Showing papers in "Ecology in 1980"

The comparison of usage and availability measurements for evaluating resource preference

Abstract: Modern ecological research often involves the comparison of the usage of habitat types or food items to the availability of those resources to the animal. Widely used methods of determining preference from measurements of usage and availability depend critically on the array of components that the researcher, often with a degree of arbitrariness, deems available to the animal. This paper proposes a new method, based on ranks of components by usage and by availability. A virtue of the rank procedure is that it provides comparable results whether a questionable component is included or excluded from consideration. Statistical tests of significance are given for the method. The paper also offers a hierarchical ordering of selection processes. This hierarchy resolves certain inconsistencies among studies of selection and is compatible with the analytic technique offered in the paper. Central to the study of animal ecology is the usage an animal makes of its environment: specifically, the kinds of foods it consumes and the varieties of habitats it occupies. Many analytic procedures have been de- vised to treat data on the usage of such resources, particularly in relation to information on their avail- ability to the animal, for the purpose of determining "preference." The objectives of this report are to de- scribe the problem of determining preference by com- paring usage and availability data, to illustrate a seri- ous shortcoming in the routine application of most procedures for comparing these data, and to suggest a new method that resolves this difficulty. The pro- posed technique results in a ranking of the components on the basis of preference, and permits significance tests of the ranking.

Importance of organic debris dams in the structure and function of stream ecosystems

Abstract: Removal of all organic debris dams from a 175-m stretch of second-order stream at the Hubbard Brook Experimental Forest in New Hampshire led to a dramatic increase in the export of organic carbon from this ecosystem. Output of dissolved organic carbon ( 1 mm) export increased 138%. Measurement of the standing stock of coarse particulate organic matter on streambeds of the Hubbard Brook Valley revealed that organic debris dams were very important in accumulating this material. In first-order streams, debris dams contain nearly 75% of the standing stock of organic matter. The proportion of organic matter held by dams drops to 58% in second-order streams and to 20%o in third-order streams. Organic debris dams, therefore, are extremely important components of the small stream ecosys- tem. They retain organic matter within the system, thereby allowing it to be processed into finer size fractions in headwater tributaries rather than transported downstream in a coarse particulate form.

Harmonic mean measure of animal activity areas

Abstract: A new method of calculating centers and areas of animal activity is presented based on the harmonic mean of an areal distribution. The center of activity is located in the area of greatest activity; in fact, more than one “center” may exist. The activity area isopleth is related directly to the frequency of occurrence of an individual within its home range. The calculation of home range allows for the heterogeneity of any habitat and is illustrated with data collected near Corvallis, Oregon, on the brush rabbit (Sylvilagus bachmani.)

Response to fertilization by various plant growth forms in an Alaskan tundra: nutrient accumulation and growth

Abstract: The fertilization responses of six tundra species belonging to three plant growth forms were compared to test the hypothesis that species of the same plant growth form are more similar to one another than to other growth forms in their response to a controlled perturbation. The controlled perturbation was a complete factorial NPK fertilization experiment in tussock tundra at Eagle Creek, Alaska, USA. We compared deciduous shrubs, evergreen shrubs, and functionally deciduous gra- minoids in terms of mineral and total nonstructural carbohydrate (TNC) concentrations, and annual production per stem or tiller. Species differed in the extent to which nutrient and TNC concentrations were altered by fertil- ization, although concentrations were usually changed in the same direction in all species in response to a given nutrient addition. Growth forms were not consistently different from each other in the responses of nutrient and TNC concentration, and frequently the two species from the same growth form responded differently. Growth per stem or tiller was stimulated most strongly by N and/or N + P fertilization in five of six species, with small and usually insignificant differences in magnitude but not direction of the response among species. Nutrient concentrations generally responded without interaction between fertilization treatments but the growth response was usually characterized by the N*P interaction, indicating that individual nutrient concentrations may vary widely and independently but that growth at Eagle Creek requires a balanced plant nutrition with first N and then P as the principal limiting factors. Decreases in TNC with fertilization suggested that carbon supply was not strongly limiting to plant growth. We conclude that species respond individually to fertilization in terms of nutrient and TNC ac- cumulation, but that the species or growth forms studied are not distinctive from each other on the basis of limiting nutritional factors for growth.

Habitat Structure, Patchiness, and Avian Communities in North American Steppe Vegetation: A Multivariate Analysis

Abstract: We investigate the relationships between vegetational structure, spatial heterogeneity, avian community structure, and the ecological responses of the breeding bird populations in the structurally simple steppe vegetation of North America. We examined the effect of variation in several measures of both horizontal and vertical patchiness in vegetation distribution on bird distribution and abundance, then integrated these measures into a smaller subset of variables using Principal Component Analysis (PCA). We interpreted the new PCA variables as reflective of proximate niche parameters to which bird species responded. Out of 550 structure/bird abundance bivariate correlations, 18.5% achieved a significant level of at least P <.05. Virtually all individual species responses to vegetation habitat measurements were straightforward: all typical tallgrass prairie birds attained highest abundances on plots with highest percent coverage of grass, typical shrubsteppe birds attained their highest abundances on plots...

