Showing papers in "Ecology in 1966"

Use of Glucose and Acetate by Bacteria and Algae in Aquatic Ecosystems

Abstract: New methods employing C14—Labeled organic compounds measure the uptake of organic solutes by planktonic microorganisms. By testing uptake over a wide range of substrate concentrations, two separate mechanisms can be differentiated and their kinetics measured. Using filtration and evidence from experiments with laboratory cultures of planktonic bacteria and algae, specific transport systems effective at very low substrate concentrations were traced to the bacteria, and a diffusion mechanism, effective only at higher substrate concentrations, to the algae. Studies have shown that V, the maximum velocity of uptake by bacterial transport systems, gives information about the size and function of the bacterial populations. A diffusion constant, kd, gives information on the rate of uptake of solutes by the algal populations. Turnover times, derived from kinetic parameters, indicate that the algal uptake of glucose and acetate in Lake Erken, Sweden, is always less than 10% of the bacterial uptake, even though the algal biomass may be orders of magnitude greater than the bacterial. Two new types of bioassay employ the kinetics of bacterial uptake systems as the measuring reaction. Acetate and glucose were found in 1—10 mg/liter concentrations in the several natural waters tested. At these very low concentrations, algal uptake of glucose and acetate is so low that effective heterotrophy is impossible. In contrast, the bacteria effectively remove substrate from solution at these low levels and probably keep the substrate at these low concentrations. By doing this, the bacteria may prevent heterotrophic growth of algae in nature.

Mortality Patterns in Mammals

Abstract: Methods of obtaining life table data are outlined and the assumptions implicit in such treatment are defined. Most treatments assume a stationary are distribution, but published methods of testing the stationary nature of a single distribution are invalid. Samples from natural populations tend to be biased in the yound age classes and therefore, because it is least affected by bias, the mortality rate curve (qx) is the most efficient life table series for comparing the pattern of mortality with age in different populations. A life table and fecundity table are presented for females of the ungulate Hemitragus jemlahicus, based on a population sample that was first tested for bias. They give estimates of mean generation length as 5.4 yr, annual mortality rate as 0.25, and mean life expectancy at birth as 3.5 yr. The life table for Hemitragus is compared with those of Ovis aries, O. dalli, man, Rattus norvegicus, Micortus agrestis, and M. orcadensis to show that despite taxonomic and ecological differences the life table have common characteristics. This suggests the hypotheses that most mammalian species have life tables of a common form, and that the pattern of age—specific mortality within species assumes an approximately constant form irrespective of the proximate causes of mortality.

The Influence of Habitats on Mating Systems of North American Passerine Birds

Abstract: Regardless of sex ratio, a polygynous mating is expected to be adaptive for the females as well as for the male. Two possible selective bases for the evolution of polygyny are considered: 1) One male may make it advantageous to several females to mate with him by appropriating a large share of a limited number of nest sites. 2) When a large share of the food for the young is obtained from the territory, local variations in food availability could influence the mating system. Given a sufficient minimum level of food supply, the difference in food availability between two males' territories may be great enough to permit a female to rear more young on the better territory, unaided by her mate, than she could on the poorer one even with full assistance from her mate. In this case, selection would favor a female pairing with the male on the better territory, even if this meant establishing a polygynous association. Fourteen of 291 species of North American passerine birds have been reported to be regularly polygynous or promiscuous. Thirteen of the 14 breed in marshes, prairies, or savannah—like habitats, where productivity resulting from solar energy is concentrated into a narrow vertical belt; that in a forest is spread unevenly over a broad vertical belt. Thus, the density of productivity–and of avian food sources–is potentially much greater in marshes and prairies than in forests. Hence, marshes and prairies are more likely than forests to present the minimum requisite food supply and sufficiently great differences in available food between territories for selection to favor polygyny.

Convexity, Desert Lizards, and Spatial Heterogeneity

Abstract: The number of lizard species in the flatland desert habitat is correlated with several different structural attributes of the vegetation. It is shown that both the horizontal and vertical components of spatial heterogeneity are correlated with the number of lizard species. The habits of the twelve component species are considered briefly as they relate to the partitioning of the biotope space. Three species are food specialists, eight display various substrate specificities, and only one species appears to be truly convex. Two tests of the present interpretation of these results are proposed, and some speculations concerning Australian flatland desert lizards are made. See full-text article at JSTOR

