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Showing papers in "Oecologia in 1993"


Journal ArticleDOI
TL;DR: A simple thermodynamic measure of the order and disorder apparent in the nested patterns of species distribution patterns is described and a method of identifying idiosyncratic species and sites is described.
Abstract: Species distribution patterns within naturally fragmented habitat have been found to often exhibit patterns of pronounced nestedness. Highly predictable extinction sequences are implied by these nested species distribution patterns, thus the patterns are important to both the philosophy and practice of conservation biology. A simple thermodynamic measure of the order and disorder apparent in the nested patterns is described. The metric offers (i) a measure of the uncertainty in species extinction order, (ii) a measure of relative populational stabilities, (iii) a means of identifying minimally sustainable population sizes, and (iv) an estimate of the historical coherence of the species assemblage. Four presumptions govern the development of the metric and its theory: (i) the fragmented habitat was once whole and originally populated by a single common source biota, (ii) the islands were initially uniform in their habitat heterogeneity and type mix, and have remained so throughout their post-fragmentation history, (iii) no significant clinal (latitudinal) gradation exists across the archipelago so as to promote species turnover across the archipelago, and (iv) all species of interest are equally isolated on all islands. The violation of these conditions promotes species distributions which are idiosyncratic to the general extinction order expected in fragmentation archipelagos. While some random variation in extinction order is to be expected, idiosyncratic distributional patterns differ from randomness and are readily segregatable from such noise. A method of identifying idiosyncratic species and sites is described.

1,009 citations


Journal ArticleDOI
TL;DR: The results obtained demonstrated the existence of a general positive, linear relationship between plant decomposition rates and nitrogen and phosphorus concentrations, and reflect the coupling of phosphorus and nitrogen in the basic biochemical processes of both plants and their microbial decomposers.
Abstract: The strength and generality of the relationship between decomposition rates and detritus carbon, nitrogen, and phosphorus concentrations was assessed by comparing published reports of decomposition rates of detritus of photosynthetic organisms, from unicellular algae to trees. The results obtained demonstrated the existence of a general positive, linear relationship between plant decomposition rates and nitrogen and phosphorus concentrations. Differences in the carbon, nitrogen, and phosphorus concentrations of plant detritus accounted for 89% of the variance in plant decomposition rates of detritus orginating from photosynthetic organisms ranging from unicellular microalgae to trees. The results also demonstrate that moist plant material decomposes substantially faster than dry material with similar nutrient concentrations. Consideration of lignin, instead of carbon, concentrations did not improve the relationships obtained. These results reflect the coupling of phosphorus and nitrogen in the basic biochemical processes of both plants and their microbial decomposers, and stress the importance of this coupling for carbon and nutrient flow in ecosystems.

872 citations


Journal ArticleDOI
TL;DR: Hydraulic lift need not only occur in arid or semi-arid environments where chronic water deficits prevail, but can be important in relatively mesic environments when subjected to periodic soil water deficits, that plants neighboring trees which conduct hydraulic lift can use a significant proportion of this water source, and that the HLW source can effectively ameliorate the influence of drought on the performance and growth of neighboring vegetation.
Abstract: During drought periods, sugar maple (Acer saccharum) demonstrates “hydraulic lift”; nocturnal uptake of water by roots from deep soil layers that is released from shallow roots into upper soil layers. Using standard water relations methods and stable hydrogen isotope analysis of both source-water and plant-water, I investigated (1) the magnitude and radial extent of hydraulic lift by mature, relatively open-grown trees, of A. saccharum, (2) the proportion of hydraulically-lifted water (HLW) used by shallow-rooted neighbors growing at different distances from target trees, and (3) the influence that this water source had on stomatal conductance to water vapor (g), water balance and growth of these neighbors. Soil water potentials (ψs) at −20 and −35 cm showed a distinct diel fluctuation. Soil pits dug beneath three mature trees revealed a distinct hard-pan (e.g. fragipan) layer at a depth of approximately 50 cm. Examination of root distributions obtained from soil cores and soil pits revealed that some larger diameter roots (1.9–3.7 cm) did penetrate the fragipan and were established in the ground water table. The presence of the fragipan indicated that the “rewetting” of the upper soil layer during the night could not be explained by capillary rise from the shallow water table; it was the trees that were taking up ground water and then redepositing it at night into the upper 35 cm of soil, above the fragipan. The greatest fluctuations in ψs occurred within 2.5 m of trees and only extended out to approximately 5 m. Application of a two-end-member linear mixing model which used stable hydrogen isotopic data obtained from environmental water sources and xylem-sap demonstrated that all neighbors used some fraction (3–60%) of HLW supplied by sugar maple trees. Plants that used a high proportion of HLW (e.g. rhizomatous or stoloniferous perennials) maintained significantly higher leaf water potentials and g, and showed greater aboveground growth when compared with (i) neighbors that used little or no HLW or (ii) conspecifics found growing at distances greater than about 3 m away from maple trees. Three important conclusions can be drawn from the results of this investigation that have not been demonstrated before: (1) hydraulic lift need not only occur in arid or semi-arid environments where chronic water deficits prevail, but can be important in relatively mesic environments when subjected to periodic soil water deficits, (2) that plants neighboring trees which conduct hydraulic lift can use a significant proportion of this water source, and (3) that the HLW source can effectively ameliorate the influence of drought on the performance and growth of neighboring vegetation. The results are also discussed in terms of their influence on plant nutrient relations (including plant-mycorrhizal associations), the nature of plant-plant interactions and the water balance of individuals, communities and floristic regions.

