Showing papers in "Oecologia in 1987"
TL;DR: Redeployment of leaf nitrogen to the top of the canopy with ageing should be more effective in increasing total canopy photosynthesis in a stand with a dense canopy than in aStand with an open canopy.
Abstract: A model of daily canopy photosynthesis was constructed taking light and leaf nitrogen distribution in the canopy into consideration. It was applied to a canopy of Solidago altissima. Both irradiance and nitrogen concentration per unit leaf area decreased exponentially with increasing cumulative leaf area from the top of the canopy. The photosynthetic capacity of a single leaf was evaluated in relation to irradiance and nitrogen concentration. By integration, daily canopy photosynthesis was calculated for various canopy architectures and nitrogen allocation patterns. The optimal pattern of nitrogen distribution that maximizes the canopy photosynthesis was determined. Actual distribution of leaf nitrogen in the canopy was more uniform than the optimal one, but it realized over 20% more photosynthesis than that under uniform distribution and 4.7% less photosynthesis than that under the optimal distribution. Redeployment of leaf nitrogen to the top of the canopy with ageing should be more effective in increasing total canopy photosynthesis in a stand with a dense canopy than in a stand with an open canopy.
692 citations
TL;DR: The soil water potential data and experimental results suggest that at night water absorbed from moist soil by deeper roots is transported to and lost from roots into drier upper soil layers.
Abstract: Diel soil water potential fluctuations reflected daytime depletion and nocturnal resupply of water in upper soil layers. Transpiration suppression experiments demonstrated that water absorption by roots caused the daytime depletion. The soil water potential data and experimental results suggest that at night water absorbed from moist soil by deeper roots is transported to and lost from roots into drier upper soil layers. The deeper roots appear to absorb and transport water both day and night. Implications for the efficiency of deep roots and water storage, nutrient uptake and water parasitism in upper soil layers are discussed.
680 citations
TL;DR: It is concluded that future investigations on transference of heavy metals to fish must take into more careful consideration the specific ecological situation of a given environment.
Abstract: 1. The uptake of heavy metals via the alimentary tract can be an important factor for the metal budget of fish. 2. Concepts such as biomagnification, bioaccumulation, biotransference, or concentration factors, convey little information about the real threat originating from heavy metals in an aquatic food chain. 3. In polluted aquatic ecosystems the transfer of metals through food chains can be high enough to bring about harmful concentrations in the tissues of fish. This relationship is called the food chain effect. 4. Two kinds of ecological factors influence the food chain effect: firstly, high levels of contamination of the food, and, secondly, the reduction of species diversity. When susceptible species are eliminated, metal-tolerant food organisms may become dominant. Their tolerance may be based either on their ability to accumulate excessive amounts of metals or to exclude heavy metals from the tissues. These two strategies represent feedback mechanisms which may enhance or weaken the food chain effect. 5. It is concluded that future investigations on transference of heavy metals to fish must take into more careful consideration the specific ecological situation of a given environment.
371 citations
TL;DR: Environmental effects on the isotopic composition of plants will extend upwards to the animals and humans they support and have important consequences for the use of nitrogen isotope data in the study of the dietary habits and trophic structures of modern and prehistoric communities.
Abstract: Data are presented for the 15N/14N ratios of 140 indigenous terrestrial plants from a wide variety of natural habitats in South Africa and Namibia. Over much of the area, from high-rainfall mountains to arid deserts, the δ 15N values of plants lie typically in the range -1 to +6‰; with no evident differences between C3 plants and C4 grasses. There is a slight correlation between δ 15N and aridity, but this is less marked than the correlation between the δ 15N values of animal bones and aridity. At coastal or saline sites, however, the mean δ 15N values for plants are higher than those at nearby inland or non-saline sites-e.g.: arid Namib coast (10‰ higher than inland Namib); wet Natal beach (5‰ higher than inland Natal); saline soils 500 km from coast (4‰ higher than non-saline soils). High values were also found at one site where there were no marked coastal or saline influences. These environmental effects on the isotopic composition of plants will extend upwards to the animals and humans they support. They therefore have important consequences for the use of nitrogen isotope data in the study of the dietary habits and trophic structures of modern and prehistoric communities.
369 citations
TL;DR: PERS was found to be inextricably linked to outcrossing plants, whose breeding systems promote genetic variability, and the traditional concept of a positive growth regulator feedback system based on the number of surviving ovules in an ovary can account for subsequent fruit survivorship.