On foraging time allocation in a stochastic environment

Abstract: A model of foraging in a stochastic environment is presented. Drawing on statistical decision theory, this paper suggests that maximization of expected utility may be a useful optimization criterion for ecological problems. The common notion of feeding preferences is extended to prefer- ences over the probability distributions of net energetic benefits associated with available resources. By incorporating a forager's response to environmental variation, the model can lead to predictions that contrast with those of most deterministic feeding models.

Algal Zonation in the New England Rocky Intertidal Community: An Experimental Analysis

Abstract: Zonation patterns of plants, including marine algae, have commonly been attributed solely and directly to physical factors. Experimental investigations of the factors affecting zonation of macroscopic, benthic algae in the New England rocky intertidal region demonstrated that biological factors set the lower limits of these plants. The mid zone at all but very exposed sites is usually dominated by brown fucoid algae. These plants are virtually absent in the low zone, which is dominated by the red alga Chondrus crispus (Irish moss). Total removal of Chondrus (including the prostrate holdfast) results in establishment of Fucus vesiculosus or F. distichus ssp. edendatus in the low zone. Fucus grows faster in the low than in the mid zone, appears healthy, and reproduces. Thus competition from Chondrus sets the lower limit of Fucus, not changes in light intensity or immersion time, per se, as previously assumed. If herbivores (primarily the perinwinkle snail Littorina littorea) are absent where Chondrus is removed, Fucus can settle very densely and occupy 100% of the space. If herbivores are present, Fucus colonizes, but less abundantly. Thus competition is the primary determinant of the zonation pattern (affecting presence or absense) and herbivory is of secondary importance (affecting abundance). Other experimental evidence suggests that the upper limit of Chondrus is determined by desiccation. The lower limit of Chondrus has not been investigated except where a sharp lower limit exists at the low intertidal—shallow subtidal interface. Experiments demonstrate that this is due to the grazing by sea urchins (Stronglyocentrotus droebachiensis) where they are locally abundant. Normally, Chondrus extends well into the subtidal region. These results parallel experimental studies of animal zonation in rocky intertidal regions in which biotic factors also set lower bathymetric limits. It is suggested that biogeographic ranges of some species may be similarly affected by biotic factors.

Age structure and disturbance history of a southern appalachian virgin forest

Abstract: The frequency and intensity of natural disturbances in a montane old-growth forest of coniferous (Tsuga) and broad-leaved deciduous trees (Liriodendron, Betula, Castanea, Fagus, Acer) were investigated by means of diameter frequency distributions, diameter-age regressions, and anal- ysis of radial growth patterns. The seven species investigated are represented by all-aged populations regardless of the degree of shade tolerance. The age distributions of most species, however, are irregular with several prominent peaks. Apparent coincidence of these peaks among some species suggests a common underlying influence. Radial growth patterns reveal higher than average numbers of abrupt and sustained increases in growth rate in most decades of peak recruitment which, along with other evidence, suggest that disturbance was the principal causal factor. There is evidence for eight partial disturbances in the past 250 yr, each of which generally removed < 10%o of the overstory trees. These fairly light disturbances appear to be sufficient to cause major deviations from the theoretical equilibrium diameter distribution of the forest.

Trophic Basis of Production Among Net-Spinning Caddisflies in a Southern Appalachian Stream

Abstract: Life histories and annual production were determined for six species of net-spinning caddisflies in a headwater stream of the Tallulah River in north Georgia, USA. Five species in the family Hydropsychidae were univoltine, whereas the sixth, a member of the Philopotamidae, had at least two generations per year. Combined annual production, as determined by the Hynes method, was 1.0 g/m2 (ash-free dry mass). Seventy-five percent of the production was concentrated in the two largest species, Arctopsyche irrorata and Parapsyche cardis. The remaining production from highest to lowest percent, was contributed by Dolophilodes distinctus, Hydropsyche sparna, Diplectrona modesta, and Hydropsyche macleodi. Analysis of gut contents alone indicated that detritus was the most important food source. How- ever, food preference and food-specific ecological efficiencies were utilized to calculate the amount of production attributable to each major food category. Surprisingly, almost 80% of all caddisfly production was attributed to animal food, 13% to detritus, and 8% to algae. Actual annual consumption required to account for this production was 2.28 g/m2 animals, 2.54 g/m2 detritus, and 0.51 g/m2 algae. We attempt to quantify the role that net-spinning caddisflies play in the "spiralling" of seston in mountain streams. Our results show that the omnivorous caddisflies are not the major consumer of detritus and algae, and that they produce more detritus in their feces than they consume, thus ap- pearing to lower the food quality of the seston. Net-spinning caddisfly production in this mountain stream appears to be limited by the amount of high quality food available in the seston.

