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Conservation strategies for populations of woodland caribou Rangifer tarandus caribou frequently emphasize the importance of predator–prey relationships and the availability of lichen-rich late seral forests, yet the importance of summer diet and forage availability to woodland caribou survival is poorly understood. In a recent article, Wittmer et al. ( Can J Zool 83:407–418, ) concluded that woodland caribou in British Columbia were declining as a consequence of increased predation that was facilitated by habitat alteration. Their conclusion is consistent with the findings of other authors who have suggested that predation is the most important proximal factor limiting woodland caribou populations (Bergerud and Elliot in Can J Zool 64:1515–1529, ; Edmonds in Can J Zool 66:817–826, ; Rettie and Messier in Can J Zool 76:251–259, ; Hayes et al. in Wildl Monogr 152:1–35, ). Wittmer et al. ( Can J Zool 83:407–418, ) presented three alternative, contrasting hypotheses for caribou decline that differed in terms of predicted differences in instantaneous rates of increase, pregnancy rates, causes of mortality, and seasonal vulnerability to mortality (Table 1, p 258). These authors rejected the hypotheses that food or an interaction between food and predation was responsible for observed declines in caribou populations; however, the use of pregnancy rate, mortality season and cause of mortality to contrast the alternative hypotheses is problematic. We argue here that the data employed in their study were insufficient to properly evaluate a predation-sensitive foraging hypothesis for caribou decline. Empirical data on seasonal forage availability and quality and plane of nutrition of caribou would be required to test the competing hypotheses. We suggest that methodological limitations in studies of woodland caribou population dynamics prohibit proper evaluation of the mechanism of caribou population declines and fail to elucidate potential interactions between top-down and bottom-up effects on populations. [ABSTRACT FROM AUTHOR]

Canopy structure and physiology of paper birch are examined in the field during two growing seasons. Leaves are found to be steeply inclined in the upper canopy levels and foliage mass is located higher in the canopy than frequently observed in more shade tolerant species. Functional relationships between the controlling environmental factors and the physiological processes of leaf resistance to water vapor diffusion and gross photosynthesis are used in a computer simulation model to examine production and water use aspects of paper birch. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This study examined the independent and interactive effects of elevated carbon dioxide (CO) and ozone (O) on the foliar quality of two deciduous trees species and the performance of two outbreak herbivore species. Trembling aspen ( Populus tremuloides) and paper birch ( Betula papyrifera) were grown at the Aspen FACE research site in northern Wisconsin, USA, under four combinations of ambient and elevated CO and O. We measured the effects of elevated CO and O on aspen and birch phytochemistry and on gypsy moth ( Lymantria dispar) and forest tent caterpillar ( Malacosoma disstria) performance. Elevated CO nominally affected foliar quality for both tree species. Elevated O negatively affected aspen foliar quality, but only marginally influenced birch foliar quality. Elevated CO slightly improved herbivore performance, while elevated O decreased herbivore performance, and both responses were stronger on aspen than birch. Interestingly, elevated CO largely offset decreased herbivore performance under elevated O. Nitrogen, lignin, and C:N were identified as having strong influences on herbivore performance when larvae were fed aspen, but no significant relationships were observed for insects fed birch. Our results support the notion that herbivore performance can be affected by atmospheric change through altered foliar quality, but how herbivores will respond will depend on interactions among CO, O, and tree species. An emergent finding from this study is that tree age and longevity of exposure to pollutants may influence the effects of elevated CO and O on plant-herbivore interactions, highlighting the need to continue long-term atmospheric change research. [ABSTRACT FROM AUTHOR]