735 citations


Journal ArticleDOI
TL;DR: A model of whole-canopy photosynthesis was used to show that observed or hypothetical canopy mass distributions toward higher LMA (and hence higher N/area) in the upper portions of the canopy tended to increase integrated daily canopy photosynthesis over other LMA distribution patterns.
Abstract: Canopy structure and light interception were measured in an 18-m tall, closed canopy deciduous forest of sugar maple (Acer saccharum) in southwestern Wisconsin, USA, and related to leaf structural characteristics, N content, and leaf photosynthetic capacity. Light attenuation in the forest occurred primarily in the upper and middle portions of the canopy. Forest stand leaf area index (LAI) and its distribution with respect to canopy height were estimated from canopy transmittance values independently verified with a combined leaf litterfall and point-intersect method. Leaf mass, N and A max per unit area (LMA, N/area and A max/area, respectively) all decreased continuously by over two-fold from the upper to lower canopy, and these traits were strongly correlated with cumulative leaf area above the leaf position in the canopy. In contrast, neither N concentration nor A max per unit mass varied significantly in relation to the vertical canopy gradient. Since leaf N concentration showed no consistent pattern with respect to canopy position, the observed vertical pattern in N/area is a direct consequence of vertical variation of LMA. N/area and LMA were strongly correlated with A max/area among different canopy positions (r2=0.81 and r2=0.66, respectively), indicating that vertical variation in area-based photosynthetic capacity can also be attributed to variation in LMA. A model of whole-canopy photosynthesis was used to show that observed or hypothetical canopy mass distributions toward higher LMA (and hence higher N/area) in the upper portions of the canopy tended to increase integrated daily canopy photosynthesis over other LMA distribution patterns. Empirical relationships between leaf and canopy-level characteristics may help resolve problems associated with scaling gas exchange measurements made at the leaf level to the individual tree crown and forest canopy-level.

708 citations


Journal ArticleDOI
TL;DR: It is concluded that intraguild predation in this system is wide-spread and has potentially important influences on the population dynamics of a key herbivore.
Abstract: We evaluated the influence of intraguild predation among generalist insect predators on the suppression of an herbivore, the aphid Aphis gossypii, to test the appropriateness of the simple three trophic level model proposed by Hairston, Smith, and Slobodkin (1960). We manipulated components of the predator community, including three hemipteran predators and larvae of the predatory green lacewing Chrysoperla carnea, in field enclosure/exclosure experiments to address four questions: (1) Do generalist hemipteran predators feed on C. carnea? (2) Does intraguild predation (IGP) represent a substantial source of mortality for C. carnea? (3) Do predator species act in an independent, additive manner, or do significant interactions occur? (4) Can the experimental addition of some predators result in increased densities of aphids through a trophic cascade effect? Direct observations of predation in the field demonstrated that several generalist predators consume C. carnea and other carnivorous arthropods. Severely reduced survivorship of lacewing larvae in the presence of other predators showed that IGP was a major source of mortality. Decreased survival of lacewing larvae was primarily a result of predation rather than competition. IGP created significant interactions between the influences of lacewings and either Zelus renardii or Nabis predators on aphid population suppression. Despite the fact that the trophic web was too complex to delineate distinct trophic levels within the predatory arthropod community, some trophic links were sufficiently strong to produce cascades from higher-order carnivores to the level of herbivore population dynamics: experimental addition of either Z. renardii or Nabis predators generated sufficient lacewing larval mortality in one experiment to release aphid populations from regulation by lacewing predators. We conclude that intraguild predation in this system is wide-spread and has potentially important influences on the population dynamics of a key herbivore.

447 citations


Journal ArticleDOI
TL;DR: It is speculated that isoprene emission may help plants cope with stressful conditions, with a 10°C increase in temperature causing a eight-fold increase in the rate of isopren emission.
Abstract: Kudzu (Pueraria lobata (Willd) Ohwi.) is a vine which forms large, monospecific stands in disturbed areas of the southeastern United States. Kudzu also emits isoprene, a hydrocarbon which can significantly affect atmospheric chemistry including reactions leading to tropospheric ozone. We have studied physiological aspects of isoprene emission from kudzu so the ecological consequences of isoprene emission can be better understood. We examined: (a) the development of isoprene emission as leaves developed, (b) the interaction between photon flux density and temperature effects on isoprene emission, (c) isoprene emission during and after water stress, and (d) the induction of isoprene emission from leaves grown at low temperature by water stress or elevated temperature. Isoprene emission under standard conditions of 1000 μmol photons·m-2·s-1 and 30°C developed only after the leaf had reached full expansion, and was not complete until up to two weeks past the point of full expansion of the leaf. The effect of temperature on isoprene emission was much greater than found for other species, with a 10°C increase in temperature causing a eight-fold increase in the rate of isoprene emission. Isoprene emission from kudzu was stimulated by increases in photon flux density up to 3000 μmol photons·m-2·s-1. In contrast, photosynthesis of kudzu was saturated at less than 1000 μmol·m-2·s-1 photon flux density and was reduced at high temperature, so that up to 20% of the carbon fixed in photosynthesis was reemitted as isoprene gas at 1000 μmol photons·m-2·s-1 and 35°C. Withholding water caused photosynthesis to decline nearly to zero after several days but had a much smaller effect on isoprene emission. Following the relief of water stress, photosynthesis recovered to the prestress level but isoprene emission increased to about five times the prestress rate. At 1000 μmol photons·m-2·s-1 and 35°C as much as 67% of the carbon fixed in photosynthesis was reemitted as isoprene eight days after water stress. Leaves grown at less than 20°C did not make isoprene until an inductive treatment was given. Inductive treatments included growth at 24°C, leaf temperature of 30°C for 5 h, or witholding water from plants. With the new information on temperature and water stress effects on isoprene emission, we speculate that isoprene emission may help plants cope with stressful conditions.

406 citations


Journal ArticleDOI
TL;DR: Although plants in small and large populations were similar in size, seed production per plant was much lower in small populations, indicating that even in large populations seed production may still not be at its maximum.
Abstract: All individuals of all known populations of Banksia goodii were assessed for seed production. Small populations produced no or only a few seeds per unit canopy area. Effects of population size on seed production per unit area and seed production per plant were present over the whole range of population sizes, indicating that even in large populations seed production may still not be at its maximum. Resource differences could not explain this disproportionate decrease in seed production with decline in population size, because there were no differences in soil properties and understorey or overstorey cover between the small and large populations. Although plants in small and large populations were similar in size, seed production per plant was much lower in small populations. This was not because plants in small populations produced fewer cones but because the fraction of these cones that was fertile was much lower. Five of the nine smallest populations (<200 m2) produced no fertile cones over the last 10 years. The number of seeds per fertile cone did not depend on population size. The results are discussed in relation to pollination biology.

