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THE STRUGGLE
FOR
EXISTENCE
Z 3
THE STRUGGLE
FOR
EXISTENCE
BY
G. F. GAUSE
Zoological Institute of the University of Moscow
BALTIMORE T;HE WILLIAMS & WILKINS COMPANY
1934
Copyright, 1934 the williams a wilkins company
Made in the United States of America Published December, 1934
COMPOSED AND PRINTED AT THE
WAVERLT PRESS, INC.
FOR
THE WILLIAMS & WILKINS COMPANY BALTIMORE, MD., U. S. A.
M»
FOREWORD
For three-quarters of a century past more has been written about natural selection and the struggle for existence that underlies the selective process, than perhaps about any other single idea in the whole realm of biology. We have seen natural selection laid on its Sterbebett, and subsequently revived again in the most recent times to a remarkable degree of vigor. There can be no doubt that the old idea has great survival value.
The odd thing about the case, however, is that during all the years from 1859, when Darwin assembled in the Origin of Species a masterly array of concrete evidence for the reality of the struggle for existence and the process of natural selection, down to the present day, about all that biologists, by and large, have done regarding the idea is to talk and write. If ever an idea cried and begged for experimental testing and development, surely it was this one. Yet the whole array of experimental and statistical attempts in all these years to produce some significant new evidence about the nature and consequences of the struggle for existence is pitifully meager. Such contributions as those of Bumpus, Weldon, Pearson, and Harris are worthy of all praise, but there have been so very, very few of them. And there is surely something comic in the spectacle of laboratories overtly em- barking upon the experimental study of evolution and carefully thereafter avoiding any direct and purposeful attack upon a pertinent problem, the fundamental importance of which Darwin surely estab- lished.
At the present time there is abundant evidence of an altered atti- tude; and particularly among the younger generation of biologists. The problem is being attacked, frontally, vigorously and intelligently. This renewed and effective activity seems to be due primarily to two things : first, the recrudescence of general interest in the problems of population, with the accompanying recognition that population problems are basically biological problems; and, second, the realiza- tion that the struggle for existence and natural selection are matters concerning the dynamics of populations, birth rates, death rates, interactions of mixed populations, etc. These things were recognized
VI FOREWORD
and pointed out by Karl Pearson many years ago. His words, how- ever, went largely unheeded for a long time. But in the last fifteen years we have seen more light thrown upon the problems of popula- tion by the work of such mathematicians as Lotka and Volterra, such statisticians as Yule, and such experimentalists as Allee and Park, than in the entire previous history of the subject. There can be no doubt of the fact that population problems now constitute a major focal point of biological interest and activity.
The author of the present treatise, Dr. G. F. Gause (who stands in the front rank of young Russian biologists, and is, it gives me great pleasure and satisfaction to say, a protege of my old student and friend, Prof. W. W. Alpatov) makes in this book an important con- tribution to the literature of evolution. He marshals to the attack on the old problem of the consequences of the struggle for existence the ideas and the methods of the modern school of population stu- dents. He brings to the task the unusual and most useful equipment of a combination in his own person of thorough training and com- petence in both mathematics and experimental biology. He breaks new ground in this book. It will cause discussion, and some will disagree with its methods and conclusions, but no biologist who desires to know what the pioneers on the frontiers of knowledge are doing and thinking can afford not to read it. I hope and believe that it is but the beginning of a series of significant advances to be made by its brilliant young author.
Raymond Pearl. Department of Biology,
School of Hygiene and Public Health, The Johns Hopkins University.
AUTHOR'S PREFACE
This book is the outcome of a series of experimental investigations upon which I have been engaged for several years past. In these experiments an attempt was made to make use of all the advantages of the controlled study of the struggle for existence in the laboratory with various organisms low in the evolutionary scale. It became evident that the processes of competition between different species of protozoa and yeast cells are sometimes subject to perfectly definite quantitative laws. But it has also been found that these processes are extremely complicated and that their trend often do not harmo- nize with the predictions of the relatively simple mathematical theory. There is also a continued need for attack upon the problems of the struggle for existence along the lines of experimental physiology and biology, even though the results obtained cannot yet be adequately expressed in mathematical terminology.