Light with decreased red:far-red (R:FR) ratios may signal neighbor presence and trigger plant developmental responses. There is some evidence that plant canopies forage towards increased R:FR ratios, but it is unclear to what extent R:FR versus the total amount of photosynthetically active radiation (PAR) influences canopy foraging responses among forest trees. The objective of this study was to examine the relative importance of PAR and R:FR as photosensory cues leading to tree canopy foraging responses. Seedlings of Betula papyrifera Marshall (paper birch) were grown in an experimental garden. Each seedling was germinated and grown in its own shading structure and exposed to two spatially separated light environments, in a factorial design of PAR and R:FR. Plant canopy foraging was evaluated at the end of one growing season in terms of canopy displacement, canopy area, leaf number, direction of stem lean, petiole aspect, and lamina aspect with respect to experimental light treatments. Leaf number and canopy area were greater on the high PAR sides of plants, irrespective of the R:FR treatment. Seedling canopies were displaced towards the direction of high PAR, but this relationship was not significant across all treatments. Petiole aspect was random and showed no significant directedness towards any of the light treatments. Lamina aspect and the direction of stem lean were distributed towards the direction of high PAR, irrespective of the R:FR treatment. Overall, first-year B. papyrifera seedlings used PAR, rather than R:FR ratio, as a photosensory cue for canopy light foraging. [ABSTRACT FROM AUTHOR]

Rising atmospheric CO2 may stimulate future forest productivity, possibly increasing carbon storage in terrestrial ecosystems, but how tropospheric ozone will modify this response is unknown. Because of the importance of fine roots to the belowground C cycle, we monitored fine-root biomass and associated C fluxes in regenerating stands of trembling aspen, and mixed stands of trembling aspen and paper birch at FACTS-II, the Aspen FACE project in Rhinelander, Wisconsin. Free-air CO2 enrichment (FACE) was used to elevate concentrations of CO2 (average enrichment concentration 535 µl l–1) and O3 (53 nl l–1) in developing forest stands in 1998 and 1999. Soil respiration, soil pCO2, and dissolved organic carbon in soil solution (DOC) were monitored biweekly. Soil respiration was measured with a portable infrared gas analyzer. Soil pCO2 and DOC samples were collected from soil gas wells and tension lysimeters, respectively, at depths of 15, 30, and 125 cm. Fine-root biomass averaged 263 g m–2 in control plots and increased 96% under elevated CO2. The increased root biomass was accompanied by a 39% increase in soil respiration and a 27% increase in soil pCO2. Both soil respiration and pCO2 exhibited a strong seasonal signal, which was positively correlated with soil temperature. DOC concentrations in soil solution averaged ~12 mg l–1 in surface horizons, declined with depth, and were little affected by the treatments. A simplified belowground C budget for the site indicated that native soil organic matter still dominated the system, and that soil respiration was by far the largest flux. Ozone decreased the above responses to elevated CO2, but effects were rarely statistically significant. We conclude that regenerating stands of northern hardwoods have the potential for substantially greater C input to soil due to greater fine-root production under elevated CO2. Greater fine-root biomass will be accompanied by greater soil C efflux as soil respiration, but leaching losses of C will probably be unaffected. [ABSTRACT FROM AUTHOR]

We examined the effects of CO-mediated changes in the foliar chemistry of paper birch ( Betula papyrifera) and white pine ( Pinus strobus) on performance of the gypsy moth ( Lymantria dispar). Trees were grown under ambient or enriched CO conditions, and foliage was subjected to plant chemical assays and insect bioassays. Enriched CO atmospheres reduced foliar nitrogen levels and increased condensed tannin levels in birch but not in pine. Foliar carbohydrate concentrations were not markedly altered by CO environment. Gypsy moth performance was significantly affected by CO level, species, and the CO x species interaction. Under elevated CO conditions, growth was reduced for larvae fed birch, while development was prolonged for larvae fed pine. Although gypsy moths performed better overall on birch than pine, birch-fed larvae were influenced more by CO-mediated changes in host quality. [ABSTRACT FROM AUTHOR]

Mature growth-phase internodes of Alaska paper birch ( Betula resinifera) are preferred by the snowshoe hare ( Lepus americanus) over juvenile growth-phase internodes due to the low food value of the latter. While the mature over juvenile preferencec cannot be explained by the levels of inorganic nutrients or gross chemical fractions (resins or phenols), it can be explained by the striking differences in secondary metabolites of the two growth phases. The principle compound which renders the juvenile phase internodes unpalatable is papyriferic acid, a triterpene which is a demonstrated feeding deterrent to snowshoe hares and which is present in juvenile internodes at concentrations 25 times greater than those in mature internodes. [ABSTRACT FROM AUTHOR]