391 citations


Journal ArticleDOI
TL;DR: Canopy-scale evaporation rate (E) and derived surface and aerodynamic conductances for the transfer of water vapour (gs and ga, respectively) are reviewed for coniferous forests and grasslands andBoundary-line relationships between gs and light and air saturation deficit (D) vary considerably.
Abstract: Canopy-scale evaporation rate (E) and derived surface and aerodynamic conductances for the transfer of water vapour (gs and ga, respectively) are reviewed for coniferous forests and grasslands. Despite the extremes of canopy structure, the two vegetation types have similar maximum hourly evaporation rates (Emax) and maximum surface conductances (gsmax) (medians = 0.46 mm h-1 and 22 mm s-1). However, on a daily basis, median Emax of coniferous forest (4.0 mm d-1) is significantly lower than that of grassland (4.6 mm d-1). Additionally, a representative value of ga for coniferous forest (200 mm s-1) is an order of magnitude more than the corresponding value for grassland (25 mm s-1). The proportional sensitivity of E, calculated by the Penman-Monteith equation, to changes in gs is >0.7 for coniferous forest, but as low as 0.3 for grassland. The proportional sensitivity of E to changes in ga is generally ±0.15 or less.

378 citations


Journal ArticleDOI
TL;DR: Five potential mechanisms for generating the observed body-size patterns are discussed: the heat-conservation hypothesis, two hypotheses concerning phylogenetic history, the migration-ability hypothesis, and the starvation-resistance hypothesis.
Abstract: Using published distributions of 65 species from the British Isles and northern Europe, we show that ant assemblages change with latitude in two ways. First, as commonly found for many types of organisms, the number of ant species decreased significantly with increasing latitude. For Ireland and Great Britain, species richness also increased significantly with region area. Second, although rarely demonstrated for ectotherms, the body size of ant species, as measured by worker length, increased significantly with increasing latitude. We found that this body-size pattern existed in the subfamily Formicinae and, to a lesser extent, in the Myrmicinae, which together comprised 95% of the ant species in our study area. There was a trend for formicines to increase in size with latitude faster than myrmicines. We also show that the pattern of increasing body size was due primarily to the ranges of ant species shifting to higher latitudes as their body sizes increased, with larger formicines becoming less represented at southerly latitudes and larger myrmicines becoming more represented at northerly latitudes. We conclude by discussing five potential mechanisms for generating the observed body-size patterns: the heat-conservation hypothesis, two hypotheses concerning phylogenetic history, the migration-ability hypothesis, and the starvation-resistance hypothesis.

348 citations


Journal ArticleDOI
TL;DR: Comparison of mangrove and upland forest litterfall data shows similar trends with latitude but indicates thatMangrove litterfall is higher than upland Forest litterfall, suggesting that the patterns of organic matter partitioning differ according to latitude.
Abstract: A major paradigm in biosphere ecology is that organic production, carbon turnover and, perhaps, species diversity are highest at tropical latitudes, and decrease toward higher latitudes. To examine these trends in the pantropical mangrove forest vegetation type, we collated and analysed data on above-ground biomass and annual litterfall for these communities. Regressions of biomass and litterfall data show significant relationships with height of the vegetation and latitude. It is suggested that height and latitude are causally related to biomass, while the relationship with litterfall reflects the specific growing conditions at the respective study sites. Comparison of mangrove and upland forest litterfall data shows similar trends with latitude but indicates that mangrove litterfall is higher than upland forest litterfall. The regression equations allow the litterfall/biomass ratio to be simulated, and this suggests that the patterns of organic matter partitioning differ according to latitude.

322 citations


Journal ArticleDOI
TL;DR: Analyzing the observed changes in reproductive strategy of female North Sea plaice since 1900 supported the hypothesis that fishing caused a genetical change in Lmat, although an unequivocal interpretation is not possible from a descriptive study.
Abstract: This paper attempts to interpret the observed changes in reproductive strategy of female North Sea plaice since 1900 in the light of possible genetical selection exerted by the fisheries. Somatic growth of juvenile plaice increased between the 1950s and the 1980s, probably as a response to an increase in food availability. Adult growth rate was constant, except during a period of increased population abundance when somatic growth decreased. Both length (Lmat) and age at first sexual maturity decreased since 1990. No firm evidence was obtained for a change in total reproductive investment, although size-specific fecundity was reduced in the period of increased population abundance, suggesting a trade-off between egg numbers and egg size. Analysis of the phenotypic response of maturation to an increase in juvenile growth suggested that only a part of the decrease in Lmat could be ascribed to the observed increase in juvenile growth. The unexplained part of the change in Lmat corresponded with the predicted change due to genetical selection by the fisheries. This supported the hypothesis that fishing caused a genetical change in Lmat, although an unequivocal interpretation is not possible from a descriptive study.

Journal ArticleDOI
TL;DR: The results suggest that destruction of the cryptobiotic crust may ultimately result in ecosystem degradation through elimination of the predominant source of nitrogen input.
Abstract: We examined the content and isotopic composition of nitrogen within soils of a juniper woodland and found that a cryptobiotic crust composed of cyanobacteria, lichens, and mosses was the predominant source of nitrogen for this ecosystem. Disturbance of the crust has resulted in considerable spatial variability in soil nitrogen content and isotopic composition; intercanopy soils were significantly depleted in nitrogen and had greater abundance of 15N compared to intra-canopy soils. Variations in the 15N/14N ratio for inter- and intra-canopy locations followed similar Rayleigh distillation curves, indicating that the greater 15N/14N ratios for inter-canopy soils were due to relatively greater net nitrogen loss. Coverage of cryptobiotic crusts has been reduced by anthropogenic activities during the past century, and our results suggest that destruction of the cryptobiotic crust may ultimately result in ecosystem degradation through elimination of the predominant source of nitrogen input.