Abstract: Reproductive success is divided into two phases: preemergent (the number of viable seeds that enter the ambient environment) and postemergent (the percentage of progeny that survive to reproduce). We studied preemergent reproductive success (PERS) in flowering plants by measuring the fruit/flower (Fr/Fl) ratio and the seed/ovule (S/O) ratio in a number of species of outcrossing and inbreeding plants, where PERS=the product of (Fr/Fl) and (S/O). In order to determine the influence of the ambient environment (including resource availability) we studied pairs of outcrossing and inbreeding species occurring in the same habitat. Among outcrossing species PERS averaged about 22%, whereas in inbreeding species the average was approximately 90%. The progeny/zygote (P/Z) ratio was studied in hand-pollinated populations in Epilobium angustifolium (a strongly outcrossing species) from populations in Oregon and Utah, by direct observation of embryogenesis at twoday intervals throughout the course of seed development. The P/Z ratio in both populations averaged near 30%, and the developing embryos showed a surprising array of abnormalities that resulted in embryo death. During early development >95% of the ovules had normally developing globular embryos, but beginning with differentiation (cotyledon formation) about 70% of the original globular embryos aborted during the course of embryogenesis and seed development. The clustering of developmental lethals during peroids of major differentiation events parallels the animal model of development. We found little evidence that PERS was limited by the ambient environment (including resource availability), pollination, or factors associated with the inbreeding habit. Instead, PERS was found to be inextricably linked to outcrossing plants, whose breeding systems promote genetic variability. The high incidence of developmental lethals in E. angustifolium and the resulting low P/Z ratio (ca. 30%) is attributed to genetic load (any lethal mutation or allelic combination) possibly working in combination with developmental selection (interovarian competition among genetically diverse embryos). Examples of maternally controlled, fixed patterns of ovule abortion with respect to position or number are discussed. However, we found no need to employ “female choice” as a hypothesis to explain our results for the extensive, seemingly random patterns of embryo abortion in E. angustifolium and other outcrossing species. A more parsimonious, mechanistic explanation based on genetic load-developmental selection is sufficient to account for the differential survivorship of embryos. Likewise, the traditional concept of a positive growth regulator feedback system based on the number of surviving ovules in an ovary can account for subsequent fruit survivorship.
355 citations
TL;DR: It is concluded that whilst the overall level of phenolics in extracts can be estimated with some confidence the information imparted by more specific assays is very dependent on the procedures employed, particularly when dealing with extracts from taxonomically highly diverse sources.
Abstract: Summary. A series of seventeen taxonomically diverse plant extracts rich in phenolic materials, including condensed and hydrolysable tannins, have been subjected to a series of biochemical analyses in an attempt to gather ecologically significant information about their interaction with proteins and amino acids. Methods employed were (i) protein-precipitation, using bovine serum albumin as substrate, followed by computation of specific activities of the tannins present in the extracts, and (ii) the inhibition of cellulase activity by tannin extracts bound to the cellulose substrate and free in solution. Both techniques revealed that all extracts contained tannin material. However, attempts to relate the results of the two procedures and in turn to relate them to information reported previously on the chemical properties of these extracts revealed that there was little correlation between any of the chemical or biochemical properties examined. From this analysis it would seem that whilst the analytical procedures available for studying tannins may generate ecologically useful information it is at present impossible, at least where plant material that is taxonomically diverse is being examined, to extrapolate from one type of measure to anticipate what would be observed from another type of measure. In addition to the above three other observations arose from this study. First, it appears to be generally true that there is not an absolute positive correlation between the level of protein precipitation and the incorporation of tannin in the tannin-protein precipitate. As relative protein concentration increases the proportion of tannin bound in the precipitate decreases, leading to less stable precipitates. Second, it is confirmed that some basic amino acids will precipitate with tannins, a phenomenon that could potentially influence amino acid balance in the diet. Third, complexation between tannin and protein absorbed on a cellulose substrate is able to interfere with the digestion of that cellulose by cellulase enzymes. Cellulose masking of this type may potentially effect the efficiency of cellulolytic activity in the rumen and if so suggests another subtle variation in the potential antifeedant properties of tannins.
294 citations
TL;DR: It is confirmed that some basic amino acids will precipitate with tannins, a phenomenon that could potentially influence amino acid balance in the diet and complexation between tannin and protein absorbed on a cellulose substrate is able to interfere with the digestion of that cellulose by cellulase enzymes.
Abstract: A series of seventeen taxonomically diverse plant extracts rich in phenolic materials, including condensed and hydrolysable tannins, have been subjected to a series of biochemical analyses in an attempt to gather ecologically significant information about their interaction with proteins and amino acids. Methods employed were (i) protein-precipitation, using bovine serum albumin as substrate, followed by computation of specific activities of the tannins present in the extracts, and (ii) the inhibition of cellulase activity by tannin extracts bound to the cellulose substrate and free in solution. Both techniques revealed that all extracts contained tannin material. However, attempts to relate the results of the two procedures and in turn to relate them to information reported previously on the chemical properties of these extracts revealed that there was little correlation between any of the chemical or biochemical properties examined. From this analysis it would seem that whilst the analytical procedures available for studying tannins may generate ecologically useful information it is at present impossible, at least where plant material that is taxonomically diverse is being examined, to extrapolate from one type of measure to anticipate what would be observed from another type of measure. In addition to the above three other observations arose from this study. First, it appears to be generally true that there is not an absolute positive correlation between the level of protein precipitation and the incorporation of tannin in the tannin-protein precipitate. As relative protein concentration increases the proportion of tannin bound in the precipitate decreases, leading to less stable precipitates. Second, it is confirmed that some basic amino acids will precipitate with tannins, a phenomenon that could potentially influence amino acid balance in the diet. Third, complexation between tannin and protein absorbed on a cellulose substrate is able to interfere with the digestion of that cellulose by cellulase enzymes. Cellulose masking of this type may potentially effect the efficiency of cellulolytic activity in the rumen and if so suggests another subtle variation in the potential antifeedant properties of tannins.
