Showing papers in "Ecological Monographs in 1974"

Biological Accommodation in the Benthic Community at McMurdo Sound, Antarctica

Abstract: Studies of the benthos between 30 and 60 m at Cape Armitage, McMurdo Sound. Antarctica, reveal an epifaunal community in which sponges and their asteroid and nudibranch predators predominate. Field experiments demonstrated that, with the exception of Mycale accrata, the growth rates of the sponges are too slow to measure in one year. Mycale, however, was observed to increase its mass as much as 67%. Because of its more rapid growth rate, Mycale appears to be the potential dominant in competition for substratum space, the resource potentially limiting to the sessile species. This conclusion is supported by observations of Mycale growing over and, in some cases, apparently having smothered many other sessile species representing at least three phyla. The densities and size frequency distributions of all the predators were measured; numerous feeding observations allowed an accurate appraisal of dietary compositions. Because of the predators' very slow consumption rates, however, direct measures of ingestion and its impact on prey populations were not possible. Estimates of the ingestion rates were derived from measurements of predator respiration rates, growth rates, and gonad growth. Data from the field surveys and the energetics studies suggest that Mycale is prevented from dominating the space resource by the predation of two asteroids. Perknaster fuscus antarcticus and Acodontaster conspicuus. Adult Perknaster specialize on Mycale, and the sponge provides a small proportion of the diet of A. conspicuus. Acodontaster conspicuus and the dorid nudibranch Austrodoris memurdensis are the most important predators on three species of rossellid sponges (Rossella racovitzae, R. nuda, and Scolymastra joubini). Despite this relatively heavy consumption and despite the fact that none of these sponges has a refuge in growth from potential mortality from A. conspicuus, very large standing crops of the rossellid sponges have accumulated. This accumulation appears to result from predation on larval and young A. conspicuus and Austrodoris by Odontaster validus, which is primarily a detrital feeder and apparently acts as a filter against the settlement and survival of the A. conspicuus and Austrodoris larvae. In addition, predation upon adult A. conspicuus by O. validus and the actinian urticinopsis antarcticus annually kills approximately 3.5% of the A. conspicuus population. This mortality exceeds the apparent rate at which A. conspicuus escape the larval filter.

The Hubbard Brook Ecosystem Study: Forest Biomass and Production

Abstract: A small watershed in the White Mountains of New Hampshire bearing meso- phytic, cool-temperate, broadleaf-deciduous forests was studied. Acer saccharum, Betula lutea, and Fagus grandifolia are dominant, but toward higher elevations Picea rubens and A bies balsamea also occur and indicate the transition toward subalpine climate. The stands are young (following cutting in 1909-17) but contain older trees; stand composition is thought reasonably representative of the climax. For application of the Brookhaven system of forest dimension analysis, 93 sample trees of major species were cut and roots excavated. Mean dimensions of sample trees, and the constants for the system of logarithmic regressions relating volume, surface, mass, and growth to diameter at breast height and other independent vari- ables, show decrease in tree sizes and height/diameter ratios toward higher elevations. Stand characteristics, based on application of the regressions to forest samples, show trends of decrease for the elevation belts from low to high: stem basal area 26.3, 23.7, and 22.0 m2/ha, weighted mean tree height 16.9, 16.7, and 10.8 m, weighted mean age 124, 95, and 83 yr, stem wood volume 176, 155, and 103 m3/ha, aboveground biomass (dry matter) 162, 152, and 102 t/ha, estimated volume increment 379, 365, and 223 cm3/m2/yr, aboveground net primary productivity (1956-60) 1127, 1041, and 790 g/m2/yr, and leaf area ratio 6.2, 5.7, and 5.5 m2/m2. Biomass (and, presumably, production) of root systems is 18%-21% of that aboveground. Different estimations suggest that a mean climax biomass for the watershed may be around 350 t/ha, aboveground. Net ecosystem production (i.e., addition to the pool of woody biomass in the community) is estimated as 350 g/m2/yr aboveground and 85 below- ground for 1956-60, 238 and 52 g/m2/yr for 1961-65. Analysis of stem wood volume incre- ments reveals an abrupt and striking (18%) decrease in volume growth and productivity from 1956-60 to 1961-65. The net primary productivity of the former period, with a weighted mean for the watershed of 1110 g/m2/yr above and below the ground, is thought more nearly normal for the forest. Both drought and effects of increasing air pollution (notably increasing acidity of rainfall) may be responsible for the recent decrease in productivity.