Contrary to Kuno's (1981) contention, dispersing does not help and individual to get a larger average progeny in an unpredictable and heterogeneous but nonlimiting environment: average progeny is exactly equal for (partially) dispersing and nondispersing populations. However, the geometric time averages of pro-capita reproduction as well as geometric averages over replicates of final progeny size after a fixed number of years differ, just as Kuno asserts. Moreover, if populations of the two types are grown in mixed culture it is the disperser who will win in the long run. This even applies if dispersal means the incurring of some additional mortality. Models with partial dispersal are much more complicated to deal with than models with either a complete redistribution each generation or no dispersal at all, contrary to the assertion of e.g. Venable and Lawlor (1980). Partial dispersers will win from nondispersers, but the optimal amount of dispersal unfortunately seems to depend sensitively on the details of the model specification, except that it has to be small if the number of independent patches is large. [ABSTRACT FROM AUTHOR]

Evidence from tropical forests suggests understory habitats are associated with a high risk of disease, which may prevent the establishment of vulnerable tree species; in contrast, canopy gaps can act as refuges from these pathogens. However, few studies have investigated the impacts of pathogens on regeneration in temperate forests. To determine whether losses to fungi of seeds of Betula papyrifera, a light-loving species, varied between habitats that differed in their degree of openness, we applied fungicide to seeds buried in old fields, treefall gaps, and forest understory sites. We found that the application of fungicide significantly reduced losses in all habitats, relative to control values. This effect was habitat-dependent: the benefit of fungicide was greater in forest understory than in openings. This suggests that B. papyrifera is prevented from establishing in understory environments in part by its susceptibility to pathogen attack, and not solely because of a high light requirement. [ABSTRACT FROM AUTHOR]

Herbivory can influence ecosystem productivity, but recent evidence suggests that damage by herbivores modulates potential productivity specific to damage type. Because productivity is linked to photosynthesis at the leaf level, which in turn is influenced by atmospheric CO concentrations, we investigated how different herbivore damage types alter component processes of photosynthesis under ambient and elevated atmospheric CO. We examined spatial patterns in chlorophyll fluorescence and the temperature of leaves damaged by leaf-chewing, gall-forming, and leaf-folding insects in aspen trees as well as by leaf-chewing insects in birch trees under ambient and elevated CO at the aspen free-air CO enrichment (FACE) site in Wisconsin. Both defoliation and gall damage suppressed the operating efficiency of photosystem II (ΦPSII) in remaining leaf tissue, and the distance that damage propagated into visibly undamaged tissue was marginally attenuated under elevated CO. Elevated CO increased leaf temperatures, which reduced the cooling effect of gall formation and freshly chewed leaf tissue. These results provide mechanistic insight into how different damage types influence the remaining, visibly undamaged leaf tissue, and suggest that elevated CO may reduce the effects of herbivory on the primary photochemistry controlling photosynthesis. [ABSTRACT FROM AUTHOR]

We argue that tests for density dependence are useful in analyses of population dynamics and suggest guide lines for their use and interpretation of results which avoid many of the problems discussed by Wolda and Dennis (1993). Processes other than density dependence per se can cause statistical tests to indicate the presence of density dependence (Wolda and Dennis 1993 and unpublished simulations). Tests for density dependence cannot reveal the mechanism of regulation, but they do indicate the nature of long-term population dynamics. Tests for density dependence give misleading results if sampling is not at generation intervals; however, this problem is avoided if we only use tests on data collected in each generation (Holyoak 1993a). Similarly, species should be semelparous. Non-delayed density dependence should not be considered without looking for delayed density dependence, since the presence of delayed density dependence can lead to over-detection of non-delayed density dependence (Woiwod and Hanski 1992; Holyoak 1993b). The partial autocorrelation function and knowledge of life-history are more useful than tests for density dependence for indicating whether any density dependence is delayed or not (Royama 1992; Holyoak 1993b). Estimation error with a constant upper size limit causes tests for density dependence to overestimate the frequency of delayed density dependence; however we do not know whether estimation error is bounded in real populations. Work in progress suggests that 20-40 generations (depending on the nature of population dynamics) gives a moderate level of accuracy with tests for density dependence, and >40 generations are necessary for tests to be accurate in their assessment of the strength of density dependence. We conclude that tests are useful indicators of whether density dependence, or other feedback mechanisms are likely to be acting. [ABSTRACT FROM AUTHOR]