271 citations
TL;DR: The results indicate that future studies of woody plant/insect defoliator interactions must consider plant carbon/nutrient balance as a potentially important control over the nutritional value of foliage for insect herbivores.
Abstract: We investigated the effects of nitrogen fertilization upon the concentrations of nitrogen, condensed tannin and phenolic glycosides of young quaking aspen (Populus tremuloides) leaves and the quality of these leaves as food for larvae of the large aspen tortrix (Choristoneura conflictana), a Lepidopteran that periodically defoliates quaking aspen growing in North America. Nitrogen fertilization resulted in decreased concentrations of condensed tannin and phenolic glycosides in aspen leaves and an increase in their nitrogen concentration and value as food for the large aspen tortrix. These results indicate that plant carbon/nutrient balance influences the quality of aspen leaves as food for the large aspen tortrix in two ways, by increasing the concentrations of positive factors (e.g. nitrogen) and decreasing the concentrations of negative factors (eg. carbon-based secondary metabolites) in leaves. Addition of purified aspen leaf condensed tannin and a methanol extract of young aspen leaves that contained condensed tannin and phenolic glycosides to artificial diets at high and low levels of dietary nitrogen supported this hypothesis. Increasing dietary nitrogen increased larval growth whereas increasing the concentrations of condensed tannin and phenolic glycosides decreased growth. Additionally, the methanol extract prevented pupation. These results indicate that future studies of woody plant/insect defoliator interactions must consider plant carbon/nutrient balance as a potentially important control over the nutritional value of foliage for insect herbivores.
263 citations
TL;DR: The length of stay after a trans-Gulf flight was related to the extent of fat-depletion upon arrival: lean birds stayed longer than fat migrants and within each species, most individuals gained weight at a rapid rate, though a few individuals lost or maintained weight during their stay.
Abstract: Length of stopover and rate of weight gain (fat deposition) were studied in several species of passerine birds that stopped in southwestern Louisiana along the northern coast of the Gulf of Mexico after a trans-Gulf flight. Fatdepleted birds were more common among the birds that arrived at our study site in southwest Louisiana, though variability characterized our samples. Migrants that landed after encountering opposing winds or rain over the northern Gulf of Mexico were, on average, fatter than migrants that landed when weather was favorable for continued migration. Some of the variation in the energetic condition of arrivals may be explained by the location where migrants initiated crossings. Our simulation of flight over the Gulf of Mexico showed that with following winds a warbler can cross the Gulf of Mexico from Yucatan with fat reserves to spare, and stronger tailwinds make flights from as far south as Honduras energetically permissible. The length of stay after a trans-Gulf flight was related to the extent of fat-depletion upon arrival: lean birds stayed longer than fat migrants. Migrants stopped over for 1-7 days and replenished energy reserves at rates that varied from 0.19 g/d for Hooded Warblers (Wilsonia citrina) to 0.87 g/d for Ovenbirds (Seiurus aurocapillus). Within each species, most individuals gained weight at a rapid rate, though a few individuals lost or maintained weight during their stay.
260 citations
TL;DR: It is suggested that infected plants may have a selective advantage in populations with uninfected members because of the physiological mechanism of enhancement of growth.
Abstract: Many grasses are infected by endophytic fungi that grow intercellularly in leaves, stems, and flowers and are transmitted maternally by hyphal growth into ovules and seeds. The seed biology and seedling growth of endophyte-infected and uninfected perennial ryegrass (Lolium perenne) and tall fescue (Festuca arundinacea) were investigated under controlled environmental conditions. The percentage of filled seeds produced by infected tall fescue was over twice of uninfected tall fescue; infected and uninfected perennial reegrass had similar percentages. Weights of seeds from infected and uninfected plants were similar in both species. Seeds from infected plants of both species exhibited a higher rate of germination than seeds from uninfected plants. Shoot growth in the greenhouse was compared by making three sequential harvests of above-ground plant parts from infected and uninfected plants of both species. Infected perennial ryegrass plants produced significantly more biomass and tillers than uninfected plants after 6 and 10 weeks of growth and significantly more biomass after 14 weeks of growth. Infected tall fescue plants produced significantly more biomass and tillers than uninfected plants after 10 and 14 weeks of growth. The physiological mechanism of enhancement of growth is not known. The results of this study suggest that infected plants may have a selective advantage in populations with uninfected members.
247 citations
TL;DR: A computerized technique for the analysis of hemispherical photographs of sites within plant canopies reliably predicted both daily PFD and the temporal pattern of PFD including predicting suflecks lasting more than a few minutes, however, temporal patterns of P FD predicted from photographs differ in potentially important details from measured patterns.