I wish to express my sincere thanks to Professor W. W. Alpatov for interest in the experimental investigations and for valuable sugges- tions. To Professor Raymond Pearl I am deeply indebted for great assistance in the publication of this book, without which it could never have appeared before the American reader. I am also grate- ful to the Editors of The Journal of Experimental Biology and Archiv fur Protistenkunde for permission to use material previously published in these periodicals.
G. F. Gause. Laboratory of Ecology, Zoological Institute, University of Moscow,
Malaia Bronnaia 12, Kv. S3. November, 1984
vn
CONTENTS
CHAPTER I The Problem 1
CHAPTER II The Struggle for Existence in Natural Conditions 12
CHAPTER III
The Struggle for Existence from the Point of View of the Mathe- maticians 27
CHAPTER IV
On the Mechanism of Competition in Yeast Cells 59
CHAPTER V
Competition for Common Food in Protozoa 90
CHAPTER VI The Destruction of One Species by Another 114
Appendix 1 143
Appendix II 149
Bibliography 154
Index 161
46027
IX
Chapter I
THE PROBLEM
(1) The struggle for existence is one of those questions which were very much discussed at the end of the last century, but scarcely any attempt was made to find out what it really represents. As a result our knowledge is limited to Darwin's brilliant exposition, and until quite recently there was nothing that we could add to his words. Darwin considered the struggle for existence in a wide sense, includ- ing the competition of organisms for a possession of common places in nature, as well as their destruction of one another. He showed that animals and plants, remote in the scale of nature, are bound together by a web of complex relations in the process of their struggle for existence. "Battle within battle must be continually recurring with varying success," wrote Darwin, and "probably in no one case could we precisely say why one species has been victorious over another in the great battle of life. ... It is good thus to try in imagina- tion to give to any one species an advantage over another. Probably in no single instance should we know what to do. This ought to convince us of our ignorance on the mutual relation of all organic beings; a conviction as necessary as it is difficult to acquire. All that we can do, is to keep steadily in mind that each organic being is striving to increase in a geometrical ratio; that each at some period of its life, during some season of the year, during each generation or at intervals, has to struggle for life and to suffer great destruction" ('59, pp. 56-57).
(2) But if our knowledge of the struggle for existence has since Darwin's era increased to an almost negligible extent, in other do- mains of biology a great progress has taken place in recent years. If we look at genetics, or general physiology, we find that a decisive advance has been made there, after the investigators had greatly simplified their problems and taken their stand upon the firm basis of experimental methods. The latter presents a particularly interest- ing example about which we would like to say a few words. We mean the investigations of the famous Russian physiologist J. P. Pavlov, who approached the study of the nervous activity of higher
1
2 THE STRUGGLE FOR EXISTENCE
animal by thoroughly objective physiological methods. As Pavlov ('23) himself says, it is "the history of a physiologist's turning from purely physiological questions to the domain of phenomena usually termed psychical." The higher nervous activity presents such a complicated system, that without special experiments it is difficult to obtain an objective idea of its properties. It is known, firstly, that there exist constant and unvarying reflexes or responses of the organ- ism to the external world, which are considered as the especial "ele- mentary tasks of the nervous system." There exist besides other reflexes variable to an extreme degree which Pavlov has named "conditional reflexes." With the aid of carefully arranged quantita- tive experiments in which the animal was isolated in a special cham- ber, all the complicating circumstances being removed, Pavlov discovered the laws of the formation, preservation and extinction of the conditional reflexes, which constitute the basis for an objective conception of the higher nervous activity. "I am deeply, irrevocably and ineradicably convinced, says Pavlov, that here, on this way lies the final triumph of the human mind over its problem — a knowledge of the mechanism and of the laws of human nature."
(3) The history of the physiological sciences for the last fifty years is very instructive, and it shows distinctly that in studying the struggle for existence we must follow the same lines. The compli- cated relationships between organisms which take place in nature have as their foundation definite elementary processes of the struggle for existence. Such an elementary process is that of one species devouring another, or when there is a competition for a common place between a small number of species in a limited microcosm. It is the object of the present book to bring forward the evidence, firstly, that in studying the relations between organisms in nature some investigators have actually succeeded in observing such elementary processes of the struggle for existence and, secondly, to present in detail the results of the author's experiments in which the elementary processes have been investigated in laboratory conditions. The experiments made it apparent that in the simplest case we can give a clear answer to Darwin's question: why has one species been victorious over another in the great battle of life?