This paper combines published and original data on sexual size dimorphism, reproductive behavior, and habitat types in turtles. Our major finding is that observed patterns of sexual size dimorphism correlate with habitat type and male mating strategy. (1) In most terrestrial species, males engage in combat with each other. Males typically grow larger than females. (2) In semiaquatic and 'bottom-walking' aquatic species, male combat is less common, but males often forcibly inseminate females. As in terrestrial species, males are usually larger than females. (3) In truly aquatic species, male combat and forcible insemination are rare. Instead, males utilize elaborate precoital displays, and female choice is highly important. Males are usually smaller than females. We interpret these correlations between sexual behavior and size dimorphism in terms of sexual selection theory: males are larger than females when large male size evolves as an adaptation to increase success in male combat, or to enable forcible insemination of females. In contrast, males are usually smaller than females where small size in males evolves to increase mobility (and hence, ability to locate females), or because selection for increased fecundity may result in increased female size. In turtle species with male combat or forcible insemination, the degree of male size superiority increases with mean species body size. [ABSTRACT FROM AUTHOR]

Geographic patterns of intraspecific variations in traits related to photosynthesis and biomass were examined in two separate common garden experiments using seed collected from 26 Sitka alder (Alnus sinuata Rydb.) and 18 paper birch (Betula papyrifera Marsh.) populations from climatically diverse locations in British Columbia, Canada. Exchange rates of carbon dioxide and water vapour were measured on 2-year-old seedlings to determine the maximum net instantaneous photosynthetic rate, mesophyll conductance, stomatal conductance, and photosynthetic water use efficiency. Height, stem diameter, root and shoot dry mass and fall frost hardiness data were also obtained. Mean population maximum photosynthetic rate ranged from 10.35 to 14.57 µmol CO2 m–2 s–1 in Sitka alder and from 14.76 to 17.55 µmol CO2 m–2 s–1 in paper birch. Based on canonical correlation analyses, populations from locations with colder winters and shorter (but not necessarily cooler) summers had higher maximum photosynthetic rates implying the existence of an inverse relationship between leaf longevity and photosynthetic capacity. Significant canonical variates based on climatic variables derived for the seed collection sites explained 58% and 41% of variation in the rate of photosynthesis in Sitka alder and paper birch, respectively. Since growing season length is reflected in date of frost hardiness development, an intrinsic relationship was found between photosynthetic capacity and the level of fall frost hardiness. The correlation was particularly strong for paper birch (r=–0.77) and less strong for Sitka alder (r=–0.60). Mean population biomass accumulation decreased with increased climate coldness. These patterns may be consequential for evaluation of the impact of climate change and extension of the growing season on plant communities. [ABSTRACT FROM AUTHOR]

Lichens are well-suited organisms for experimental herbivory studies because their secondary compounds, assumed to deter grazing, can be non-destructively extracted. Thalli of 17 lichen species from various habitats were cut in two equal parts; compounds were extracted from one part by acetone, the other served as a control. These two pieces were offered as a paired choice to the generalist herbivore snailCepaea hortensis. Control thalli of all lichens were consumed at a low rate regardless of their investments in acetone-extractable lichen compounds; naturally compound-deficient lichen species were not preferred compared to those with high contents. However, for extracted thalli, there was a highly significant positive correlation between rate of consumption and the extracted compound contents. These data imply that herbivore defence has evolved in different directions in different lichens. Studied members of Parmeliaceae, common in oligotrophic habitats, have high contents of carbon-rich acetone-soluble compounds; these lichens became highly palatable to snails subsequent to acetone rinsing. Extracted lichen compounds were applied to pieces of filter paper and fed to snails. Extracts from members of the Parmeliaceae significantly deterred feeding on paper. Such data suggest that generalist herbivores may have shaped evolution in the widespread and highly diverse Parmeliaceae towards high investments in lichen compounds. On the other hand, lichens belonging to the Physciaceae and Teloschistales, common in nutrient-enriched habitats, are deficient in, or have low concentrations of, lichen compounds. Such lichens did not become more palatable after acetone rinsing. The orange anthraquinone compound parietin, restricted to the Teloschistales, and which has previously been found to protect against excess light, did not deter grazing. [ABSTRACT FROM AUTHOR]