The Role of Pin Cherry (Prunus pensylvanica L.) in the Maintenance of Stability in Northern Hardwood Ecosystems

Abstract: A bhstrt. The design of the life cycle of pin cherry (Prlmlits Ape(/vasO i(mi L. ), a successional xpecies common on disturbed sites througlhout Much of the northern hardwood and boreal forest ecosystems, assures that its occurrence is integrated into the pattern of disturbance in the climax ecosysten. The combination of burlied seed strategy and the nobility offered through avian consuLmption of fruits ensures reasonably large populations of buried, viable seeds in the soils of forests well after the disappearance of pin cherry from a particular site. Soil sampling data indicate that Sufficient numbers of viable pin cherry seeds reside in the soils of second-growth forests in central New Hampshire to account 'for the dense stands frequently observed after cutting or burning. Further, germination of these bur-ied seeds is apparently triggered by some factor(s) associated with formation of a large gap. By ,age 25 or 30, when pin cherry individuals are dying rapidly, Sufficient numbers of seeds have been produced and disseminated in a dormant condition for the cycle to renew itself with the Subsequent occurrence of major disturbance. In high density stands pin cherry grows rapidly, with early attainment of canopy closure (high leaf area index), and rapid attainment of high values of net annual production and nutrient, accunmulaltion.

Populations and Local Extinctions of Birds on Barro Colorado Island, Panama

Abstract: A 1960—71 study of populations of color—banded ant—following antbirds of three species on a tropical—forested lowland reserve, Barro Colorado Island, showed that the small species (Spotted Antbird, Hylophylax naevioides) remained stable at about 20 pairs/km2. A medium—sized species, the Bicolored Antbird (Gymnopithys bicolor), decreased from about 3 pairs to 1.5 pairs/km2. A large species, the Ocellated Antbird (Phaenostictus mcleannani), declined from 1.5 pairs/km2 to near extinction–only one female remained in early 1971. Two of three other species that regularly follow army ants showed relatively stable populations, but a third large species (Barred Woodcreeper, Dendrocolaptes certhia) declined from two pairs to local extinction. Prior to 1960 a very large ground—cuckoo that follows ants had already become extinct there. Thus, the three largest of the seven original species that regularly followed ants were gone or nearly gone by 1970. The decrease in numbers of regular ant—following birds was not made up by increases in occasional followers. Detailed studies of antbirds showed no clear reasons for declines, except that annual mortalities of adults were high in Ocellated Antbirds (about 30%) compared to Spotted Antbirds (15%—17%) and nest losses perhaps higher in the former (96% compared to 91%). Nest mortalities were slightly lower (88%) and adult mortalities intermediate (about 25%) in Bicolored Antbirds. Female Ocellated Antbirds had higher mortalities than males. The antbirds renest repeatedly during long nesting seasons, up to 14 times per year for Ocellated Antbirds. However, to replace females of this species under Barro Colorado conditions 19 nestings per year would be needed. Concurrent listing of all birds of the island showed that 45 species of breeding birds, 22% of the avifauna present when the island was made a reserve, had disappeared by 1970. No new species replaced them. Of the lost species 13 are forest birds, in danger if forests are cut elsewhere. The other species, second—growth and forest—edge birds, have been crowded out by growth of the forest. Loss of species from this tropical reserve, especially the part apparently caused by the small size and isolation of the reserve, poses problems for conservation and ecological studies of tropical biotas. It is suggested that large future reserves have corridor zones to each other, that is, that intensive human use not preempt too much area nor interrupt immigration of animals or plants from one refuge to another.

Polychaete Abundance Patterns in a Marine Soft-Sediment Environment: The Importance of Biological Interactions

Abstract: Samples of infauna and measurements of temperature, oxygen, salinity, and algal cover were taken from January 1969 to December 1970 at —1.2—ft tidal elevation in a mud flat dominated by polychaetes in Mitchell Bay, San Juan Island, Washington. Mortality of adults after spawning and variable larval settlement success probably explained much of the variation in population numbers of the four large and numerically important polychaete species, Lumbrineris inflata, Axiothella rubrocincta, Platynereis bicanaliculata, and Armandia brevis. No correlations were found between the abundances of numerically important species and physical factors. Exclosures constructed of 3—mm mesh plastic screening placed on the flat became covered with diatoms. Settling juveniles of tube—building species, such as P. bicanaliculata, Axiothella rubrocincta, and L. inflata, built tubes in this layer of diatoms and thus did not reach the enclosed sediment, while settling juveniles of a burrowing species, Armandia brevis, burrowed through the diatom layer and reached the sediment. Thus, cleaning the cage surfaces or removing the cage after settlement reduced abundances of tube—building species without disturbing the sediment since adults of all three numerically important tube builders experience mortality after spawning. The manipulation of tube—builder abundances showed that the burrowing species responded to space vacated by tube builders by increased settlement success. Results from experimental variation of A. brevis numbers per unit volume of sediment in the laboratory and abundance data from unmanipulated natural areas also demonstrated the presence of interspecific and intraspecific competition for space. Changes in physical factors due to algal cover had some impact on population levels but the competitive interactions and behavior patterns, revealed only by observations on the behavior of living organisms and manipulation of the infauna, demonstrated the importance of biological interactions to the determination of species abundance patterns in a soft—sediment environment.