Abstract: We describe a computerized technique for the analysis of hemispherical photographs of sites within plant canopies. The analysis yields estimates of canopy openness, a summary of the spatial distribution of canopy openings, estimates of direct and diffuse site factors, predicted times and durations of sunflecks for any date, and predicted daily courses of photosynthetically active photon flux density (PFD) for any date and for horizontal as well as inclined surfaces. By comparing estimates from photographs with measured values in 32 tropical forest microsites, we evaluate this technique and assess its potential and limitations. In sites of widely varying light availability, and with predictions specific to leaves at various angles, measured daily PFD correlated strongly (r=0.908) with predictions from photographs. Measured daily PFD was also closely correlated with estimates, from the photographs, of diffuse and direct site factors (r>0.8). Differences between predictions and measurements were proportionately largest in heavily shaded sites. Under clear conditions, this technique reliably predicted both daily PFD and the temporal pattern of PFD including predicting suflecks lasting more than a few minutes. However, temporal patterns of PFD predicted from photographs differ in potentially important details from measured patterns.
TL;DR: Using experimental data on an aquatic food web, it is shown that body size can lead to the type of trophic hierarchy used in the cascade model, suggesting that many food web patterns may be a product of body size.
Abstract: It has been suggested by Cohen and Newman (1985) that many of the patterns in published food webs can be derived from a stochastic model in which the species are arranged in a trophic hierarchy (the ‘cascade model’). We suggest that, if predators are larger than their prey, a trophic hierarchy can be generated on the basis of body size Empirical evidence from the literature shows that there is a positive relationship between predator and prey size for a range of invertebrates and that predators are usually larger than their prey. Using experimental data on an aquatic food web we show that body size can lead to the type of trophic hierarchy used in the cascade model, suggesting that many food web patterns may be a product of body size. This conclusion is discussed with respect to the limitations of the food web data and the relationship between ‘static’ and ‘dynamic’ models of web structure.
TL;DR: There was no apparent pattern between the degree of heritability of plant resistance and the destructiveness, feeding method, breadth of host range, or taxonomic group of the insects, and the lack of marked heritable variation in resistance to some insects may be the result of reduced variation caused by strong selection during prolonged or repeated insect outbreaks.
Abstract: Genetic variation in resistance to 16 species of herbivorous insects was studied in 18 clones of Solidago altissima growing in an old field near Ithaca, New York, USA. Resistance to each insect, defined as the abundance of a species attacking a particular host genotype relative to other genotypes, was measured in both the natural stand and in two experimental gardens. The heritability of resistance was estimated by parent-offspring regression and sibcorrelation. The primary result was that clones differed in resistance to 15 of 16 insect species. The resistance of genotypes to these insect species remained relatively constant over the four years of the study. However, for only 10 of these resistances were the heritability estimates significantly different from zero. Thus the common assumption of plant-insect studies — that phenotypic variation in insect abundance is closely correlated with underlying genetic variation — is only conditionally true. There is heritable variation in resistance to many insects, but not all. The insects for which we observed heritable variation in plant resistance represent five different orders and several functional groups, including leaf chewers, phloem and xylem feeders, and gall formers. There was no apparent pattern between the degree of heritability of plant resistance and the destructiveness, feeding method, breadth of host range, or taxonomic group of the insects. The lack of marked heritable variation in resistance to some insects may be the result of (a) reduced variation caused by strong selection during prolonged or repeated insect outbreaks, and (b) genotype-environment interactions that obscure differences among genotypes.
TL;DR: If reduced locomotor performance results in greater risk of predation and/or lowered foraging ability, then natural selection may result in important constraints on both clutch size and relative clutch mass in squamates.
Abstract: Recent studies suggest that lower survival among gravid squamate reptiles may be partially the result of decreased locomotor ability during gestation. In this study, we compared the speed and endurance of female garter snakes (Thamnophis marcianus), before, during, and after pregnancy. Gravid snakes had significantly lower locomotor performance than did non-gravid females, and performance varied among stages of gestation, reaching a minimum 0–6 weeks prior to parturition. Both number of offspring and relative clutch mass were inversely correlated with locomotor performance; as females increased these traits, locomotor ability decreased. If reduced locomotor performance results in greater risk of predation and/or lowered foraging ability, then natural selection (operating via differential mortality or feeding rates of gravid females) may result in important constraints on both clutch size and relative clutch mass in squamates.
TL;DR: As soon as favourable temporary aquatic sites were established, reproductive activities decreased at the permanent pond and increased at temporary breeding sites, and the reproductive periods of the species were highly affected by the rainfall distribution.
Abstract: At Panguana, a study in the upper Amazon basin, 7 different aquatic breeding sites of anurans were investigated from Nov. 1, 1981 to Oct. 31, 1982. Rainfall in this area is seasonal. Only 20% of the total annual precipitation was maesured during the dry period, which lasted from mid-March until mid-September 1982. The reproductive periods of the species were highly affected by the rainfall distribution. Calling males and/or gravid females of 46 species were found during the study period. 37.0% of the anuran species were exclusively, and 43.5% predominantly, active during the rainy season. Plots of 100 m2 were established at 6 aquatic sites in primary rainforest. A total of 2126 frogs was counted; 93.2% of these were made during the rainy season. A maximum of 378 individuals were found during one check in the plot at the permanent pond. At this site high concentrations of individuals and large choruses were found at the beginning of the rainy season. As soon as favourable temporary aquatic sites were established, reproductive activities decreased at the permanent pond and increased at temporary breeding sites.