The Metabolism of Fossorial Rodents: A Study of Convergence

Abstract: Temperature regulation and rate of metabolism were studied in five species of fossorial rodents: Geomys pinetis, Spalax leucodon, Tachyoryctes splendens, Heliophobius kapeti, and Heterocephalus glaber. The burrows of these rodents are in well—drained soils of poor water—holding capacity. Burrow atmospheres have an oxygen concentration of 15 to 20%, a carbon dioxide concentration of 0.5 to 2.0%, and limited temperature fluctuations; they are saturated with water vapor. Body temperature are somewhat low (35 to 37°C) in four species; temperature regulation is good in these species at ambient temperature down to 5 or 10°C. Heterocephalus, however, has a much lower body temperature (about 32°C) and the poorest capacity for thermoregulation of any known mammal. Fossorial rodents have low basal rates of metabolism, high conductances, and high ranges of thermoneutrality. Lethal ambient temperatures are inversely proportional to the normal levels of body temperature. Analysis of basal rates of metabolism (M), conductances (C), and body temperature suggests that the first two are determined in relation to environmental characteristics, and that body temperature, "goodness" of thermoregulation, midpoint of thermoneutrality, and lethal ambient temperature are dependent upon the ratio M/C. It is concluded that the lowest body temperature in homoiotherms compatible with good thermoregulation is about 35°C. A comparison of Heterocephalus and lizards illustrates the energetic similarities and differences between endotherms and ectotherms. The reduction of M and the increase of C in fossorial rodents are directly related to the mean burrow temperatures. These modifications reduce the probability of overheating in an environment where evaporative and convective cooling are greatly reduced. Tachyoryctes and Heliophobius use forced evaporative cooling for emergency thermoregulation, in spite of the saturated burrow atmosphere. Heterocephalus facilitates heat loss during heat stress by increasing peripheral circulation over its naked body; Geomys does this by increasing the circulation to its naked tail and, possibly, naked feet. Geomys may lose up to 30% of its heat production via its tail. As a result of these modifications, the lethal ambient temperature is proportional to the mean burrow temperature. Geomys responds to geographic variation in heat loading either by a decrease in body size or by an increase in tail length. The smaller size of nonpregnant, female gophers helps to maintain their heat production during pregnancy below the limits established by the environment for males. Gopher distribution appears to be limited to soils with high rates of gas exchange. Interaction among species of gophers has, to some extent, a physiological basis. It is suggested that the physiological characters concerned with energetics are readily modified, in an evolutionary sense, to conform to the requirements imposed by the environment.

Soil Phosphate and Its Role in Molding Segments of the Australian Flora and Vegetation, with Special Reference to Xeromorphy and Sclerophylly

Abstract: It is postulated that the Australian angiosperm flora ins derived from a sample of a one—time world flora of about 180 families that entered Australia by a land bridge from Asia. Many of the genera that entered still persist. New genera evolved in all habitats, many from lineages which were adapted to soils of low fertility determined primarily by phosphate level. Soil parent materials with the highest phosphate content are derived mainly from the Tertiary basalts in the east and support rainforests (except in cold regions). Soil phosphate levels decrease from east to west across the continent. The number of rainforest genera in any community is usually correlated with soil P. The converse holds for xeromorphic genera. Some rainforest genera therefore are more readily adapted to aridity than to low fertility conditions. Expansion and contraction of the rainforest is determined partly by climate and partly by soil fertility levels. A sudden expansion of the rainforest in the Oligocene occurred when the basalt flows covered part of eastern Australia. Since that time the rainforests have contracted, partly as a result of the removal of basaltic material and partly because of the development of colder climates in the ranges. The absence of rainforest and the paucity of rainforest genera in southwest Western Australia are accounted for by low fertility levels rather than by a decimation of rainforest genera by a past dry climate. Xermorphy, a result of high lignification, heavy cutinization, silicification, or a combination of these may occur in rainforest species. Adaptation to low fertility accentuates xeromorphic characters through a reduction in leaf size. The low fertility xeromorphs are not xerophytes. Anatomical features have little significance in their survival, which is determined by the ability of the plants to withstand long periods of mineral starvation. Experimental work indicates that the degree of xeromorphy can be reduced in many taxa by the addition of phosphorus and nitrate; four different types of leaf response have been identified.

An ecological significance of evergreenness

Abstract: Percentage of evergreenness in 158 forest stands from seven community types in north-central Florida ranged from 0 to 100. In general, the evergreen species are more im- portant on dry sterile sites and the deciduous species on mesic fertile sites. Nine environ- mental variables or combinations of variables accounted for approximately 51% of the variation in stand evergreenness. The segregation of evergreen species to dry sterile sites may be related to the gradual return of nutrients to the soil or the. establishment of a more closed mineral cycle through (1) the leaching of nutrients from leaves and (2) year-round leaf fall. A study of 60 southern mixed hardwood forest stands from north-central Florida revealed that evergreenness ranged from 7% to 100% (Monk 1965). Preliminary analyses suggested that deciduous species were important on mesic fertile sites while evergreen species were more important on dry sterile sites. Since the initial study, data have been obtained from 98 additional stands repre- senting six different forest community types. An attempt was made to determine the relationships between stand evergreenness and certain environmental factors and to de- velop a hypothesis explaining a possible ecological sig- nificance of evergreenness.

