Abstract The common coactions between plant and animal populations can readily be studied in the normal Drosophila melanogaster culture, and from this laboratory model it may be possible to obtain some indication of the significant variables affecting the growth of similar populations in nature. This model m ay also be used to test the validity of the assumption inherent in the various mathematical theories of population growth. The present experiments are concerned only with what various circumstances determine oviposition rates and fecundity of the parents. The growth of the population is determined in the first place by changes of this rate. Abstract Under optimum conditions the daily egg output of the flies changes as they age. This change in time is described for flies fed with different quantities of food (yeast) and also for flies fed on food of different qualities. Flies fed only on sugars lay a minimum number of eggs, and fecundity increases in proportion to the relative amount of food supplied to the flies. Only a very small amount of yeast is necessary to maintain normal oviposition. Abstract The rate of oviposition is also determined by the quality of the yeast supplied to the flies. Baker's yeast produces a higher rate of egg output than brewer's yeast, and live yeast gives a greater output than autoclaved yeast. This difference is due to the amount of specific substances, probably vitamins, in the food. But total fecundity is but little affected by these differences of quality, since flies which lay less rapidly live longer and produce the same total number of eggs as those which lay more rapidly. Total fecundity is approximately proportional to the size of the flies on hatching. However, the quality of food available to them during their larval stage does affect their oviposition rates. Abstract In population cultures, rate of egg output is suboptimal after the second to third day. The changes of the quality of yeast in the cultures is probably the significant ecological agency which produces this lowering. After the sixth day, destruction of the medium surface by larval activity and disappearance of yeast from that surface m ay be of some importance. The course of daily egg output in such cultures must lead to a phase of 'family grow th' which might be described by a logistic curve. Abstract Crowding of adults leads to only a slight lowering of fecundity when the flies are adequately fed. There is also little evidence of competition for oviposition space within the limits tested. So the decrease of fecundity demonstrated by Pearl (1932) can take place only when there is competition for food and is the direct result of this competition. This is the correct explanation of his results. These findings are discussed in the light of ecological theory. I n t r o d u c t i o n (a) General. Various attempts have been made to describe the development of populations from generation to generation and to formulate generalized principles of population growth in mathematical terms (Kostitzin 1937; Volterra 1931; Pearl 1926; and others) . Interpretations of this kind must rely on simple postu lates, and the conclusions to which they lead do not necessarily give a unique explanation of any set of data: for goodness of fit may be due to the formal properties of the mathematical edifice, and these may have scant relevance to biological phenomena, as Teissier (1937) , Kavanagh & Richards (1934) and Gray [ 258 ] (1929) have shown. Consequently the simple postulates which have been used as a basis for such generalizations as the 'logistic law' (Pearl & Reed 1920) depend on numerical relations which, if valid, might arise from a variety of circumstances each quite distinct from the biological point of view. Indeed, Hammond's (1939) recent review makes it abundantly clear that the significant ecological determinants underlying population growth are different for different species and environments. This is what should be expected. Since this is so, the search for general principles of population growth must be guided by an intimate knowledge of all the relevant biological phenomena involved in each situation. Generalization is premature until more of such information is available. The object of the present series of investiga tions is to undertake this task for a simple ecological system, the Drosophila culture, already studied by Pearl and his co-workers (Pearl 1927) . I n t r o d u c t i o n (a) In nature animal population growth necessarily involves a balanced reaction between at least two populations. In the last resort one component must be a species which is not an animal, and as such is capable of utilizing inorganic sub stances to build up protoplasm. In the relation between flies and yeasts present in a Drosophila culture, the most, elementary situation which embodies the essential features common to more complex conditions of population growth is found. The yeasts grow on the culture medium and form the major food source of the flies (Baumberger 1919) . Further, the fly has several advantages for experimental work: it is easy to culture and its life cycle is short. It has also been the subject of many genetic and other investigations, and much is already known of its population growth. I n t r o d u c t i o n (a) Present knowledge of the Drosophila-je^st system is far from complete. Previous workers have based their observations primarily on counts of adult flies and have not paid due regard to the separate effects of external conditions on other stages of the life cycle. This may invalidate much of the theory based on that work. Bodenheimer's (1938) more recent inquiry, which avoids this error, neglects all changes of the food (yeast) population. And nothing has been done to examine changes which may occur during the larval feeding stage. Thus to analyse what is relevant to the total result, from the standpoint outlined above, separate information is