Ecological character displacement

Abstract: A theoretical study is made of the conditions under which competition mediated by a quantitative character in each of two species will result in the convergence or divergence of the mean values of the character. Both within— and between—species competition are considered and a general model of the genetical basis of the character is used. Both the genetical and ecological parts of the model are based on the biological assumptions that are made in discussions of ecological character displacement, which include the assumptions that there are no constraints on the evolution of the character in either species and that individuals in each species with the same value of the character use the limiting resources in the same way. One of the main results is that the usual biological assumptions do not necessarily lead to permanent and significant displacement (i.e., d/w > 1). While some previous models have led to different conclusions, it is shown that those results are due to other, implicit, assumptions in those models. In the present model, two types of conditions lead to significant character displacement under biologically reasonable assumptions. One type of condition, which would apply to species that are roughly equally abundant at equilibrium, is that there be some differences in the set of limiting resources of the two species. In that case, the equilibrium displacement would be much larger than the differences between the resource spectra. Small and possibly unmeasurable differences between species could lead to significant displacements. The other type of resource distribution that leads to significant displacements is a skewed distribution. This requires no differences between the resource spectra for the two species but it leads to an equilibrium in which one of the species is much more abundant than the other. One consequence of these results is that the relative abundances of two species thought to have undergone character displacement are important to understand the possible mechanism leading to that displacement.

Kelp Beds and Sea Otters: An Experimental Approach

Abstract: In Torch Bay, Alaska, USA, sea urchins structure shallow subtidal communities by consuming most macroscopic algae Experimental removal of urchins leads to the development, within 1 yr, of a kelp association of high biomass and diversity In the 2nd yr, however, these attributes decrease as a single kelp species, Laminaria groenlandica, becomes dominant Additional field experiments confirm the competitive superiority of this kelp These manipulations lead to predictions regarding the ecological role of sea otters, a once common (but now absent) sea urchin predator The predictions are tested and verified by examining areas into which sea otters have recently been transplanted As expected, a rapid and extensive modification of algal species composition and a dramatic increase in kelp biomass follow the return of sea otters See full-text article at JSTOR

Heteromorphic Life Histories of Certain Marine Algae as Adaptations to Variations in Herbivory

Abstract: Many of the annual or ephemeral algae of the mid to high intertidal zones have het- eromorphic life histories, existing as upright morphs during seasonal algal blooms and as crustose or boring morphs during other portions of the year. Experimental removal of herbivores on the coasts of New England and Oregon resulted in the occurrence of the upright morphs in the times of year when they were normally absent (summer in our areas), demonstrating that such uprights can survive the summertime physical regime (contrary to earlier speculation). We suggest that the upright and crustose or boring stages of these algae represent mutually exclusive adaptations to fluctuations in grazing pressure: the upright stages are adapted for high rates of growth and reproduction when grazing pressure is low, and the crustose and boring stages are adapted for surviving through times of high grazing pressure. We predict isomorphic species of algae would predominate in these sorts of habitats if grazing pressure were more constant.

Vesicular-Arbuscular Mycorrhizae Affect Lowland Tropical Rain Forest Plant Growth

Abstract: In experiments with plants grown in pots, vesicular—arbuscular mycorrhizal increased seedling growth of 23 to 28 species from a lowland tropical rain forest region. Mycorrhizae improved survival in six species and cotyledon retention in five species. Mycorrhizae also increased the size of bacterial nodules and the proportion of nodulated plants among three leguminous species. Growth of seedlings lacking mycorrhizae slowed greatly or ceased after attainment of sizes correlated with average seed dry mass. Removing cotyledons from individuals of two species that are dependent on mycorrhizae, however, did not increase their response to inoculation. Thus, seed reserves are important for mycorrhiza formation as well as for seedling growth before infection. Large seeds are advantageous to plants that depend on vesicular—arbuscular mycorrhizae because they provide mineral reserves upon which the seedling can draw while awaiting infection. Seedlings of some species could not grow without mycorrhizae, but inoculation did not affect the growth of other species. Species that are least dependent on mycorrhizae have light seeds and colonize disturbed habitats.

