Showing papers in "Oecologia in 1985"

Fruit characters as a basis of fruit choice and seed dispersal in a tropical forest vertebrate community.

Abstract: Interactions between a large community of vertebrate frugivore-granivores (including 7 species of large canopy birds, 19 species of rodents, 7 species of ruminants, and 6 species of monkeys), and 122 fruit species they consume, were studied for a year in a tropical rainforest in Gabon. The results show how morphological characters of fruits are involved in the choice and partitioning of the available fruit spectrum among consumer taxa. Despite an outstanding lack of specificity between fruit and consumer species, consideration of simple morphological traits of fruits reveals broad character syndromes associated with different consumer taxa. Competition between distantly related taxa that feed at the same height is far more important than has been previously supposed. The results also suggest how fruit characters could have evolved under consumer pressure as a result of consumer roles as dispersers or seed predators. Our analyses of dispersal syndromes show that fruit species partitioning occurs more between mammal taxa than between mammals and birds. There is thus a bird-monkey syndrome and a ruminant-rodent-elephant syndrome. The bird-monkey syndrome includes fruit species on which there is no pre-dispersal seed predation. These fruits (berries and drupes) are brightly colored, have a succulent pulp or arillate seeds, and no protective seed cover. The ruminant-rodent-elephant syndrome includes species for which there is pre-dispersal predation. These fruits (all drupes) are large, dull-colored, and have a dry fibrous flesh and well-protected seeds.

Gradients in density variations of small rodents: the importance of latitude and snow cover

Abstract: Microtine rodents are known to show extreme population variations (cycles) but non-cyclic populations have also been recognized during recent years. The cyclic populations have been widely thought to be regulated by intrinsic mechanisms. However, such predictions for cyclic populations are usually not applicable to non-cyclic ones and extrinsic factors may have to be included in any explanation.A hypothesis that the degree of fluctuations in small rodent numbers is related to the sustainable number of generalist predators was tested on mainly literature data by computing "indices of cyclicity" for local populations. These indices were related to latitude and snow cover (two measures) as these variables will affect the amount of alternative prey available for these generalists. Within Fennoscandia such indices for Clethrionomys glareolus and Microtus agrestis were clearly positively related to latitude and snow cover. The fraction of populations with summer declines in numbers, characterizing highly cyclic populations, increased in the same way. Cyclicity indices in Great Britain were similar to those in southern Fennoscandia, both areas being poor in snow, but were higher at the same latitudes in eastern Europe with more snow. Indices of density variations were generally low in North American Clethrionomys species and very variable in Microtus species.The gradients observed and differences between continents are interpreted as due to microtine-vegetation interactions in northern European areas poor in generalist predators but with important small mustelid predation, and to similar snowshoe hare-vegetation interactions in mainly Canada-Alaska, where small rodents may serve as alternative prey for numerically fluctuating hare predators, at least in the forests. Western European microtine populations, and probably many others, seem to be regulated by generalist predators.

The responses of stomata and leaf gas exchange to vapour pressure deficits and soil water content : II. In the mesophytic herbaceous species Helianthus annuus