Mosaics of exclusive foraging territories, produced by intra-and interspecific competition, are commonly reported from arboreal ant communities throughout the tropics and appear to represent a recurring feature of community organization. This paper documents an ant mosaic within mangrove forests of Panama and examines the behavioral mechanisms by which one of the common species, Azteca trigona, maintains its territories. Most of the mangrove canopy is occupied by mutually exclusive territories of the ants A. trigona, A. velox, A. instabilis, and Crematogaster brevispinosa. When foraging workers of A. trigona detect workers of these territorial species, they organize an alarm recruitment response using pheromonal and tactile displays. Nestmates are attracted over short distances by an alarm pheromone originating in the pygidial gland and over longer distances by a trail pheromone produced by the Pavan's gland. Recruits are simultaneously alerted by a tactile display. No evidence was found for chemical marking of the territory. Major workers are proportionally more abundant at territory borders than on foraging trails in the interior of the colony. The mechanisms of territory defense in A. trigona are remarkably similar to those of ecologically analogous ants in the Old World tropics. [ABSTRACT FROM AUTHOR]

In a recently published food utilization study of larvae of the European pine sawfly, Neodiprion sertifer (Larsson and Tenow, Oecologia (Berl) 43 (1979) 157-172), it was concluded that a high consumption rate was part of a suite of presumed adaptations exhibited by these larvae for feeding on a food with a low nitrogen content. In this note it is shown that larvae of N. sertifer exhibit a relative consumption rate which is comparable to that of other sawflies and intermediate in comparison to other herbivorous insects. [ABSTRACT FROM AUTHOR]

In a recently published energy budget for the larvae of the leafcutter bee (Wightman and Rogers, Oecologia (Berl.) 36 (1978) 245-257) respiration as estimated by respirometry amounted to only 67% of the respiration as estimated from the difference between assimilation and production. In this note it is shown that this discrepancy seems to result from an incorrect value of the oxycalorific equivalent and that a more reasonable value makes the two estimates of respiration agree. [ABSTRACT FROM AUTHOR]

Effects of CO on plants are often confounded with effects of nutrition. Coleman et al. (1993) showed that nitrogen concentrations in plants grown under different CO levels did not differ if plants of equal sizes were compared but varied at equal times and equal sizes for different levels of nitrogen additions. I will suggest why this must occur under the conditions of the experiment. I will also suggest why nitrogen productivity should be used to interpret experimental results rather than the dubious nitrogen use efficiency. [ABSTRACT FROM AUTHOR]

Recently, there is an increase in number of studies concerned with the effect of various types of limitations on species local population size and distribution pattern at the landscape scale. The terminology used to describe these limitations is, however, very inconsistent. Since the different terms often appear in conclusions of the papers, the inconsistency in their use obscures the message of these papers. In this study, we review the current uses of these terms, identify the basic concepts involved in the discussion of a limitation and link the concepts with the currently used terms. Finally, we discuss the experimental approaches used to assess the different types of limitations. We differentiated four basic concepts resulting from the combination of limitation by environment versus ability to grow and spread, and two spatial scales (local and regional scale). The two concepts at each spatial scale are expected to form a gradient of all possible combinations of the two respective types of limitations. In the considerations of various experimental approaches used to assess these limitations, we conclude that sowing experiments, i.e. seed addition into existing populations or seed introduction into unoccupied habitats, are the only reliable types of evidence for the different types of limitations. [ABSTRACT FROM AUTHOR]