TL;DR: In this paper, the authors investigated the evolution of the carrion arthropod community as a continuum of gradual change: rapid at first, slow during peak activity, and erratic in the final days as carcass resources become depleted.
Abstract: Patterns of species change, computed day to day community similarity, polar ordination, and other statistics were used to discern successional trends and correspondence with named decay stages from eleven carrion arthropod studies The carrion arthropod community develops primarily as a continuum of gradual change: rapid at first, slow during peak activity, and erratic in the final days as carcass resources become depleted In only five studies the ordination analysis revealed recognizable clusters representing discrete faunal seres; none of these, however, completely supported a stage-based view of faunal succession Collectively, the authors of published studies identified 29 decay stage boundaries; of these, only 14 were associated with major faunal changes We found at least 27 additional episodes involving compositional differences in the fauna not recorded by the authors Named decay stages may have descriptive utility in carrion studies However, ecologists and forensic entomologists should be alerted to the inadequacies of decay stages in summarizing patterns of faunal succession in carrion arthropod investigations
TL;DR: During the last 20 years this replacement of Erica-dominant communities by monocultures of Molinia has been observed in almost all wet heathlands in The Netherlands along with a strong increase in nitrogen availability.
Abstract: The term relative nutrient requirement is introduced in order to measure and to compare the nutrient losses from different perennial plant populations and the amount of nutrient that they need to absorb to compensate these losses. The relative nutrient requirement (L) is defined as the amount of the growth-limiting nutrient that must be taken up to maintain or replace each unit of biomass during a given time interval (e.g., mgN g-1 biomass year-1). It is derived that in a plant community with two competing perennial plant populations, species1 will become dominant if the relative competition coefficient k 12 (sensu De Wit 1960) exceeds the ratio between the relative nutrient requirements of the two species (L 1/L 2), whereas species 2 will become dominant, if k 12 is below this critical ratio. The above-ground litter production was measured inwet heathland communities dominated by Erica tetralix or by Molinia caeruleain order to estimate N and P losses from theaboveground biomass and to calculate the relative N and P requirements of these species. Molinia lost during one year 63% and 34%, respectively, of the amount of N and P present in the above-ground biomass at the end of the growing season. These losses were in Erica 27% and 31%, respectively. The relative N requirements of the two species show the same difference: 7.5 and 2.6 mg N g-1 yr-1, respectively, in Molinia and in Erica. Also the relative P requirement of Molinia is higher as well as that of Erica (0.18 versus 0.08 mg P g-1 yr-1). The relative competition coefficient of Molinia with respect to Erica (k me ) is equal to unity under unfertilized conditions but increases with increasing nutrient supply. Under nutrient-poor conditions k me is below the critical ratio of the relative nutrient requirements of the two species (L m /L e =2.9 or 2.3), so that Erica will be the dominant species. After an increase in nutrient availability k me increases and exceeds this critical limit which results in Molinia replacing Erica. During the last 20 years this replacement of Erica-dominant communities by monocultures of Molinia has been observed in almost all wet heathlands in The Netherlands along with a strong increase in nitrogen availability.
TL;DR: It is concluded that the physical and chemical changes in the leaf substrate which result from fragmentation and digestion by isopods do not necessarily accelerate the subsequent decomposition of the litter very significantly.
Abstract: The indirect contribution terrestrial isopods make to decomposition processes by stimulating microbial activites has been quantified in laboratory experiments. The extent to which microbial metabolism is enhanced as a result of the passage of Betula pendula leaf litter through the alimentary system of isopods was measured for both freshly fallen and decayed leaves. Faeces derived from 1 g freshly fallen litter lost 75 mg g-1 D.W. more than did intact leaves, as a result of enhanced microbial metabolism. Faeces derived from 1 g of previously decayed leaves, which were shown to be the preferred food of isopods, lost only 17.5 mg g-1 D.W. more than intact decaying leaves. The isopod's direct contribution to soil metabolism was calculated to be 151 mg and 138 mg g-1 litter ingested when fed on freshly fallen and decayed leaves respectively. It is concluded that the physical and chemical changes in the leaf substrate which result from fragmentation and digestion by isopods do not necessarily accelerate the subsequent decomposition of the litter very significantly. Fungal propagule density was 3.2x and 3.6x higher in faeces derived from freshly fallen and decayed leaves respectively than in the intact litter. Numbers of viable bacteria were correspondingly 126x and 34x higher in faeces than in the freshly fallen and the decayed leaves. Levels of microbial inhibitors were lower in the faeces than in the leaves but levels of free amino acids stayed higher for longer in the faeces than they did in intact litter. In the field the physical removal of litter by the soil macrofauna from surface to deeper and moister microsites may be the most important indirect contribution that they make to decomposition processes.
TL;DR: Comparisons along artificial gradients of soil water availability and light transmittance indicated that the cytotypes had distinct physiological requirements which probably originated in metabolic and more general genetic differentiation and could be directly attributable to ploidy.
