Showing papers in "Oecologia in 1983"

Fractionation and turnover of stable carbon isotopes in animal tissues: Implications for δ13C analysis of diet

Abstract: The use of stable carbon isotopes as a means of studying energy flow is increasing in ecology and paleo- ecology. However, secondary fractionation and turnover of stable isotopes in animals are poorly understood pro- cesses. This study shows that tissues of the gerbil (Meriones unguienlatus) have different 613C values when equilibrated on corn (C4) or wheat (C~) diets with constant 13c/1ac contents. Lipids were depleted 3.0%o and hair was enriched 1.0%o relative to the C4 diet. Tissue 6t3C values were ranked hair > brain > muscle > liver > fat. After changing the gerbils to a wheat (C3) diet, isotope ratios of the tissues shifted in the direction of the 6a3C value of the new diet. The rate at which carbon derived from the corn diet was replaced by carbon derived from the wheat diet was ade- quately described by a negative exponential decay model for all tissues examined. More metabolically active tissues such as liver and fat had more rapid turnover rates than less metabolically active tissues such as hair. The half-life for carbon ranged from 6.4 days in liver to 47.5 days in hair. The results of this study have important implications for the use of 6~3C values as indicators of animal diet. Both fractionation and turnover of stable carbon isotopes in animal tissues may obscure the relative contributions of isotopically distinct dietary components (such as C3 vs. C,, or marine vs. terrestrial) if an animal's diet varies through time. These complications deserve attention in any study using stable isotope ratios of animal tissue as dietary indicators and might be minimized by analysis of several tissues or products covering a range of turnover times.

Allocating leaf nitrogen for the maximization of carbon gain: Leaf age as a control on the allocation program.

Abstract: Simulations using a biochemically-based model of leaf photosynthesis make it possible to predict the distribution of leaf nitrogen contents that maximizes photosynthetic carbon gain over the canopy of an entire plant. In general, the optimal nitrogen content increased with increasing daily photosynthetically active photon irradiance.

Analyzing insect movement as a correlated random walk.

Abstract: This paper develops a procedure for quantifying movement sequences in terms of move length and turning angle probability distributions. By assuming that movement is a correlated random walk, we derive a formula that relates expected square displacements to the number of consecutive moves. We show this displacement formula can be used to highlight the consequences of different searching behaviors (i.e. different probability distributions of turning angles or move lengths). Observations of Pieris rapae (cabbage white butterfly) flight and Battus philenor (pipe-vine swallowtail) crawling are analyzed as a correlated random walk. The formula that we derive aptly predicts that net displacements of ovipositing cabbage white butterflies. In other circumstances, however, net displacements are not well-described by our correlated random walk formula; in these examples movement must represent a more complicated process than a simple correlated random walk. We suggest that progress might be made by analyzing these more complicated cases in terms of higher order markov processes.

Compromises between water-use efficiency and nitrogen-use efficiency in five species of California evergreens

Abstract: In five California evergreen trees and shrubs cooccurring in this study but most common in habitats of different moisture availability, leaf nitrogen was a major determinant of photosynthetic capacity. Within each species, stomatal conductance was highly correlated with photosynthetic capacity, resulting in little variation in the concentration of CO2 in the intercellular spaces. Among species, intercellular CO2 concentrations varied significantly. Under controlled conditions, the leaves that realized the highest photosynthesis per unit of leaf nitrogen tended to realize the lowest photosynthesis per unit of water transpired. The ratio of photosynthesis to transpiration, an instantaneous measure of intrinsic water-use efficiency, was highest in the species commonly found in the direst habitats and lowest in the species most common in the wettes habitats.

The structural complexity of old field vegetation and the recruitment of bird-dispersed plant species.

Abstract: The input of bird-disseminated seeds into four old fields of different structural complexity was examined. Seed input was greatest along the edges of fields. Significantly more seeds were found in a 13 year old field that had structurally complex vegetation, than in a 3-year-old field with a single layer of vegetation. The lower input into the latter field was a function of both low fruit availability and low structural complexity of the field. Similarly, more seeds were found in a 2-year-old field which had artificial structures, simulating saplings, placed in it than in an adjacent control field of the same age. The shape of the structures was not a significant factor in the input of seeds. Timing of seed deposition was correlated with fruit ripening times, relative nutritional value of the fruit and the movements of frugivorous birds. The input of bird-disseminated seeds into fields appears to be directly related to the structural complexity of the vegetation. Woody plants increase the structural complexity of the old fields and serve as recruitment foci for bird-disseminated seeds. Thus, seed deposition by birds influences vegetation pattern, and conversely, the presence of recruitment foci in the vegetation may influence bird dispersal patterns of bird-disseminated seeds.