(4) It would be incorrect to fall into an extreme and to consider the complicated phenomena of the struggle for life in nature as simply a sum of such elementary processes. Leaving aside the existence in
THE PROBLEM 6
nature of climatic factors which undergo rhythmical time-changes, the elementary processes of the struggle for life take place there amid a totality of most diverse living beings. This totality presents a whole, and the separate elementary processes taking place in it are still insufficient to explain all its properties. It is also probable that changes of the totality as a whole put an impress on those processes of the struggle for existence which are going on within it.
Nobody contests the complexity of the phenomena taking place in the conditions of nature, and we will not enter here into a discussion of this fact. Let us rather point out all the importance of studying the elementary processes of the struggle for life. At present our position is like that of biophysicists in the second half of last century. First of all it had been necessary to show that separate elementary phenomena of vision, hearing, etc., can be fruitfully studied by physi- cal and chemical methods, and thereupon only did the question arise of studying the organism as a system constituting a whole.
(5) Certain authors at the close of last century occupied themselves with a purely logical and theoretical discussion of the struggle for existence. They proposed different schemata for classifying these phenomena, and we will now examine one of them in order to give just a general idea of those elementary processes of the struggle for life with which we will have to deal further on. To the first large group of these processes belongs the struggle going on between groups of organisms differing in structure and mode of life. In its turn this struggle can be divided into a direct and an indirect one. The struggle for existence is direct when the preservation of life of one species is connected with the destruction of another, for instance that of the fox and the hare, of the ichneumon fly and its host larva, of the tuberculosis bacillus and man. In the chapter devoted to the experimental analysis of the predator-prey relations we will turn our attention to this form of the struggle. In plants, as Plate ('13) points out, the direct form of the struggle for existence is found only in the case of one plant being a parasite of the other. Among plants it is the indirect competition, or the struggle for the means of livelihood that predominates; this has also a wide extension among animals. It takes place in the case when two forms inhabit the same place, need the same food, require the same light. We will later give a great deal of attention to the experimental study of indirect competition. To the second group of phenomena of the struggle for life belongs the
4 THE STRUGGLE FOR EXISTENCE
intraspecies struggle, between individuals of the same species, which in its turn can be divided into a direct and an indirect one.
(6) In this book we are interested in the struggle for existence among animals, and it is just in this domain that exact data are almost entirely lacking. In large compilative works one may meet an indication that the struggle for existence "owing to the absence of special investigations has become transformed into a kind of logical postulate," and in separate articles one can read that "our data are in contradiction with the dogma of the struggle for existence." In this respect zoologists are somewhat behind botanists, who have accumu- lated already some rather interesting facts concerning this problem.
What we know at present is so little that it is useless to examine the questions: what are the features common to the phenomena of competition in general, and what is the essential distinction between the competition of plants and that of animals, in connection with the mobility of the latter and the greater complexity of relations into which they enter? What interests us more immediately is the practi- cal question: what are the methods by means of which botanists study the struggle for existence, and what alterations do these meth- ods require in the domain of zoology?
First of all botanists have already recognized the necessity of having recourse to experiment in the investigation of competition phenomena, and we can quote the following words of Clements ('24, p. 5) : ' 'The opinions and hypotheses arising from observation are often interesting and suggestive, and may even have permanent value, but ecology can be built upon a lasting foundation solely by means of experiment. ... In fact, the objectivity afforded by compre- hensive and repeated experiment is the paramount reason for its constant and universal use."
However, the experiments so far made by botanists are devoted to the analysis of plant competition from the viewpoint of ontogenic development. The competition began when the young plantlets came in contact with one another, and all the decisive stages of the competition took place in the course of development of the same plants.