Journal ArticleDOI
TL;DR: The results suggest that the spatial pattern and scale of soil variability can differ markedly among edaphically identical sites and that these differences can be related to disturbance history.
Abstract: The spatial distributions of selected soil properties in two adjacent sites in southwest Michigan were examined to evaluate the potential effects of chronic disturbance on resource heterogeneity. One site was a cultivated field that had been cleared, plowed, and cropped annually for decades prior to sampling while the other, uncultivated field was cleared of original forest in 1960 after which it was mown annually but never plowed or cropped. We took replicate samples from a 330-point unaligned grid across the sites for soil pH, gravimetric moisture, inorganic phosphorus, total carbon, and net nitrification and nitrogen mineralization potentials. Soils in the cultivated site contained less than half as much carbon as in the uncultivated site, but had higher levels of inorganic phosphorus and moisture, and higher soil pH. Potential net nitrogen mineralization and nitrification rates did not differ between sites. Geostatistical analysis showed that almost all properties examined were strongly autocorelated within each site; structural variance as a proportion of sample variance ranged from 30–95% for all properties, and for any given property differed little between sites. The distance over which this dependence was expressed, however, was for all properties but pH substantially less in the uncultivated site (7–26 m) as compared to the tilled site (48–108m), especially for total C and net nitrification and N mineralization. These results suggest that the spatial pattern and scale of soil variability can differ markedly among edaphically identical sites and that these differences can be related to disturbance history.

Journal ArticleDOI
TL;DR: There was little epiphyte response to treatment during the fall, a result possibly of high ambient nutrient concentrations and low grazing pressure, and the relative influence of epipHYtes on macrophyte production may have been related to seasonally changing water temperature and macrophytes requirements for light and inorganic carbon.
Abstract: The independent and interactive effects of nutrient concentration and epiphyte grazers on epiphyte biomass and macrophyte growth and production were examined in Zostera marina L. (eelgrass) microcosms. Experiments were conducted during early summer, late summer, fall, and spring in a greenhouse on the York River estuary of Chesapeake Bay. Nutrient treatments consisted of ambient or enriched (3× ambient) concentrations of inorganic nitrogen (ammonium nitrate) and phosphate. Grazer treatments consisted of the presence or absence of field densities of isopods, amphipods, and gastropods. epiphyte biomass increased with both grazer removal and nutrient enrichment during summer and spring experiments. The effect of grazers was stronger than that of nutrients. There was little epiphyte response to treatment during the fall, a result possibly of high ambient nutrient concentrations and low grazing pressure. Under low grazer densities of early summer, macrophyte production (g m−2 d−1) was reduced by grazer removal and nutrient enrichment independently. Under high grazer densities of late summer, macrophyte production was reduced by enrichment only with grazers absent. During spring and fall there were no macrophyte responses to treatment. The relative influence of epiphytes on macrophyte production may have been related to seasonally changing water temperature and macrophyte requirements for light and inorganic carbon.

Journal ArticleDOI
TL;DR: Early-successional, shade-intolerant Betula species had high relative growth rates (RGR) and high rates of photosynthesis, nitrogen uptake and respiration when grown in high light, while fire-adapted Quercus rubra had intermediate photosynthetic rates, but had the lowest RGR and leaf area ratio and the highest root weight ratio of any species.
Abstract: The physiology, morphology and growth of first-year Betula papyrifera Marsh., Betula alleghaniensis Britton, Ostrya virginiana (Mill.) K. Koch, Acer saccharum Marsh., and Quercus rubra L. seedlings, which differ widely in reported successional affinity and shade tolerance, were compared in a controlled high-resource environment. Relative to late-successional, shade-tolerant Acer and Ostrya species, early-successional, shade-intolerant Betula species had high relative growth rates (RGR) and high rates of photosynthesis, nitrogen uptake and respiration when grown in high light. Fire-adapted Quercus rubra had intermediate photosynthetic rates, but had the lowest RGR and leaf area ratio and the highest root weight ratio of any species. Interspecific variation in RGR in high light was positively correlated with allocation to leaves and rates of photosynthesis and respiration, and negatively related to seed mass and leaf mass per unit area. Despite higher respiration rates, early-successional Betula papyrifera lost a lower percentage of daily photosynthetic CO2 gain to respiration than other species in high light. A subset comprised of the three Betulaceae family members was also grown in low light. As in high light, low-light grown Betula species had higher growth rates than tolerant Ostrya virainiana. The rapid growth habit of sarly-successional species in low light was associated with a higher proportion of biomass distributed to leaves, lower leaf mass per unit area, a lower proportion of biomass in roots, and a greater height per unit stem mass. Variation in these traits is discussed in terms of reported species ecologies in a resource availability context.

Journal ArticleDOI
TL;DR: In this article, coarse woody debris (CWD) was found to provide refuge from predation in aquatic habitats, and it was shown that CWD was the dominant above-bottom physical structure in shallow water, ranging in size from small branches (50 cm diameter) to large branches (100 cm diameter).
Abstract: This study demonstrates experimentally that coarse woody debris (CWD) can provide refuge from predation in aquatic habitats. In the Rhode River subestuary of Chesapeake Bay, Maryland, (USA), we (1) measured the abundance of CWD, (2) examined the utilization of CWD by mobile epibenthic fish and crustaceans, and (3) tested experimentally the value of CWD as a refuge from predation. CWD was the dominant above-bottom physical structure in shallow water, ranging in size from small branches ( 50 cm diameter). In response to experimental additions of CWD, densities of common epibenthic cpecies (Callinectes sapidus, Fundulus heteroclitus, Fundulus majalis, Gobiosoma bosc, Gobiesox strumosus, Palaemonetes pugio, and Rithropanopeus harrisii) increased significantly compared to control sites without CWD. In laboratory experiments, grass shrimp (P. pugio) responded to predatory fish (F. heteroclitus and Micropogonias undulatus) by utilizing shelter at CWD more frequently than in absence of fish. Access to CWD increased survivorship of grass shrimp in laboratory and field experiments. These experimental results (1) support the hypothesis, commonly proposed but untested for freshwater habitats, that CWD can provide a refuge from predation for epibenthic fish and invertebrates and (2) extend the recognized functional importance of CWD in freshwater to estuarine and marine communities. We hypothesize that CWD is an especially important refuge habitat in the many estuarine and freshwater systems for which alternative physical structure (e.g., vegetation or oyster reefs) are absent or in low abundance.