Plant-Herbivore Interactions in a North American Mixed-Grass Prairie. I. Effects of Black-Tailed Prairie Dogs on Intraseasonal Aboveground Plant Biomass and Nutrient Dynamics and Plant Species Diversity

Abstract: Seasonal dynamics of soil nematodes and root biomass were examined from under western wheatgrass (Agropyron smithii) and little bluestem (Andropogon scoparius) from a heavily grazed prairie dog (Cynomys ludovicianus) colony occupied for 5 to 10 years and an adjacent lightly grazed, uncolonized area in Wind Cave National Park, South Dakota, USA. Nematodes were differentiated into classes of plant-parasitic Tylenchida and Dorylaimida and nonparasitic Dorylamida and Rhabditida. Root-feeding nematodes were generally more numerous from A. smithii than from A. scoparius, while nonparasitic populations were not different in soil from beneath the two plant species. Rhabditida, parasitic Dorylaimida and Tylenchida (from A. scoparius only) were more numerous on the prairie dog colony than from the uncolonized site, but nonparasitic Dorylaimida populations did not differ between the two areas. Mean total (live plus dead) root biomass beneath A. scoparius and A. smithii on the prairie dog colony averaged 71% and 81%, respectively, of values from the uncolonized area. Estimated consumption by root-feeding nematodes averaged 12.6% and 5.8% of annual net root production in the upper 10 cm from the prairie dog colony and uncolonized site, respectively. We conclude that, because of microhabitat modification or reductions in plant resistance to nematodes, heavy grazing by aboveground herbivores apparently facilitates grazing by belowground herbivores. Because heavily grazed plants have less roots than lightly grazed or ungrazed plants, the impact of root-feeding nematodes on primary producers is likely to be greatest in heavily grazed grasslands.

Leaf age and seasonal effects on light, water, and nitrogen use efficiency in a California shrub.

Abstract: Photosynthetic capacity, leaf nitrogen content, and stomatal conductance decreased with increasing leaf age in the chaparral shrub, Lepechinia calycina, growing in its natural habitat Efficiency of resource use for three resources that potentially limit photosynthesis did not, however, decrease with increasing leaf age Light-use efficiency, given by the quantum yield of photosynthesis at low light intensities, was unaffected by leaf aging but decreased slightly through the winter and spring growing season Water-use efficiency, the ratio of photosynthesis to transpiration at light saturation and with a constant water vapor concentration gradient, was not affected by leaf aging or seasonal change Nitrogen-use efficiency, the ratio of photosynthesis at light saturation to leaf nitrogen content did not change with leaf age but was lower in the leaves with the highest specific weights This ensemble of leaf-age effects is consistent with the hypothesis that aging represents resource redistribution and not uncontrolled deterioration

The distribution of C3 and C4 grasses in Australia in relation to climate.

Abstract: All but four of 833 native and 292 naturalised Australian grasses (Poaceae) have been assigned as having the C4 or C4 photosynthetic pathway. In conjunction with comprehensive species composition data for 75 geographic subdivisions Australiawide, this has permitted the construction of distribution maps for C3 and C4 grasses. C3 and C4 grass distributions have been considered (i) independently, using subdivisional native species numbers; and (ii) relatively, using 'subdivisional % C4'. C3 species are most numerous in the Southern Tablelands (New South Wales), the QZ subdivision (Victoria), and Tasmania; C4 species in the northern Northern Territory and northern Queensland, including the Cook subdivision where 54% of Australia's native C4 grasses can be found. C3 species predominate only in south-west Western Australia, parts of southern South Australia, the Tablelands, central and south coast, and south western slopes of New South Wales, Victoria and Tasmania. C4 species predominate over 80-85% of the continental area. For 16 temperature and rainfall variables, subdivisional area-weighted means were calculated, and correlation analyses performed. C3 species number correlates most highly with January average maximum temperature (-ve) and spring rainfall (+ve); C4 species number with October average minimum temperature (+ve) and February median rainfall (+ve); '%C4' with January average minimum temperature (+ve). Predictive multiple linear regression equations were generated using climatic variables. In general, C4 grass species, like C3 species, increase in number with increasing rainfall, in their preferred temperature regime. C4 species are most numerous where the summer is hot and wet; C3 species where the spring is cool and wet. C4 species numbers decline with decreasing temperature and/or decreasing summer rainfall; C3 species numbers decline with increasing temperature and/or decreasing spring rainfall. Results are also considered in relation to the taxonomic and physiological heterogeneity of grasses and to Australia's geobotanic history.