In such circumstances the question as to the causes of the victory of certain forms over others presents itself in the following aspect: By the aid of what morphological and physiological advantages of the process of individual development does one plant suppress another
THE PROBLEM 5
under the given conditions of environment? Clements has character- ized this phenomenon in the following manner: "The beginning of competition is due to reaction when the plants are so spaced that the reaction of one affects the response of the other by limiting it. The initial advantage thus gained is increased by cumulation, since even a slight increase of the amount of energy or raw material is followed by corresponding growth and this by a further gain in re- sponse and reaction. A larger, deeper or more active root system enables one plant to secure a larger amount of the chresard, and the immediate reaction is to reduce the amount obtainable by the other. The stem and leaves of the former grow in size and number, and thus require more water, the roots respond by augmenting the absorbing surface to supply the demand, and automatically reduce the water content still further and with it the opportunity of a competitor. At the same time the correlated growth of stems and leaves is produc- ing a reaction on light by absorption, leaving less energy available for the leaves of the competitor beneath it, while increasing the amount of food for the further growth of absorbing roots, taller stems and overshading leaves" (Clements, '29, p. 318).
(7) It is not difficult to see that for the study of the elementary processes of the struggle for existence in animals we need experiments of another type. We are interested in the processes of destruction and replacing of one species by another in the course of a great num- ber of generations. We are consequently concerned here with the problem of an experimental study of the growth of mixed populations, depending on a very great number of manifold factors. In other words we have to analyze the properties of the growing groups of individuals as well as the interaction of these groups. Let us make for this purpose an artificial microcosm, i.e., let us fill a test tube with a nutritive medium and introduce into it several species of Protozoa consuming the same food, or devouring each other. If we then make numerous observations on the alteration in the number of individuals of these species during a number of generations, and analyze the fac- tors that directly control these alterations, we shall be able to form an objective idea as to the course of the elementary processes of the struggle for existence. In short, the struggle for existence among animals is a problem of the relationships between the components in mixed growing groups of individuals, and ought to be studied from the viewpoint of the movement of these groups.
6 THE STRUGGLE FOR EXISTENCE
For the study of the elementary processes of the struggle for exist- ence in animals we can have recourse to experiments of two types. We can pour some nutritive medium into a test tube, introduce into it two species of animals, and then neither add any food nor change the medium. In these conditions there will be a growth of the num- ber of individuals of the first and second species, and a competition will arise between them for the common food. However, at a cer- tain moment the food will have been consumed, or toxic waste prod- ucts will have accumulated, and as a result the growth of the popula- tion will cease. In such an experiment a competition will take place between two species for the utilization of a certain limited amount of energy. The relation between the species we will have found at the moment when growth has ceased, will enable us to establish in what proportion this amount of energy has been distributed between the populations of the competing species. It is also evident that one can add to the species "prey" growing in conditions of a limited amount of energy the species "predator," and trace the process of one species being devoured by the other. Or, in the experiments of the second type, we need not fix the total amount of energy as a determined quantity, and only maintain it at a certain constant level, continually changing the nutritive medium after fixed intervals of time. In such an experiment we approach more closely to what takes place in the conditions of nature, where the inflow of solar energy is maintained at a fixed level, and we can study the process of competition for com- mon food, or that of destruction of one species by another, in the course of time intervals of any duration we may choose.
(8) Experimental researches will enable us to understand the mechanism of the elementary process of the struggle for existence, and we can proceed to the next step: to express these processes mathematically. As a result we shall obtain coefficients of the struggle for existence which can be exactly measured. The idea of a mathematical approach to the phenomena of competition is not a new one, and as far back as 1874 the botanist and philosopher Nageli attempted to give "a mathematical expression to the suppression of one plant by another," taking for a starting point the annual increase of the number of plants and the duration of their life. But this line of investigation did not find any followers, and the experimental researches on the competition of plants which have appeared lately
THE PROBLEM 7
are as yet in the stage of nothing but a general analysis of the proc- esses of ontogenesis.
In past years several eminent men were deeply conscious of the need for a mathematical theory of the struggle for existence and took definite steps in this domain. It often happened that one investi- gator was ignorant of the work of another but came to the same con- clusions as his predecessor. Apparently every serious thought on the process of competition obliges one to consider it as a whole, and this leads inevitably to mathematics. A simple discussion or even a quantitative expression of data often do not suffice to obtain a clear idea of the relationships between the competing components in the process of their growth.