needed concerning the circumstances which affect (a) fecundity, i.e. total eggs produced by a female and changes of oviposition rate; (b) pre-natal mortality, i.e. the difference between fecundity and fertility, the latter defined as the total number of viable eggs produced by a female (Norris 1932) ; (c) larval and pupal mortality; and ( d) adult mortality. Finally, one must also find how such c stances are changing as a population develops. I n t r o d u c t i o n (a) The relevant circumstances which must be examined fall into three categories: (a) Changes of the yeast population. This is, changes of the quantity of yeast available to the flies and such qualitative differences of yeast strains as may modify fecundity or mortality by virtue of their total food content or their content of specific food substances (vitamins) . I n t r o d u c t i o n (a) (b) Changes of the physical structure or chemical state of the agar gel medium produced by the activities of the larvae and yeasts. That is, the investigation will be Ecological determinants of population growth 259 chiefly concerned with the effects of larval feeding behaviour on the structure of the gel in so far as this provides physical support for them and for the flies, with such concomitant changes as may affect food and oxygen availability, and with effects produced by the accumulation of metabolic by-products of the two species. I n t r o d u c t i o n (a) (c) The effects of mutual stimulation of flies and of larvae arising from their various motor activities. I n t r o d u c t i o n (a) These categories are not independent of each other. For example, the physical and chemical changes produced in the medium by larval activity may affect, on the one hand, the availability of the yeast, and on the other may create conditions favourable to the selection of physiologically different yeast strains, and so on. Hence the significant interrelations between categories during the different phases of population growth must also be found. I n t r o d u c t i o n (a) (6) Fecundity. This communication deals with some circumstances which affect the fecundity of Drosophila melanogaster Meig. Recorded data on this topic are not consistent. The estimated number of eggs laid by a female during a life-time ranges from 100-300 (Lutz 1914 ) to over 2000 (Hyde 1920 . Similarly, recorded daily oviposition rates vary from 3-3 (Adolf 1920 ) to 52 eggs per day (Hanson & Ferris 1929) . This inconsistency is not surprising. For it has been shown that genotype (Wentworth 1913; Alpatov 1932; Shapiro 1932; and Carver 1937) , temperature (Kaliss & Graubard 1936; Harries 1939) , humidity (Bodenheimer 1938) , nutrition (Guyenot 1913 a, 6; Mickey, Carpenter, Cumley & Burdette 1941) , oviposition stimuli (Veillet 1936; Spencer 1937; Reed 1938; and Mickey et al. 1941 ) and the presence of other individuals (Hanson & Ferris 1929; Pearl 1932 ) all modify fecundity. There is apparently no periodicity of oviposition (Donald & Lamy 1936) . Most workers agree that oviposition starts on the second day after hatching, increases up to the sixth to tenth days, declines thereafter and ceases just before death of the fly. This is true only of flies kept under more or less optimal conditions. It may not be true of flies kept in a developing population culture. So the circum stances affecting oviposition rates and which of these may be significant ecological determinants during population growth in such artificial cultures must be deter mined. One must also find the extent to which the character of population growth depends on changes of oviposition rates. M a t e r i a l a n d m e t h o d s An inbred Oregon strain of Drosophila melanogaster (Meig.) was the material for most experiments. The mutant ' Antennaless ' was also used when examining certain problems; when employed it will be referred to specifically in the text. Stock cultures were 2 x 2 or 3 x 3 matings reared on Pearl's S 101 medium (modified)* * The m edium is the same as th a t given by Pearl & Pennim an (1926) , with this difference: th e sugar is replaced by 30 g. molasses and 15 g. dextrose per litre. This formula gave optimum yeast grow th in test experim ents. Presum ably the molasses supplies some of the growth sub stances (bios) required by th e yeasts. except in one experiment where we used normal cornmeal-molasses-agar (see Gordon & Sang 1941 ) and a similar medium in which oatmeal and malt extract replaced the cornmeal and molasses. Only flies hatching during the first or first 2 days were used for oviposition experiments. This ensured a greater similarity of their culture conditions and reduced variability of results. All cultures were kept at 25 ± 1° C. Alpatov's (1932) technique was used when measuring fecundity, and a modi fication of it was employed when collecting large numbers of eggs for other work (figure 1). Except where actively growing yeast was used 1 c.c. acetic acid and 2 c.c. ethanol were added to each 100 c.c. of the oviposition medium (Pearl's S101 modified) . This addition compensates for the alcohol and acetic acid produced when live yeast is growing on the medium-both these substances tend to encourage oviposition and also have an adverse effect on yeast growth. Except in experi ments described in tables 1, 2, 9 and 10 the yeast was supplied in the form of a moist pasty mass in the centre of the oviposition medium. Under these conditions yeast growth is inhibited over the experimental period. So variable factors resulting from active yeast metabolism are cut out. Physical consistency of the food can be standardized and egg counts are facilitated, since most of the medium surface is free of yeast growth. Unless otherwise stated food was always supplied in gross excess. The surface of the medium was roughened by slicing off the upper layer. Flies which died during experiments were either replaced by others with a similar history or the cultures were