Circadian Activity Pattern with Two Peaks

Abstract: The 24—hr rhythms of behavior and other physiological functions are based upon an endogenous self—sustained oscillation. This is proven by experiments in which animals, kept in constant conditions, show a freerunning circadian period which deviates to a certain degree from that of the earth's rotation. The 24—hr rhythms of activity are furthermore characterized by typical patterns of which the one with two peaks in one period is most common. Under natural conditions, these two peaks of activity are often closely related to specific stimuli in the environment; e.g. to dawn and dusk. They have therefore been described as being directly caused by the environment. However, one may consider not only the circadian period but also the pattern part of an endogenous system. This hypothesis has been tested in experiments with finches. The results are: 1) Birds kept in artificial light—dark cycles show the same two—peak pattern either in cycles with interposed twilight or in cycles with rectangular changes between light and dark, 2) Under constant conditions, the second peak may disappear; but more often, it either remains or becomes more pronounced tan under conditions of a light—dark cycle. 3) With different levels of constant intensity of illumination, the birds show different circadian periods and different durations of activity; under those conditions, the two peaks keep the same proportional relationship to the activity time regardless of its duration. It is concluded that the basic two—peak pattern of locomotor activity is a persistent property of the circadian oscillating system.

Decomposition of Leaf Litter in Relation to Environment, Microflora, and Microbial Respiration

Abstract: Bacterial and fungal counts, mycelial growth, microbial evolution of Co2, and substrate moisture and temperature in bags with litter of either mulberry, redbud, white oak, loblolly pine, or beech were measured biweekly over the period November 1960—November 1961 in oak, pine, and maple stands at Oak Ridge, Tennessee. Serial dilution plate method and closed—box technique were effective for measuring microbial densities and microbial respiration respectively. Microbial densities, microbial respiration, and annual weight losses of species declined in the order mulberry, redbud, white oak, pine, beech, ad were significantly positively correlated. Stand effects were not significant possibly due to partial exclusion of stand effects in the bags. Microbial densities and moisture conditions of the various leaf species converged with time and their short—term fluctuations decreased indicating homogenation of substrate and stabilization of microbial populations. Microbial respiration was controlled in decreasing order by temperature (T), bacterial density (B), moisture (M/D), and the number of weeks (W) since leaf drop. An effective model for prediction of microbial respiration (C) is C = 46.5 + 3.2T + 26.9 M/D + 11.4 log B — 0.6W. Mean CO2 production was 0.17 liters/g substrate decomposed. Production was higher for rapidly decomposing leaf species dominated by relatively inefficient bacterial flora than for slowly decaying litter with predominantly more efficient fungi. Loss of weight and respiration were highly correlated with a microbial population estimate, combining bacterial and fungal counts.

Forest Dimensions and Production in the Great Smoky Mountains

Abstract: Samples based on stand measurements and borings of trees, and clippings of undergrowth, were taken from 24 forests and forest heaths in the Great Smoky Mountains. Samples were taken also from heath balds and grassy balds and a California coastal redwood forest. Available information on ratios and regressions of forest tree and shrub dimensions were used to estimate stand biomass and net annual production above ground. Mature climax forests of mesic environments below 1,400 m are characterized by: wood basal areas of 50—64 m2/ha and basal area increments of 0.3—0.6 m2/ha/yr, stem wood volumes (parabolic estimate) of 750—900 m3/ha and estimated wood volume increments of 530—590 cm3/m2/yr, aboveground biomasses of 500—610 t/ha and aboveground net annual productions of 1,000—1,200 g/m2, and biomass accumulation ratios of 40—50. These and other stand dimensions decrease along the moisture gradient to xeric sites and decrease toward higher elevations. Above—ground net annual productions of forest heaths of xeric slopes and forests of highest elevations are 420—650 g/m2. Evergreen spruce—fir forests are more productive than deciduous forests above 1,400 m. Among mesic high—elevation beech and fir forests, production is higher on south slopes than on north slopes. Production and biomass of steady—state forests were estimated from multiple correlations using elevation and weighted—average index of site moisture as independent variables. Apart from some high—elevation stands, production is not significantly correlated with evergreen vs. deciduous foliage or with direction of exposure affecting incident sunlight. When unstable stands are excluded, production and biomass are strongly correlated with each other and with mean tree height and other stand dimensions. A wide range of temperate—zone climax forests of relatively favorable environments have net annual productions above and below ground of 1,200—1,500 g/m2. Productions of stable closed heath balds are lower, 700—1,200 g/m2, but productions of heath balds and less productive forests overlap broadly. Production of unstable forests, both successional stands and those opened by removal of large trees, exceeds that of steady—state forests of similar environments. Canopy coverage and light penetration are not strongly correlated with forest production. Light penetration to the herb stratum ranges from 1.4—7.0% of incident sunlight in more open forests and forest heaths to 0.3—0.9% in cove forests. Foliage live/dry ratios decrease along the moisture gradient from mesic to xeric stands–from about 5.0 to 2.8 in shrub clippings, 7.6 to 2.8 in herb clippings. Undergrowth production and biomass are trivial compared with the tree stratum in many forests. Shrub production is generally higher in xeric environments and is 20—145% of tree production in forest heaths. Herb production is higher at the extremes of the moisture gradient (exceeding 3% of total aboveground production in mesic and in open xeric stands) than in intermediate stands (below 1%). Apart from such differences, her and thallophyte biomass and production increase with elevation to maximum values in fir forests of the highest summits.