Experimental analyses of the structure and dynamics of mid-shore rocky intertidal communities in New South Wales.

Abstract: At mid-shore levels on rocky shores in New South Wales, grazing gastropods are the dominant species in sheltered areas. Where wave-exposure is great, barnacles occupy most of the space. At intermediate levels of waveexposure, there are mixtures of grazing gastropods and barnacles, and the patterns of occupancy of space, and structure of the community change from time to time. The major species found in these areas are the coronuloid barnacle Tesseropora rosea, the patellid limpet Cellana tramoserica, the smaller acmaeid limpet, Patelloida latistrigata, which is mostly confined amongst barnacles, and the predatory whelk Morula marginalba. The roles of each of these species in determining the structure and persistence of intertidal communities were investigated by experimental manipulations of the densities of each of these organisms. In most experiments, a range of densities of limpets and barnacles was used, rather than the simple removal of all of one species.

The light environment and growth of C3 and C4 tree species in the understory of a Hawaiian forest.

Abstract: The light environment in the understory of a Hawaiian forest containing a C4 tree species, Euphorbia forbesii, was characterized using photosynthetic photon flux density sensors connected to portable data acquisition systems and a strip chart recorder, and hemispherical “fisheye” photographs of the canopy During July 1980, 86 μmol cm2 day1 was received in the understory of which approximately 40% was contributed by sunflecks The understory received 24% of the light reaching the top of the canopy Nearly all sunflecks had peak photon flux densities greater than 250 μmol m2 s1, but two-thirds were less than 05 min in length The number of minutes of sunflecks received per day at any site was highly variable, depending on cloudiness and the overstory canopy structure On a relatively clear day a 10-fold difference in the number of minutes of sunflecks was observed between sample sites Estimates obtained from hemispherical photographs were used to calculate the annual mean potential number of minutes of sunflecks per day received by saplings of Euphorbia and a C3 tree species, Claoxylon sandwicense The growth of saplings of both species was highly correlated with the estimates of the minutes of sunflecks and was similar for both species Although C4 photosynthesis is usually found in plants native to high-light environments, it does not appear to confer any disadvantage in terms of growth to Euphorbia forbesii in the low-light conditions of the forest understory

The role of adult feeding in egg production and population dynamics of the checkerspot butterfly Euphydryas editha

Abstract: Carbohydrate intake increases longevity, body weight maintenance and egg production in female Euphydryas editha. Amino acid intake leads to heavier eggs, larvae from which are more likely to survive. Females fed nectar produce more eggs in later masses than females which are not fed. During years of normal and below normal precipitation, larvae emerging from these later eggs are unlikely to reach obligatory size for diapause before their food dries up. On Jasper Ridge, where mortality is density-independent, nectar plays an important role increasing production of late egg masses during years of greater than normal rainfall when larvae from these masses are likely to reach diapause. The resulting large population increases, though infrequent, are probably important in maintaining population sizes large enough to reduce the chances of extinction during dry years.

The effect of body size on animal abundance.

Abstract: Although it is a commonplace that small animals are more abundant than large ones, few attempts have been made to quantify this and none for non-mammalian species. This study uses estimates of animal density and body mass culled from 12 journals published between 1961 and 1978 to test and extend Damuth's relationship between population density and body size of herbivorous mammals. In general, his analysis is supported, for density usually declines roughly as W -0.75 and poikilotherms maintain higher densities than homeotherms. However the residual variation is higher than Damuth's regressions might suggest and significant differences exist among animal groups. In particular, birds maintain much lower, and aquatic invertebrates much higher abundances than a general curve for all species would suggest. Carnivores are significantly rarer than herbivores. These relationships may be used to compare the average relative contributions of species of different size to community structure and function. Such relations also provide a necessary basis both for more complete empirical analyses of the determinants of animal abundance and for the construction of more realistic conceptual models in theoretical ecology.