Journal ArticleDOI
TL;DR: The relatively ineffective screening of UV-B by foliage of many deciduous plants suggests they may be more responsive to enhancedUV-B than evergreen species.
Abstract: The ultraviolet-B radiation (UV-B, 300 nm) screening effectiveness of foliage of a diverse group of plants was examined by measuring epidermal transmittance and depth of penetration of UV-B with a fiberoptic microprobe. In addition, the concentration of UV-B-absorbing compounds and various anatomical characteristics were measured to assess whether they were useful predictors of UV-B screening. Sun foliage of naturally growing individuals of seven species were sampled in each of six life forms comprising two evergreen groups (gymnosperms and angiosperms) and four deciduous angiosperm groups (trees, shrubs and vines, herbaceous dicotyledons, and grasses). There was significant life-form variation in epidermal transmittance and depth of penetration of UV-B, concentration of UV-B-absorbing compounds (leaf-area basis), epidermal (including cuticle and hypodermis) thickness, and specific leaf area. Values of these parameters tended to be related to leaf longevity, with the most notable differences apparent between evergreen and deciduous species. The mean epidermal transmittance and depth of penetration of UV-B in foliage averaged 4% and 32 μm in evergreens, compared to 28% and 75 μm in deciduous species. These values are conservative estimates since the microprobe was oriented in foliage such that much of the side- and backscattered UV-B was ignored. The strongest predictors of epidermal transmittance and depth of penetration were epidermal thickness and the concentration of absorbing compounds, which averaged 32 μm and 1.50 A cm−2 in evergreens, but only 19 μm and 0.99 A cm−2 in deciduous foliage. However, the variation found in these relationships implies that additional factors warrant consideration in assessing UV-B-screening effectiveness. The relatively ineffective screening of UV-B by foliage of many deciduous plants suggests they may be more responsive to enhanced UV-B than evergreen species.

Journal ArticleDOI
TL;DR: The mechanism by which A. lanipes influences algae and benthic insects is evaluated by comparing patterns of algal biomass, taxonomic composition, and insect abundance between shrimp-exclusion and shrimp-presence treatments both with and without manual sediment removal, supporting the hypothesis that sediment removal enhances the biovolume of understory algal taxa.
Abstract: Freshwater shrimp dominate the faunal biomass of many headwater tropical streams: however, their role in community organization is unclear. Enclosure/exclosure experiments in a montane Puerto Rican stream examined direct and indirect effects of two dominant taxa of atyid (Atyidae) shrimp, Atya lanipes Holthuis and Xiphocaris elongata Guerin-Meneville. Both shrimp taxa caused significant reductions in sediment cover on rock substrata, reducing sedimentation and enhancing algal biovolume on clay tiles in cages. When tiles incubated in shrimp exclosures for 2 wks were placed outside of cages, atyid shrimp removed 100% of the sediment cover within a 30 min observation period. Atyid shrimp appear to play an important role in stream recovery after high discharge events by rapidly removing sediments and detritus deposited on benthic substrata in pools. We evaluated the mechanism by which A. lanipes influences algae and benthic insects by comparing patterns of algal biomass, taxonomic composition, and insect abundance between shrimp-exclusion and shrimp-presence treatments both with and without manual sediment removal. The shrimp exclusion treatment without manual sediment removal bad significantly lower algal biomass and greater sedimentation than all other treatments. The treatment in which shrimp were excluded but sediment was manually removed, however, accrued almost the same algal biovolume as the shrimp enclosure treatment, supporting the hypothesis that sediment removal enhances the biovolume of understory algal taxa. Algal community composition was similar between stream bottom bedrock exposed to natural densities of shrimp and all experimental treatments for both Atya and Xiphocaris: a diatom community strongly dominated (78–95%) by the adnate taxon, Achnanthes lanceolata Breb ex. Kutz. Atyid shrimp are important in determining the distribution and abundance of benthic insects through both direct and indirect effects. Sessile, retreat-building chironomid larvae (Chironomidae: Diptera) are negatively affected by both A. lanipes and X. elongata, through direct removal by foraging activities and/or indirectly through depression of sediment resources available to larvae for the construction of retreats. In constrast, the mobile grazer, Cloeodes maculipes (Baetidae: Ephemeroptera) was not adversely affected and atyid shrimp have the potential to exert positive indirect effects on this taxon by facilitating its exploitation of algal resources and/or through enhancement of understory algal food resources through sediment removal.

Journal ArticleDOI
TL;DR: The freshwater crayfish was one of the few taxa that appeared to obtain its biomass carbon from detrital material, and the enormous biomass of littoral and fringing vegetation could contribute to metazoan food webs in these billabongs only if an additional highly 13C-depleted source was consumed simultaneously.
Abstract: We used the stable isotopes of carbon and nitrogen to examine the food webs of three small flood-plain lakes (billabongs) in south-eastern Australia. With few exceptions, stable carbon isotope analysis could not be used to discriminate among the conspicuous potential sources of fringing, emergent or floating vegetation or benthic detritus. These primary sources showed little spatial or temporal variation in δ13C values, with means ranging from-28.5 to-26.8‰ in spring and-29.1 to-25.4‰ in late summer. Submerged vegetation had similar δ13C values to the above sources in spring but showed greater spatial variation and were less 13C-depleted, considerably so in some species, in late summer. Epiphytes and algae were 13C-depleted in spring compared with the other primary sources but became more 13C-enriched in late summer. Mean δ13C values for primary and secondary consumers were not only far more variable (-37.4 to-22.7‰) but in general were more negative than the potential food sources, particularly in spring. Using the combined information from stable carbon and nitrogen isotope analysis, we could narrow down the list of potential primary sources driving food webs in these billabongs. The freshwater crayfish (Cherax) was one of the few taxa that appeared to obtain its biomass carbon from detrital material. Gastropods and leptocerid caddis larvae on emergent or submerged vegetation obtained a mixture of carbon from epiphytes and macrophytes; in both taxa, epiphytes contributed more to biomass carbon than did the macrophytes. However, other common grazers and collector/gatherers sampled from macrophytes, e.g. baetid mayflies, chironomid larvae and atyid shrimps, were often too 13C-depleted even to have derived their biomass carbon solely from epiphytes. Many other primary consumers, including zooplankton, and mussels (Velesunio), and most of the secondary consumers, including water mites (Hydracarina), phantom midge larvae (Chaoborus) and fish, were also 13C-depleted. The enormous biomass of littoral and fringing vegetation could contribute to metazoan food webs in these billabongs only if an additional highly 13C-depleted source was consumed simultaneously. Methane released from billabong sediments could provide such a 13C-depleted carbon source that is re-introduced into metazoan food webs via the consumption of methanotrophic bacteria. Alternatively, food webs in these water bodies are largely driven by an unknown and inconspicuous 13C-depleted primary producer, such as planktonic Chlorophyta.

