Abstract
Very frequently the autotrophic and heterotrophic components of an ecosystem are partially separated in space in that they are stratified one above the other (vegetation-soil on land, phytoplankton-sediments in water). Also, the basic functions are usually partially separated in time in that there may be a considerable delay in the heterotrophic utilization of a large portion of the net production of autotrophic organisms. Consequently, between the first and second trophic levels the energy flow of the community is often divided into two broad streams resulting in two types of primary consumption: (1) direct and immediate utilization of living plant tissues by herbivores and plant parasites, and (2) delayed utilization of dead tissues and stored food by other consumers. In most ecosystems many species of primary consumers rely exclusively on one or the other types of food source while other species utilize both, or shift from one to-the other seasonally.
In addition to the problem of time-lag in utilization, one of the difficulties in developing a satisfactory comparative population ecology of heterotrophs is the fact that different species, and also life history stages of the same species, associated together in a given area of the earth's surface differ greatly in size and rate: of
COMPARISON OF POPULATION ENERGY FLOW OF A HERBIVOROUS AND A DEPOSIT-FEEDING INVERTEBRATE IN A SALT MARSH ECOSYSTEM* BY EUGENE P. ODUM AND ALFRED E. SMALLEYf UNIVERSITY OF GEORGIA, ATHENS
Communicated by G. Evelyn Hutchinson, February 16, 1959 Very frequently the autotrophic and heterotrophic components of an ecosystem are partially separated in space in that they are stratified one above the other (vegetation-soil on land, phytoplankton-sediments in water). Also, the basic functions are usually partially separated in time in that there may be a considerable delay in the heterotrophic utilization of a large portion of the net production of autotrophic organisms. Consequently, between the first and second trophic levels the energy flow of the community is often divided into two broad streams resulting in two types of primary consumption: (1) direct and immediate utilization of living plant tissues by herbivores and plant parasites, and (2) delayed utilization of dead tissues and stored food by other consumers. In most ecosystems many species of primary consumers rely exclusively on one or the other types of food source while other species utilize both, or shift from one to-the other seasonally.
In addition to the problem of time-lag in utilization, one of the difficulties in developing a satisfactory comparative population ecology of heterotrophs is the fact that different species, and also life history stages of the same species, associated together in a given area of the earth's surface differ greatly in size and rate: of Numbers of grasshoppers were at a peak in late May, when numerous small nymphs hatched out from overwintering eggs, and then declined rapidly during the season. Two periods of heavy mortality were followed by accelerated growth of survivors, and also by a small amount of recruitment (new eggs hatching in early summer, immigration in late summer). A definite homeostasis or self-adjustment in the population was evident since energy flow fluctuated only 2-fold while numbers and biomass varied 5-to 6-fold. Since metabolism-per-gram of small nymphs was several times greater than that of adults, the high number but small biomass population of spring was about equal to the low number but large biomass population of late summer. Note also that the peak in energy flow (and hence the time of greatest consumption of marsh grass) did not correspond with either maximum numbers or maximum biomass (although more nearly with the latter than the former), but occurred when the population was composed of a medium number of medium-sized nymphs all growing rapidly.
The (Fig. 2) (Fig. 2) . Only about 10 per detritus (including associated microflora) to the high marsh cent of the annual total energy provides one potential food source for Littorina. flow of the snal potation flow of the snail population was production (i.e., 90 per cent was respiration) while more than a third of the grasshopper energy utilization was production.
In Figure 3 the broad annual patterns of energy flow of the grasshopper and the snail populations are compared. Shown also are measurements of net production of the low marsh Spartina on which the grasshopper feeds, and our preliminary estimate of the rate of formation of Spartina detritus which comprises at least a part of the food of the snail population. Actually we do not have a good estimate of detritus "production," but the rate of disappearance of dead grass from quadrats may provide a rough estimate-probably an overestimate since some decomposed grass is undoubtedly lost to estuaries and deeper sediments. The point to emphasize is that detritus is produced more or less continually throughout the year; more dead grass is present in the colder months, but this is counterbalanced by the more rapid decomposition during the warmer months.-Littorina undoubtedly also consumes algae and other aufwuchs; the proportion of different food items actually utilized in nature is unknown. Pomeroy6 has shown that net production of "mud algae" (algae living on and in the soft sediments alternately exposed and covered by tides), while not as great as that of marsh grass, makes a major contribution to the total primary productivity of the salt marsh ecosystem. The rate proved to be almost constant the year around with production occurring in summer chiefly when the sediments are covered by the high tide, and in winter when the sediments are exposed and quickly warmed by the sun. Consequently, Littorina has a more or less constant supply of both detritus and algae available throughout the year. It is evident from Figures 1-3 that the population structure of both Littorina and Orchelimum is such that the pattern of population energy flow of each species is well adjusted to the utilization of the particular sources of food potentially available to each population.