Abstract: The microdistribution of diploid and tetraploid plants of Dactylis glomerata L. was examined and related to their immediate environment in several sites in central Galicia, where morphologically indistinguishable individuals of both ploidies grow in sympatry. The two related cytotypes differed in habitat preference. Diploids were mainly confined to the low-density forest-floor habitat in woodlands of mostly ancient origin, whereas tetraploids were widespread in varied habitats but clearly predominant in open areas, particularly in disturbed anthropic sites. The in situ comparison of plant performance showed that where plants of each ploidy were more common they produced more tillers, panicles and seeds. This habitat preference closely reflected differences in life-history characteristics. The tetraploids had an early and short flowering time almost always completed before the aestival drought, whereas the diploids began to flower several weeks later and flowered throughout the drought. Comparisons along artificial gradients of soil water availability and light transmittance indicated that the cytotypes had distinct physiological requirements which probably originated in metabolic and more general genetic differentiation and could be directly attributable to ploidy. Habitat differentiation increases the species' colonizing ability. It also amplifies divergence in reproductive strategy between diploids and tetraploids, which reduces ineffective crossing between cytotypes and thereby permits them to coexist in sympatry. The effect of hybridization at the polyploid level on the differentiation between cytotypes was assessed from the recent introduction of a foreign tetraploid entity into the study area. Hybridization between the two distinct tetraploids was found to increase habitat differentiation between the diploids and the tetraploids, but the major part of this differentiation is probably attributable to ploidy itself.
TL;DR: The greater sensitivity of Alaska paper birch defensive chemistry and palatability to snowshoe hares in comparison to green alder is in agreement with the hypothesis that early successional woody plants that are adapted to high resource availability are more plastic in their chemical responses to the physical environment than are species from less favorable environments.
Abstract: Plant carbon/nutrient balance has been implicated as an important factor in plant defensive chemistry and palatability to herbivores. We tested this hypothesis by fertilizing juvenile growth form Alaska paper birch and green alder with N, P and N-plus-P in a balanced 2x2 factorial experiment. Additionally, we shaded unfertilized plants of both species. Fertilization with N and N-plus-P increased growth of Alaska paper birch, reduced the concentration of papyriferic acid in internodes and increased the palatability of birch twigs to snowshoe hares. Shading decreased birch growth, decreased the concentration of papyriferic acid in internodes and increased twig palatability. These results indicate that the defensive chemistry and palatability of winter-dormant juvenile Alaska paper birch are sensitive to soil fertility and shade. Conversely the defensive chemistry and palatability of green alder twigs to snowshoe hares were not significantly affected by soil fertility or shade. The greater sensitivity of Alaska paper birch defensive chemistry and palatability to snowshoe hares in comparison to green alder is in agreement with the hypothesis that early successional woody plants that are adapted to high resource availability are more plastic in their chemical responses to the physical environment than are species from less favorable environments.
TL;DR: The results suggest that the success of perennial herbaceous plants at high elevations does not necessarily depend on a large below ground biomass fraction, and that the increased fine root length at high altitude may substitute for reduced mycorrhizal infection.
Abstract: Partitioning patterns in 22 exclusively low and 27 exclusively high altitude perennial herbaceous species were examined in order to test the hypothesis that plants of high altitudes allocate more dry matter to below-ground parts and in particular to storage organs, than typical low altitude plants. Our results raise some doubts about the general validity of this hypothesis. The mean fractions of total dry matter allocated to green leaves (22±2% s.e. at low and 24±2% at high altitude) and special storage organs (28±4% at both altitudes) do not differ significantly among sites. The mean relative portions of total dry matter allocated to above-ground plant parts amount to 57±3% at low and 42±3% at high elevation (P=0.002) and differ less than often assumed. The greater below-ground fraction at high altitude results from reduced stem and proportionally increased fine root compartments. At high altitude specific root length is increased by 50% and mean individual rooting density is tripled. Fine root length per unit leaf area is 4.5 times greater (P<0.001). However, interspecific variation in all these quantities is considerable and species with quite contrasting partitioning patterns coexist at both elevations. This suggests that the success of perennial herbaceous plants at high elevations does not necessarily depend on a large below ground biomass fraction. The increased fine root length at high altitude may substitute for reduced mycorrhizal infection. Figure 1 provides a graphical summary.
TL;DR: Succession at carcasses is viewed as being inherently different from the traditional concept as the habitat is non-replenishing and does not lead to a climax community.
Abstract: Carcasses are temporary resources which are unbredictable and inconsistent in their availability and locality. A recognisable community of interacting user arthrocods comprising sarcophages, coprophages, dermatophages, keratophages, detritivores, predators and parasites has evolved to exploit the carcass habitat. The large number of arthropods, close confinement, and limited duration of resources necessitates aggressive utilisation. The trophic relations, competition and successionary pattern of these arthropods is discussed. Several pathways to reduce competitive conflict are described. Succession at carcasses is viewed as being inherently different from the traditional concept as the habitat is non-replenishing and does not lead to a climax community.
TL;DR: Abundance patterns during 6–7 years and orchid visitation were determined for 51 species of the 57 local euglossine bees and stability and abundance were positively associated, but year-to-year population stability and the degree of seasonality were not correlated.