The effects of size of opening in vegetation and litter cover on seedling establishment of goldenrods (Solidago spp.)

Abstract: We investigated the effects of size of opening in the vegetation and litter cover on seedling establishment of two species of goldenrods (Solidago spp.) in an abandoned field in southwestern Michigan, U.S.A. Seeds of S. canadensis and S. juncea were sown into clipped plots, ranging from 0 cm (control, unclipped) to 100 cm in diameter, with and without litter. Seedling emergence, survival and growth were followed for one year. Soil moisture was not significantly different among the opening sizes, but, within a size, tended to be lower when litter was removed. Light intensity at the soil surface was positively related to opening size early in the growing season, but later in the growing season reached a maximum in intermediate-sized openings and then leveled off.

Local movement in herbivorous insects: applying a passive diffusion model to mark-recapture field experiments.

Abstract: A simple passive diffusion model is used to analyze the local within-habitat dispersal of twelve species of herbivorous insects. The data comprise field mark-recapture studies in relatively homogeneous habitats. For eight of the species, the cumulative frequency distributions of dispersal distances are consistent with a model of movement by passive diffusion. The observed departures from passive diffusion indicate the directions in which we need to modify our mathematical descriptions of movement if we are to develop realistic models of population dynamics and dispersal. The analyses also synthesize in a standard way the relative dispersal rates of several ecologically similar species. The variation both within and between species in diffusion coefficients is striking-certainly sufficient to generate significant consequences for population dynamics and interactions.

Estimates of N2-fixation from variation in the natural abundance of 15N in Sonoran desert ecosystems.

Abstract: The 15N abundance of tissues of five Prosopis specimens at our primary study site (a Prosopis woodland at Harper's Well in the Sonoran desert of Southern California) was determined over two growing seasons 1980 and 1981. The 15N abundance of soil and of tissues of presumed non-N2-fixing (control) plants was also measured. Prosopis tissues were significantly lower in 15N than either soil N or corresponding tissues of presumed non-N2-fixing plants which derive their N entirely from soil. Soil N was also significantly higher in 15N than atmospheric N2. We conclude that it is feasible to use variations in the natural abundance of 15N as an index of N2-fixation in this kind of ecosystem, and that N2-fixation is of considerable importance to Prosopis growing at this site.We also determined the 15N abundance of leaf tissue of presumed N2-fixing and control plants growing at the same site at six additional sites (five in the Sonoran desert of southern California and one in Baja California, Mexico near the town of Catavina). Four of these additional sites were dominated by Prosopis and two were mixed communities. There were statistically significant differences between the 15N abundances of the pooled legume population and control plants at all sites, although not every legume specimen exhibited this difference. From 15N abundance data we estimated the fractional contribution of biologically fixed N to the N economy of desert legumes. We concluded that N2-fixation is very important to Prosopis at six of seven sites in the Sonoran Desert. At the site where Prosopis did not appear to be fixing N2, N2-fixation was important only for legumes of the sub-family Papilionoideae, Lupinus, Dalea, Astragalus and Lotus.

Wound induced defences in plants and their consequences for patterns of insect grazing.

Abstract: Three scales of wound-induced chemical responses in plants are identified: (1) highly localised chemical changes associated with disruption of cell compartmentation; (2) changes induced in cells surrounding the damaged area, forming a kind of halo around the damage, and (3) more widely-dispersed changes which may affect an entire organ, branch or plant. A brief review of the literature reveals that such chemical responses are very widespread in plants, and many of the substances formed are known to affect adversely the growth, development, or reproduction of insects. It is argued that wound-induced changes in plant chemistry represent for insects a powerful selective pressure for the dispersal of grazing. Levels and patterns of invertebrate grazing in a range of herbaceous and deciduous woody plants sampled at the end of the growing seasons were examined. Leaves of many species exhibited a strikingly evident over-dispersion of grazing initiations, and in some cases the arrangement of holes appeared close to regularity. The pattern of damage between leaves was, in most cases, heavily biased towards a large proportion of leaves receiving a low level of grazing. These highly dispersed patterns of grazing damage are consistent with the hypothesis that wound-induced responses play an important role in determining patterns of insect feeding. They have important implications for the expected levels of insect exploitation of host plants and for the advantages to the plant of distributing grazing damage evenly through the canopy.