The Export of Nutrients and Recovery of Stable Conditions Following Deforestation at Hubbard Brook

Abstract: We report the effects of deforestation on the export of particulate matter, erodibility of the ecosystem, and the relative importance of dissolved substances and particulate matter in exported materials. The mature forested ecosystem is little affected by erosion, with an average annual particulate matter export of only 2.5 MT (metric tons) km—2 yr—1. Deforestation and repression of growth for 3 yr increased export to a maximum of 38 MT km—2 yr—1, but the increase in export was exponential with rather minor increases in the first 2 yr after cutting and a sharp increase in the 3rd yr. This resulted because the ecosystem continued to exercise considerable control over erodibility for 2 yr after cutting without annual renewal of biotic regulation by primary productivity. Increases in particulate matter export are primarily due to increases in erodibility rather than increased flow rates. In the mature ecosystem, the average ratio of annual net export of dissolved substance to particulate matter is 2.3. Deforestation shifts this ratio to >8.0 during the first 2 yr after cutting. This shift results because the export trends, for dissolved substances and particulate matter following deforestation, are not synchronous. The first response to deforestation is mobilization of nutrients from the available nutrient and organic matter compartments and leakage in stream water. After 2 yr, particulate matter output rises sharply as biotic control of erodibility weakens, while dissolved substance export declines, probably because of diminution of readily available nutrients stored within the system. This nonsynchrony, coupled with the action of natural species adapted to take advantage of the abundance of nutrients and water that results immediately after disturbance, may be considered as part of a homeostatic mechanism that allows rapid recovery of a forest ecosystem while minimizing the effects of erosion. Thus, following destruction of the vegetation, the ecosystem maintains a residual resistance to erosion and has a high potential for repair through successional productivity. Nutrient flux and erosion losses return to previous levels as the intrasystem aspects of the hydrologic—nutrient cycle interaction and erodibility are increasingly regulated by biotic factors. These results relative to stable conditions and their reestablishment following disturbance may have applicability to a wide range of terrestrial ecosystems.

Relationships of ring widths in arid-site conifers to variations in monthly temperature and precipitation'

Abstract: Two multivariate techniques for evaluation of ring-width and climatic relation- ships are described. One technique employs multiple regression on principal components of climate. Response functions are obtained which express in mathematical form the relative effect of monthly temperature and monthly precipitation during a 14-month period on ring- width variations. Response functions are calculated for 127 coniferous tree sites in western North America. The other technique is a cluster analysis which is used to identify similarities among the response functions and classify them on the basis of their similarities and differ- ences. Higher-than-average precipitation most commonly results in higher-than-average growth, though on cold sites the effects of precipitation during the cooler parts of the year are some- times lacking or inverse. Precipitation is directly related to growth throughout the entire year for 32% of the sites, representing many that have been used in dendrochronology. In the remaining 68% of the sites the effects of precipitation vary from season to season. Tempera- ture is most commonly inversely related to ring width during autumn, spring, and summer. Inverse relationships occur in mid-winter for trees on the warmest sites and on certain high- altitude sites. However temperature effects are often direct ones, especially during winter and for many sites at high altitudes, high latitudes, or on north-facing slopes. Site factors appear most responsible for variations in the growth responses. Aspect of slope appears to be the most critical, followed by altitude and latitude. There are fewer differences between species than between factors of the site, although certain species such as bristlecone pine have a more or less unique growth response. Some speculations are offered on the effects of the sites and their microenvironments on biological processes linking the climatic variables and growth. The variety of ring-width responses to variations in climatic factors suggest that more physiological study of trees on extreme sites may reveal unique growth-environment rela- tionships that have not been observed on more optimum sites. The median percent of tree- growth variance accounted for by climate is approximately 60 to 65%. With such a high percentage of variation related to climate, ring-width variability, if adequately sampled, dated, and calibrated with climate, can be used successfully to estimate past climate, even though all linkages of cause and effect have not been demonstrated by physiological research. It is pro- posed for reconstruction of past climate that many ring-width chronologies be used with diverse tree-growth responses, because the differences can be handled by multivariate tech- niques and provide more information on various climatic factors than chronologies with the same growth response. It is also proposed that response functions have applications in the modeling of productivity in forest ecosystems. They can be used to ascertain the effects of climatic variation on tree growth and to help in extrapolating information gathered in one area to other tree sites.