The increasing popularity of stable isotope analysis (SIA) as an ecological research tool and the ease of automated analysis have created a knowledge gap between ecologists using SIA and the operators of isotope ratio mass spectrometry (IRMS) equipment. This has led to deterioration in the understanding of IRMS methodology and its proper dissemination in the ecological literature. Of 330 ecological research papers surveyed, 63 (19%) failed to report any form of analytical error associated with IRMS. Of the 267 papers that reported analytical error, there was considerable variation both in the terminology and approach used to quantify and describe error. Internal laboratory standards were often used to determine the analytical error associated with IRMS, so chosen because they are homogenous and have isotopic signatures that do not vary over time. We argue that true ecological samples collected in the field are complex bulk mixtures and often fail to adhere to these two criteria. Hence the analytical error associated with samples is potentially greater than that of standards. A set of standard data run over time with a precision typically reported in the ecological literature (1 standard deviation: 1SD=0.26‰) was simulated to determine the likelihood of spurious treatment effects depending on timing of analysis. There was a 90% likelihood of detecting a significant difference in the stable nitrogen ratio of a single sample (homogenized bovine liver) run in two time periods when n>30. Minor protocol adjustments, including the submission of blind replicates by researchers, random assignment of sample repeats within a run by analytical labs, and reporting 1SD of a single sample analyzed both within and between runs, will only serve to strengthen the interpretation of true ecological processes by both researchers and reviewers. [ABSTRACT FROM AUTHOR]

Many ant partners of tropical ant-plants prune the leaves and shoot tips of other plants growing around their hosts. According to the hypothesis proposed by Davidson et al. (Ecology 69:801–808), this specialized behaviour not only protects the host plants against overgrowth, but it also conveys a direct benefit to the ant colony as it removes contact points to the neighbouring vegetation where invasions of enemy ants could occur. Here we test this hypothesis by comparing pruning intensity in five closely related Crematogaster (subgenus Decacrema) plant-ant species (and one species of Technomyrmex) that differ in their exposure to competition by other ants. Pruning intensity was quantified by measuring the area loss of paper tape pieces wrapped around the stems of Macaranga host plants. All Crematogaster (Decacrema) ants tested but not Technomyrmex sp. pruned, but the intensity of the behaviour varied strongly between and within species. Pruning was significantly weaker in the three tested Crematogaster species inhabiting Macaranga host plants with a slippery, waxy stem surface, which functions as a mechanical barrier protecting the specific ant partners against generalist competitors. Pruning was generally stronger on more densely ant-populated trees. Even though the number of ants per twig length was lower in associations of ants with glaucous Macaranga hosts, only part of the variation of pruning activity could be explained by "ant density". When corrected for ant density, "wax-running" Crematogaster (Decacrema) ants still pruned more weakly than their congeners inhabiting non-glaucous Macaranga hosts. Pruning is obviously most important when an ant-plant is potentially accessible to intruders, but less necessary when the ant colony is isolated by a protective wax barrier. Our results support the hypothesis that "selfish" defence against invasions is the major selective pressure that has led to the development and maintenance of pruning behaviour in weakly competitive plant-ants. [ABSTRACT FROM AUTHOR]

The potential effects of increased ultraviolet-B radiation (UV-B, 280–320 nm) simulating stratospheric ozone depletion in field studies with vascular plants have previously been summarized only in narrative literature reviews. In this quantitative synthesis, we have assessed the significance of solar UV-B enhancement for ten commonly measured variables involving leaf pigmentation, plant growth and morphology, and photosynthesis using meta-analytic statistical methods. Of 103 papers published between 1976 and mid-1999 from field studies, more than 450 reports from 62 papers were included in the database. Effects of UV-B were most apparent for the case of UV-B-absorbing compounds with an average increase of approximately 10% across all studies when comparing the ambient solar UV-B control to the treatment (involving ambient UV-B plus a UV-B supplement from special UV lamps). Some morphological parameters such as plant height and leaf mass per area showed little or no response to enhanced UV-B. Leaf photosynthetic processes (leaf gas exchange and chlorophyll fluorescence) and the concentration of photosynthetic pigments (total chlorophylls and carotenoids) were also not affected. Shoot biomass and leaf area showed modest decreases under UV-B enhancement. The reduction in shoot biomass occurred only under very high levels of simulated ozone depletion and leaf area was affected only when studies inappropriately used the plant (i.e., the subreplicate) rather than the plot as the experimental replicate. To the best of our knowledge, this review provides the first quantitative estimates of UV-B effects in field-based studies using all suitable published studies as a database. [ABSTRACT FROM AUTHOR]