discarded. So recorded data refer only to cultures containing the stated number of females for the entire experimental period. Sterile cultures were prepared by the method given by Gordon & Sang (1941) . , In the tables recording experimental results the mean total number of eggs laid by the flies is given in the form of means accompanied by their standard error. Where necessary significance is estimated by use of the test (Fisher & Yates I93^)-E x p e r i m e n t a l Comparison of oviposition rates under optimum conditions and in population cultures We have already suggested that the daily egg output of flies may change for two reasons. First, the physiological make-up of the flies varies as they age, and this will be reflected in their oviposition rates. Secondly, the environment in popula tion cultures will change as larvae develop and feed in the medium. But these environmental changes may, or may not, significantly modify the rate changes produced by ageing. So to find if environmental changes do play a part in regu lating population growth comparison must be made between the number of eggs laid under optimum conditions with the number laid in population cultures. The difference between the two will give a measure of the importance of the environ ment. For this purpose, then, 'optimum' conditions need only be the best set-up which can be handled conveniently and which will give a standard reference curve for changes of oviposition rate with age. To guard against unforeseen variables the majority of experiments are also internally controlled. Comparison of oviposition rates under optimum conditions and in population cultures Standard conditions chosen were half-pint milk bottles supplied with fresh ovi position lids daily (figure 1). A portion of live brewer's yeast is placed in the centre of each lid, so each day the flies have an excess of food and a fresh surface on which to oviposit. Brewer's yeast was chosen as food, since samples are more stan dardized than those of other yeast available. Normal culture conditions were reproduced by setting up similar matings in dipping jars (figure 1), the lids of which are filled with 25 c.c. of medium (S101 modified). This medium is seeded with live yeast. Each day the lids are removed, the eggs counted, and the lids replaced. In this way the cultures develop with a minimum of interference, and the egg counts are made with greater accuracy than when the medium is removed from normal culture bottles. In the normal culture group actual fecundity is probably slightly less than recorded, since no allowance was made for recount of unhatched eggs. Comparison of oviposition rates under optimum conditions and in population cultures The average oviposition rate of a mated female kept under standard conditions shows that the flies lay no eggs until the second day after they emerge (table 1) . Thereafter, maximum daily output is rapidly reached and maintained during the fourth to sixth days. This peak is followed by a slow and irregular decline of oviposition rate similar to that described by Hanson & Ferris (1929) . On any day there is a considerable variability of oviposition rate, and this is not related to age of female. Fecundity is also variable; the mean whole eggs laid by twenty-seven flies over 10 days was 420*4 + 20*3. We have been unable to reduce this variability in spite of standardization of stocks and technique. Comparison of oviposition rates under optimum conditions and in population cultures Normal population cultures are seeded with baker's yeast, so the above experi ment was repeated using this in place of brewer's yeast (table 1) . Detailed differ ences between the two experiments need not be considered here, but it is necessary to note that the quality of yeast may influence the rate of oviposition. Baker's yeast was also used to seed the dipping jar cultures. These more or less normal population conditions produce a strikingly different oviposition rate. The numbers of eggs laid during the first 2 days are practically the same in the three sets of experiments, but in the population cultures there is a considerable fall after the third day. Fecundity is consequently reduced in the latter cultures, and by the tenth day the flies have stopped laying. Observation shows that by the fifth to sixth days large numbers of larvae are present in the lids and that these have ploughed up the medium to some extent. The yeasts are also growing below the medium surface after the fifth day, so the food available to the flies must be less than on preceding days. Repetition of this experiment confirmed the general statements made above (see Sang 1944 ), but they did not always show as drastic a lowering of fecundity. Growth of moulds, excessive moisture in the cultures and other technical difficulties account for these differences. The lowered oviposition rate in population cultures is probably produced by at least five changing environmental circumstances: Comparison of oviposition rates under optimum conditions and in population cultures {a) The larvae in the medium may disturb the flies as they feed and oviposit. Pearl (1932) has suggested that the act of oviposition is readily inhibited by inter ference of this sort. But experiments reported below show that this suggestion is incorrect; and preliminary experiments indicated that larval interference was of minor importance. Comparison of oviposition rates under optimum conditions and in population cultures (b) The larvae may break up the medium surface and so make it unfavourable for oviposition. Again, preliminary experiments suggested that this is not of great importance, at least until the last stages of population growth are reached. Comparison of oviposition rates under optimum conditions and in population cultures (c) The first lowering of oviposition is found about the third day in the life of the culture. At this stage there is a thick layer of yeast on the