Loss of mass and chemical change in decaying boles of a subalpine balsam fir forest

Abstract: Decay of balsam fir (Abies balsamea) boles was examined in an upper subalpine forest of the White Mountains, New Hampshire, USA. Fifty percent of the initial mass was lost in 23 yr; 90% was lost in 77 yr. High decay rates were attributed to the small diameters of the boles, ample moisture, and a nitrogen-rich environment. Average dead wood mass in this forest was 4.9 kg/M2, representing 25% of the sum of dead wood, live plant biomass, and forest floor organic matter. Changes in density and moisture and in the concentrations and content of various chemical com- ponents of the boles were traced over the decay sequence. Changes in the content of cellulose, lignin, carbon and sodium followed loss of mass during decay. Contents of calcium, magnesium, potassium and phosphorus decreased faster than loss of mass in the early stages of decay. Much of this initial loss was acribed to sloughing of nutrient-rich bark which in these small boles comprised 13% of dry mass. Later in decay, the loss rates of calcium, magnesium and potassium were about the same or slightly less than the loss rate of mass. After a steep initial drop, phosphorus content of the boles remained approximately constant between years 12 and 33. Thereafter the loss rate paralleled loss of mass. Nitrogen content was approximately constant in the first 33 yr after which it declined in parallel with loss of mass.

Experimental Acidification of a Stream in the Hubbard Brook Experimental Forest, New Hampshire

Abstract: Incident precipitation in the northeastern United States averages about pH 4 as a result of increased pollution from sulfuric and nitric acids. To determine the effect of this increased acidity on the ecology of aquatic ecosystems, dilute concentrations of sulfuric acid were added to Norris Brook, a stream in the Hubbard Brook Experimental Forest, West Thornton, New Hampshire, USA. The stream was maintained at pH 4 from April to September 1977. With increased acidity stream water concentrations of Al, Ca, Mg, K, and probably Mn, Fe, and Cd were elevated; no change in dissolved organic carbon (DOC), Na, NO,, NH4, Ni, Pb, Cu, or Zn occurred at the lower pH. Emergence of adult mayflies (Ephemeroptera), some stoneflies (Plecoptera), and some true flies (Diptera) decreased at the lower pH. Larger numbers of immature aquatic inver- tebrates in the collector, scraper, and predator functional groups were found in drift samples from the experimental area during the 1st wk after acid addition. After the 1st wk of increased acidity total numbers of organisms drifting in the experimental area were similar to values obtained in the reference area. Emergence of adult collectors and invertebrate density in the benthos decreased in the treatment area. Periphyton biomass increased at the low pH, but hyphomycete fungal densities decreased. A basidiomycete fungus increased in the experimental area relative to the reference section. Brook trout showed no morphological signs of stress at the low pH. Stream acidification decreased species di- versity, increased representation of community dominants. and decreased the complexity of the food web.

Effects of predation and competition on survivorship, fecundity, and community structure of desert annuals'

Abstract: Effects of rodent and ant granivores, watering, and thinning were studied in a diverse community of winter annuals in the Sonoran Desert. Seed predation considerably reduced plant densities, but ants and rodents had qualitatively different effects on the plant community. Ants in- creased species diversity of annuals by differentially harvesting the numerically dominant species, while rodents preyed selectively on species which dominated the community in terms of biomass. Competition among annual plants reduced growth rate, biomass, and fecundity, but had no significant effect on mortality. One species, Euphorbia polycarpa, apparently increased in density when the density of competing Erodium species was reduced by rodent predation.

Predator-Prey Interactions between Stoneflies and Mayflies: Behavioral Observations

Abstract: Field experiments were conducted at two streams to determine the mechanisms by which mayfly prey detect and avoid stonefly predators, and to quantify the effect of stonefly predators upon mayfly behavior, distribution, and activity levels. Direct observations of the interactions be- tween stoneflies and mayflies were made at Otter Creek, Sauk County, Wisconsin, USA and the East River, Gunnison County, Colorado, USA using plexiglass boxes with screen ends and an artificial substrate. Ephemerella subvaria and Stenonemafuiscum, Otter Creek, avoided the stonefly predator Acroneuria lycorias given only noncontact chemical stimuli. Ephemerella infrequent and Baetis bicaudatus, East River, responded to long-range chemical stimuli from Megarcys signata and Ko- gotus modestus, East River. Baetis phoebus and Heptagenia hebe, Otter Creek. and Cinygmula sp., East River, did not respond to stonefly predators given long-range chemical cues alone. Differences among responses of different mayflies may be due to differential predation pressure or effectiveness of predator evasion tactics. None of the species of mayflies tested responded to the presence of stonefly predators given only visual stimuli. The Ephemerella species assumed a "scorpion"-like display posture upon encounter with the stoneflies. This posture may increase the apparent size of the mayflies which are then rejected by the tactile predators. The Baetis species characteristically swam or drifted in response to contact with the stonefly predators. Predator evasion could partially explain the high incidence of this genus of mayflies reported in stream drift. The remaining mayflies, of the family Heptageniidae, showed an effective crawling evasion upon contact with stonefly predators. Ephemerella infrequens and B. bicaudatus differentially responded to contact with three different stonefly predators in the East River. Both species of mayflies evaded contact with Pteronarcella badia, a large omnivorous stonefly which morphologically resembles M. signata, significantly less frequently than they evaded M. sig- nata and K. modestus upon contact. The mechanism by which this differentiation occurs could be chemotactile. Cinygmula sp. avoided all stoneflies equally regardless of size or relative threat of consumption. Tactile stimuli are probably responsible for this behavior.