Some aspects of the ecology of migrant shorebirds

Abstract: During migration, shorebirds form dense multispecific aggregations within rela- tively uniform and limited marine littoral habitats. The amount of available feeding space in the habitats frequented fluctuates widely with the daily and seasonal changes in the tidal rhythm. Shorebird species broadly overlap in their periods of peak abundance, inter- and intrahabitat distributions, and in the food organisms preyed upon. However, the totality of species differ- ences and the transient character of migratory assemblages apparently minimizes interspecific interactions that might result in competitive exclusion. The staggering of peak population densities and differences in distribution is most pronounced among morphologically similar species. The number of individuals and species that occur in an area is apparently determined by the amount of available feeding space and the physical diversity of the habitat. Food appears to be generally abundant relative to the requirements of the birds in all the habitats studied. The environmental conditions encountered during migration and the interactions with other individuals have evidently been important factors in the evolution of morphological and be- havioral differences among shorebird. species. Individuals must be as able to survive during the nonbreeding season as they are during the breeding season.

The clonal growth habit of American Aspens.

Abstract: Bigtooth and trembling aspens (Populus grandidentata and P. tremuloides) typically occur in Michigan and throughout much of their native range in natural clones of several to many genetically identical individuals. A clone is initiated by the establishment of a seedling (the ortet); suckers (ramets) arise from the root system of the ortet. Death of ramets and decay of root connections may result in formation of several independent root systems within the same clone. Average size of individual clones of both species on the research sites in Michigan was approximately 0.007 acre. Male and female clones did not differ in size. Clones are established, expand, intergrow, and coalesce depending upon the ease of seedling establishment, rapidity of root expansion, inherent suckering ability, and amount of disturbance. Although the aspens are typically recognized as pioneer species, they may be more permanently part of the habitat than most of their non—clonal associates. Recognition of the clonal structure of most aspen stands is important in many aspects of research.

Recent pollen spectra from the deciduous and coniferous-deciduous forests of Northeastern Minnesota: a study in pollen dispersal

Abstract: Pollen samples were taken along nine transects across local vegetational belts bordering bogs or ponds in overall deciduous and coniferous-deciduous forest regions. Three types of pollen rain are distinguished: local, extralocal, and regional. Local pollen rain is derived from plants that grow at or very close to the sampling point. High local values are common in all sorts of vegetation types: lakes, bogs, marshes, and forests. Extralocal pollen rain is derived largely from trees that grow on the slopes and upland adjacent to the sampling site, but not extensively over larger areas. When excluded from the pollen sum along with the local types, its curves increase slightly, but noticeably, along the transect as the source of dispersal is approached. The regional pollen rain is derived from plants common far beyond the immediate basin slopes. When the extralocal and local types are excluded from the sum of upland pollen types, the regional pollen rain differs little from site to site. Its recognition permits comparison of modern and presettlement pollen rain with the composition of the recent regional vegetation.