Seasonal variation in composition, plant biomass, and net primary productivity of a tropical grassland at kurukshetra, india'

Abstract: The variation in composition, plant biomass, and net primary productivity was analyzed in a tropical grassland situated within the campus of the Kurukshetra University, India, at 290 58' N latitude and 760 51' E longitude. A study of life forms indicated a thero- cryptophytic flora. Detailed phytosociological values of constituent species of the vegetation were studied at monthly intervals (May 1970 to May 1971) through tiller analysis. Most of the species were found to be contagiously distributed. The changes throughout the year in the aboveground plant biomass, standing dead, litter, and belowground biomass showed a maximum aboveground biomass in September (1,974 g/m2) and maximum belowground biomass in November (1,167 g/m2) . Examination of vertical distribution of the aboveground biomass of total vegetation as well as of the individual species indicated that different layers of vegetation are dominated by different species in different months. The aboveground net primary production was maximum during the rainy season (1,706 g/m2), and the belowground maximum occurred during the winter season (785 g/m2). Total annual net primary production is estimated to be 3,538 g/m2. The system transfer functions revealed that productivity was more aboveground-directed during the wet period and more belowground-directed during the dry period. Annual efficiency of energy capture by the primary producers was calculated to be 1.66% on the basis of half total incident solar radiation.

Vegetation, Soil, and Climate on the Green Mountains of Vermont

Abstract: The Appalachian extension of the Boreal Forest, dominated by balsam fir, red spruce, and white birch, extends down the slope of the Green Mountains to about 2,600 ft (792 m), where it merges with the Eastern Deciduous Forest dominated by sugar maple, beech, and yellow birch. These two forest formations occur as well—developed horizontal bands on the mountains, with a distinctive tension zone forest between them. In this midslope forest the species of neither the deciduous nor boreal forest are able to form well—developed long—lived stands. The contact between the deciduous and boreal forest is climatically, not edaphically, controlled. The current vegetation and soil development is the result of the long—term effects of a vertical climatic discontinuity expressed as a nonlinear decline in the length of the frost—free period across the midslope transitional forest and as a marked increase in the frequency of the cloud base at and above ca. 792 m. This results is increased moisture from fog drip and frequen...

Primary Production in the Plankton Community of a Tropical Lake

Abstract: The primary production of Lake Lanao, Philippines, was studied over a 15—mo period by in situ application of C—14 and oxygen—difference techniques. Supporting data include weather, water chemistry, light penetration, and standing crop of both autotrophs and heterotrophs. A statistical treatment of production estimates precedes the presentation of data. Extensive comparison of the oxygen and C—14 methods indicates that the C—14 method as applied in Lake Lanao measures net primary production. Data from time—course experiments show no evidence of diurnal rhythms in the efficiency of photosynthesis per unit area of lake surface. Heterogeneity studies based on transect data indicate that at low to moderate levels of production, the probability that production at an index station will differ from the average for the lake on a given date by more than 30% is less than .05, while the comparable probability for high levels of production is .35. There is no significant difference between stations in mean primary production for the study period. Vertical profiles of photosynthesis exhibit light inhibition on all but the most overcast days. The threshold for inhibition at the surface is near 133 kerg/cm2°s during calm weather and somewhat lower in windy weather. The mean threshold for inhibition 1 m or more below the surface is lower than at the surface (101 kerg/cm2°s). The lake is exceptionally transparent (mean extinction coefficient, 0.38) considering its high productivity and has a vertical dispersion of production that is similar to temperate oligotrophic lakes. The characteristic is explained in terms of the low amounts of dissolved and suspended matter in the euphotic zone, high production per unit of standing crop, and great amount of mixing in the upper water column. Net primary production average 1.7 gC/m2°day, and gross primary production is 2.6 gC/m2°day. Autotrophs account for 80% of respiration in the euphotic zone. Factors controlling seasonal variation are related to resource supply rather than to temperature or biomass removal. Between 12 and 30% of seasonal variation in production can be accounted for by variations in incident light. Light limitation also occurs due to thickening of the zone of mixing during the circulation period and during storms. Nutrient supply is the dominant controlling factor during stratification. Nutrient depletion is relieved at frequent intervals by changes in the depth of mixing associated with storms. High sustained production on a low nutrient base is explained by rapid transfer of nutrients from the zone of decomposition back to the euphotic zone. General conclusions are drawn concerning the relative importance of seasonal and aperiodic variation in regulating the resource supply of temperate and tropical plankton communities.