(9) About thirty years ago mathematical investigations of the struggle for existence would have been premature, or in any case sub- ject to great difficulties, due to the absence of the needed preliminary data. Of late years, owing to the publication of a number of investi- gations, these difficulties have disappeared of themselves. What is it that these indispensable preliminary researches represent?
There is no doubt that a rational study of the struggle for existence among animals can be begun only after the questions of the multipli- cation of organisms have undergone a thoroughly exact quantitative analysis. We have mentioned that the struggle for existence is a problem of the relationships between species in mixed growing groups of individuals. We must therefore begin by analyzing the laws of growth of homogeneous groups consisting of individuals of one and the same species, and the competition between individuals in such homogeneous groups. During the second half of the last century and the beginning of the present much has been said about multiplica- tion, and "equations of multiplication" have even been proposed of the following type: the coefficient of reproduction — the coefficient of destruction = number of adults. (Vermehrungsziffer — Vernich- tungsziffer = Adultenziffer; see Plate ('13) p. 246.) Usually, how- ever, things did not go any further, and no attempts were made to formulate exactly all these correlations. Recently the Russian geo- chemist, Prof. Vernadsky, has thus characterized from a very wide viewpoint the phenomena of multiplication of organisms ('26, p. 37 and foil.): "The phenomena of multiplication attracted but little the attention of biologists. But in it, partly unnoticed by the natu-
8 THE STRUGGLE FOR EXISTENCE
ralists themselves, several empirical generalizations became estab- lished to which we have become so accustomed that they appear to us almost self-evident.
"Among these generalizations the following must be recorded. Firstly, the multiplication of all organisms can be expressed by geometric progressions. This can be evaluated by a uniform formula:
2bf = Nt
where t is time, b the exponent of progression and Nt the number of individuals existing owing to multiplication at a certain time t. Param- eter b is characteristic for every kind of living being. In this formula there are included no limits, no restrictions either for t, for b, or for Nt. The process is conceived as infinite as the progression is infinite.
"This infinity of the possible multiplication of organisms can be considered as the subordination of the increase of living matter in the biosphere to the rule of inertia. It can be regarded as empirically estab- lished that the process of multiplication is retarded in its manifestation only by external forces ; it dies off with a low temperature, ceases or becomes weaker with an insufficiency of food or respiration, with a lack of room for the organisms that are being newly created. In 1858 Darwin and Wallace expressed this idea in a form that had been long clear to naturalists who had gone into these phenomena, for instance, Linnaeus, Buff on, Humboldt, Ehrenberg and von Baer: if there are no external checks, every organism can, but at a different time, cover the entire globe by its multiplication, produce a progeny equal in size to the mass of the ocean or of the earth's crust.
"The rate of multiplication is different for every kind of organisms in close connection with their size. Small organisms, that is organisms weighing less, at the same time multiply much more rapidly than large organisms (i.e., organisms of a great weight).
"In these three empirical generalizations the phenomena of multi- plication are expressed without any consideration of time and space or, more precisely, in geometrical homogeneous time and space. In reality life is inseparable from the biosphere, and we must take into consideration terrestrial time and space. Upon the earth organisms live in a limited space equal in dimensions for them all. They live in a space of definite structure, in a gaseous environment or a liquid environment penetrated by gases. And although to us time appears
THE PROBLEM 9
unlimited, the time taken up by any process which takes place in a limited space, like the process of multiplication of organisms, cannot be unlimited. It also will have a limit, different for every kind of organisms in accordance with the character of its multiplication. The inevitable consequence of this situation is a limitation of all the parameters which determine the phenomena of multiplication of organisms in the biosphere.
'Tor every species or race there is a maximal number of individuals which can never be surpassed. This maximal number is reached when the given species occupies entirely the earth's surface, with a maximal density of its occupation. This number which I will hence- forth call the 'stationary number of the homogeneous living matter' is of great significance for the evaluation of the geochemical influence of life. The multiplication of organisms in a given volume or on a given surface must proceed more and more slowly, as the number of the individuals already created approaches the stationary number."