Crassulacean acid metabolism in australian vascular epiphytes and some related species

Abstract: The occurrence of crassulacean acid metabolism (CAM) among epiphytes and related plant species from tropical and subtropical rainforests in Eastern Australia was investigated. As judged from δ13C value and the absence of Kranz anatomy, indications of CAM were found in 66 species belonging to the families, Polypodiaceae (3), Orchidaceae (55), Asclepiadaceae (6) and Rubiaceae (2).Two thirds of orchidaceous plants examined appeared to use CAM. Those species with thicker leaves generally had less negative δ13C values, as did those species growing on more exposed sites; leaves thicker than about 1 mm in most species yielded δ13C values indicative of pronounced CAM. Two leafless species, Chiloschista phyllorhiza and Taeniophyllum malianum, which depend on chloroplast-containing, stomata-less roots for photosynthesis also showed δ13C values typical of CAM. Pseudobulbs and swollen stems, a characteristic of many orchids, were usually somewhat enriched in 13C compared to corresponding leaves.In Polypodiaceae CAM was found in the genus Pyrrosia. While δ13C values were generally less negative with increasing frond thickness, the leaf morphology was extremely variable within species. Pyrrosia confluens plants from shaded habitats had long, relatively thin and darkgreen fronds whereas specimens from sun-exposed sites were characterized by short, thick, bleached fronds. Both types showed the capacity for nocturnal accumulation of titratable acidity and exhibited continuous net CO2 fixation during 12 h light/12 h dark cycles under laboratory conditions. Shade-fronds showed this capacity even when irradiance was lower than 2% of full sunlight during the 12 h light period.In Asclepiadaceae CAM was found in species of two genera which usually have fleshy leaves, Hoya and Dischidia. In Rubiaceae CAM was recorded in two genera of epiphytic ant plants, Hydnophytum and Myrmecodia.It is concluded that CAM is widespread in Australian epiphytes. It is most prevalent in species found in exposed microhabitats where the growing conditions are characterised by relatively high light intensities and short but frequent periods of water stress.

The effects of density dependent resource limitation on size of wild reindeer.

Abstract: A density-dependent decrement in size for wild reindeer from 12 different Norwegian herds at 16 different densities was shown using lower jawbone-length as the criterion of size. This criterion was tested and found to adequately predict body size of both bucks and does. Lactation in does did not affect jaw length but significantly affected dressed weights. A decrement in the size of does as a result of gross density was found. This size decrement was further analysed in relation to the habitat densities in winter (R 2=0.85) and in summer (R 2=0.75) separately, in order to estimate the relative effects of each factor. For herds with adequate food in winter (no signs of overgrazing of lichens) density in relation to summer habitat and mires yielded the highest predictive power in a multiple regression. For herds with adequate summer pastures, densities per winter habitat and lichen volumes showed likewise a highly significant correlation. The inclusion of the lichen volume data in the regression increased its predictive power. The major effect of resource limitation was to delay the time of calving because a maternal carry-over effect allowed the calf a shorter period of growth to be completed during its first summer. Neonate size at birth was highly correlated with maternal size regardless of the mean calving date although the latter was significantly delayed for small-sized does in food resource-limited herds. Likewise the postnatal growth rate of all calves were not significantly different during 50 days postpartum regardless of maternal conditions in winter feeding. The summer growth rates of bucks ≧1 year did not vary significantly between herds. The age of maturity of food resource-limited does was delayed by one year and growth ceased after the initiation of reproduction. This shows that under conditions of limited resources the does with delayed births of calves allocated less energy to body growth simply because they had less time to replenish body reserves once they were freed of the energetic demands of lactation. The overriding effects of such limitation of food resources is thus to produce a time-lag for the completition of all the important life-history events, such as growth, maintenance and reproduction. From a theoretical point of view, i.e. according to the reproductive effort model their only option is to try to overcome this time limitation to reproductive success.

Induction of host preference in Drosophila melanogaster.