Role of resource allocation, aggression and spatial heterogeneity in coexistence of two competing intertidal starfish'

Abstract: A bstract Previous field experiments indicated that the rocky intertidal starfish Pisaster ochraceus and Leptasterias hexactis compete for food Here, the method of resource partitioning and mechanisms of competition and of coexistence are examined Comparisons between the diets, feeding activities (high tide), and location at low tide are made for co-occurring Leptasterias (001-115 g wet weight) and small (001-115 g), medium (115-100 g), and large ( 100-550 g) Pisaster These comparisons suggest that ( 1 ) there is no spatial or temporal subdivision of the habitat while feeding, (2) there is some microhabitat separation at low tide, with Leptasterias farther back both in crevices and under rocks, and (3) although the diets of the two species overlap greatly in both size and species of prey eaten, each obtains most of its energy from a different combination of prey size and species Leptasterias obtains most of its energy from small- to medium-sized mobile prey; small Pisaster depend on small, sessile prey, and medium to large Pisaster depend on larger and more mobile prey Thus, the food resource is divided along a combination of resource dimensions and not along a single resource "axis" Laboratory experiments indicate (1) that Pisaster is highly aggressive toward Leptasterias, using its pedicellariae to pinch the latter, and (2) the effect of this aggression is to reduce the feeding rate of Leptasterias Field experiments confirm this result This aggression is somewhat surprising in light of the complete spatial overlap of the competitors Coexistence is possible because in nature the smaller seastar evidently acclimates relatively quickly to changes in Pisaster abundance Leptasterias is not aggressive towards Pisaster, and its location at low tide suggests it avoids close contact with the larger asteroid The primary competitive advantages of the larger asteroid thus are the ability to reach a larger size (and thus to capture "better" prey), and aggression The competitive advantage of the smaller starfish is suggested by calculations of the number of calories of prey consumed per gram wet weight of starfish per tidal cycle These comparisons indicate Leptasterias has a higher rate of energy intake than Pisaster Since this advantage could conceivably give Leptasterias a great initial size advantage over small Pisaster, aggression may function to reduce the foraging time of the former As Pisaster recruitment is low, small individuals of this species are rare Leptasterias may coexist with Pisaster by ( I ) reproducing at a small size (2 g wet weight vs 70-90 g in Pisaster) and (2) using a spatial portion of the intertidal (deeper parts of crevices) not normally utilized by Pisaster during low tide when the two species would be in closest association Pisaster's aggressive superiority may render invasion of this system by large asteroid predators similar to Pisaster impossible

Interactions between a Sea Pen and Seven of Its Predators

Abstract: The sea pen Ptilosarcus gurneyi Gray provides a major source of food for at least seven predator species in Puget Sound: the asteroids Hippasteria spinosa, Dermasterias imbricata, Crossaster papposus, and Mediaster aequalis, and the nudibranchs Armina californica, Tritonia festiva, and Hermissenda crassicornis. Ptilosarcus is long—lived (15 yr +), takes 5 or 6 yr to reach sexual maturity, and has a spatially clumped pattern of recruitment. It never grows large enough to avoid predation by four of its predators, and the refuges and escape mechanisms of Ptilosarcus are not sufficient to explain its abundance in the face of this predation. Determination of rates of critical life history processes of species involved in this association, e.g., recruitment, growth, and predation, permitted estimates of the effects of the activities of each species on the others in the association. Particular attention was paid to age structure and to the natural history during the periods shortly following metamorphosis of each species. In dense recruitment patches, 97% of Ptilosarcus mortality occurs during the 1st year because of predation by Hermissenda, Tritonia, and Crossaster. Adult pens are eaten by Hippasteria, Dermasterias, Mediaster, and Armina, which together remove 2 pens/m2°yr, or 3.1% from the 10 adult year classes combined. The latter four predator species selectively prey upon the largest pens available. The time required for capture and consumption increases at a slower rate with prey size than does prey biomass. Recruitment of the long—lived food specialist Hippasteria seems to be impaired by the spatially unpredictable and clumped nature of Ptilosarcus recruitment. Although the adults have an opulent food supply, the young Hippasteria require small Ptilosarcus to feed on and rarely find enough. The generalists, which are not dependent upon one prey during their recruitment and are able to maintain themselves without Ptilosarcus, could in theory increase until they have reduced the sea pen populations to below the level needed by the specialists and thus outcompete them. However, predation on the generalists by a higher order predator, the asteroid Solaster dawsoni, removes each year at least 9.4% of the adult Mediaster and 38% of the adult Crossaster, a rate at least as great as, and usually greater than, the rate of recruitment into the populations of adult prey. The characteristic organization of the Ptilosarcus association and the existence of Hippasteria may depend upon S. dawsoni.