Some comments on measuring niche overlap

Abstract: This article discusses the usefulness of niche overlap and niche overlap measures in studying competition. It is argued that (1) neither niche overlap nor niche overlap measures can be used to estimate the intensity of competition, but (2) niche overlap can be used to determine the relative amounts of inter— and intraspecific competition, while niche overlap measures are not useful in this regard. Before using overlap to estimate the relative intensities of inter— and intraspecific competition, it is necessary to have a model of the consumer—resource interaction, and some knowledge of the parameters in that model. The niche overlap and competition coefficient formulae proposed by Hurlbert (1978) are considered in some detail. Previously proposed niche overlap measures appear to be more useful than Hurlberts if one is interested in using the niche overlap measure for purely descriptive purposes. See full-text article at JSTOR

The Role of Substratum in Benthic Macroinvertebrate Microdistribution and Litter Decomposition in a Woodland Stream

Abstract: This experiment tested the effect of substratum particle size, in the absence of velocity variation, on the determination of macroinvertebrate microdistribution and decomposition in wood- land streams. Replicate baskets of three different substrata were placed in a single riffle of New Hope Creek, North Carolina, USA. The substrata were fine gravel (=1 cm diameter), pebbles (=2.5 cm diameter), and large cobbles (=8.5 cm diameter). Half the baskets had leaf packs attached to their upper surfaces. Replicate samples were removed on five dates at 2-wk intervals and analyzed for macroinvertebrates and leaf pack biomass. Colonization and distribution patterns of individual taxa were used to assess substratum preferences, which were correlated with physical properties of the sediments and the presence or absence of the leaf covering. Overall, litter decomposition did not vary among substrata. The fraction of animal populations in leaf packs was proportional to the biomass remaining. Animals showed substratum preferences even when velocity differences were eliminated. Preferences of common taxa were unaffected by the presence or absence of leaf packs on the substratum. Common taxa showed strong preferences for either leaves or substrata. Abundance data (the number of individuals per basket) strongly contradict the density data (number per square metre of substratum surface). The latter measure offers more biological insight. These results emphasize the importance of substratum size as a prime determinant of the structure of lotic macroinvertebrate communities.

Energy flow, nutrient cycling, and ecosystem resilience.

Abstract: The resilience, defined here as the speed with which a system returns to equilibrium state following a perturbation, is investigated for both food web energy models and nutrient cycling models. Previous simulation studies of food web energy models have shown that resilience increases as the flux of energy through the food web per unit amount of energy in the steady state web increases. Studies of nutrient cycling models have shown that resilience increases as the mean number of cycles that nutrient (or other mineral) atoms make before leaving the system decreases. In the present study these conclusions are verified analytically for general ecosystem models. The behavior of resilience in food web energy models and nutrient cycling models is a reflection of the time that a given unit, whether of energy or matter, spends in the steady state system. The shorter this residence time is, the more resilient the system is.

Fruit Set, Herbivory, Fruit Reduction, and the Fruiting Strategy of Catalpa Speciosa (Bignoniaceae)

Abstract: Catalpa speciosa regularly initiates more fruits than it matures. Most of the fruits that fail to mature are abscised from 2-5 wk after the flowering period. Catalpa speciosa has a host- specific herbivore, Ceratomia catalpae (Sphingidae), which severely defoliates some branches while causing little or no damage to leaves of other branches. The pattern of fruit abscission was examined in relation to the principal herbivore. Field observations and experiments show: (1) the larger the infructescence, the higher the probability that a given fruit will abort; (2) branches that have expe- rienced simulated herbivory have significantly more abortions than control branches; (3) if one fruit is aborted from an infructescence, the remaining fruits have a higher probability of maturing; (4) fruits accumulate <10% of their mature dry weight and total protein during the 1st 5 wk of growth and the remaining 90W in the next 4 wk (after the period of abortion). These data suggest that (1) each branch supplies the energy necessary to mature the fruits that it bears, (2) environmental stress, especially herbivory, reduces the resources available for fruit production, (3) fruit abortion is a response to limited resources, and (4) the pattern of fruit growth reduces the "cost" of abortion. The overall pattern of fruit reduction is interpreted as an adaptive trade-off between seed number and seed quality which permits the parent plant to match fruit production with available resources.