Abstract: Individuals of Drosophila melanogaster were maintained as larvae and/or as adults on media made from one of the following natural breeding sties: apple, tomato, banana, or squash. The oviposition site preference of adults was not affected by the type of food they had developed on as larvae. However, prior exposure to either apple or tomato enhanced their preference for that food when subsequently given a choice between apple and tomato for oviposition. In a second experiment, adult flies were kept on media made from either apples, oranges, grapes, tomatoes, or onions; their oviposition site preferences were subsequently tested in a 5-choice situation. Prior exposure to a particular food did in some cases increase the acceptability of that food to flies. More remarkably, it sometimes modified the flies' responses to other foods, a phenomenon referred to here as cross-induction.

Algae-grazing minnows (Campostoma anomalum), piscivorous bass (Micropterus spp.), and the distribution of attached algae in a small prairie-margin stream

Abstract: Campostoma anomalum is an algae-grazing minnow, abundant in many streams of the central and eastern United States. In a small stream in south-central Oklahoma, Campostoma has a marked impact on standing crops of attached algae. Pools with schools of Campostoma are barren, while pools in which Campostoma are apparently excluded by bass (Micropterus salmoides or M. punctulatus) support large standing crops of filamentous green algae (predominantly Spirogyra sp. and Rhizoclonium sp.). Campostoma grazed actively on algae-covered cobbles transferred into their pools, and visibly reduced standing crops within one hour. After 24 h of exposure to Campostoma, standing crops of attached algae on cobbles were reduced from 22.0 to 6.3 mg ash-free dry weight cm-2. When a largemouth bass was tethered in a pool with Campostoma, the minnows did not graze on algae-covered cobbles within 30-50 cm of the bass, but fed actively on cobbles that were more than 1.3 m away. These results indicate that interactions of Campostoma and their predators may be an important factor contributing to pool-to-pool variation in attached algae in small streams of the central and eastern United States.

Effects of light regime on the growth, leaf morphology, and water relations of seedlings of two species of tropical trees

Abstract: An experiment was conducted with Heliocarpus appendiculatus, a pioneer or large gap species of tropical moist forest in Costa Rica, and Dipteryx panamensis, a small gap species. Seedlings were grown in full sun, partial (80%) shade, and full (98%) shade. After one month of growth they were switched between environments and grown for two more months.Growth in height of Heliocarpus was greatly affected by irradiance, being increased in response to full shade and decreased in full sun. Height of Dipteryx was unaffected by irradiance level. Survival of Heliocarpus seedlings was only 49% in full shade, whereas Dipteryx had 100% survival. Biomass of Heliocarpus was not significantly greater in full sun than in partial shade whereas it was for Dipteryx. The response of root: shoot ratio was similar for both species. They were lowest in full shade and highest in full sun. Heliocarpus exhibited greater changes in leaf thickness, specific leaf weight, and stomatal density than did Dipteryx. Stomatal conductance of both species was lower in full shade and full sun than in partial shade.The results of the experiment indicate that growth of Heliocarpus is more plastic than that of Dipteryx in response to changes in irradiance. Previous environment did not affect the response to the present environment in either species. Both species responded positively to increases in irradiance.

Plant-herbivore interactions in a North American mixed-grass prairie : II. Responses of bison to modification of vegetation by prairie dogs.

Abstract: Studies were conducted during the 1979 growing season to examine how North American bison (Bison bison) use prairie dog (Cynomys ludovicianus) colonies in Wind Cave National Park, South Dakota. Objectives included (1) determining whether bison selected for prairie dog towns parkwide; (2) characterizing in greater detail bison use patterns of a 36-ha colony in Pringle Valley as a function of time since prairie dog colonization; and (3) relating these bison use patterns to measured changes in structure and nutritional value of vegetation on and off the dog town.During midsummer, prairie dog towns were one of the most frequently used habitats by bison parkwide. Day-long observations at Pringle Valley revealed that bison exerted strong selection (nearly 90% of all habitat use and feeding time) for the dog town, which occupied only 39% of the valley. While there, they partitioned their use of the colony by grazing in moderately affected areas (occupied 26 years occupation).Prairie dogs facilitate bison habitat selection for a shortgrass successional stage in this mixed-grass community by causing a broad array of compositional, structural, and nutritional changes in the vegetation.