An Analysis of Variation Between Upland and Lowland Switchgrass, Panicum Virgatum L., in Central Oklahoma

Abstract: Mass field collections and plants grown under various environmental conditions were analyzed to determine the extent of the morphological differences occurring between upland and lowland types of switchgrass, Panicum virgatum L., in McClain County, Oklahoma. The bases for these differences were also investigated. A comparison of field populations revealed differences in clonal habit, in the size of clones, and in the gross morphology of their vegetative organs. Lowland plants were larger in most aspects than those of upland areas although the differences between them were somewhat nullified by variations within each of the two types. Genetically controlled morphological differences became evident when the effects of environmental differences were eliminated. Although upland and lowland switchgrasses were somewhat modified under common greenhouse conditions, they exhibited essentially the same morphological differences as did field populations. A physiological difference in water requirement also occurred between the two types. Lowland switchgrass grew best under flooded conditions, whereas upland plants reached their greatest development under more moderate conditions of soil water. The morphological differences between field populations occurred largely, therefore, as a result of the morphological expression of this physiological difference in water requirement which was superimposed on lesser genetic differences. Upland and lowland switchgrasses also differed in their requirement for nitrogen, the latter having a somewhat lower requirement than the former. This factor probably did not contribute significantly to the existing morphological differences. The results of reciprocal transplants substantiated these conclusions. Plants of the lowland type were tetraploids. Hexaploids and octoploids occurred in the upland with hexaploids being the most common.

Spectral Distribution of Light in the Forest

Abstract: Spectral distributions of shade light between 400 and 740 mm were measured under corn, sugar maple, oaks, pines, and spruce with a portable recording spectrophotometer. Differences were found between hardwoods and softwoods and between clear cloudy days. An energy maximum at 550 nm, a minimum at 670 to 680 nm, and a very high maximum in the near infrared occurred under all species. Four components of light within a plant canopy can be distinguished: both beam solar radiation and diffused sky radiation are transmitted both directly and indirectly (reflected and scattered). Separate consideration of each of these components leads to great understanding of similarities and differences between light regimes in different stands. See full-text article at JSTOR

Behavioral Thermoregulation in Butterflies

Abstract: Behavioral thermoregulation is well studied in reptiles but has not previously been known to exist in butterflies, where it seems to be widespread and perhaps universal. Like reptiles, butterflies are heliotherms, deriving their heat almost exclusively from the sun. For reception of heat they make much use of their wings. Blood circulates in the wings, making them effective heat exchangers whose efficiency is improved by modifications of structure, color and pattern. Behavioral acts (often group—specific) for heat gain include dorsal, lateral and body basking, and ground contact. Behavioral devices for heat loss include shade seeking and probably normal and excessive respiration (evaporative cooling). In optimal air temperatures a series of minor adjustment devices permits normal activity with little concession to temperature control: sum minimizing, irradiation balancing, and wing opening. Five major thermoregulatory behavior patterns can be distinguished: minimal activity, matutinal warming, vesper warming, vernier control and cooling dominant. One or another of these is in operation at all times. Butterflies and reptiles share many behavior devices but moths differ considerably from both. They are myothermus, their major heat source being muscular energy. Heliothermic butterflies and myothermic moths are compared and contrasted with respect to the ecological significance of these specializations, particularly in feeding habits. Some of the many remaining problems are discussed, most important of which is the necessity for obtaining actual body temperatures to amplify the observational data of the present paper.

Ecology of Vertebrate Animals in Relation to Chaparral Fire in the Sierra Nevada Foothills

Abstract: Chaparral fire brings decided changes in the species composition and density of both plant and animal populations in the Sierra Nevada foothills. Some species decrease whereas others increase following a burn, but not species is totally eliminated, nor is there any apparent diminution of total life on a burn after plant growth resumes. These conclusions were reached in the course of a 4—year study of adjoining burnedand unburned areas near Glennville, Kern County, California. Field work began in 1953 at which time study plots were selected and plant and vertebrate population were censused. A year later part of the study area was burned, and ensuing investigation compared populations on the burned and check areas for a period of 3 years, terminating in 1957. At the time of the fire, temperatures were recorded in sites both above and below ground, and the actions of animals were observed. There was very little evidence of direct mortality among any of the vertebrates, most of them escaping the heat in one way or another. The woodrat was perhaps the most vulnerable species because of its dependence on houses made of dry twigs. However, in the bare ash after the fire many species were severely exposed to predation, and populations of the most small mammals and some brush—dwelling birds decreased rapidly. Predatory birds and mammals increased, as did some seed—eating birds that found good foraging on the exposed earth. When the rains stimulated new plant growth, a very different habitat developed in the area of burned chaparral. Most of the original trees sustained little damage, although the pitchy digger pines were largely eliminated. However, the extensive brush stands were reduced by almost 90%, and there was a corresponding increase in invading grasses and forbs. Birds and mammals that normally exhibit a strong preference for chaparral habitat were substantially reduced in numbers in the years following the burn. Conversely, some of the birds that normally prefer grassland or oak woodland increased in number. The fire resulted in an overall increase in densities of nesting birds. None of the small mammals increased in numbers but some of the larger predators, such as the coyote and badger, moved into the burn during the months following the fire.