Abstract: The responses of leaf water potential, leaf conductance, transpiration rate and net photosynthetic rate to vapour pressure deficits varying from 10 to 30 Pa kPa-1 were followed in Helianthus annuus as the extractable soil water decreased. With a vapour pressure deficit of 25 Pa kPa-1 around the entire plant as the soil water content decreased, the leaf conductance and transpiration rate showed a strong closing response to leaf water potential at a value of-0.65 MPa, whereas with a vapour pressure deficit of 10 Pa kPa-1 around the entire plant, the rate of transpiration and leaf conductance decreased almost linearly as the leaf water potential decreased from-0.4 to-1.0 MPa. Increasing the vapour pressure deficit from 10 to 30 Pa kPa-1 in 5 Pa kPa-1 steps decreased the leaf conductance by a similar proportion at all extractable soil water contents. At high soil water contents, the decrease in conductance with leaf water potential was greater when the vapour pressure deficit was increased than when it was not, indicating a direct influence of vapour pressure deficit on the stomata. The rate of net photosynthesis decreased to a smaller degree than the leaf conductance when the vapour pressure deficit around the leaf was varied. Overall, the net photosynthetic rate decreased almost linearly from 20 to 25 μmol m-2 s-1 at-0.3 MPa to 5 μmol m-2 s-1 at-1.2 MPa. As the soil water decreased, the internal carbon dioxide partial pressure was maintained between 14 and 25 Pa.No unique relationship between leaf conductance, transpiration rate or photosynthetic rate and leaf water potential was observed, but in all experiments leaf conductance and the rate of net photosynthesis decreased when about two-thirds of the extractable water in the solid had been utilized irrespective of the leaf water potential. We conclude that soil water status, not leaf water status, affects the stomatal behaviour and photosynthesis of H. annuus.

Carnivore body size: Ecological and taxonomic correlates.

Abstract: Variation in body size (weight) is examined across the order Carnivora in relation to taxonomy (phylogeny), latitude, habitat, zonation, activity cycle, diet, prey size, and prey diversity. Significant differences in body weight are observed with respect to family membership. Some of these differences may be explained by phylogenetic history and/or dietary effects. Body weight is not correlated with habitat, zonation, activity cycle or latitudinal gradients. Significant differences in body weight are found among insectivorous, herbivorous and carnivorous species, and some of these differences may relate to energetic constraints. Among predatory carnivores, prey size and diversity increases with body weight. The adaptive significance, both intra- and inter-specifically, of prey characteristics (size, availability, diversity) and carnivore body weight qualities (strength, endurance, hunting technique) is discussed.

Fine root turnover in forest ecosystems in relation to quantity and form of nitrogen availability: a comparison of two methods.

Abstract: Two methods of estimating fine root production and turnover are compared for 13 forest ecosystems exhibiting a wide range in form (NH4+ vs. NO3-) and quantity of available nitrogen. The two methods are by comparison of seasonal maxima and minima in biomess and by nitrogen budgeting. Both methods give similar results for stands with low rates of nitrification. The budgeting method predicts higher fine root turnover and productivity than the max-min method for systems with significant rates of nitrification.

Canopy transpiration and water fluxes in the xylem of the trunk of Larix and Picea trees - a comparison of xylem flow, porometer and cuvette measurements

Abstract: Leaf gas exchange, transpiration, water potential and xylem water flow measurements were used in order to investigate the daily water balance of intact, naturally growing, adult Larix and Picea trees without major injury. The total daily water use of the tree was very similar when measured as xylem water flow at breast height or at the trunk top below the shade branches, or as canopy transpiration by a porometer or gas exchange chamber at different crown positions. The average canopy transpiration is about 12% lower than the transpiration of a single twig in the sun crown of Larix and Picea. Despite the similarity in daily total water flows there are larger differences in the actual daily course. Transpiration started 2 to 3 h earlier than the xylem water flow and decreased at noon before the maximum xylem water flow was reached, and stopped in the evening 2 to 3 h earlier than the water flow though the stem. The daily course of the xylem water flow was very similar at the trunk base and top below the lowest branches with shade needles. The difference in water efflux from the crown via transpiration and the water influx from the trunk is caused by the use of stored water. The specific capacitance of the crown wood was estimated to be 4.7 x 10-8 and 6.3 x 10-8 kg kg-1 Pa-1 and the total amount of available water storage was 17.8 and 8.7 kg, which is 24% and 14% of the total daily transpiration in Larix and Picea respectively. Very little water was used from the main tree trunk. With increasing transpiration and use of stored water from wood in the crown, the water potential in the foliage decreases. Plant water status recovers with the decrease of transpiration and the refilling of the water storage sites. The liquid flow conductance in the trunk was 0.45 x 10-9 and 0.36 x 10-9 mol m-2s-1 Pa-1 in Larix and Picea respectively. The role of stomata and their control by environmental and internal plant factors is discussed.