Stability in relation to nutrient enrichment in arthropod consumers of old-field successional ecosystems'

Abstract: Two adjacent, abandoned hayfields in central New York State were subjected to nutrient enrichment perturbation by means of a single application of 10-10-10 N, P, K fertilizer early in the growing season of 1970. Aboveground arthropod herbivores and carni- vores were monitored with respect to net productivity (dB/dt) and diversity for one growing season (1970) in the younger field (6 yr old), and for two growing seasons (1970 and 1971) in the older field (17 and 18 yr old). Sampling of arthropods was done with a gasoline- powered suction sampler between 1200 and 1600 h, twice weekly. Stability of the old-field arthropods was defined as resistance to change imposed by external (fertilizer) perturbation and was measured in terms of (1) magnitude (amplitude of deflection from ground state, (2) rate of initial response to perturbation, and (3) rate of damping or return to ground state. The degree of deflection from ground state was determined by com- paring fertilized (treated) plots with unfertilized (control) plots in each field. During the first growing season the older, more species-rich field was less stable with respect to magnitude and rate of productivity deflection than the younger, less species-rich field at both arthropod consumer levels. In addition, the magnitude of productivity deflection from ground state was lower in the carnivore level than in the herbivore trophic level. By the second growing season, the productivity of the arthropod consumers in the treated plots of the older field had not yet returned to control levels. The productivity of arthropod herbivores in the older field exhibited a relatively lower magnitude of response in 1971 than in 1970, while the arthropod carnivores in the older field exhibited a significantly greater difference from ground state in 1971 than in 1970. It was concluded that (1) the magnitude of deflection caused by the enrichment perturba- tion was decreased up the trophic levels from the herbivore to the carnivore level, (2) the rate of response to perturbation decreased up the trophic levels, and (3) although in control plots diversity increased and productivity decreased with increasing successional age for both trophic levels, stability was not positively related to either higher diversity or to increasing age in either consumer trophic level in this old-field successional ecosystem.

Interspecific competition between tadpoles of rana pipiens and rana sylvatica: an experimental field study'

Abstract: The wood frog, Rana sylv'atica, and the leopard frog, Rana pipiens, occur sym- patrically over much of northern North America. Within this area the two appear to show habitat separation. Previous studies have given circumstantial evidence that competition leading to the habitat separation might be occurring between tadpoles of these species. In a pond on the University of Michigan's Edwin S. George Reserve, tadpoles were raised in enclosures where species composition and density of experimental populations were con- trolled. A preliminary experiment was designed to determine the influence of density and mixing of species on tadpole survival and production of biomass. In a second experiment, in addition to density, food and predation levels were manipulated to determine which were influential in the responses observed. The behavior of tadpoles in the 2 yr was markedly different. In the first experiment tadpoles behaved as if there were no limiting carrying capacity to the pond; mean survivorship was about 14%. There was no evidence of competition. In the second experiment the pond's carrying capacity appeared to be near the lowest density level tested. Both species reacted strongly to each other, behaving like ecological equals. Tadpole survivorship proved sensitive to both predation and food levels. At high predation levels, the effect of mixing species was reduced. Although mixed populations performed less well than those raised alone, I was unable to alter the interaction by adding food to experimental populations even though intraspecies competitive effects were reduced. The original concept of this ecological system was that it would be controlled either by food or by predators. The results of both experiments suggest that this concept was erroneous. Both factors act jointly. The role of food must not be studied without recognition of the extent to which predators can change the impact of tadpoles on their food supply, and vice versa. The results of the first experiment suggest that interspecific competition for food under natural conditions can eventually be demonstrated, because food levels can be sufficiently increased to negate the influence of one species on the other. As ecological systems are feedback systems, future field studies must become more experimental if they are to reveal the underlying mechanisms determining the structure and function of ecosystems.

Models for Age‐Specific Interactions in a Periodic Environment

Abstract: In 1954 Hutchinson and Slobodkin suggested that populations can be viewed as feedback systems that could resonate in response to environmental fluctuation with a frequency of approximately one generation time. In this paper we pursue this observation by constructing several mathematical models based on the demographic equations governing population age structure. The existence of this and several other effects of ecological importance are investigated for populations coupled by age—specific interactions.