Perception of Infrared Radiation from Forest Fires by Melanophila Acuminata de Geer (Buprestidae, Coleoptera)

Abstract: When Melanophila acuminata De Geer was stimulated with infrared radiation, responses were obtained in the range of from 2.0 to 6.0@m. The highest sensitivity was found to be, however, in the 2.5 to 4.0@m range at an energy level of 6.0 x 10@o5 watts per cm2. The organs which detect infrared radiation are found on the mesothorax adjacent to the coxal cavities. Through consideration of the physical characteristics of infrared radiation it has been possible to estimate the distances from which forest fires of various sizes could be detected by M. acuminata. For instance, the radiation from a 50 acre fire in mountainous terrain could be detected by this insect from 5 km away but the same size fire on flat land could only be detected from 1 km. The ecological implications of the habits of this insect are also discussed. See full-text article at JSTOR

Effects of Microclimate on Spring Flowering Phenology

Abstract: Sixteen microclimatic stations with differences in slope, exposure, vegetation cover, and seasonal change were established in a heavily dissected 180—acre Indiana tract. Correlations, based on cumulative air temperature duration—summations, were made between microclimatic differences and variation in phenological events. Nine widespread species of spring wildflowers had a collective mean range in dates of first flowering of 7.2 days for all stations. The maximum range for a single species was 11 days. Flowering dates of nine species of a large gorge were retarded an average of 6.0 days in the north—facing slope with respect to the opposing south—facing slope. This 6.0—day difference between gorge slopes 150 ft apart is equal to the expected to occur in about 110 miles of latitude, assuming standard exposures. Six species of the north—facing slope in a small gorge were retarded an average of 2.8 days with respect to the opposing south—facing slope. Flowering on north—facing gorge slopes was retarded more t...

The Regulation of Density in a Natural Population of the Pond Snail, Lymnaea Elodes

Abstract: The role and mechanisms of density regulation were studied in a natural population of the pond snail, Lymnaea elodes, in a small permanent pond in southern Michigan. A portion of the snail's habitat along the margin of the pond was subdivided into 28 artificial snail—proof enclosures (pens). In two groups of four pens each, adult densities were altered to about 1/5 and 5 time initial spring density estimates (roughly 1,000/pen). A third group of four pens was left unaltered as a control. Sampling in these pens verified that alterations in adult density were maintained. The percentage of dead adults did not vary significantly among the three treatments, indicating the absence of any regulation to the numbers of adults through differential survivorship. On three dates in July the pens were sampled. Each time an inverse relationship was found between the densities of adults and young. A later, more extensive set of samples was taken after the margin of the pond was dry and the snail population in estivation. These samples showed no significant difference in the number of young snails (roughly 5,000/pen) among the three groups of pens. While the population was reproducing, a time—limited search for eggs was made which showed no significant difference in the total number of eggs per pen among the three groups. Thus treatment effects (alterations in adult density) had entirely disappeared in the numbers of eggs and young snails. In other pens predators were added or excluded without their having any apparent effect on the numbers of young or adults. Additions of food in the form of frozen spinach to two pens at regular intervals resulted in a dramatic (25—fold) increase in adult fecundity and an increase (4— and 9—fold on two separate dates in July) in the numbers of young. The differences between fed and unfed pens were taken as strong evidence for the presence of food limitation in the snail population, but there were indications that the limitation was one of a relative and not an absolute nature. There was an abundance of vegetation and coarse debris, but a lack of high quality food necessary for maximum fecundity and growth. Concurrent with the pen experiments, the general dynamics of a portion of the snail population not under enclosure was followed. Generally 12 to 24 samples were taken weekly from transects outside the pens. When the field data on growth and reproduction are compared to similar data from animals raised in the laboratory with superabundant food, it is obvious that field animals are realizing only a portion of their potential for fecundity and growth. These differences are attributed to food limitation. Field estimates of young snail mortality ranged from about 93 to 98%, but there is no evidence that this mortality was regulatory. Within the framework of the 20—fold differences in adult density in the altered pens, there is no indication that juvenile mortality contributed to the final observed convergence in densities of young. There are indications from other sources that mortality may be a potential regulatory force. The regulation of density in the pond snail was far more complete and occurred more rapidly than was thought possible. Although the mechanisms of the regulation are not as clear as the demonstration of their existence, the data strongly suggest that it was mediated through food limitation as expressed in fecundity.