Population dynamics of the ribbed mussel, Geukensia demissa: The costs and benefits of an aggregated distribution.

Abstract: Although mussel beds are common in many intertidal habitats, the ecological significance of the aggregated distribution of mussels has not been examined. The ribbed mussel, Geukensia demissa, is found in dense aggregations on the seaward margin of many salt marshes in New England. Here, we examine the population structure of G. demissa in a New England salt marsh and investigate experimentally the costs and benefits of aggregation.

Spatial variation in larval concentrations as a cause of spatial variation in settlement for the barnacle, Balanus glandula.

Abstract: Settlement rates of the high intertidal barnacle, Balanus glandula, were monitored at three sites in the rocky intertidal zone in Central California simultaneously with measurements of larval concentrations in the adjacent water column. In both 1983 and 1984, settlement rates onto vacant substrate differed among the sites by nearly two orders of magnitude. For all sampling dates, this spatial variation in settlement mirrored the spatial distribution of Balanus glandula cyprid concentration in the water column. A perfect rank correlation was found between cyprid concentrations near a site and subsequent settlement. A noteworthy observation was that the sites switched rank in their settlement rates from 1983 to 1984. This change in settlement rankings matched a switch in rankings for cyprid concentrations. Settlement itself appears to be an important cause of the spatial pattern of cyprid concentrations. Comparing the rates of settlement to estimates of the number of cyprids available at a site suggests that settlement causes a large drain on the cyprid population as a water mass passes over successive sites. No consistent spatial patterns were found in the distribution of other major plankton groups (calanoid copepods) that are similar in size to Balanus cyprids but do not settle. The large differences in settlement rates among these sites were previously shown to be a leading cause of large differences in the structure of benthic barnacle populations. The close correspondence shown here between these large differences in settlement and differences in larval concentrations suggests that nearshore oceanic processes affecting larval arrival contribute to the control of benthic community structure.

Maximizing feeding efficiency and minimizing time exposed to predators: a trade-off in the black-capped chickadee.

Abstract: Animals often must feed away from protective cover, sometimes at a considerable risk of being preyed upon. Feeding at the maximum rate while away from cover may simultaneously minimize the time spent exposed to predators, but this is not always the case. Under some circumstances, carrying prey items to protective cover before they are consumed will minimize the time spent exposed to predators, whereas feeding at maximum efficiency (staying to eat prey where they are found) will actually increase the time spent exposed to predators. Whether or not there is a conflict between maximizing foraging efficiency and minimizing exposure time, depends upon the travel time to cover relative to the handling time of a prey item; short handling times and/or long travel times are associated with the no-conflict situation, whereas the conflict situation is associated with long handling times and/or short travel times to cover. Free-ranging chickadees foraging at an artificial patch at various distances from cover can distinguish between these two foraging situations. When there is no conflict, they stay and eat at the patch. Their behavior in the conflict situation indicates that they are tradingoff foraging considerations against the risk of predation. When the cost of carrying is low and the benefit gained is high, the chickadees elect to carry items to cover; they tend to stay and eat at the patch when the relative magnitudes of costs and benefits are reversed.

The role of habitat in avian community composition: physiognomy or floristics?

Abstract: It has been proposed that within rather broad habitat types the distribution and abundance of bird species may be more closely associated with plant taxonomic composition than with the structure and configuration of the vegetation. Birds from a sample of eight representative grassland habitats in middle and western North America are consistent with this hypothesis. Over half (55%) of the variation in bird community composition was associated with floristic variation, but only a third (35%) was associated with physiognomy. Separating the interacting effects of floristics and physiognomy from each other served to accentuate the difference between them with respect to the avifauna. It is postulated that bird species/plant taxa associations, especially within similar habitat types, are mediated by the specific food resources that different plant taxa provide. Summary indices such as diversity measures obscure the taxonomic information content of plant or animal assemblages, and the use of such indices has likely impeded detection of the relationships described here.