medium surface overlaid to a considerable extent by the fungus Oidium lactis. This fungus grows rapidly on yeast and reduces the availability of the latter to the flies. Oidium is commonly found on our S 101 cultures but is usually absent on maize and oatmeal cultures. Comparison of oviposition rates under optimum conditions and in population cultures (d) With removal of excess Oidium by feeding larvae oviposition would be expected to rise, since the yeast is again more accessible. This does not occur. The further decline of oviposition is explicable if it is assumed that the yeasts change qualitatively and that this leads to lowering of oviposition. This explanation is suggested by fecundity differences among flies fed on different strains of yeast (tables 4, 5) . Comparison of oviposition rates under optimum conditions and in population cultures (e) After the sixth day the quantity of yeast on the surface may be insufficient to maintain egg production. All these and perhaps other agencies may affect oviposition, and each may do so to a different extent during the various phases in the life of a culture. Comparison of oviposition rates under optimum conditions and in population cultures To test the possibility outlined in ( d) flies from (1) non-growing live yeast, (2) yeast which had been allowed to grow [36] [37] [38] [39] [40] hr. at 25 C and which was largely overlaid by Oidium at the time of application, (3) yeast which had been treated as in (2) but for the addition of approximately twenty young larvae per lid at the time of seeding (table 2) . Flies in group (1), i.e. on non-growing yeast, lay more eggs than do those in group (2), in which the covering of Oidium restricts feeding opportunity. Flies in both these groups lay many more eggs than do those in group (3). In the third group, however, growth of Oidium has been restricted by the larvae. Since yeast is always visible on the surface of the medium throughout the experimental period, and since experiments showed that this type of medium surface does not have a marked inhibitory effect on the act of oviposition, the results can be explained by assuming that the quality of the yeast changes in the presence of larvae. In the experiments reported below we have therefore confined ourselves primarily to an examination of the nutritional changes which take place in a culture. N u tritio n a n d fecund Comparison of oviposition rates under optimum conditions and in population cultures We have suggested that both the quantity and quality of yeast available to the flies will change as population cultures age. The work of Gordon & Sang (1941) lends credibility to this suggestion. However, there is no measure of these changes or of the quantity of yeast necessary to maintain oviposition. So the limit within which food quantity can modify fecundity must be found, and by supplying yeast in excess of this one can ensure that the flies are more than adequately fed when yeasts of different quality are tested. Comparison of oviposition rates under optimum conditions and in population cultures It is difficult to regulate or to measure the quantity of food consumed by flies, so a first estimate was made by allowing the flies to feed for different times. The oviposition medium contained alcohol-acetic mixture and was seeded with baker's yeast. The first group of flies was not allowed to feed and was supplied with yeastfree lids. The second group was fed on yeast for 1 hr. each day and then supplied with a yeast-free lid; similarly, the third group was allowed to feed for 6 hr. daily. The fourth group was fed normally. All eggs laid during the day, whether on feeding or yeastless lids, were counted; the results are summarized in table 3 and the individual counts shown in figure 2. Increased feeding time raises fecundity and alters rate of oviposition. When the flies are fed the number of eggs they lay in 10 days is proportional to the length of time they are permitted to feed. Apparently unfed flies would lay considerably fewer eggs than flies fed for only a very short time. Unfed flies lay the majority of their eggs by the sixth day (figure 2), and these 'larval' eggs, i.e. eggs formed from food materials stored during the larval period, can therefore be produced without further intake of food, except perhaps sugars. Once these eggs are laid further oviposition depends on feeding. A small quantity of food permits a con tinuous output at a low level (4-17 % series), and a greater quantity presumably leads to a certain degree of storage and therefore to bursts of oviposition (25 % series). Excess food throughout the entire period gives the normal result. Since Alpatov (1932) and Shapiro (1932) have tried to describe changes of oviposition by simple mathematical formulae, it is worth emphasizing how drastically in adequate nutrition may affect such curves. It will be seen later that food quality has similar effects. 3. The influence of food quality on rates The experiments of Gordon & Sang (1941) suggest that the yeasts change quali tatively as population cultures develop. That is, the food content of the yeasts is altered either by selection of the strains or by modification of the strains as en vironment changes. Both possibilities exist, since it is known that different strains have a different food content (Fixen & Roscoe 1938) and that the chemical com position of a strain may be modified by growing it on different media (see below). The rapid growth of the yeasts in our cultures would permit both changes to occur: differentiation between them is not necessary here. In either case the changes may alter the fecundity of the parents. This is suggested by the data given in table 1. To test this possibility further flies emerging from the same cultures were separated into two groups and set up as paired matings. One group was supplied with baker's yeast and the other with brewer's yeast throughout the experiment. The