Abstract: Abundance patterns during 6-7 years and orchid visitation were determined for 51 species of the 57 local euglossine bees. Male bees were counted at 3 chemical attractants presented in the same manner each month. Sites were separated by 75 km but included wet Atlantic forest at 500 m elevation, moist forest at 180 m near Barro Colorado Island, and cloud forest at 900 m near the Pacific ocean. 1. From 15 to 30 euglossine species of 4 genera were active in each month and site; monthly species number and general bee abundance were positively correlated. Many species had 3 annual abundance peaks (range 1-4) and were active throughout the year, but peak annual abundances rarely occurred during late wet or early dry seasons. In contrast, Eufriesea generally were present as adults only 1-2 months in a year. 2. Euglossine populations were exceptionally stable. Species at each site were more stable than any known insect population, and stability and abundance were positively associated. However, year-to-year population stability and the degree of seasonality were not correlated. Among the three sites, the more diverse (species rich) bee assemblages displayed lower stability; these were the wetter and upland sites. 3. The most abundant bees visited more orchid species. Eg. and El. each visited and average of 4 orchid species (range 0-13); Ex. and Ef. visited 0-3. Stable populations did not visit more or fewer orchid species than did unstable populations. 4. Less than 68% of species at each site visited orchid flowers; less than a few dozen of the 100-800 bees counted in a day carried orchid pollinaria. Over 20% of the euglossine species never were seen with pollinaria at any site and probably seldom visit orchids in central Panama. 5. Most bee species visited 1 or no fragrance orchids in a given habitat. Orchids tended to utilize common pollinators that seldom included more than 1 species, and they utilized stable or unstable, seasonal or aseasonal bees. However, the most stable and abundant bee, Eg. imperialis, rarely pollinated orchids; fewer than 10 of ca. 20000 bees carried pollinaria. 6. Orchids may interact primarily with discrete seasonal bee population peaks-probably the emerging adults. Although specialized orchid preferences are implicated for species that visit few or no local orchids but pollinate other species and carry pollinaria in other areas, euglossine bees do not need orchids to survive or reproduce.
TL;DR: In this paper, the authors show correlations of soil nitrogen, vegetation, and gopher mounds that are consistent with this indirect mechanism by which gophers affect local species composition and old field succession.
Abstract: Pocket gophers (Geomys bursarius: Geomyidae Rodentia) are shown to affect soil resources and thus, indirectly, vegetation. Gophers reduce average soil nitrogen near the surface and increase point-to-point heterogeneity of soil nitrogen by moving nitrogen-poor subsurface soil to the soil surface. Data from 22 old fields at Cedar Creek Natural History Area, Minnesota, USA show correlations of soil nitrogen, vegetation, and gopher mounds that are consistent with this indirect mechanism by which gophers affect local species composition and old field succession.
TL;DR: Several simple mathematical models are used to argue that effects on short-term population growth rate, long term population size, and short term relative fitness of interactants may differ qualitatively.
Abstract: The classification of interspecific interactions can have an important impact on ecologists' world views. Previous classifications have often been incomplete, have suffered from ambiguously defined categories, and/or have wrongly equated categories of population level effects with particular mechanisms of interaction. I use several simple mathematical models to argue that effects on short-term population growth rate, long term population size, and short term relative fitness of interactants may differ qualitatively. Equating all (--) effects with competition and all (+-) effects with predation may have caused ecologists to ignore a variety of potentially important interaction mechanisms. Failure to define the type of effect used in classifying interactions has led to confusion about the nature of interactions; several controversies regarding competition have apparently been caused or exaccerbated by problems with definition or clasification. In applying classification schemes, ecologists should realize that the classification of an interaction between two populations may change with the sizes of those populations or of other populations with which they interact.
TL;DR: The difference in the responsiveness of leaf specific mass to light indicates that the broad-ranging species maintains access to modes of response little utilized by the open-site specialist, and there is no evidence that the open site specialist is better suited than the generalist to high-light sites.
Abstract: Piper auritum (H.B. & K.), a pioneer tree restricted to open sites and Piper hispidum (Swartz), a shrub common in sites ranging from recent clearings to shaded understory, both adjust photosynthetic characteristics in response to light availability during growth. The sensitivity of photosynthetic capacity to light availability during growth was indistinguishable for the two species growing in their natural habitat. Photosynthetic capacity was strongly correlated with leaf nitrogen in both species, and the relationship was similar between species. Dark respiration and leaf specific mass were more sensitive to light during growth in P. hispidum, the species with the broad habitat ange, than in P. auritum. In general, similarities between the species were more striking than differences between them. The differences in dark respiration could have important implications for carbon balance. The difference in the responsiveness of leaf specific mass to light indicates that the broad-ranging species maintains access to modes of response little utilized by the open-site specialist. We did not and, in the gas exchange characteristics, any evidence that the open site specialist is better suited than the generalist to high-light sites.
TL;DR: The principle conclusion is that the medfly is a successful generalist frugivore because of its developmental ability to offset the effect of the value for a host-specific trait with one that tends to increase r, the net result of which is to maintain a relatively high r.