Seedlings of paper birch ( Betula papyrifera Marsh.), green ash ( Fraxinus pennsylvanica Marsh.), and red pine ( Pinus resinosa Ait.) fumigated with 0.2 ppm SO for 30 h at 30° C had higher leaf diffusive conductances (LDC) and absorbed more sulfur than seedlings fumigated at 12° C. Comparisons among the three species fumigated at the same temperature, however, do not support the view that a plant with higher LDC should absorb more SO than a plant with lower LDC. Mean relative growth rates ( $$\overline {RGR} $$ ) of seedlings grown at 21° C after fumigation were variously affected by SO. $$\overline {RGR} $$ of green ash was not inhibited by SO, but $$\overline {RGR} $$ of roots of red pine seedlings was reduced by SO, with greater inhibition in seedlings fumigated at 30° C. Root and shoot $$\overline {RGR} $$ of paper birch seedlings were lowered by SO, and effects of SO were about equal at both exposure temperatures. The data indicate that temperature can affect mechanisms of SO avoidance, tolerance, or both to various degrees in different species. Thus generalizations on the influence of exposure temperature on resistance of plants to SO may be inappropriate. [ABSTRACT FROM AUTHOR]

This paper tests the model of Meats (1974) by comparing its simulations with the real data of Milne et al. (1965) and of Laughlin (1967). Simulations are also made for conditions not previously investigated. [ABSTRACT FROM AUTHOR]

After decades of misusing the term diversity in community ecology, over the last 5 years some papers have offered important advances toward developing a more rigorous mathematical background, which allows us to achieve more clarity in the terminology for the vast range of biological phenomena that have been placed under the umbrella of this term. Some points have been clearly stated in previous papers of this Views and Comments section, and new terms have even been proposed for specific cases, but other issues, such as the need for the prefix true have not been discussed. Our aim is to clarify some of the terms and concepts, the proper use of which appears still to remain unclear, and to provide biologists with a simplified version of the general framework resulting from recent contributions, with an emphasis on identifying points of consensus in the field. We also comment on the possibility of extending the basics of this general framework to other facets of the broad term biodiversity, such as functional or phylogenetic diversity. [ABSTRACT FROM AUTHOR]

This paper summarizes the recent research on problems of the so-called colonization cycle as one aspect of the migration and flight behaviour of freshwater insects. New results are presented and the applied methods described and discussed. In general the research on this phenomenon shows that the colonization cycle can not only be seen as a type of compensatory flight behaviour, but also as an expression of the winter survival strategy of water insects in high mountainous areas. It opens up the possibility of exploiting ecologically different biotopes such as living in coastal streams and oligohaline brackish water estuaries. [ABSTRACT FROM AUTHOR]

The sea urchin, Centrostephanus coronatus, exhibits a relatively broad diet. Evidence presented in this paper suggests that evolution of dietary breadth is favored by two mechanisms. First, individual urchins occur in habitat types that support somewhat different foods. Urchins consume principally the most preferred food where it is common but are forced to eat other food types where it is rare. Second, overgrazing by individual urchins reduces the local abundance of the most preferred food, thereby forcing them to add less preferred items to their diet. Previous work indicates that the escape behavior of Centrostephanus from its fish predator, Pimelometopon pulchrum, results in different individuals being exposed to different foods and in reduction of the size of urchin grazing ranges to the point that overgrazing occurs. Hence, by the two mechanisms discussed in this paper, the presence of the fish predator favors the broad diet of its sea urchin prey. [ABSTRACT FROM AUTHOR]

The general conclusion of this paper is that heteromyids do not select seed sizes on the basis of their body size. My conclusion comes from the analysis of new data from central New Mexico, analysis of data in the literature on food habits of heteromyids, and a reanalysis of Brown and Lieberman (1973) and Brown (1975). All of these sources agree that no seed size selection exists. Although no pattern of simple seed size selection was found, interesting differences were noticed among species. First, the tendency to husk appears to be related to the size of the rodent. Second, large heteromyids may depend on fruiting heads made up of small seeds. This may or may not have significance to the coexistece of heteromyid communities. At present, insufficient data are available to make conclusions along these lines. Intuitively satisfying hypotheses, such as seed size allocation by heteromyids, are normally very hard to lay to rest. It is my hope that this paper demonstrates the weight of evidece is against seed size allocation in heteromyids. Those who wish to maintain this hypothesis, or reveal that it or related hypotheses have anything to do with heteromyid coexistence, must now produce data to support their position. [ABSTRACT FROM AUTHOR]