results are given in table 4 and figure 3. The flies used in this experim ent were reared on three different types of medium, and random samples from th e first d a y 's hatch of each were fed on th e two yeasts. No matter what the previous history of the flies, the group fed on live baker's yeast always lays more eggs on any day than does the corresponding group fed on brewer's yeast. This difference in the rate of egg output is usually apparent by the second day, and the difference of means over the 9-day experimental period is significant in all cases (table 4) . That is, nutritional changes can alter rate of oviposition and can do so sufficiently rapidly to account for the lowering found in a growing population. But the experimental population was seeded with baker's not brewer's yeast, and therefore should have given a higher yield than the standard brewer's yeast culture (table 1) . This difference between experiments may be due to the modification of the strain as well as to selection in the population cultures. The possibility of such modification was examined next. The influence of food quality on rates Two strains of brewer's yeast, S and H, each a pure line descended from a single spore, were cultured on two media of different composition. Medium A was an artificial salt solution containing a minimal quantity of growth factors (bios); medium W was normal beer wort. For yeast W is the more nutritive medium of the two. After the yeast seedings had grown they were separated from the medium by centrifuging and fed to the flies in the normal way. The results confirm the statement made above, namely, that differences of strain, even of the same yeast, may alter fecundity (table 5) . Further, the same strain may produce a different oviposition rate of Drosophila when it is cultured on a different medium in spite of its presumed genetic homogeneity (figure 4). When cultured on artificial media both yeast strains give a better egg output than when they are grown on wort. The influence of food quality on rates When the yeast fed to the flies is killed by autoclaving their rate of oviposition is lowered (table 6), and the form of the oviposition-time graph also changed. But again, it is found that baker's yeast gives a higher rate of oviposition than brewer's yeast. The lowering of the rate of oviposition may be due to a real decrease of the number of eggs laid or to a slower reproductive rate which would manifest itself as a decrease during the 10-day experimental period. So changes during the entire life period have also been examined. For flies fed on living baker's yeast and on a portion of the same yeast which had been autoclaved the difference of means for: (a) life period = 6-16, standard error of the difference -45*50; (6) 7-day period -71*12, standard error of the difference -25*66. The latter difference is significant; the former is not. Thus, potential fecundity is independent of adult nutrition while oviposition rate is determined by it. The influence of food quality on rates Longevity of females fed on dead baker's yeast is significantly greater than that of females fed on live yeast. No such difference is found for males. That is, feeding on dead yeast slows down the rate of egg production and this is correlated with longevity. This confirms Alpatov's (1932) findings for flies kept on five yeast. Flies which are fed only on sugars and therefore produce a very small number of eggs live longer than normally fed flies (Glaser 1924) . This further supports the conclusion that longevity is associated with rate of egg production. The influence of food quality on rates Since food is excessive, it may be assumed that the flies are adequately and equally fed in these experiments. It then follows that dead yeast is qualitatively Ecological determinants of population growth different from live yeast. Heating yeast may: (a) change it physically; (6) partially destroy some substance or substances necessary to maintain the high fecundity found with live yeast. The distinction between these possibilities is more apparent than real, for in both cases it means that less of some necessary substance or substances is available to the flies. These substances are, presumably, the ones which are present in greater or less amounts in the different yeast strains and in 1 ' 2 F ig u r e 4. Two pure-yeast strains give a different ra te of egg production when they are fed to Drosophila. The solid line gives the yields for strain H and the broken line the yields for strain S. The circles show th e yield when these strains are cultured on w ort and the crosses give th e corresponding values when they are cultured on artificial medium. The values are th e average num ber of eggs laid by a female in 10 days when fed on the yeast type shown. F. W. Robertson and J. H. Sang the yeasts cultured in different media. It is the quantitative changes of these which account, at least in part, for the changes of oviposition rates in normal cultures. . The significance of specific food substances To distinguish between the two possibilities stated in the last paragraph, baker's yeast was autoclaved normally (20 min.) and for three times that period. At the same time a further portion of the same sample was extracted with water in the autoclave at 120° C. Table 7 shows that dry baker's yeast autoclaved for 20 and for 60 min. produces substantially the same figures for fecundity. It is again lower than when live yeast is used. This suggests that the difference between live and dead yeast is a physical one, inasmuch as availability of fecundity substance is decreased. Since fecundity is lower after 60 min. than after 20 min. extraction, heating may destroy this substance to some extent. But this is more likely to be an extraction effect. Therefore the fecundity substance can be extracted from yeast with water. This behaviour suggests that the fecundity substance is related to the vitamin