Are big mammals simply little mammals writ large

Abstract: Populations are regulated intrinsically (self-regulated) when the animals lower their rate of increase behaviorally or physiologically as a reaction to rising density They are regulated extrinsically if the equilibrium is a mechanical consequence of interaction between the population and the organisms providing its food We suggest that, at least for mammalian herbivores, self-regulation is unlikely to evolve unless the population's intrinsic rate of increase exceeds about 045 on a yearly basis That value corresponds to a body weight of about 30 kg, the intrinsic rate being related inversely to body weight by r m=15 W(-036) with W in kgThe two dynamic strategies, self-regulation and extrinsic regulation, should enforce a bimodality of the frequency distribution of observed intrinsic rates of increase This in turn might be reflected in a bimodality of body sizes, the smaller herbivores constituting the lower mode generally showing intrinsic regulation and the larger herbivores of the upper mode generally being regulated by extrinsic mechanisms There is some empirical support for these predictions but it is by no means clearcutMechanisms of self-regulation can evolve either by individual or group selection Individual selection may act in two ways By inhibiting their neighbours with some form of interference, individuals may increase their relative fitness without increasing their reproductive rate Alternatively, individual selection may raise the absolute fitness of individuals and thereby raise the populations's intrinsic rate of increase The population is destabilized if that process continues beyond a certain threshold and the population is then at significant risk of extinction at the troughs of the consequent oscillations Selection between such populations will favour those carrying the beginnings of a self-regulating mechanism, and with that mechanism strengthened and fixed by continuing group selection, individual selection is again freed of the dynamic restraints on raising further the intrinsic rate of increase

Ecophysiology of Amaranthus palmeri, a sonoran desert summer annual

Abstract: Basic photosynthetic and water relations characteristics in relation to soil water availability were investigated in Amaranthus palmeri, a common C4 summer annual of the Sonoran Desert Under conditions of high soil water availability, photosynthetic capacity exceeded 70 μmol CO2 m-2 s-1 at a leaf temperature optimum of 42°C, and photosynthesis was not light saturated at an irradiance of 20 mmol m-2 s-1 (400–700 nm) Leaves of A palmeri exhibited diaheliotropic movements, allowing them to take advantage of their high photosynthetic capacity In response to a long term drought cycle, both photosynthetic rate and leaf conductance to water vapor decreased, reaching minima at an approximate leaf water potential of-29 MPa Active leaf osmotic adjustment appeared to play an important role in allowing leaves to maintain gas exchange activities down to these low leaf water potentials The photosynthetic rate became light saturated at the lower leaf water potentials Although the two parameters decreased in concert, the decreased photosynthetic rate was not due to increased stomatal diffusion limitations, since intercellular CO2 concentrations remained constant over the range of leaf water potentials Instead it appeared that during the drought cycle, both intrinsic photosynthetic capacity and leaf protein content decreased as well These results suggested a coordination of gas exchange parameters during long term drought such that the intercellular CO2 concentration remained constant

Carbon dioxide exchange of C3 and C4 tree species in the understory of a Hawaiian forest.

Abstract: Field measurements of photosynthetic CO2 exchange were made on saplings of a C4 tree species, Euphorbia forbesii, and a C3 tree species, Claoxylon sandwicense, in a shaded mesic forest on Oahu, Hawaii. Both species had light responses typical of those generally found in shade plants. Light saturated photosynthetic rates were 7.15 and 4.09 μmol m2 s1 and light compensation points were 6.3 and 1.7 μmol m2 s1 in E. forbesii and C. sandwicense, respectively. E. forbesii maintained a higher mesophyll conductance and a higher water use efficiency than C. sandwicense as is typically found in comparisons of C4 and C3 plants. Under natural light regimes, both species maintained positive CO2 uptake rates over essentially the entire day because of low respiration rates and light compensation points. However, photosynthesis during sunflecks accounted for a large fraction of the daily carbon gain. The results show that the carbon-gaining capacity of E. forbesii is comparable to that of a C3 species in a moderately cool, shaded forest environment. There appears to be no particular advantage or disadvantage associated with the C4 photosynthetic pathway of E. forbesii in this environment.

The effect of physical disturbance on the relative abundances of two filter-feeding insects in a small stream.

Abstract: We examined the importance of disturbance in determining the relative abundances of two lotic filter-feeders, Simulium virgatum and Hydropsyche oslari, in a small, coastal stream in southern California, USA.