Abstract: Demographic parameters for the Mediterranean fruit fly reared on each of twenty four different hosts from sixteen different plant families are reported. These include cohort parameters of development, survival, pupal sizc, and fecundity as well as population parameters such as intrinsic rate of population increase (r) and mean generation time. Major findings include the following: i) no consistent quantitative relationships existed between r and its chief demographic determinants such as preadult developmental time and adult fecundity; and ii) few correlations existed among the cohort life history parameters themselves. The principle conclusion is that the medfly is a successful generalist frugivore because of its developmental ability to offset the effect of the value for a host-specific trait that tends to lower r with one that tends to increase r, the net result of which is to maintain a relatively high r.
TL;DR: In this paper, the authors investigated the diet of a Canis familiaris dingo in response to measured fluctuations of prey populations over a period of seven years and concluded that the diet was functionally related to the respective abundances of all major prey species, but the relationship shifted during drought when predation on low populations was most severe.
Abstract: Changes in the diet of dingoes (Canis familiaris dingo) in response to measured fluctuations of prey populations were followed over 7 years The study began after great rains had broken a long drought Eruptions of rodents and rabbits followed, but some prey were always either relatively abundant (live cattle) or scarce (red kangaroo, lizards, birds) Cattle carcasses were increasingly available during a subsequent drought Small and medium-sized prey, rodents (26%), lizards (12%) and rabbits (56%) were preferred, probably because they were easily caught Only rabbits were caten consistently regardless of density By contrast, large prey were eaten in relatively large amounts only during drought, with initial emphasis on red kangaroos (15% overall) and then cattle (17%) mostly as carcasses The diet was functionally related to the respective abundances of all major prey species, but the relationship shifted during drought when predation on low populations was most severe There was evidence that growth of resurging prey populations were suppressed by predation Diets of dingoes did not differ significantly with age or sex An hypothesis of 'alternation of predation' is presented: dingoes feed sequentially on prey of increasing size (rodents, rabbits, red kangaroos, cattle) in response to rainy periods and subsequent droughts, meanwhile always concentrating on the staple prey (rabbits) The fluctuating abundances of small and medium-sized prey determined not only their own relative availabilities but also that of large prey, and hence determined the diet of the dingo at any time Prey availability (catchability, accessability) appeared to be more important than prey abundance (numbers, biomass), and the dingo's flexible social organisation allowed versatility in hunting strategies and defence of resources We conclude that dingoes do not always forage most efficiently as optimal foraging models predict because of the constraints imposed by the capricious environment in arid Australis, where prey availability fluctuates greatly and becomes limited and clumped in drought, so that dingoes may be faced with outright starvation Instead we conclude that dingoes utilise a conservative feeding strategy and adopt any behaviour which provides at least a threshold quantity of energy or nutrient as part of a trade-off with other competing ecological requirements
TL;DR: It is indicated that the observed growth decreases of surviving red spruce trees at northeastern sites with high mortality have been anomalous during the past 20 to 25 years with respect to both historical annual growth patterns and past relationships to climate or stand development at these sites.
Abstract: Long-term growth patterns of red spruce (Picea rubens Sarg.) were analyzed from increment cores collected from over 1000 trees at 48 sites in the eastern United States. Principal objectives were the evaluation of the distribution, timing, and uniqueness of observed patterns of decreasing radial growth during the past 25 years and the examination of stand competition and climate as factors contributing to observed changes. Our analyses focused on historical records of spruce mortality and approximately 200 years of radial growth data to search for historical precedents for current trends. In this work we have used time series analysis to detect the temporal frequency of significant negative or positive shifts in radial growth rates, an analysis of relationships between a stand competition index and observed changes in growth and mortality, and modeling of past growth-climate relationships to determine whether recent growth changes could be predicted based on climate. Collectively, these analyses indicate that the observed growth decreases of surviving red spruce trees at northeastern sites with high mortality have been anomalous during the past 20 to 25 years with respect to both historical annual growth patterns and past relationships to climate or stand development at these sites. In general, reductions in radial increment that have also been noted at southern high elevation sites but not at low elevations occurred 5 to 10 years later than at northern sites and represent less substantive departures from growth trends predicted by linear climate models. These results suggest that regional and not local stresses have triggered the observed decline in radial growth of red spruce at these sites. While climatic change may have contributed to observed changes, the degree of radial growth suppression observed is greater than would be expected based on past growth-climate relationships. This unique relationship of growth to climate suggests the influences of either recent, unique combinations of climatic stresses or the possibly interactive intervention of other regional-scale stresses, such as atmospheric pollution.
TL;DR: Investigation of the role of nutrient supply in the interactions between Calluna vulgaris and Molinia caerulea concluded that increased nutrient availability favours the competitive vigour of Molinia in interaction with Calluna.
Abstract: A combination of a removal and fertilization experiment in the field and a replacement experiment in containers in a cold frame was carried out to investigate the role of nutrient supply in the interactions between Calluna vulgaris and Molinia caerulea. It is concluded that the growth of Calluna as well as that of Molinia increased with increasing nutrient availability. However, the increase in biomass of Molinia was much larger than that of Calluna. It is also concluded that increased nutrient availability favours the competitive vigour of Molinia in interaction with Calluna.