Community Evolution and the Origin of Mammals

Abstract: The evolutionary course from primitive pelycosaurian reptiles through therapsids to mammals can be profitably studied in relationship to modifications of the structure of the communities in which these reptiles existed. For this purpose the community is defined in very board terms. Three types of communities are recognized upon the basis of the nature of the food chain. Each has an important tetrapod component. Early phases of the evolution that culminated in mammals took place in communities that were strongly tied to water by the structure of the food chain. The physiological bases of the development of mammals appear to have been related to this environmental restriction. In successive pulses, however, the pelycosaur—therapsid communities developed terrestrial reptilian herbivores and thereby broke with the water—based food chain. More strictly terrestrial communities developed concurrently, with the insects, which were a food source for the reptiles, as the principal herbivores. From this sort of community came the terrestrial lepidosaurian—archosaurian reptilian radiation. The terrestrial communities so developed came into competition. In this competition the therapsid lines were temporarily unsuccessful, leaving only small, but very mammal—like, representatives in the late Triassic. After a long period with relatively little adaptive radiation, these remnants provided the basis for the radiations of mammals that led to the great successes of the Cenozoic era.

Rates of Carbon Dioxide Evolution From the Forest Floor

Abstract: Evolution of CO2 from the forest floor measured by the inverted—box method was related to moisture, temperature, and age of the litter, and to CO2 evolution from oak leaves in litter bags measured by the closed—box method. Rates of CO2 evolution measured by the inverted— and closed—box methods did not differ significantly (P>0.2). Respiration rates from bagged oak leaves were significantly correlated (P 0.05). See full-te...

Season of Birth in Man. Contemporary Situation with Special Reference to Europe and the Southern Hemisphere

Abstract: The season of birth in man appears to be meteorologically controlled though minor minima of cultural origin are also apparent. The chance of conception may be tempera- ture dependent. The periodicity in the Southern Hemisphere is the reverse of that of the northern one. The Moslem Mediterranean region exhibits patterns similar to southern Europe, as does Central America. The United States, Canada and Puerto Rico have a pattern that appears to be anomalous compared to the rest of the world. The seasonality of birth is a sensitive indicator of certain cultural and biological phenomena. The amplitude is increased by illegitimacy, death, belonging to a race other than Caucasian, war, depression, lack of industrialization, poverty and rural living. There is some indication, over time, that the mere process of urbanization and industrialization can cause the amplitude to behave in an erratic fashion. An hypothesis has been suggested to explain changes that have occurred during the past century. It is thought that the effect of urbanization is responsible for the change and in part the improvement of the standard of living has also been an influence. Spain and Sweden have shown no major variation in their season of birth for the period of time for which there are data.

Production Rate, Population Density, and Drift of a Stream Invertebrate

Abstract: Production rates of the stream mayfly, Baetis vagans McDunnough (Ephemeroptera: Baetidae), were estimated by two different methods, compared, and related to population density and drift rate. In the year of study, this mayfly had two summer generations and one winter generation. The two production rate methods yielded estimates of 9.1 and 12.6 g/m2 year as the sum of the three generations, although both estimates were minimal. Production rates for the summer generations were higher than for the winter generation, although winter production was still at a significant level. The summer generations drifted in large quantities, up to about 22 g/day, but the winter generation appeared to drift little, if any, until near the end of the generation in early spring. The relation between annual production rate and mean population density, or turnover ratio, was 9.7, or 3.2 times the number of generations. See full-text article at JSTOR

Light and Temperature Requirements for Diapause Development and Release in Daphnia

Abstract: The light and thermal requirements for diapause development and release in Daphnia pulex were determined for the ephippia from a pseudo—sexual strain cultured in the laboratory and from an autumnal diapausing, bisexual strain in Paul Lake, Michigan. Light was essential for termination of diapause in the laboratory—cultured strain regardless of the temperature or duration of ephippia storage. Ephippia from the lake population were activated by light, but prolonged storage in constant dark eliminated the requirement for light, and thereby implicated photoperiodic control of diapause release. The laboratory population completed diapause development within a period of 3 to 6 weeks when stored in constant dark at 22°C. Storage at 3.5°C in constant dark porlonged diapause. In the Paul Lake strain, low temperature was a requirement for diapause development, and at 3.5°C the eggs were in diapause for a period of 5 or 6 months. The contrasting light and thermal requirements are discussed in the context of environments regulating the duration of diapause in summer and winter diapausing populations of Daphnia.