A Population Study of Irradiated Desert Rodents

Abstract: An experiment was begun in 1963 to evaluate effects of long—term low—level radiation exposure on desert populations of plants and animals. Monthly censuses of four species of heteromyid rodents in three 9—ha enclosures and one non—enclosed area indicate great year—to—year changes in densities of the nonirradiated populations and even greater fluctuations in the irradiated populations. Although magnitude of density changes varied, timing of fluctuations was similar. In a good growing season a population may increase by a factor of 5 or more. In a poor growing season there may be no reproduction. Chronic radiation exposure to Perognathus formosus was measured to be 211—360 r/year. This reduced survival, particularly before the age of 6 mo. The instantaneous rate of death for the irratiated population during this period was 0.219, and for the control populations was .075 and 0.104. Future life expectancy at age 1 mo in the irradiated population was 9.2 mo, and for the control populations was 11.4 and 14.4 mo. The computed intrinsic rate of increase for the irradiated population was 0.314, compared to 0.493 and 0.498 for the two control populations. Demographic and histologic evidence indicate an increased birth rate in the irradiated population. A population consisting of animals 3 yr old and older, having been subjected to a lifetime of such radiation exposure, would likely be incapable of reproduction. Therefore, only in the most unfavorable periods, when reproduction is curtailed by natural factors, will the threshold levels of artificial radiation insult on the natural population be discernible. A rapid turnover of generations prevents a population from accumulating damaging effects of low—level exposure. The diets of rodents were analyzed, and the production of important foods was measured over 3 yr. Seeds and vegetative parts of plants made up approximately equal portions of the diet. All parts of the small annual plant Thelypodium were highly selected at all seasons. Total production of food shows a lag effect in relation to precipitation. Estimates of food requirements of the rodent population, based on laboratory feeding trials, suggest that food supply may be an important limiting factor. Populations respond strikingly to periods of favorable vegetative growth. Desert rodent populations are adapted to withstanding periods of unfavorable conditions. They have comparatively long life—spans and maintain fertility. These adaptations make them vulnerable to changed conditions, such as environmental contamination.

The Habitats of 17-Year Periodical Cicadas (Homoptera: Cicadidae: Magicicada Spp.)

Abstract: Three species of 17—yr periodical cicadas–Magicicada septendecim (Linnaeus), M. cassini (Fisher), and M. septendecula Alexander and Moore–have ranges that are coextensive except around the edges. All the cicadas occurring together are the same age and emerge in the same 17th yr (hence they are "periodical"), yet populations in different parts of the range (called "broods") are out of phase with one another. Within the same brood, the three species are always perfectly synchronized, but they are separated microspatially by having different habitats within the same woodland. Magicicada septendecula prefers ovipositing in hickories and walnuts, and emerges in higher proportions under those trees than under comparable oaks. Both M. septendecim and M. septendecula occur together in upland woods, but septendecim exhibits much less host specificity than septendecula. The latter species is much rarer than septendecim; it can usually be heard chorusing in local patches within a woods occupied by septendecim. Magicicada cassini is a species of floodplain woods, and characteristically can be seen to replace septendecim and septendecula as one moves down a wooded slope leading to a stream. Over much of the eastern United States, however, the original forest has been extensively disturbed. Periodical cicadas survive and reproduce surprisingly well in cutover, scrubby second growth. Tree species characteristic of floodplains, like American elm, are often a component of upland second growth, and, especially in such situations, cassini, septendecim, and septendecula become intermixed though they remain reproductively isolated. The present lack of microspatial separation in many situations, then, is an artifact of human disturbance. Even so, on the edges of the range where only one species occurs, it does not invade the habitat of the other, even though its "competitor" is absent. These assertions are based on 4 yrs of summer field studies, from 1962—65 on Broods II, III, IV, and V primarily in Virginia, Iowa, Kansas, Oklahoma, and Ohio, and later, more cursory observations in 1968—70 on Broods VIII, IX, and X in Pennsylvania, West Virginia, and Indiana, as well as on previous studies by ourselves and others reported in the literature.

Analysis of a North Dakota Gallery Forest: Vegetation in Relation to Topographic and Soil Gradients