Condensed tannins deter feeding by browsing ruminants in a South African savanna

Abstract: The palatability of 14 species of woody plant was assessed for three species of browsing ruminant, namely kudus, impalas and goats. Results show that palatability was most clearly related to leaf contents of condensed tannins. The effect was a threshold one, with all plants containing more than 5% condensed tannins being rejected as food during the wet season period. In contrast palatability was not influenced by concentrations of protein-precipitating polyphenols, and only weakly related to contents of nitrogen, phosphorus, cations, fibre components and other secondary metabolites. Insect herbivory shows a different pattern. These findings support the hypotheses that (i) condensed tannins function to protect plant cell walls against microbial attack; (ii) hydrolyzable tannins function to inactivate the digestive enzymes of insect herbivores. Large mammalian herbivores are influenced by condensed tannins due to their dependance upon microbial fermentation of plant cell walls for part of their energy needs.

Community and population dynamics of serpentine grassland annuals in relation to gopher disturbance.

Abstract: This study examines the effects of soil disturbance by gophers on patterns of species abundance in an annual grassland community on serpentine soil. We assessed production, dispersal and storage of seed, germination, survivorship and growth of the most abundant species in undisturbed vegetation and on gopher mounds. Fewer seeds of the dominant species were dispersed onto gopher mounds due to the limited movement of seeds from within the closed vegetation. Species with taller flowering stalks were more likely to colonise gopher mounds. The timing of gopher disturbance in relation to the timing of seed fall determined which species could colonise mounds. Lower numbers of seeds falling onto gopher mounds resulted in lower seedling densities of several species compared with undisturbed areas. Survivorship of the commonest species differed between undisturbed areas and gopher mounds formed at different times of year. This resulted in characteristic spectra of species abundance on the different microhabitats, giving rise to distinct spatial patterning in the community. Plants growing on gopher mounds were generally larger and produced more seed than plants in undisturbed vegetation. We suggest that continued gopher disturbance is a factor allowing several species, including perennial grasses, to persist in this community.

Use of nest material as insecticidal and anti-pathogenic agents by the European Starling.

Abstract: Passerine birds that reuse nest sites face an increased parasite and pathogen load. They also are more likely to use fresh green vegetation during nest construction. The present results demonstrate that at least one passerine, the European Starling: (a) selects a small subset of available plant species for inclusion in nest material; and (b) chooses plants whose volatiles are more likely to inhibit arthropod hatching and bacterial growth relative to a random subset of available vegetation. The results also show that preferred plants possess greater numbers of mono- and sesqueter-penes at higher concentrations relative to a random subset of available plants. These findings strongly suggest that starlings use chemicals in fresh vegetation as fumigants against parasites and pathogens.

Differences in chemical composition of plants grown at constant relative growth rates with stable mineral nutrition

Abstract: Leaf chemistry of a willow clone (Salix aquatica Smith) differed significantly when grown at constant relative growth rates depending upon the relative availability of nutrients and light Concentration of amino acids and nitrate were high in plants grown with a relative surplus of nutrients Concentrations of starch, tannin, and lignin, on the other hand, were high in plants grown with a relative surplus of carbon Photosynthetic rates, expressed per unit leaf area, were similar when plants were grown under high light conditions, regardless of nutrient availability Dark respiration was much higher in plants supplied with abundant nutrients than in those with a more limited supply, reflecting differences in nitrogen concentration of the tissue The experimental approach allows plants to be grown to a standard size with differing, but highly uniform chemistry Plants grown in such a manner may provide good experimental material to evaluate interactions between herbivores or pathogens and their hosts

Neighborhood predictors of plant performance.