Leaf Ultraviolet Optical Properties Along a Latitudinal Gradient in the Arctic-Alpine Life Zone

Abstract: Leaf epidermal transmittance of terrestrial solar ultraviolet-B radiation (295 to 320 nm) was examined along a latitudinal gradient of solar uv-B radiation. In high uv-B radiation zones, e.g., equatorial and tropical regions, mean epidermal transmittance for the species examined was less than 2%. At higher latitudes, mean epidermal transmittance exceeded 5%. Although this latitudinal solar uv-B gradient represents more than a seven-fold difference in daily integrated uv-B irradiance, the calculated mean effective uv-B irradiance at the mesophyll of low-latitude species is not substantially different from that of species at higher latitudes. Species in high uv-B radiation environments appear to attenuate this radiation more effectively than those in lower irradiance environments. In most cases, absorption of uv-B in the epidermis is the major parameter effecting low transmittance. Reflectance from glabrous leaves is generally less than 10%. In some species, pubescent or glaucous leaf surfaces can reflect more than 40% of the uv-B radiation incident on a horizontal leaf, although such surface characteristics do not necessarily indicate high uv-B reflectance. Under controlled conditions, epidermal transmittance in Pisum sativum L. decreased in response to uv-B irradiation. The modification of epidermal transmittance, resulting in lower uv-B irradiance at the mesophyll, may represent a mechanismmore » of plant acclimation to uv-B radiation. Such acclimation may have occurred in several wildland species of temperate-latitude origin that have invaded high uv-B irradiance equatorial and tropical regions.« less

Flow analysis of models of the Hubbard Brook Ecosystem.

Abstract: Several models of energy and nutrient flow on the Hubbard Brook Ecosystem, New Hampshire, USA, were analyzed for the pattern of flow through the models, cycling index CI, path length PL, and straight—though path length PLs. CI represents the proportion of flow that cycles through components of the system. PL is the average number of components that a unit of flow passes through on its way from inflow to outflow. PLs is that portion of PL attributable to flow passing straight through the system without cycling at all. The energy model had low values for PL, PLs, and CI, compared to nutrient models. Flow measures for nutrients varied greatly. The order of the elements in terms of CI and K > Na > N > Ca > P > Mg > S. This could be explained by the mobility and biological role of each element, although Na is somewhat anomalous. PL was very large for K (24.3) but small for S (5.2). PLs was near 4 for all elements but Na (2.6) and S (2.5). This was a reflection of different flow patterns for Na, which flows primarily between available nutrients and belowground biomass, and S, which flows primarily between available nutrients, and below— and aboveground biomass. Sulfur returns to available nutrients via stemflow and throughflow, bypassing the forest floor. The remaining elements cycle between above— and belowground biomass, forest floor, and available nutrients. Three different models of Ca flow were compared. When all three models were analyzed using the same nutrient flux data, flow measures become very close, despite structural differences in the models. For these models, flow values were more important than the architecture of the models in determining cycling and flow characteristics.

Influence of Amphibian Larvae on Primary Production, Nutrient Flux, and Competition in a Pond Ecosystem

Abstract: In a pond ecosystem near St. Louis, Missouri, natural variations in tadpole biomass during 1971-1972 were accompanied by shifts in patterns of nutrient cycling and primary production, particularly when metamorphoses caused abrupt removal of these transient consumers. In the field, increased tadpole biomass was associated with: (I) reduced standing crop of suspended particles, including phytoplankton, the tadpoles' major food source; (2) a shift in the state of nitrogen from largely particulate to largely dissolved; (3) reduced rates of primary production, from both H14CO:t uptake and diurnal oxygen methods; (4) a nonlinear effect on phytoplankton specific growth rates; (5) a shift in phytoplankton community structure away from filamentous blue-green algae; and (6) a reduced proportion of active chlorophyll a in the photosynthetic pigments of phytoplankton. From laboratory experiments, the potential impact of tadpoles on nitrogen flux, through feeding and nutrient release, was estimated. Several conclusions were made: (1) Suspension feeding by tadpoles reduced concentrations of suspended particles. Under condi- tions of low particles:high tadpoles, the specific growth rates of tadpoles were reduced. Recruitment was absent except under conditions of high particles:low tadpoles; the most diverse community of new tadpole recruits (four species) was observed under such conditions. All four species (three genera) had similarly sized particles in their guts. These field observations are consistent with an hypothesis of competition among the tadpoles. (2) Tadpoles apparently were regulatory consumers; they became a large component with respect to phytoplankton. Nitrogen flux through tadpoles was within the same order of magnitude, and some- times exceeded the estimated N uptake by phytoplankton. (3) Tadpoles probably regulated primary production by both reducing standing crop and altering specific growth rates of algae. At maximum tadpole biomass, suspended particle concentrations were stabilized near the laboratory-determined threshold concentration for feeding by these Rana tadpoles. When metamorphosis removed these transient consumers, rates of primary production increased dramatically. (4) Interactions within the pond ecosystem apparently determined aquatic-terrestrial nutrient bal- ances for the amphibian communities. Some species deposited more nutrient in their eggs than was assimilated by larvae, but the community as a whole extracted nutrient from the ecosystem. Nutrient input in eggs was much less than that assimilated by autotrophs.