Journal ArticleDOI
TL;DR: Top predatory nematodes were sometimes strongly related to the microbial but not microbial-feeding trophic levels, indicating that microbial biomass may directly influence top predator numbers, and that the intermediate level may simply serve as a conduit by which resources pass from the bottom to top trophIC levels.
Abstract: The relative importance of predation and competition (resource limitation) in influencing the components of a below-ground food-web consisting of three trophic levels (bacteria and fungi; bacterial-feeding and fungal-feeding nematodes; and top predatory nematodes) was estimated using microbial biomass and nematode frequency data collected throughout a 1-year period in two agro-ecosystems. The study suggested that bacterial and fungal biomass were likely to be regulated by grazing and competition respectively, and that these differences were likely to be attributed to the biological (probably morphological) differences between bacteria and fungi, in contrast to the predictions of the hypothesis of Hairston et al. (1960). Top predatory nematodes were sometimes strongly related to the microbial but not microbial-feeding trophic levels, indicating that microbial biomass may directly influence top predator numbers, and that the intermediate level may simply serve as a conduit by which resources pass from the bottom to top trophic levels. This study also suggests that the detritus food-web acts as two distinct (bacterial-and fungal-based) compartments.

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TL;DR: It is suggested that large-scale increase in forest fragmentation affects the interaction between these natural enemies and forest tent caterpillar populations, and increased clearing and fragmentation of boreal forests, by agriculture and forestry, may be exacerbating outbreaks of this forest defoliator.
Abstract: I examined historical data (1950–1984) on the duration of outbreaks of the forest tent caterpillar (Malacosoma disstria) in northern Ontario, Canada. Outbreak duration was compared to host tree species dominance and forest structure over large areas of boreal forest partially cleared for agriculture. Abundance of the principal host tree species Populus tremuloides had no consistent effect on duration of outbreak within forest districts, and was negatively correlated with duration of outbreaks among the eight forest districts examined. The amount of forest edge per km2 was the best, and most consistent, predictor of the duration of tent caterpillar outbreaks both within individual forest districts and among forest districts. Because forest tent caterpillar populations are driven largely by the impact of parasitoids and pathogens, results here suggest that large-scale increase in forest fragmentation affects the interaction between these natural enemies and forest tent caterpillar. Increased clearing and fragmentation of boreal forests, by agriculture and forestry, may be exacerbating outbreaks of this forest defoliator.

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TL;DR: This paper quantifies the influences of maternal age on egg size and offspring performance of the bruchid beetle, Callosobruchus maculatus, and examines whether nutrients transferred during copulation reduce the magnitude of maternalAge effects on egg Size and larval performance when mothers are nutrient-stressed.
Abstract: Maternal age influences offspring quality of many species of insects. This observed maternal age influence on offspring performance may be mediated through maternal age effects on egg size, which in turn may be directly influenced by the female's nutritional state. Thus, behaviors that influence a female's nutritional status will indirectly influence egg size, and possibly offspring life histories. Because males provide nutrients to females in their ejaculate, female mating frequency is one behavior which may influence her nutritional status, and thus the size of her eggs and the performance of her offspring. In this paper, I first quantify the influences of maternal age on egg size and offspring performance of the bruchid beetle, Callosobruchus maculatus. I then examine whether nutrients transferred during copulation reduce the magnitude of maternal age effects on egg size and larval performance when mothers are nutrient-stressed. Egg size and egg hatchability decreased, and development time increased, with increasing maternal age. Multiple mating and adult feeding by females both resulted in increased egg size. This increase in egg size of females mated multiply did not translate into reduced development time or increased body size and egg hatchability, but did correlate with improved survivorship of offspring produced by old mothers. Thus, it appears that because the influence of mating frequency on egg size is small relative to the influence of maternal age, the influence of nutrients derived from multiple mating on offspring life history is almost undetectable (detected only as a small influence on survivorship). For C. maculatus, female multiple mating has been demonstrated to increase adult female survivorship (Fox 1993a), egg production (Credland and Wright 1989; Fox 1993a), egg size, and larval survivorship, but, contrary to the suggestion of Wasserman and Asami (1985), multiple mating had no detectable influence on offspring development time or body size.

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TL;DR: Results indicate that a CO2-induced reduction in plant nitrogen concentration may not be due to physiological changes in plantnitrogen use efficiency, but is probably a size-dependent phenomenon resulting from accelerated plant growth.
Abstract: Plants often respond to elevated atmospheric CO2 levels with reduced tissue nitrogen concentrations relative to ambient CO2-grown plants when comparisons are made at a common time. Another common response to enriched CO2 atmospheres is an acceleration in plant growth rates. Because plant nitrogen concentrations are often highest in seedlings and subsequently decrease during growth, comparisons between ambient and elevated CO2-grown plants made at a common time may not demonstrate CO2-induced reductions in plant nitrogen concentration per se. Rather, this comparison may be highlighting differences in nitrogen concentration between bigger, more developed plants and smaller, less developed plants. In this study, we directly examined whether elevated CO2 environments reduce plant nitrogen concentrations independent of changes in plant growth rates. We grew two annual plant species. Abutilon theophrasti (C3 photosynthetic pathway) and Amaranthus retroflexus (C4 photosynthetic pathway), from seed in glass-sided growth chambers with atmospheric CO2 levels of 350 μmol·mol−1 or 700 μmol·mol−1 and with high or low fertilizer applications. Individual plants were harvested every 2 days starting 3 days after germination to determine plant biomass and nitrogen concentration. We found: 1. High CO2-grown plants had reduced nitrogen concentrations and increased biomass relative to ambient CO2-grown plants when compared at a common time; 2. Tissue nitrogen concentrations did not vary as a function of CO2 level when plants were compared at a common size; and 3. The rate of biomass accumulation per rate of increase in plant nitrogen was unaffected by CO2 availability, but was altered by nutrient availability. These results indicate that a CO2-induced reduction in plant nitrogen concentration may not be due to physiological changes in plant nitrogen use efficiency, but is probably a size-dependent phenomenon resulting from accelerated plant growth.