Abstract: We developed models of inter-individual interference to predict the fecundity of individuals in populations of the annual plant species, Arabidopsis thaliana. An individual plant is modeled as having a neighborhood which is a circular area of fixed radius with the plant at its center. Other plants which share the circle with the focal plant are termed neighbors of the focal plant. We developed an index of neighborhood interference which is the independent variable in a non-linear regression model that predicts individual plant fecundity. We present methods of exploratory data analysis that are useful in determining a best neighborhood radius, defined as that radius which minimizes residual sum of squares, and in deciding on the functional form of the interference index. In developing the interference index for Arabidopsis, we focus on aspects of the spatial distribution of neighbors: their number, distance and angular dispersion. We found that a best (or optimal) neighborhood radius can be resolved, which provides the best predictor of plant performance. Fecundity predictors based on adult neighbors were noticeably better than those based on neighbors at the seedling stage. Rosettes of Arabidopsis may change location during development (they fall over) and the new “fallen” positions do provide some improvement in the predictor. Taking into account distance to neighbors within the neighborhood provided only negligible improvement in the model. Finally, the incorporation of angular dispersion in the crowding index produced a considerably better fit. The fecundity predictor that included number of neighbors and angular dispersion in the crowding index explained about 70% of the variation in individual seed set.

Moose-wolf dynamics and the natural regulation of moose populations.

Abstract: In southwestern Quebec, non-harvested moose populations stabilize at a density of ≃0.40 animal·km-2. In an attempt to test whether or not moose were regulated by predators, we investigated wolf predation near this equilibrium density (0.37) and at 2 lower densities (≃0.23, 0.17). Scat analysis in summer and feeding observations in winter indicated a greater use of alternative food resources by wolves at lower moose densities. Each wolf pack killed on average 5.3, 1.8, 1.1 moose·100 days in the area of 0.37, 0.23, and 0.17 moose·km-2, respectively. Consumption of moose per wolf was 2.8, 1.7, and 1.6 kg/day, respectively. January wolf densities were estimated at 1.38, 0.82, and 0.36 animals·100 km-2, respectively. Year-long predation rates proved to be density-dependent, increasing with moose density from 6.1 to 19.3% of the postnatal populations. We conclude that moose populations in southwestern Quebec are regulated largely by predators (wolves and maybe black bears) at a density where competition for forage produces no detrimental effect. We support the concept that wolf predation can have an important regulatory effect at low moose densities but also a depensatory (inversely density-dependent) effect at higher densities.

Experimental disturbance and the maintenance of species diversity in a stream community.

Abstract: In order to test the role of disturbance and the effects of disturbance frequency on stream communities, an experiment was conducted in New Hope Creek, North Carolina, USA. Patches of cobbles were tumbled 0, 1 or 2 times in a 6 week span. These tumbling disturbances lasted only 30 seconds. The recovery of the macroinvertebrates was monitored.

Drastic effects of El Niño on Galapagos pinnipeds.

Abstract: Population dynamics of pinnipeds living in the tropical upwelling ecosystem of the Galapagos were strongly influenced by the 1982–83 Southern Oscillation-El Nino (EN) event which was the strongest recorded in this century. The Galapagos fur seal (Arctocephalus galapagoensis) population lost the four youngest year classes (1980–1983) almost entirely and approximately 30% of the adult females and non-territorial males. Mortality of large territorial males was almost 100%. Most of the 1982 year class of Galapagos sea lions (Zalophus californianus wollebaeki) died and there was a much lower pup production in the breeding season following EN. Recurrent EN events must strongly influence age structure and average population size of these and other otariid species depending on tropical upwelling ecosystems.

The importance of contaminated food for the uptake of heavy metals by rainbow trout (Salmo gairdneri): a field study.