These general notions on the multiplication of organisms have lately received a rational quantitative expression in the form of the logistic curve discovered by Raymond Pearl and Reed in 1920. The logistic law mathematically expresses the idea that in the conditions of a limited microcosm the potentially possible "geometric increase" of a given group of individuals at every moment of time is realized only up to a certain degree, depending on the unutilized opportunity for growth at this moment. As the number of individuals increases, the unutilized opportunity for the further growth decreases, until finally the greatest possible or saturating population in the given conditions is reached. The logistic law has been proved true as regards populations of different animals experimentally studied in laboratory conditions. We shall have an opportunity to consider all these problems more in detail further on. Let us now only note that the rational quantitative expression of growth of groups consisting of individuals of the same species represents a firm foundation for a further fruitful study of competition between species in mixed popu- lations.
(10) Apart from a great progress as regards the mathematical expression of the multiplication of organisms, an important advance has taken place in the theory of competition itself. The first step in this direction was made in 1911 by Ronald Ross, who at this time was interested in the propagation of malaria. Considering the
10 THE STRUGGLE FOR EXISTENCE
process of propagation Ross came to the conclusion that he was deal- ing with a peculiar case of a struggle for existence between the malaria Plasmodium and man with a participation of the mosquito. Ross formulated mathematically an equation of the struggle for existence for this case, which closely approached in its conception those equa- tions of the struggle for existence which the Italian mathematician Volterra proposed in 1926 without knowing the investigations of Ross.
Whilst Ross was working on the propagation of malaria the Ameri- can mathematician Lotka ('10, '20a) examined theoretically the course of certain chemical reactions, and had to deal here with equa- tions of the same type. Later on Lotka became interested in the problem of the struggle for existence, and in 1920 he formulated an equation for the interaction between hosts and parasites ('20b), and gave a great deal of interesting material in his valuable book, Ele- ments of Physical Biology ('25). Without being acquainted with these researches the Italian mathematician Vito Volterra proposed in 1926 somewhat similar equations of the struggle for existence. At the same time he advanced the entire problem considerably, investi- gating for the first time many important questions of the theory of competition from the theoretical point of view. Thus three distin- guished investigators came to the very same theoretical equations almost at the same time but by entirely different ways. It is also interesting that the struggle for existence only began to be experi- mentally studied after the ground had been prepared by purely theo- retical researches. The same has already happened many times in the fields both of physics and of physical chemistry: let us recollect the mechanical equivalent of heat or Gibbs' investigations.
(11) The study of the struggle for existence will undoubtedly rapidly progress in the future, but it will have to overcome a certain gap between the investigations of contemporary biologists and mathe- maticians. There is no doubt that the struggle for existence is a biological problem, and that it ought to be solved by experimentation and not at the desk of a mathematician. But in order to penetrate deeper into the nature of these phenomena we must combine the experimental method with the mathematical theory, a possibility which has been created by the brilliant researches of Lotka and Vol- terra. This combination of the experimental method with the quantitative theory is in general one of the most powerful tools in the hands of contemporary science.
THE PROBLEM 11
The gap between the biologists and the mathematicians represents a significant obstacle to the application of the combined methods of research. Mathematical investigations independent of experiments are of but small importance due to the complexity of biological sys- tems, narrowing the possibilities of theoretical work here as compared with what can be admitted in physics and chemistry. We are in com- plete accord with the following words of Allee ('34): "Mathematical treatment of population problems is necessary and helpful, particu- larly in that it permits the logical arrangement of facts and abbre- viates their expression by the use of a sort of universal shorthand, but the arrangement and statement may lead to error, since for the sake of brevity and to avoid cumbersome expressions, variables are omitted and assumptions made in the mathematical analyses which are not justified by the biological data. Certainly there is room for the mathematical attack on population problems, but there is also con- tinued need for attack along the lines of experimental physiology, even though the results obtained cannot yet be adequately expressed in mathematical terminology."