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TL;DR: Phenological data suggest that certain species of columnar cacti and at least one group of paniculate Agaves on the Mexican mainland provide a spatio-temporally predictable nectar corridor along which nectarivorous bats may migrate in the spring and fall, respectively.
Abstract: Three species of nectar-feeding bats migrate from tropical and subtropical Mexico into the Sonoran and Chihuahuan deserts during the spring and summer months. We examined geographic and seasonal changes in the diet of one migrant species, Leptonycteris curasoae, using carbon stable isotope techniques to determine the relative importance of C3 and CAM (Cactaceae, Agavaceae) plants in its diet. We also examined the diet of a non-migratory nectar-feeding bat, Glossophaga soricina, from southern Mexico using the same techniques. We found that L. curasoae feeds extensively or exclusively on CAM plants during migration and in the northern part of its range and feeds mostly on C3 plants in southern Mexico. This bat is a year-round resident on Baja California where it is a CAM specialist. The non-migrant G. soricina feeds mostly on C3 plants year-round. Phenological data suggest that certain species of columnar cacti and at least one group of paniculate Agaves on the Mexican mainland provide a spatio-temporally predictable nectar corridor along which nectarivorous bats may migrate in the spring and fall, respectively. Different flowering schedules of Agaves in Baja California appear to promote year-round dietary specialization and perhaps non-migratory behavior in nectar-feeding bats living there.

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TL;DR: Identifying the major determinants of variation in growth (e.g. LWR*Amass) across light environments, species and ontogeny contributes to the establishment of a framework for exploring limits to productivity and the nature of ecological success as measured by growth.
Abstract: The influence of ontogeny, light environment and species on relationships of relative growth rate (RGR) to physiological and morphological traits were examined for first-year northern hardwood tree seedlings. Three Betulaceae species (Betula papyrifera, Betula alleghaniensis and Ostrya virginiana) were grown in high and low light and Quercus rubra and Acer saccharum were grown only in high light. Plant traits were determined at four ages: 41, 62, 83 and 104 days after germination. In high light (610 μmol m−2 s−1 PPFD), across species and ages, RGR was positively related to the proportion of the plant in leaves (leaf weight ratio, LWR; leaf area ratio, LAR), in situ rates of average canopy net photosynthesis (A) per unit mass (Amass) and per unit area (Aarea), and rates of leaf, stem and root respiration. In low light (127 μmol m−2 s−1 PPFD), RGR was not correlated with Amass and Aarea whereas RGR was positively correlated with LAR, LWR, and rates of root and stem respiration. RGR was negatively correlated with leaf mass per area in both high and low light. Across light levels, relationships of CO2 exchange and morphological characteristics with RGR were generally weaker than within light environments. Moreover, relationships were weaker for plant parameters containing a leaf area component (leaf mass per area, LAR and Aarea), than those that were solely mass-based (respiration rates, LWR and Amass). Across light environments, parameters incorporating the proportion of the plant in leaves and rates of photosynthesis explained a greater amount of variation in RGR (e.g. LWR*Amass, R2=0.64) than did any single parameter related to whole-plant carbon gain. RGR generally declined with age and mass, which were used as scalars of ontogeny. LWR (and LAR) also declined for seven of the eight species-light treatments and A declined in four of the five species in high light. Decreasing LWR and A with ontogeny may have been partially responsible for decreasing RGR. Declines in RGR were not due to increased respiration resulting from an increase in the proportion of solely respiring tissue (roots and stems). In general, although LWR declined with ontogeny, specific rates of leaf, stem, and root respiration also decreased. The net result was that whole-plant respiration rates per unit leaf mass decreased for all eight treatments. Identifying the major determinants of variation in growth (e.g. LWR*Amass) across light environments, species and ontogeny contributes to the establishment of a framework for exploring limits to productivity and the nature of ecological success as measured by growth. The generality of these relationships both across the sources of variation we explored here and across other sources of variation in RGR needs further study.

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TL;DR: Examination of the effect of native large herbivores on aboveground primary production of nonforested habitat in Yellowstone National Park, Wyoming suggests that stimulation of aboveground production by ungulate may be, in part, due to the migratory behavior of native ungulates that track young, high quality forage as it shifts spatially across the Yellowstone ecosystem.
Abstract: We examined the effect of native large herbivores on aboveground primary production of nonforested habitat in Yellowstone National Park, Wyoming Productivity of vegetation grazed by elk (Cervus elaphus) and bison (Bison bison) was compared with that of ungrazed (permanently fenced) vegetation at four sites Two methods were used that, we believed, would provide the most accurate measurements under the different grazing regimes encountered in the study Production of ungrazed vegetation in permanent exclosures (10×10 m or 15×15 m, 3 per site) and that of vegetation that was grazed only in the winter was taken as peak standing crop Production of vegetation grazed during the growing season was the sum of significant increments (P<005) in standing crop inside temporary exclosures (15×15 m, 6 per site) moved every four weeks to account for herbivory Aboveground productivity of grazed vegetation was 47% higher than that of ungrazed vegetation across sites (P<00003) This result could be explained by either a methodological or grazer effect We believe it was the latter Results from a computer simulation showed that sequential sampling with temporary exclosures resulted in a slight underestimation of production, suggesting that the reported differences between treatments were conservative We suggest that stimulation of aboveground production by ungulates may be, in part, due to the migratory behavior of native ungulates that track young, high quality forage as it shifts spatially across the Yellowstone ecosystem