Abstract: 1. The concentrations of Zn, Cu, Cd, Pb, Cr, Ni, and Mn have been measured in water, sediments, and organisms from two contaminated rivers situated in an industrial area and near a motorway on the southern city boundary of Bolzano, Italy. 2. The low metal contents of water are in contrast to the elevated concentrations of sediments indicating a chronic rather than an acute contamination of the rivers. 3. The metal concentrations of the dominant moss Fontinalis antipyretrica are higher than those of sediments, the lower branches of the plant being more concentrated than its upper parts. 4. Among benthic invertebrates the dominant animals are the isopod Asellus aquaticus and the snail Lymnaea truncatula. Both species feed on contaminated moss or detritus and accumulate the heavy metals to an even greater extent than the water plants. 5. The dominant predator is the rainbow trout (Salmo gairdneri) which feeds mainly on metal enriched isopods and snails. 6. As a consequence, there is an absorption of heavy metals from lumen of fish into the gut tissue and a flux to other organs such as liver, kidney, and muscle. 7. Since the metal concentrations of water are low it is suggested that the absorption through the gills of fish may be of secondary importance compared with the excessive supply through the food.

Higher survival of an aposematic than of a cryptic form of a distasteful bug

Abstract: An experiment was performed to assess the relative survival of two forms of 5th instar larvae of Lygaeus equestris (Heteroptera, Lygaeidae) — the normal red form, called aposematic, and a mutant grey form, called cryptic — when given to hand-raised great tits (Parus major). Sixteen birds were presented with aposematic larvae and 16 were presented with cryptic larvae in 10 consecutive trials. One attack per trial was allowed. Both larval forms were presented against a background matching the grey larvae, but since both prey types were presented in a specific place known to the predator, detection rate for both was assumed to be unity. Birds learned to avoid both prey types. However, the survival of the aposematic larvae was higher than that of the cryptic ones due to three aspects of predator behaviour: i) a greater initial reluctance to attack, ii) a more rapid avoidance learning, and iii) a lower frequency of killing in an attack, when the prey was aposematic. Moreover, a greater number of birds learned to avoid prey without killing any individual, when the prey was aposematic. This result is considered to be due to prey coloration alone, since, in a separate test, no difference in prey distastefulness could be detected. This experiment shows that individual prey can benefit from being aposematic and indicates that individual selection can be a sufficient explanation for the evolution of aposematic coloration. It was concluded that, since the survivorship was 6.4 times higher for the aposematic prey, it could have a detection rate that is correspondingly higher than the cryptic in order for the two forms to have equal fitness.

Body weights and fat deposition of Palaearctic passerine migrants in the central Sahara.

Abstract: 1. Data on body weight, fat score and length of stopover of Palaearctic passerine migrants trapped at two sites in the central Algerian Sahara in autumn 1983 are presented. 2. Birds found grounded in oases were in good condition. They were not critically short of fat reserves. 3. Some of the migrant species found there have probably terminated their migration whereas other species do only stopover for fattening and do in fact continue their migration if they have restored sufficient fat for a further flight. 4. The daily activity pattern of caged migrants depended on their fat reserves. 5. The available food supplies were high enough for fattening.

Phenolic compounds of willow bark as deterrents against feeding by mountain hare.

Abstract: Overwintering mountain hares (Lepus timidus) fed selectively on the shoots of a number of northern willow (Salix spp.) species. The hares preferred certain species over others and mature shoots over juvenile ones. There was a negative correlation between the phenolic glycoside concentration in the twigs and the hares' feeding. This correlation was substantiated by feeding experiments in which oat grains treated with purified phenoglycoside and catechin fractions of willow bark were offered along with untreated control oat grains to free-ranging mountain hares. Both fractions in concentrations normally found in willow twigs inhibited hare feeding. The results suggest that these phenolic compounds play a crucial role in the building up of resistance patterns among the willows. The decline in resistance in mature shoots of tall willows indicates that the juvenile resistance can be, perhaps secondarily, an adaptation against mammals browsing from ground level. Accordingly, low willow species retain a high level of resistance also in maturity.

