. For this reason, it is crucial to recognize which variables are of primary importance and not include more than are necessary. It is also because of too many variables that it is essential for ecology to become as analytical as possible. This is one of the most difficult points to understand. The large number of variables leads many people to believe that it is hopeless to attempt physical and mathematical analysis of such complex problems. But actually, the contrary is true: analysis must be used. There are two important reasons for this. The first is the fact that, during the last 15 years, analytical techniques have been successful in yielding answers to many previously unanswered problems. The second reason is that modern science can contribute very much more to the study of ecology than it has to date. Although systems ecology has given us some very significant advances, there is still a serious lack of understanding concerning mechanisms by which plants and animals interact with their environments. Relatively few persons have really addressed the problems of ecology using techniques commensurate with those of modern physics, chemistry, engineering, mathematics, and other analytical disciplines. The limitation is not one of technique or equipment but is due rather to a lack of understanding of what must be done.

Organization of the Book

This book is organized to give the reader an overall view of biophysical ecology, together with examples of its application. In order to show the necessity for the more-detailed treatment given in subsequent chapters, the introductory chapters contain simplified examples of the application of biophysical theory. Following the introductory chapters is a chapter concerning the fundamental laws of energy and radiation, including a thorough discussion of units. Radiation is the most ubiquitous environmental factor of the world in which we live, and all terrestrial organisms are subjected to radiation fields throughout their lives. (Aquatic organisms, by contrast, live in greatly attenuated radiation fields and often at night are free of radiation entirely.) Because of the enormous complexity of the topic of radiation, several chapters are required for its complete discussion. One chapter is devoted to solar radiation, one to thermal and total radiation, and one to the spectral properties of radiation and organisms. Much of the material presented in these chapters is not found in any other book, and an attempt is made to be as complete as possible.

Aquatic organisms have their energy status strongly coupled to the ambient temperature of their environment by means of the thermal conductivity of water. Terrestrial organisms are loosely coupled to air temperature by conduction and convection because of the very poor thermal conductivity of air. The subject of convective heat transfer has been thoroughly worked out by engineers, but application of these principles to biological problems is relatively recent. Those aspects of the subject important to problems of air or water flow around plants and animals are described in Chapter 9. All terrestrial organisms lose water vapor to some degree, and for many organisms, this is an important part of the energy budget. The diffusion of water vapor from an organism to the air is described in Chapter 10. The topics of radiation, convection, and evaporation are treated in the next two chapters, which give a thorough description of energy budgets for plants and animals. Chapter 13 deals with the time-dependent energetics of animals, and Chapter 14 treats the subject of photosynthesis.

The energy status of an organism determines its temperature, and whether or not an organism is too warm or too cold is important to its viability. The ability of organisms to undergo cold or heat resistance determines survival during temperature extremes. For these reasons, and to give the reader an overall view of the subject, a large chapter on the temperature tolerances of organisms (Chapter 15) is included.

Photosynthesis

One objective of this book is to understand as thoroughly as possible how a plant grows in response to environmental factors. Since plant growth is largely related to photosynthesis and respiration, it is important to understand how photosynthesis and respiration respond to environmental factors. A great deal is known about the biochemistry of photosynthesis and respiration, the structure of higher plants, the location of chloroplasts, peroxisomes, and mitochondria within the cell, and the behavior of stomates. Until recently, however, relatively little has been done to make an analytical model describing how the entire system works. A plant leaf exchanges energy and, as a result, has a certain temperature and loses a certain amount of water. While water vapor diffuses outward, carbon dioxide diffuses into the leaf. At the chloroplast, the carbon dioxide is oxidized and assimilated. Oxygen gas is released during photosynthesis and diffuses out from the leaf. The rate of photosynthesis is dependent on temperature, light, carbon dioxide concentration, oxygen concentration, and other factors such as the water status of the leaf. Leaf temperature is determined by the energy budget, and light intensity, which drives the photochemistry, is a part of the radiation term in the energy budget of the leaf. The rate of diffusion of water vapor outward is directly related to the diffusion of carbon dioxide inward, which in turn is commensurate with the rate of consumption in...

Table of contents

• Title Page
• Copyright Page
• Dedication
• Preface
• Table of Contents
• Symbols
• Chapter 1 - Introduction
• Chapter 2 - Energy and Energy Budgets
• Chapter 3 - Application to Plants
• Chapter 4 - Application to Animals
• Chapter 5 - Radiation Laws, Units, and Definitions
• Chapter 6 - Solar Radiation
• Chapter 7 - Longwave and Total Radiation
• Chapter 8 - Spectral Characteristics of Radiation and Matter
• Chapter 9 - Conduction and Convection
• Chapter 10 - Evaporation and Transpiration
• Chapter 11 - Energy Budgets of Plants
• Chapter 12 - Energy Budgets of Animals
• Chapter 13 - Time-Dependent Energetics of Animals
• Chapter 14 - Photosynthesis
• Chapter 15 - Temperature and Organisms
• Appendices
• References
• Index