Certain ratios or "ecological efficiencies" based on the total annual energy flow are shown in Table 1 . Ingestion was estimated by adding the caloric value of feces to assimilation. The most rapidly growing segment of the Littorina population (the 1955 year class) proved to have a higher assimilation efficiency but a lower growth efficiency as compared with Orchelimum. If all the snails in the population are considered the growth efficiency is even lower. Thus, the slow-growing, long-lived snails apparently assimilate a considerable part of ingested food but use a relatively small amount of the assimilated matter for growth. In contrast, the grasshopperswhich emerge, grow and die within a short period-assimilate a smaller percentage of ingested food but convert nearly 40 per cent of the assimilated material into protoplasm. A high population growth efficiency would presumedly be of survival value for a population which depends on a seasonal "bloom" of primary production, whereas a high assimilation efficiency would presumedly benefit a population whose food is produced at a lower but more continuous rate. Whether an increase in assimilation efficiency tends to result in a decrease in growth efficiency (and vice versa) is, as yet, an unanswered question.
The total annual energy flow of the grasshopper population was estimated to be about 28 kg. calories per square meter per year. During the summer the population assimilated less than 1 per cent of the net production of grass, but ingested three times as much, that is, about 84 Calories or 21 gm. dry weight of grass (4 Calories/ gm.) per square meter. The nonassimilated material (feces) does not change trophic levels but is available to other primary consumers such as bacteria or detritus feeders. The annual energy flow of the snail population was estimated to be about 290 Calories/M2/year, but during the summer the average standing crop of 700 snails/M2 assimilated only about twice as much as the average population of 10-20 grasshoppers/M2 (Fig. 3) . While it would appear that both populations utilize only a small percentage of the total net primary production of the ecosystem it should not be assumed that food is nonlimiting. The maximum portion of the net production which is actually available to the populations has not yet been determined, nor has the utilization of competing primary consumers been considered. The fact that growth rate per individual in snails declined with increasing density in the field suggests that the rate at which nourishment can be ingested and assimilated on a square meter of marsh is not unlimited.
It is evident that populations which differ greatly in life history characteristics, age structure, and metabolic rate cannot be compared on the basis of numbers and biomass (i.e., "standing crop") alone. However, through the common denominator of energy flow valid comparisons can be made, and the true role of the populations in the community can be evaluated. Energy flow analysis is the logical first step in finding out how populations really function in nature. 27, 283 (1957) . 3Smalley, Alfred E., Ph.1). Thesis, University of Georgia, 1959. Ivlev, V. S., Biochem. Ztschr., 275, 49 (1934) . Burkholder, P. R., and G. H. Bornside, Bull. Torry Bot. Club, 84, 366 (1957) . 6 Pomeroy, L. R., Limnol. and Oceanogr., 4 (in press).
THE DIFFERENCE BETWEEN SPONTANEOUS AND BASE-ANALOGUE INDUCED MUTATIONS OF PHAGE T4
BY ERNST FREESE* BIOLOGICAL LABORATORIES, HARVARD UNIVERSITY Communicated by Ernst Mayr, February 24, 1959 This paper describes studies about the induction of reverse mutations by two base analogues, 2-aminopurine (AP)' and 5-bromo-deoxyuridine (BD). It will be shown that mutants of phage T4 induced by these base analogues in the forward direction can also be induced to revert. In contrast, proflavine induced and most spontaneous mutants cannot be induced to revert by these base analogues, although

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