B complex. T a b l e 7. H e a t d e s t r u c t i o n a n d e x t r a c t i o n o f t h e f e c u n d i t y s u b s t a n c e Ecological determinants of population growth Guyenot (19136) states that peptones alone are sufficient to maintain egg production. Other water-soluble substances necessary for Drosophila nutrition (see Tatum 1941; Gordon & Sang 1941) are also found in yeast. The importance of peptone and of other substances was examined by adding (a) casein peptone, (6) a 10 % acid water extract of brewer's yeast, to heat-killed brewer's yeast as basic food. Heat-killed brewer's yeast was used because it has been shown to be the most inadequate for adult nutrition when this is measured by rate of oviposition. T a b l e 7. H e a t d e s t r u c t i o n a n d e x t r a c t i o n o f t h e f e c u n d i t y s u b s t a n c e Addition of peptone and of brewer's yeast extract both raise fecundity (figure 5). The extract is more effective than peptone. This increase is due to increased fecun dity throughout the entire experimental period. The quantity of extract used was small, just sufficient to moisten the dead yeast. The quantity of peptone was relatively great, about one-fifth of the total food. So the extract effect is of a different order from that produced by peptone. Peptones alone cannot provide an adequate diet, and this suggests that certain specific substances are required for egg production. The work of Mickey et al. (1941) confirms this conclusion. These workers find that addition of wheat germ to media increases oviposition. T a b l e 7. H e a t d e s t r u c t i o n a n d e x t r a c t i o n o f t h e f e c u n d i t y s u b s t a n c e It is likely that the high vitamin B content of this substance is the active com ponent concerned. If so, the two results suggest that some fraction of the vitamin B complex stimulates oviposition. F. W. Robertson and J. H. Sang 5. The effect of the larval diet on fecundity The filial generation of flies will not all develop under the same environmental conditions. In particular, the food available to them during the larval instar will be quantitatively and qualitatively different at different stages in the life of the culture. Details of these changes will be discussed in a later paper, the present experiments being concerned only with their effect on fecundity. The first, food shortage, leads to the development of small flies, and the number of eggs laid by such flies is roughly proportional to their size. But the results of experiments which show this are very erratic; and a previous experiment suggests an explana tion of this variability. Small flies are produced either by crowding of larvae or during the later stages of culture, and the two groups of larvae will not neces sarily have the same kind of food available to them. That is, the quality of food consumed during the larval stages may affect adult rate of oviposition. Cultures made with three different media, but all seeded with baker's yeast, yielded flies of approximately the same weight during the first day's hatch. These flies when fed on the same yeast do not all lay the same number of eggs. The detailed oviposition rates of the three groups are given in figure 3. Flies raised on maizemeal lay more eggs than flies reared on oatmeal during a 9-day period, and the latter lay more than flies raised on S 101 medium (table 8) . For the series fed with baker's yeast the difference between the maizemeal and S101 groups is statistically significant, but the differences between oatmeal and S 101, and between maizemeal and oatmeal are not. However, examination of the fecundity graphs (figure 3) suggests that the difference between maizemeal and S 101 groups is in fact significant. For the series fed with brewer's yeast only the difference between maizemeal and S101 media is significant. This result confirms our conclusion that medium quality may modify the quality of the yeast and shows that adult fecun dity is partly determined during the larval stage. 5. Another conclusion can also be drawn from this experiment. It has already been stated that there was a considerable variability between experiments in spite of efforts to standardize the experimental stocks and methods. This is evident, for example, in the figures for fecundity of flies fed on baker's yeast given in the preceding tables. Part of the variability is due to differences in the samples of yeast used, and such differences could not be avoided. It is also likely that differ ences in the culture conditions of the flies used for the* experiments accounts for the remaining differences between experiments. The larval environment of the flies which hatch on the first or first 2 days will differ from experiment to experi ment. Whether this larval environment does or does not determine potential egg output is at present uncertain. Crowding and fecundity Emergence of the filial generation of flies necessarily leads to crowding effects. Pearl & Parker (1922) and Pearl (1932) demonstrated that crowding or 'density' influences both productivity and fecundity. And this density effect leads to a drastic lowering of egg output and may be the major circumstance controlling population growth after emergence of the first filial generation. Here 'density' means numbers of adults per unit of food and space. But as food quantity is continuously changing and as food and space units have a different meaning for different species, this term has little advantage over the word 'crowding'. 