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TL;DR: It is proposed that a wasp's reproductive strategy, as opposed to hyperparasitism, is the dominant factor in aphidiid population dynamics, and that a parasitoid's potential to regulate the host population is largely determined by its foraging strategy.
Abstract: Aphidiid parasitoids (Hymenoptera: Aphidiidae) of aphids generally exploit only a small percentage of the available host resources in the field. This limited impact on aphid populations has often been explained as a consequence of hyperparasitism. We propose that a wasp's reproductive strategy, as opposed to hyperparasitism, is the dominant factor in aphidiid population dynamics. A wasp's foraging efficiency and oviposition decisions are influenced by several variables, including searching behaviour between and within patches, host choice (as modified by the aphids' defensive behaviours), and plant structural complexity. Two broadly different patterns of host exploitation have evolved in aphidiid wasps in relation to ant-aphid mutualism. Firstly, in species that are exposed to predation and hyperparasitism, a female may leave a patch before all suitable hosts are parasitized. Because predators and hyperparasitoids tend to aggregate at high aphid or aphidiid densities, or in response to aphid honeydew, this strategy enables females to reduce offspring mortality by “spreading the risk” over several host patches. Secondly, in species that have evolved mechanisms to avoid aggression by mutualistic ants, females are able to exploit a hyperparasitoid-free resource space. Such species may concentrate their eggs in only a few aphid colonies, which are thus heavily exploited. Although hyperparasitism of species in the first group tends to reach high levels, its overall impact on aphid-aphidiid population dynamics is probably limited by the low average fecundity of most hyperparasitoids. We discuss the foraging patterns of aphidiid wasps in relation to aphid population regulation in general, and to classical biological control in particular. We argue that a parasitoid's potential to regulate the host population is largely determined by its foraging strategy. In an exotic parasitoid, a behavioural syndrome that has evolved and presumably is adaptive in a more diverse (native) environment may, in a more uniform (managed) environment, result in suboptimal patch-leaving and oviposition decisions, and possibly increased resource usage.

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TL;DR: The driving force for the allocation of nitrogen within a canopy is the difference between the leaf nitrogen content that is required to bring Wc and Wj into balance and the current nitrogen content, which is very similar to that obtained using optimization theory.
Abstract: It has long been observed that leaf nitrogen concentrations decline with depth in closed canopies in a number of plant communities. This phenomenon is generally believed to be related to a changing radiation environment and it has been suggested by some researchers that plants allocate nitrogen in order to optimize total whole canopy photosynthesis. Although optimization theory has been successfully utilized to describe a variety of physiological and ecological phenomena, it has some shortcomings that are subject to criticism (e.g., time constraints, oversimplifications, lack of insights, etc.). In this paper we present an alternative to the optimization theory of plant canopy nitrogen distribution, which we term coordination theory. We hypothesize that plants allocate nitrogen to maintain a balance between two processes, each of which is dependent on leaf nitrogen content and each of which potentially limits photosynthesis. These two processes are defined as Wc, the Rubiscolimited rate of carboxylation, and Wj, the electron transport-limited rate of carboxylation. We suggest that plants allocate nitrogen differentially to, leaves in different canopy layers in such a way that Wc and Wj remain roughly balanced. In this scheme, the driving force for the allocation of nitrogen within a canopy is the difference between the leaf nitrogen content that is required to bring Wc and Wj into balance and the current nitrogen content. We show that the daily carbon assimilation of a canopy with a nitrogen distribution resulting from this internal coordination of Wc and Wj is very similar to that obtained using optimization theory.

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TL;DR: The results have important ecological implications in showing the underlying role of food in the selection of habitat by ruminants, and that simple, mechanistic models of forage intake and digestion can be scaled up to the level of animal behavioural choices.
Abstract: We tested the idea that ruminants allocate their feeding time to habitat patches in relation to foraging efficiency. We used five tame red deer (Cervus elaphus) in an enclosure planted with four treatment of timothy grass (Phleum pratense) differing in their stage of growth. Older swards offered higher biomass but lower nutritional quality than younger swards. We observed time spent feeding in each treatment during each of seven trials. We measured goodness-of-fit between observed times and predictions from two alternative hypotheses differing in optimization strategy (maximizing versus matching), and a third, null hypothesis. We tested the hypotheses using two alternative currecies: digestible protein, and digestible dry matter or energy. Although digestible protein concentration and dry-matter digestibility were highly correlated (r=0.763, P<0.001), the wider range of digestible protein made it the much more sensitive measure of forage quality. Distributions of feeding time closely matched estimated intake rates of digestible protein (R infPredsup2 =0.899) across all animals and trials. The other hypotheses were rejected. The results have important ecological implications in showing the underlying role of food in the selection of habitat by ruminants, and that simple, mechanistic models of forage intake and digestion can be scaled up to the level of animal behavioural choices.

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TL;DR: The behavior of these assemblages suggest that small ant species at La Selva potentially compete with the entire range of ant body sizes, whereas large ants forage when and where small ants are inactive; and seeds dispersed to the forest floor at dawn will be consumed or further dispersed by a larger suite of ants species than those falling in the heat of the tropical afternoon.
Abstract: The stability of tropical microclimates has left microclimate use by tropical species little unexplored. At La Selva Costa Rica, I related foraging activity at seed baits to humidity in two forests types. I recorded 38 and 35 ant species at seed baits in closed and open canopy forest. The microclimate 5 cm above the forest floor in the younger, Open Forest was warmer, drier, more variable, and more sensitive to current weather than in the older Closed Forest. Ant species within both forests foraged at different Vapor Pressure Deficits (kPa), a measure of the drying power of the air. VPD use was not confounded with diel activity patterns. Body size explained 46% of the variance in mean VPD use among ant species. Small ant species tended to forage in moist microclimates; large species tended to be microclimate generalists. Larger species were also more active in the drier Open Forest. Foraging activity by these assemblages varies 4-fold, and peaks close to the mean VPD for each habitat. The behavior of these assemblages suggest that 1) small ant species at La Selva potentially compete with the entire range of ant body sizes, whereas large ants forage when and where small ants are inactive; and 2) seeds dispersed to the forest floor at dawn will be consumed or further dispersed by a larger suite of ants species than those falling in the heat of the tropical afternoon.