Neighbors ameliorate local salinity stress for a rhizomatous plant in a heterogeneous environment

Abstract: The selection pressures experienced by clonal plants in heterogeneous environments may be significantly affected by physiological integration among ramets via rhizome connections. We experimentally examined how connections affected the response to saline soil conditions in Ambrosia psilostachya clones from natural saline basins in eastern Nebraska. Paired stems connected by uniform lengths of rhizome were grown in partitioned pots in 3 watering regimes: (1) both stems watered with tapwater, (2) both stems watered with salt water (1% NaCl), and (3) one stem watered with salt water and one with tapwater. All plants survived and grew in salt water, yet dry weight gain of salt-salt plants was only 34% of that for plants in uniform tapwater. Salt plants connected to tapwater plants had 2-fold higher dry weight gain than salt-salt plants. Their tapwater neighbors had significantly smaller biomass than pairs with both stems growing in tapwater. Measurements of leaf stomatal conductance, transpiration rate and water potential, together with root-shoot allocation patterns, suggest that rhizomes transported both water and photosynthate from tapwater plants to their neighbors in saline soil. These results indicate that ramets in a locally inferior environment can be helped by their neighbors, but at some cost to the contributing ramet. We discuss the consequences of this phenomenon for the evolution of local adaptation in populations of rhizomatous plants.

The responses of stomata and leaf gas exchange to vapour pressure deficits and soil water content : III. In the sclerophyllous woody species Nerium oleander.

Abstract: The responses of leaf conductance, leaf water potential and rates of transpiration and net photosynthesis at different vapour pressure deficits ranging from 10 to 30 Pa kPa-1 were followed in the sclerophyllous woody shrub Nerium oleander L. as the extractable soil water content decreased. When the vapour pressure deficit around a plant was kept constant at 25 Pa kPa-1 as the soil water content decreased, the leaf conductance and transpiration rate showed a marked closing response to leaf water potential at-1.1 to-1.2 MPa, whereas when the vapour pressure deficit around the plant was kept constant at 10 Pa kPa-1, leaf conductance decreased almost linearly from-0.4 to-1.1 MPa. Increasing the vapour pressure deficit from 10 to 30 Pa kPa-1 in 5 Pa kPa-1 steps, decreased leaf conductance at all exchangeable soil water contents. Changing the leaf water potential in a single leaf by exposing the remainder of the plant to a high rate of transpiration decreased the water potential of that leaf, but did not influence leaf conductance when the soil water content was high. As the soil water content was decreased, leaf conductances and photosynthetic rates were higher at equal levels of water potential when the decrease in potential was caused by short-term increases in transpiration than when the potential was decreased by soil drying.As the soil dried and the stomata closed, the rate of photosynthesis decreased with a decrease in the internal carbon dioxide partial pressure, but neither the net photosynthetic rate nor the internal CO2 partial pressure were affected by low water potentials resulting from short-term increases in the rate of transpiration. Leaf conductance, transpiration rate and net photosynthetic rate showed no unique relationship to leaf water potential, but in all experiments the leaf gas exchange decreased when about one half of the extractable soil water had been utilized. We conclude that soil water status rather than leaf water status controls leaf gas exchange in N. oleander.

Phenolic glycosides govern the food selection pattern of willow feeding leaf beetles.

Abstract: The relationship between the food selection of four leaf beetle species (Phratora vitellinae, Plagiodera versicolora, Lochmaea capreae, Galerucella lineola) and the phenolic glycosides of willow (Salix spp.) leaves was tested in laboratory food choice experiments. Four willow species native to the study area (Eastern Finland) and four introduced, cultivated willows were tested. The willow species exhibited profound differences in their phenolic glycoside composition and total concentration. The food selection patterns of the leaf beetles followed closely the phenolic glycoside spectra of the willow species. Both the total amount and the composition of phenolic glycosides affected the feeding by the beetles. Phenolic glycosides apparently have both stimulatory and inhibitory influences on leaf beetle feeding depending on the degree of adaptation of a particular insect. Very rare glycosides or exceptional combination of several glycoside types seem to provide certain willow species with high level of resistance against most herbivorous insects. Analogously the average absolute amount of leaf beetle feeding was lower on the introduced willows than on the native species to which the local herbivores have a good opportunity to become adapted.