'Den sity ' implies a constancy of food and space conditions and a change in number of individuals only. These conditions are rarely, if ever, found, as Hogben (1931) points out. The more neutral term ' crowding' will be used in its place in this paper. Crowding and fecundity (a) Importance of crowding. To repeat Pearl's (1932) experiments bottles con taining 2, 4, 8, 16 and 32 pairs of virgin flies were prepared as before and supplied with live baker's yeast food. Dead flies of both sexes were replaced from parallel cultures. Under these experimental conditions crowding lowers fecundity (table 9). The decrease is slight and most marked between four and eight pairs. Total number of eggs per bottle is roughly proportional to degree of crowding. This conflicts with Pearl's (1932) and Bodenheimer's (1938) published figures. Bodenheimer's data which give productivity but not fecundity are incomplete, and no detailed comparison can be made. On recalculating Pearl's data we find fecundity for the first 4 days falls more rapidly and is at a lower level than for a similar period in our experiments. Similarly, his yields range from 549-2 eggs per 1000 female hours (2 pairs) to 176-6 eggs per 1000 female hours (32 pairs) compared with our 2077-8 eggs per 1000 female hours (2 pairs) to per 1000 female hours (32 pairs). Fecundity is always higher under the experimental conditions described here. Pearl's flies were probably of a different strain from those used in the present experiments. The area of oviposition surface in his experiments was 314 sq.mm., in the present experiments it was 1390 sq.mm. Both differences will influence th^ results, the latter by decreasing interference between ovipositing females. How ever, in both cases the change should only be a relative one. Evidence for a crowding effect comparable to Pearl's would be expected from the present data. High fecundity in these experiments suggests that availability of food constitutes an important difference. As always yeast was supplied in excess, and it was often necessary to scrape away the surplus growth to reveal all the eggs. Pearl often made drawings from which egg counts were taken, so his yeast growth could never have been abundant. Indeed, he states in a footnote: £By the technique used here .. .flies do not get an adequate amount of food at any density.' So starved flies are much more sensitive to crowding and Pearl's fecundity-crowding curve is only valid under special conditions. Food competition, as well as interference between ovipositing flies and other coincident circumstances, must also be considered as one of the agencies modifying fecundity. F. W. Robertson and J. H. Sang (b) Oviposition area. Pearl's (1932) results and our own findings suggest that area of medium available for oviposition may have a considerable effect on fecun dity. Available surface does not play any part in producing results recorded in previous experiments, but it may be important at later stages of population growth. To examine this bottles were prepared and the oviposition area was varied by covering the medium with cellophane disks. Ten pairs of flies were placed in each bottle. Only a few eggs were laid on the cellophane. In the centre of each watchglass a food plug of 10 % live brewer's yeast in agar was inserted. The area of this plug was 38*5 sq.mm., and this is included as part of the available oviposition area. F. W. Robertson and J. H. Sang The food supplied as an agar plug containing yeast is apparently inadequate, because the yield per day is low. After 4 days the flies were themselves spreading the yeast over the medium. So the experiment was stopped. The constancy of yield during this period is surprising (table 10) . A sevenfold increase of oviposition space has no effect on fecundity. The conclusion to which this leads us is contrary to Maclagan's (1932) findings for Sitophilus. Neverthe previous experiments is clearly adequate. It is not possible to account for Pearl's (1932) results by assuming that competition with respect to oviposition takes place at the surface of the medium. This again emphasizes the importance of the food and shows that Pearl's findings are only valid under restricted specific conditions. (c) Food quantity and the crowding effect. When flies are equally but slightly inadequately fed there is no change of fecundity when the oviposition area is diminished up to a limit. Hence, the decrease of egg output with crowding recorded by Pearl (1932) cannot be produced, as Park (1941) suggests, by competition at the medium surface, at least in so far as this is competition for oviposition space. However, there may be competition for food. As has been shown (table 3) food shortage could produce a result compatible with Pearl's findings. And it has also been shown that this is so by feeding with flies on the usual medium containing alcohol-acetic mixture, which prevents yeast growth, and by seeding this with only one drop of 0*5 % baker's yeast suspension. Figure 6 shows that under conditions approaching starvation, crowding has a considerable effect on fecundity. When there is only one pair of flies per bottle the average level of oviposition is only slightly subnormal. With more than one pair of flies the number of eggs laid per female day declines, and small increases have a proportionally greater effect than a large increase of crowding. The relation ship between crowding and fecundity is approximately a hyperbolic one under these conditions. This is in good agreement with Pearl's findings, so it may be concluded that in his experiments, as in those described here, the crowding effect is brought about primarily by food competition among the flies. findings for Since the cultures containing only one pair of flies give a level of oviposition that is only slightly subnormal it can be assumed that the food supply is almost adequate. So one pair of flies requires only 0-0005-0-0006 g. of yeast (wet weight) per day to maintain oviposition. It is very unlikely that less than this quantity is available at any time during the first 10 days in the life of a population culture. Further, if comparison is made between the results of this experiment with the time of feeding experiment (table 3) , it will be found that four pairs of flies are approxi mately 25 % fed, eight pairs of flies are about 4 % fed, and so