Abstract: A bstract. Vegetation-environment relationships were studied in forest stands located on slopes and a floodplain in northeastern North Dakota. Coverage values of all vascular plants and basal area, frequency, density, and mensuration data for tree species were obtained. A total of 109 species of vascular plants, 10 of mosses, and 15 of lichens are reported for the 40 plots studied. The three most important (based on importance value) overstory species are Quercus macrocarpa, Tilia americana, and Fraxinus pennsylvanica var. subintegerrima. Less important overstory species include Ulmus americana, Acer negundo, Populus tremuloides, and Betula papyrifera. Tree-ring analysis indicates that the present forest is less than 100 yr old. Correlation analyses were made between community characteristics of vegetation and the topographic parameters, slope inclination, aspect, and elevation. Principal component analysis and Swan-Dix-Wehrhahn ordination were used to establish five abstract community types. Environmental soil parameters measured for each soil horizon within the rooting zone include available water capacity; pH; electrical conductivity; cation exchange capacity; percentage organic matter; replaceable Ca, Mg, K, Na, Sr, and Mn; available P; and, EDTAextractable Al, Cu, Fe, Mn, Ni, Pb, Si, Sr, and Zn. Topographic and soil parameters were used for stepwise elimination multiple regressions to develop predictive equations for the distribution of plant species. Correlation analysis reveals that the basal area of trees and number and coverage values of herbaceous species is highest on lower elevations and south exposures. Whereas pronounced vegetation differences occur between slopes and the floodplain, the vegetation patterns on the slopes are continuous and overlapping. Based on the results from multiple regression analysis, gradients for potential solar beam irradiation, available water capacity, elevation, linear aspect, slope inclination, and replaceable K were established. By use of coverage values, ecological modalities of selected species are demonstrated along quantified gradients. Relationships of plots in several bivariate combinations show patterns of community type resembling those obtained by stand ordination.

Energy-Exchange Analysis of the Belding Ground Squirrel and Its Habitat

Abstract: An analysis is made of energy exchange between Belding ground squirrels (Citellus beldingi beldingi) and the microhabitats which they occupy during their annual summer period of activity. A natural population was observed, and the microclimates of six different types of microhabitats which the squirrels typically occupied are quantitatively described. Methods are derived for calculating the average radiation absorbed by a geometric model which is used to represent the squirrel at different orientations to the sun. The average absorbed radiation is plotted as a function of air temperature in climate diagrams that describe the thermal conditions occurring hourly in the microhabitats on clear, partly cloudy, and stormy days. The physiological and physical properties of the animals which affect energy exchange were measured in an open—flow metabolic apparatus. Body temperature, surface temperature, metabolic rate rate of evaporative water loss, and conductance are expressed as functions of effective ambient temperature. The capabilities of the squirrels for maintaining energy balance under extreme combinations of air temperature, absorbed radiation, and wind speed were calculated from physiological data by using an energy—budget equation, and are expressed on climate diagrams similar to those describing the thermal environments of the microhabitats. By comparing a climate diagram for a microhabitat with that for a squirrel, one can determine whether or not the squirrel is capable of maintaining thermal equilibrium in that microhabitat during any hour of the day. The net gain or loss of energy can be calculated from the climate diagrams for an animal in an environment in which it cannot maintain thermal equilibrium. The rate of change in body temperature resulting from energy imbalance is discussed in terms of its significance for animals occupying thermally unsuitable habitats. A method is proposed for comparing the nearly blackbody radiation environment of a metabolic chamber to complex natural thermal environments. With this technique it is possible to use laboratory data taken for a species at different blackbody ambient temperatures to estimate the physiological responses of that species to any natural environment where the wind speed, air temperature, and absorbed radiation are known.

Role of Learning in Insect Parasitism

Abstract: Several stochastic models of the process of parasitism, each incorporating learning in a different way, are developed. The clues the parasitoid learns to use to find its hosts are divided into two types, clues to individual hosts and clues to the host's habitat. The value of clues to the habitat can be a reflection of the spatial distribution of the hosts. The conclusions suggested by the experimental test of the models with the ichneumonid wasp Nemeritis canescens are that Nemeritis can learn to hunt its hosts in a novel environment;that a model postulating the learning of clues to the hosts fits better than a model postulating the learning of one: and that learning is potentially a stabilizing factor in the dynamics of host—parasite systems. See full-text article at JSTOR

Autecological studies of three species of callitriche native in california

Abstract: Study of three species of Callitriclhe from California revealed different degrees of tolerance of varying environmental conditions which help explain the plants' occurrence. Plants were studied in nature and experimentally under sterile conditions. Callitriclhe palustris occurs over a wide geographical range in highly divergent habitats, but C. ion gipedllacullata and C. hernmaphroditica grow in more restricted conditions, the former being confined to low altitude vernal pools in central and southern California. Callitriche palitstris tolerated wider temperature ranges for seed germination and flowering than did the other two species, and tolerated lower light intensity and higher osmotic pressure for growth and flowering than did C. ion gipedmllicit- lata. At low temperatures C. palu.stris flowered in both 8- and 16-h photoperiods; C. lon gi- pedlunculata flowered only in 16-h photoperiods. Dormancy of C. palustris seed could be over- come with milder treatments than could dormancy of C. loagipeduncotlalt seed. Callitriche palustris, a highly protogynous species, was able to set normal amounts of viable seed at temperatures permitting formation of only pistillate flowers. Circumstantial evidence indicates that much of this species' reproduction in nature is apomictic. Apomixis would tend to preserve the highly tolerant genotypes this species has evolved.