A steep latitudinal gradient of solar ultraviolet-b radiation in the arctic-alpine life zone'

Abstract: Solar ultraviolet-B (280-320 nm) spectral irradiance and total shortwave radiation were measured along an arctic-alpine life zone gradient from the Arctic (70'N) at sea level to equatorial latitudes at high elevations above sea level. This series of measurements portrays the radiant flux to be anticipated during periods of maximum annual solar radiation. Along this arctic-alpine gradient, maximum daily total shortwave irradiance varies only by a factor of 1.6 and total daily shortwave radiation by less than 15% for cloudless conditions. In contrast, the maximum integrated effective UV-B irradiance can vary by a full order of magnitude and by seven-fold for total daily effective radiation. The steep UV-B radiation gradient is the result of a natural latitudinal gradient in total atmospheric ozone column thickness, prevailing solar angles at different latitudes, elevation above sea level, and an optical amplification effect, which results from a combination of highly wavelength- dependent radiation attenuation in the atmosphere and the pronounced wavelength dependence of biological action spectra. Forty to 70o of the solar UV-B flux is in the form of scattered radiation. Because of the large proportion of diffuse UV-B radiation, steeply inclined foliage would still receive a sizable fraction of the global UV-B irradiance in most situations. Snow cover can add substantially to the radiation load.

Monkey dispersal and waste of a neotropical fruit

Abstract: Foraging birds and mammals were observed at a Panamanian rain forest tree Tetra- gastris panamensis (Burseraceae) in order to determine (1) relative importance of different dispersal agents and (2) the relationship between fruit production and dispersal. The system is of interest because fruits provide little but sugar, fruits are not depleted by animals (i.e., are superabundant), and the tree is visited by a variety of vertebrates with different effects on seed dissemination. Three mammals (Alouatta palliata, Cebus capucinus, Nasua narica) account for 97% of all seeds removed from the trees; the howler monkey (Alouatta) is alone responsible for 74% of the seeds removed. Seeds from the feces of these mammals are fully viable. In addition, two diurnal mammals, three nocturnal mammals, and nine birds ingest and disperse small numbers of seeds, but these species are numerically insignificant. Two parrots (Amazona farinosa and A. autumnalis) are im- portant fruit thieves and seed predators, while three mammals and three additional birds kill or drop small numbers of seeds near the parent trees. Assemblage richness increases markedly with crop size, but most of 25 visitors are of little significance to the plant. Use by three monkeys closely reflects their relative abundance; the abundance of the coatimundi (Nasua) is unknown. This system is characterized by enormous seed waste. Of >430,000 seeds produced by 19 study trees in 1978, <4% had even a remote chance of establishment. The remainder were killed by parrots (6%), were dropped or spontaneously fell under parent trees (66%), or were doomed to intense competition in fecal clumps (24%). The three primary dispersal agents indirectly caused most seed and seedling mortality by dropping seeds under the tree crown or leaving them in fecal clumps, precluding survival of any but a small fraction of seedlings. Crop size ranged from 165 to 100,000 arillate seeds per season, but fruits appeared superabundant on all trees. Numbers of seeds dispersed increased dramatically with crop size, but the proportion dispersed was highest at plants with crops of intermediate size. The proportion of seeds killed by parrots increased with crop size. Dispropor- tionate mortality of seedlings in high densities near adult trees indicated density-dependent mortality among juveniles. Wide geographical ranges of this tree and members of the frugivore assemblage, use of other food resources by common foragers, richness of the assemblage, variation in species visitation at individual trees, irregular annual fruit production, and obvious "inefficiency" of the dominant dispersal agents from the perspective of the plant suggest that Tetragastris has a generalized dispersal strategy and is not closely coevolved with particular frugivores. A generalized dispersal system is expected from a tree, such as this one, that produces sugary fruit incapable of providing a balanced diet.

Multalism between Harvester Ants and a Desert Ephemeral: Seed Escape from Rodents

Abstract: The harvester ants Veromessor pergandei and Pogonomyrmex californicus modify the highly localized seed shadow of the Sonoran Desert ephemeral Datura discolor (Solanaceae) through (1) attraction to food bodies attached to seeds released beneath the parent plant, (2) immediate transport of diaspores to the ant nest, (3) subsequent removal of the food body in underground granaries, and (4) final expulsion of intact seeds onto the colony midden. D. discolor seed without food bodies are not attractive to ants and remain beneath the parent canopy. At Nude Wash, Cali- fornia, USA, diaspore removal distances are generally small, averaging 2.3 m away from the nearest D. discolor canopy in March-May 1977. Two lines of evidence indicate that this limited transport has a large effect on seed detection and utilization by rodents: (1) seeds placed in ant-proof dishes beneath the D. discolor canopy were about 10 times more likely to be removed by nocturnally foraging rodents than seeds placed in locations similar to those where ants locate their nests; (2) using D. discolor as bait, significantly more rodents were trapped beneath the plant canopy than in open areas where seeds were transported by ants. Manipulation of the density of D. discolor seed in these tests suggests that seed utilization by rodents is distance-responsive to the parent canopy rather than dependent on the density of seeds. Diaspore analyses indicate that investment in the transport system is relatively small and repre- sents <5% of the total investment in seeds.