on. That is, as crowding is increased the efficiency of good finding, as measured by fecundity, follows the law of diminishing returns. In the light of Nicholson's (1933) work this is a reasonable conclusion and is in accord with the other experimental work on this topic quoted by Park (1941). F. W. Robertson and J. H. Sang D i s c u s s i o n a n d C o n c l u s i o n s The oviposition rate of the parent, or parents, constitutes the basic variable in the population growth of the fly species in the yeast system. The sequence of larvae in the medium depends on this rate and so, therefore, does the competition between larvae. It is not likely that this competition will prove to be simple, but one of its results must be to affect the rate of yeast population growth. Another will be to alter the physical and chemical condition of the medium in the culture bottles. Both of these may reflect back on the parents and alter their rate of egg output. They may also modify the period of larval and pupal development and so further determine the course of emergence of the first filial generation of flies. And, when the cultures can support them, the emergence of subsequent generations will be determined in a similar but considerably more complex fashion. D i s c u s s i o n a n d C o n c l u s i o n s Under more or less optimum physical'and nutritional conditions the rate of egg output of the flies changes as they age. At first, until the flies have fed, the output is low; then as new eggs form it increases and reaches an optimum which may be maintained for a few days. Thereafter the oviposition rate falls off and egg laying ceases just before the flies die. The rate of oviposition determines, to some extent, the longevity of the females, but the average fecundity of a given batch of flies is the same whatever the rate of output. That is, oviposition rate follows a more or less regular pattern in time, and it is this pattern which is modified by the environ mental changes which take place in a population culture. These environmental changes reduce egg output after the third day and consequently produce an earlier and more distinct optimum and decrease the total eggs laid in a 10-day period. This modified course of oviposition must itself lead to subsequent increase of the adult population of the first generation which can be described, even if only roughly, by the 'logistic law'. But this is only family growth and not population growth in the strict sense. D i s c u s s i o n a n d C o n c l u s i o n s In the culture set-up described, at least four changing environmental circum stances produce the lowering of oviposition rate. The first in time is the reduction in availability of the yeast by growth of Oidium. More or less consistent with this and assuming greater importance later is probably the change in the quality of yeast on the medium surface. It has been shown that such a change may be brought about either by selection of different yeast strains by the environment or by the chemical modification of a strain or strains. Both possibilities exist, and it is difficult to distinguish between them. For the present purpose it is sufficient to record that different yeast strains and the same strain cultured on different media contain different proportions of specific substances which determine the rate of oviposition when fed to the flies. The relevant substance or substances are probably components of the vitamin B complex. Such differences of strain have a further effect. When fed to larvae they may alter their fecundity when they become adult. This will be reflected in the later growth of the population. This 'carry over' from the larval to the adult stage may partly account for the result reported by Alpatov (1932) . This worker found that flies reared at 19° C produce more eggs than flies reared at 30° C. The low-temperature group are not only larger and therefore more fecund, but they will also have passed through a qualitatively different nutritional environment. D i s c u s s i o n a n d C o n c l u s i o n s Changes of the yeasts are produced not only by their own growth on the medium but also by larval activity. After about the sixth day in a population culture larval action has considerably reduced the yeasts on the medium surface. The results presented here suggest that this decrease of food is of little significance at first; it may be important during the later stages in the life of a culture. The break up of the medium surface which happens at the same time may influence oviposition, but as the flies only require a small area for this act it is again unlikely that this circumstance is relatively important. Also, the accumulation of larval and adult excretory products may play some part in the slowing down of oviposition during the later stages. However, further experiments are necessary to confirm or disprove these suggestions. D i s c u s s i o n a n d C o n c l u s i o n s The movements of the larvae are not likely to reduce egg production to an extent greater than do the mutual contacts between flies in crowded cultures. And this effect is negligible except when the flies are starved. That is, the oviposition rate of Drosophila is determined by the quantity of food consumed by the adults, and when these are crowded fecundity is but little reduced so long as they are adequately nourished. Larvae require more food than adults. If their food supply is quanti tatively limited the larvae will develop into small adults with a potential egg output Ecological determinants of population growth 275 lower than would occur in larger flies, reared under better feeding conditions (Golightly 1940) . When the available food is restricted crowding will
{'paper_id': '84078287', 'title': 'The ecological determinants of population growth in a Drosophila culture. I. Fecundity of adult flies', 's2_url': 'https://api.semanticscholar.org/CorpusID:84078287', 'source': 'S2ORC'}