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Chapter II
THE STRUGGLE FOR EXISTENCE IN NATURAL
CONDITIONS
(1) Before beginning any experimental investigation of the ele- mentary processes of the struggle for existence we must examine what is the state of our knowledge of the phenomena of competition in nature. The regularities which it has been possible to ascertain there, and the ideas which have been expressed in their discussion, will help us to formulate correctly certain fundamental requirements for further experimental work.
In thorough field observations the fact which strikes the investiga- tor most of all is the extreme complexity of the communities of organ- isms, and at the same time their possession of a definite structure. On the one hand they undergo changes under the influence of external environment, and on the other the slightest changes of some compo- nents produce an alteration of others and lead to a whole chain of con- sequences. It is difficult here to arrive at a sufficiently clear under- standing of the processes of the struggle for existence. Elton writes for instance: "We do not get any clear conception of the exact way in which one species replaces another. Does it drive the other one out by competition? and if so, what precisely do we mean by competi- tion? Or do changing conditions destroy or drive out the first arrival, making thereby an empty niche for another animal which quietly replaces it without ever becoming 'red in tooth and claw' at all? Succession brings the ecologist face to face with the whole problem of competition among animals, a problem which does not puzzle most people because they seldom if ever think out its implications at all carefully. At the present time it is well known that the American grey squirrel is replacing the native red squirrel in various parts of England, but it is entirely unknown why this is occurring, and no good explanation seems to exist. In ecological succession among animals there are thousands of similar cases cropping up, practically all of which are as little accounted for as that of the squirrels" ('27, p. 27-28). All this suggests that an analysis must be made of com- paratively simple desert or Arctic communities where the number of
12
STRUGGLE IN NATURAL CONDITIONS
13
components is small. Such a tendency to examine certain elemen- tary phenomena is clearly seen in the following words of a Russian zoologist, N. Severtzov, written as far back as 1855: "It seems to me that the study of animal groupings in small areas, the study of these elementary faunas is the firmest point of support for drawing conclu- sions about the general laws regulating the distribution of animals on the globe."
However, besides this first possibility of studying competition phe- nomena among a small number of components, an active intervention into natural conditions by means of biotic experiments may also be very important. Among such experiments the most frequent ones consist in the transportation of animals into countries new to them, which commonly leads to a great number of highly interesting proc-
TABLE I
Number of fir trunks on a unit of surface under different conditions
From Sukatschev ('28)
20 TEARS AGE |
60 TEARS AGE |
|||
TYPE OF LIFE |
||||
CONDITIONS |
Predominant trunks |
Oppressed trunks |
Predominant trunks |
Oppressed trunks |
I |
5600 |
— |
1300 |
640 |
II |
5850 |
— |
1600 |
680 |
III |
6620 |
— |
1950 |
650 |
IV |
7480 |
— |
2280 |
720 |
V |
8400 |
— |
2780 |
760 |
esses of the struggle for existence (Thomson, '22). The second type of biotic experiments is an "exclusion" of the animal from a certain community. Further on we give some examples of the struggle for existence observed by such methods, but so far none of them have been sufficiently studied.
(2) It fell to the lot of botanists to have to deal with the simplest conditions of competition, and they arrived at a very instructive con- ception of the intensity of the struggle for existence. Foresters were the first to be confronted with the question of competition when they began to estimate the diminution in the number of tree trunks accom- panying forest growth in different conditions of environment. They characterize the struggle for existence by the percentage decrease in the number of individuals on a unit of surface in a certain unit of
14 THE STRUGGLE FOR EXISTENCE
time. At first sight one might think that the better the conditions of existence the less active is the struggle for life, and the greater the number of trunks that can survive with age on a unit of surface. Let us, however, look at the data of the foresters. For an example we will give in Table I the number of the fir trunks in the government of Leningrad (Northern Russia) corresponding to five different types of life conditions (Type I represents the best soil and ground condi- tions; V, the worst ones).
These data show, contrary to our expectations, that the better are the soil and ground conditions, the more active is the struggle for life, or in other words the smaller the number of trunks remaining on a unit of surface and, consequently, the greater the percentage of those which perish. If we think out this phenomenon, it becomes quite understandable : the more favorable the environment is for the plants' existence, the more luxuriant will be the development of each plant, the sooner will the tops of the trees begin to close above, and the earlier the oppressed individuals become isolated. Also, in better conditions of existence, every individual in the adult state will be more developed and occupy a greater space, but the individuals will be fewer in number. Investigations show that this is a general rule for all the forest species (Sukatschev, '28, p. 12).