Competition for bumblebee pollinators in rocky mountain plant communities

Abstract: Meadow plant communities at four sites in the Colorado Rocky Mountains were stud- ied to evaluate the influence of competition for bumblebee pollinators. The niche relationships among plant species were characterized by overlap with respect to species of bumblebee visitors and times of flowering. Species in each community could be divided into two to three guilds based upon the identity of the species' major bumblebee visitor. Only the members of a guild would potentially be in competition with one another for pollinators. The competition hypothesis is that guild members have segregated blooming periods which would minimize competition. The null hypothesis is that blooming periods are dispersed randomly through time. The competition hypothesis was tested by comparing the actual temporal spacing pattern for a guild with random patterns generated by computer simulation. For 10 out of 11 guilds the sequence of blooming periods was more regular than expected (either at the statistically significant level (five cases) or near it (five cases)). I also examined the relationships among guild members using another measure of the competitive effect of one species on another which is more accurate than temporal overlap. This variable, called competitive load, includes information on the overlap, abundance, and floral attractiveness of com- petitors. It is an indicator of the number of visits a species loses to competitors. In general species were found to lose approximately half the number of visits they could potentially receive during the time they were in flower. All species received approximately the same number of visits despite large differences in floral abundance. Thus the pool of available pollinator visits appears to be rather evenly divided among the species in a community. This regularity in resource partitioning is further inferential evidence of competition for pollinators. Rare species avoid the potentially heavy loss of visitors to more abundant competitors by having flowers which are more attractive to pollinators. Attractiveness appears to be a function of the nectar production rate of a species' flowers. There are some exceptions to these generalizations. Species which can also reproduce vegetatively are less attractive, and lose more visits that expected. The competitive effect that one species experiences because of the presence of another may be the result of a loss of pollinator visits (exploitation) and/or a disruption of nonspecific pollen flow (interference). Species may reduce the interference component of competition by spatial isolation. For the one guild that had a random flowering sequence with broad overlap among species, spatial isolation and competition between guild members were positively correlated. Temporal divergence and differential attractiveness are seen to be primarily a means of avoiding exploitation competition.

Effects of Plant Density and Diversity on the Population Dynamics of a Specialist Herbivore, the Striped Cucumber Beetle, Acalymma Vittata (Fab)

Abstract: To determine the effects of plant density and diversity on the population dynamics of a specialist herbivore, the striped cucumber beetle (Acalymma vittata (Fab.)), cucumbers (Cucumis sativus L.) were planted in monocultures and in polycultures with corn (Zea mays L.) and broccoli (Brassica oleracea L.). Population densities of A. vittata and its insect predators, as well as cucumber growth, survivorship, and reproduction were monitored in these experimental plots. By controlling total plant density, host plant density, and plant diversity, it was possible to distinguish the effects of these three confounding variables. Densities of A. vittata were 10-30 times greater in monocultures than in polycultures, both per plot and per plant, even when total plant density and host plant density were held constant. The lack of any effect of plant density on per-plot beetle abundances emphasized the importance of diversity per se in influencing beetle populations. These differences in beetle abundances caused by diversity resulted from differences in tenure time and movement patterns among plots, rather than from dif- ferences in colonization, reproduction, or predation. In mark-recapture studies, a greater proportion of beetles marked in monocultures than in polycultures was later found, and of those marked in monocultures, a greater number were later found in monocultures than in polycultures. All measures of growth (leaf area, growth rate, and vine length) and reproduction (fruit production and number of flowers) of Cucamis sativas were most strongly affected by diversity, but also were affected by plant density. Both per-plot and per-plant values were greater in monocultures than in polycultures. The number of beetles was strongly correlated with the total amount of plant growth and reproduction in monocultures but not in polycultures. For plots with equal amounts of leaf area, monocultures had an order of magnitude greater number of beetles; thus, differences in host plant quantity did not explain the differences in beetle abundances between monocultures and polycultures. Numbers of beetles were much more strongly related to total plot characteristics than individual plant characteristics. Although defoliation was negligible, beetles significantly decreased cucumber survi- vorship and longevity by disseminating bacterial wilt disease (Erwinia tracheiphila (E. F. Sm.)). Kevy t'ords: Acalymma vittata; Cucumis sativus; insectlplant relationships; insect population dynamics; plant diversity; plant spatial pattern; resource concentration; specialist herbivore.