Diet shifts in moose due to predator avoidance.

Abstract: On Isle Royale, Michigan, moose cows which have calves show a different distribution and diet from solitary adults and yearling moose. Solitary adults and yearlings follow a feeding pattern predicted from the location of high nutrient plant growth. In the presence of wolves, they feed on the ridges of the main island, where they take advantage of plants that leaf early; only later in the season do they move to the small outlying islands where plant phenology is delayed by the cold water of Lake Superior. Cows with calves deviate sharply from this pattern. They remain on the wolf-free small islands throughout the growing season. While on the islands, cows with calves eat a poorer quality diet than other moose. They switch to eating high quality spring and summer foods later than other moose and they eat significantly fewer high preference shrubs and significantly more herbs and low preference shrubs than other moose. This study suggests that in order to avoid predators, cows with calves sacrifice the high quality diet available on the main island. These data also suggest that the wolves not only affect prey numbers by direct kills but may also indirectly influence prey numbers by altering the diet of their prey. In this case, the reproductive cows, those individuals that contribute most directly to growth of the population, avoid predators but frequent poor feeding areas.

Plant succession and gopher disturbance along an experimental gradient.

Abstract: A field was disturbed via disking, divided into 36 plots, and fertilized with different ratios of N:Mg in 1980. High N:Mg supply ratios tended to favor the perennial grasses Agrostis scabra and Agropyron repens over annual plants. In 1981 the experimental field was invaded by a fossorial mammalian herbivore, the plains pocket gopher, Geomys bursarius. Geomys bursarius was significantly more active in plots dominated by Agrostis scabra and Agropyron repens. The gopher mounds it created were dominated by the annual grass Setaria glauca and the annual herb Polygonum convolvulus. Thus the direct effect of resource competition among these plants was counteracted by the indirect response of an herbivore to the treatments. Although nitrogen fertilization tended to increase the rate of successional replacement of annuals by perennials, the indirect response of gophers to nitrogen tended to slow the rate of succession. This suggests that herbivore responses to spatial variation in plant community composition and productivity may be an important determinant of the rate of succession and the species diversity of the community.

Defoliation responses of western wheatgrass populations with diverse histories of prairie dog grazing.

Abstract: Photosynthesis and regrowth were compared over a 10-day period following defoliation of about 75% of the tillers of western wheatgrass (Agropyron smithii) plants collected from a black-tailed prairie dog (Cynomys ludovicianus) town and a grazing exclosure at Wind Cave National Park, South Dakota. Prior to defoliation, dog town plants had more tillers, but fewer leaves per tiller, shorter and narrower leaf blades, more horizontal leaves, and higher leaf blade/leaf sheath ratios than plants from the grazing exclosure. Rates of net photosynthesis (PN) did not differ significantly among plants of the two populations, either prior to or following defoliation. From Days 2–10 following defoliation, PN of remaining undamaged leaves averaged 104% of predefoliation rates while PN of similar leaves on non-defoliated plants declined steadily with time. averaging only 79% predefoliation rates during this period. Following defoliation, transpiration rates followed similar trends to CO2 exchange, and rates did not differ between plants of the two populations. Absolute rates of leaf elongation and shoot production were greater in plants from the exclosure. However, defoliation of plants from the exclosure population resulted in a 20% reduction in their cumulative shoot dry weight, while cumulative shoot dry weight of plants from the prairie dog town was not significantly affected by defoliation. This apparent ability of plants from the prairie dog town population to withstand defoliation better than plants from the exclosure was atributed to factors such as the higher leaf blade/leaf sheath ratio and more horizontal leaf angles of plants from the former population.

Bacteria as a natural source of food for adult fruit flies (Diptera: Tephritidae)

Abstract: Bacteria isolated from crops and stomachs of adult fruit flies feeding on mulberry fruit and leaf surfaces were cultured on agar and proved highly attractive to adult fruit flies and provided them with virtually a complete breeding diet. Diets of bacteria, sugar and water gave equal longevity and increased fecundity in Dacus tryoni (Froggatt) compared with the conventional diet of autolysed brewer's yeast, sugar and water. The type and abundance of leaf and fruit surface bacteria appears to be an important factor in the life history of tropical fruit flies.