Similar data were obtained by Sukatschev ('28) in experiments with the chamomile, Matricaria inodora, on fertilized and non-ferti- lized soil. In counting up the individuals remaining at the end of summer (August 17), the following decrease of the original number of individuals was ascertained (see Table II and Fig. 1).
Here likewise in better conditions of existence competition pro- ceeds with greater intensity, and the per cent of individuals which perish is greater.
The results obtained by botanists are certainly characteristic for the ontogenetic development of plants, but at the same time they give us an approach to the quantitative appreciation of the intensity of the struggle for existence, the whole significance of which was already clearly understood by Darwin. In the next chapter we shall consider the struggle for life in animals, and there, using entirely different methods, we shall endeavor to formulate quantitatively the intensity of this struggle.
(3) In field observations the question often arises as to the struggle for existence in mixed populations, about which Darwin wrote: "As
STRUGGLE IN NATURAL CONDITIONS
15
the species of the same genus usually have, though by no means in- variably, much similarity in habits and constitution, and always in structure, the struggle will generally be more severe between them, if they come into competition with each other, than between the species of distinct genera." Lately, botanists have tried to approach this problem experimentally. It became evident that, actually, in a number of cases competition is keenest when the individuals are most
Percentage, of perished individuals
15.1%
5.8%
good Bad
conditions
Fig. 1. Intensity of the struggle for existence in the chamomile, Matricaria inodora, on fertilized and non-fertilized soil (dense culture).
TABLE II
Decrease of the number of individuals in the chamomile {Matricaria inodora)
expressed in percentage of the initial number
From Sukatschev ('28)
PERCENTAGE |
|
Dense culture (3x3 cm.): Non-fertilized soil |
5.8 |
Fertilized soil |
25.1 |
Culture of middle density (10 x 10 cm.): Non-fertilized soil |
0.0 |
Fertilized soil |
3.1 |
similar. The more unlike plants are, the greater difference in their needs, and hence some adjust themselves to the reactions of others with little or no disadvantage. This similarity must rest upon vege- tation or habitat form, and not merely upon systematic position (Clements, '29). Researches on competition in mixed populations consisting of different kinds of cultivated plants were undertaken by many investigators (e.g., Montgomery, '12). Particularly interesting
16 THE STRUGGLE FOR EXISTENCE
data concerning wild-growing plants have been recently published by Sukatschev ('27) in his "Experimental studies on the struggle for existence between biotypes of the same species." First of all he studied the competition between local biotypes of the plant, Taraxa- cum officinale Web., from the environs of Leningrad. These biotypes were cultivated in similar conditions with a fixed distance between the individuals, and the experiments led Sukatschev to the following conclusions: (a) One must rigorously distinguish the conditions of the struggle for existence in a pure population, formed by a single biotype, and in a mixed population, consisting of various biotypes. (b) It is to be noted that a biotype which shows itself to be the most resistant in an intrahiotic, struggle for existence, may turn out to be the weakest one in an interhiotic, struggle between different biotypes of the same species, (c) The increase in mutual influence of plants upon each other with an increase in density of the plant cultures, may completely reverse the relative stability of separate biotypes in the process of the struggle for existence. The biotypes yielding the great- est percentage of survivors under a small density of cultivation may occupy the last place in this respect in conditions of a dense culture. This can be illustrated by Table III.
If we arrange the biotypes mentioned in Table III according to de- creasing stability, we shall find that in the conditions of a not dense, pure culture : C > A > B, i.e., the biotype C gives the smallest percent- age of non-survivals and is the most resistant, whilst the biotype B is the weakest of all. In dense pure cultures the relations are entirely different: B > A > C, i.e., the biotype B is the most stable one. Lastly, for dense, but mixed cultures, we have: C > A > B. Almost similar data have been obtained by Montgomery ('12) in studying the competition between two races of wheat.