Nutrient content of Abutilon theophrasti seeds and the competitive ability of the resulting plants.

Abstract: Siblings of Abutilon theophrasti, were grown on a nutrient gradient. The plants grown at higher nutrient levels were larger and produced larger and more seeds than plants grown at lower soil nutrient concentrations. There were no differences in germinability of seeds, but the competitive abilities of resulting plants were markedly different. In two different competition experiments designed to eliminate the effects of genotype, seed size, and germination time, by using synchronously germinated seedlings derived from similar size seed from plants grown at different nutrient levels, we found that plants from seeds produced at higher nutrient levels consistently, outperformed plants from seeds produced at the lower nutrient levels. The dominance of seeds produced at higher nutrient levels may be explained by the fact that they had markedly higher concentrations of nitrogen than did seeds produced at lower soil nutrient levels. The additional advantage of increased seed quality to plants controlling more of the nutrient resource than their neighbors would be expected to accelerate their contributions to the gene pool of the population.

Population regulation of Serengeti Wildebeeest: a test of the food hypothesis

Abstract: The food hypothesis proposes that density dependent mortality regulates populations through food shortage. For Serengeti wildebeest, we found an empirical relationship between dry season adult mortality rate, density and food supply. This relationship predicted that: (1) the population would stabilize between 1.0 and 1.5 million animals, (2) dry season mortality would be density dependent and sufficient to account for the levelling off of this large ungulate population. Recent observations have tested and confirmed these predictions.

δ13C-variations of leaves in forests as an indication of reassimilated CO2 from the soil

Abstract: An attempt has been made to evaluate the contribution of soil respired CO2 to the total assimilation of a forest tree, by heeding the 13C-concentrations of CO2 from the free atmosphere and from mineralization processes within the soil respectively. An expression has been derived, according to which the assimilated fraction of CO2 from the soil at a particular height of a tree is given by the δ13C-value of the corresponding leaves, δ13C of atmospheric CO2, δ13C of soil respired CO2 and the physiological state of the leaves expressed as the ratio of total respiration over gross photosynthesis and internal over external CO2-concentration. In the particular case investigated, a δ13C-difference of 5‰ has been determined from bottom to top of a beech tree which results in a CO2 contribution from the soil of about 22% for the lower forest strata, while the total contribution of soil respired CO2 accounts for about 5% of the overall assimilation.

Empirical relationships between predator and prey size among terrestrial vertebrate predators.

Abstract: In an effort to make complex food web relations more tractable, published data on the food habits of terrestrial vertebrate predators were analyzed for patterns in the use of prey by size. Regressions of prey weight on predator weight were run and provided descriptions of both the relationship between mean prey weight and predator weight and the variation in prey weights taken by the predators. Separate models proved necessary for three trophic specializations: insectivores, piscivores and carnivores. Insectivores were found to take proportionately much smaller prey than carnivores. Mean prey weight tends to decrease slightly relative to predator weight among larger insectivores, while the ratio of prey weight to predator weight tends to increase with carnivore size. On average, insectivores also take a relatively wider range, of prey sizes than carnivores. In all respects, piscivores were intermediate to insectivores and carnivores. These models produce log-normal approximations to the frequency distributions (by weight) of prey sizes in the ration of predators from knowledge of the predators's body mass. Combined with allometric models of the bionergetics and productivity of animals, these relations are used to predict, that: (1) the daily kill rate declines with predator weight and (2) the upper limit to predator biomass is independent of predator weight.

Physiological influences on carbon isotope discrimination in huon pine (Lagarostrobos franklinii).

Abstract: Measurements of the light environment and stomatal and photosynthetic behaviour are reported for Huon Pine (Lagarostrobos franklinii, family Podocarpaceae) in western Tasmanian rainforest. For a variety of microenvironments, these are used in an analysis of stable carbon isotope measurements in the air, and in branch and leaf material, using a model for carbon isotope fractionation in leaves (Farquhar et al. 1982).