The Distributional Pattern of Benthic Invertebrates of the Continental Shelf off North Carolina

Abstract: The benthic macroinvertebrate fauna of the continental shelf off North Carolina may be divided into three species assemblages, each occupying a given area and corresponding to three biogeographic provinces. North of Cape Hatteras, in a wedge having its narrow end near Diamond Shoals, the fauna and the environment correspond to the Virginian Province. South of Cape Hatteras, over roughly the inshore half of the continental shelf, the Carolinian Province is represented. Offshore from these two provinces, over the outer half of the shelf, lies the Tropical Province. Conclusions regarding these faunas and these provinces are based on dredging collections including 211 species representing 16 classes from 6 phyla. One—fourth of the total number of species are new distribution records for the coasts of North Carolina. See full-text article at JSTOR

Analysis of the Species-Area Relation

Abstract: An equation of the form y = kx2 describing more precisely the species—area relation for plant species in smaller areas is proposed as a result of intensive examination of species presence in six stands of three undisturbed plant communities in the midwest. Each individual equation for each stand is based on a relatively small sample of 900 m2, but within such an area, the fit of observed data to the equations is very close. In these equations, in which y represents species numbers and x the area, k varies primarily according to species size, while the exponent z more truly reflects the species—richness of the community. Both of these constants may prove to be important measures of different community features. The equations fit rooted data more closely than covered, and it is suggested that the former type of information, being easier to obtain and probably more accurate, is preferable. Comparison of published species numbers for larger areas in floras show clearly that the above equation, so very accurate in the smaller areas actually sampled,does not hold for larger areas as the actual number of species drops far below what the equation predicts. An equation describing this relation for a communityover it entire range of area cannot be accurately constructed without considerable additional information on species numbers in larger areas. Such an equation will probably be sigmoid and more complex than the ones presented here. A method is described whereby one may objectively select an appropriate plot size for a given community based on its equation and not affected by the size of the sample from which the equation is derived. Such equations provide a useful means of comparing several floristic features as well as helping to assess historical aspects of plant communities.

Ecological Status of Introduced Brome Grasses (Bromus Spp.) in Desert Vegetation of Southern Nevada

Abstract: Of the plant species introduced on the Nevada Test site, Nye County, Nevada, Bromus rubens and B. tectorum are well established in the present vegetation mosaic. B. rubens is frequently the dominant winter species in Colegyne (blackbrush) communities at 4,000—5,000 ft. Relative density (or absence) of populations is a site characteristic, as indicated by quantitative data from 18 study sites in Yucca Flat in the Years 1963—65. Like the native species, it occurs in higher densities on disturbed sites of areas where it is already established in the undisturbed vegetation. Its success is in part due to a growth regime and environmental requirements unlike those of the native winter annuals, and perhaps to higher percentage survival to maturity as in the 1963—64 season. It is not aggressive in the region today. B. tectorum is confined to disturbed sites at the higher elevations (5,000—7,500 ft) where vegetation is Artemisi (sagebrush) or Armenisia—Pinyon—Juniper. It is numerically and really increasing with a...

Effects of Population Density on Growth Rates of Animal Populations

Abstract: The growth rate of a population at time t is defined as the change in numbers per unit population at that time period (rt = dn/Ntd). The rate usually changes with time as the population increases or decreases. The objective of this study was determine whether the growth rate of animal populations is or is not a function of the population density. In most mathematical models of populations whose changes resemble those of actual populations, the growth rate is a decreasing function of density, meaning that as the population increases the growth rate decreases and vice versa. Records of actual populations can be used to test the relation between growth rate and density by calculating correlation coefficients for these two characteristics. The methods of doing this are described and the limitations on the data are explained. The records of 111 different populations representing 71 species were analyzed by these methods. Of the 71 species, 7 were eliminated from further analysis because their census records were not significantly different from a series of random numbers, 47 had correlation coefficients that were negative and significantly different from zero, 16 had coefficients not significantly different from zero (all but one of these estimated coefficients were negative), and one, the world's human population, had a positive coefficient significantly different from zero. There were no differences between taxonomic groups (insects other invertebrates, fish, birds, mammals). The primary conclusion is that in most animal species a population's growth rate is a decreasing function of density. The explains the relative stability of animal populations, which never continue to increase at rates their fertility would allow, and rarely decrease to extinction. Tentative conclusions are presented regarding the processes regulating population numbers. Populations of herbivorous insects at low and moderate levels are regulated by disease, parasites, and predators causing mortality that is an increasing function of density. Favorable conditions may allow an increase in insect numbers so rapid that the population temporarily escapes regulation by its enemies. Nonterritorial species of vertebrates are normally controlled by predation and, when that fails, by competition; in both cases juvenile individuals are most affected. In territorial vertebrates competition for suitable territories determines the size of the breeding population. Populations of vertebrate species (expecting man) are in general regulated by the production of adult individuals being a decreasing function of population density.