According to a simple growth model, grazed and ungrazed plants may have equal absolute growth rates provided that the relative growth rate (RGR) of grazed plants increases exponentially with grazing intensity (proportion of biomass removed). This paper reports results from an experiment designed to determine whether plants of two grass species subjected to a gradient of defoliation intensities, from 0 to 100% aboveground biomass removal, showed such a response. The relationship between aboveground RGR and defoliation intensity was exponential and closely matched the theoretical relationship of equal absolute growth rate. Thus, plants showed the same aboveground growth regardless of defoliation intensity thanks to an exponential stimulation of RGR by defoliation. Belowground RGR was depressed by defoliation of more than 20% of the above-ground biomass. In spite of the drastic modification imposed by the treatments on the relative proportions of different plant parts, after a 42-day recovery period basic allometric relationships, such as root:shoot and leafarea: weight ratios, were not affected by defoliation intensity. Exponential aboveground compensatory responses represent a key feedback process resulting in constant aboveground growth regardless of defoliation intensity and appear to be a simple consequence of strong commitments to certain allometric relationships. [ABSTRACT FROM AUTHOR]

Using 12 years of monthly sweep-net data from 9-12 permanent sampling stations, we evaluated the variability and stability of the dragonfly assemblage in Bays Mountain Lake (northeastern Tennessee, USA). To do this, we adopted the view that a stable assemblage (i.e. one capable of recovering quickly from disturbances) should have low variability (i.e. high persistence of taxa, relatively constant densities, and high rank concordance), except with disturbances more intense and frequent than those in this system. Moreover, a stable assemblage should contain populations that exhibit density dependence and should tend to remain within a restricted range of densities (boundedness), shifting toward a narrow density interval between generations (attraction). To test some specific predictions derived from these views, we analyzed 12-year sequences of larval population sizes just before the onset of emergence for the 13 dominant dragonfly taxa in the lake. Most but not all of the 13 dominant taxa persisted during the 12-year period. Variabilities of taxon densities, measured as standard deviations across generations of log-transformed population sizes, were representative of the broad range for other invertebrates but somewhat higher than those of terrestrial vertebrates. There were fewer than three significant abundance trends over the 12-year period, and rank concordance between generations was high ( W=0.716). Density dependence was detected among some of the dragonfly density sequences by five different methods. Using techniques presented in the companion paper, we found strong indications of both boundedness and attraction in the whole assemblage. We conclude tentatively that an assemblage consisting of at least 11 of the 13 dominant dragonfly taxa at Bays Mountain Lake has low-to-moderate variability and is stable, but that the complete 29-species assemblage is probably not stable at the scale of this single lake. We emphasize the need for coupling such long-term descriptive analyses with studies of responses to experimental disturbances. [ABSTRACT FROM AUTHOR]

State-dependent changes in prey preference are among the phenomena to be expected in studies of predator behaviour. For example, the rate of attack on each prey type is well known to be affected by the state of satiation, the dynamics of which is often assumed to parallel that of gut fullness. An interesting question is whether satiation alone is the determinant of the attack rate or whether the particular mixture of prey types in the predator's direct environment has an additional influence by itself. To detect examples of the latter type the predictive method proposed by Cock (1978) may be useful. In the present paper the predictive tool is a model built on the assumption that gut fullness is the sole internal state variable determining the attack rate. It is provided with parameter estimates from observations in monocultures of each type and then used to predict predation in mixtures of prey types. When measured predation on these prey types differs from what is predicted, the model may be too simple in various respects, one of which is that predators change prey preference in response to their own sample estimates of the densities of each prey type and their (innate or sample) estimate of the profitability of each prey type in terms of reproductive success. Thus, the lack of fit of the model poses a challenging problem, for to explain it one must identify underlying causes, such as differences in prey quality with respect to scarce nutrients or noxious chemicals that need to be detoxified or rendered harmless in other ways. The predictive approach is illustrated by analysis of preference of predatory mites ( Phytoseiulus persimilis Athias-Henriot and Typhlodromus occidentalis Nesbitt) with respect to various stages of development of their prey, the two-spotted spider mite ( Tetranychus urticae Koch). The results show that the relation between attack rate and gut fullness might well explain prey stage preference of predatory mites when the prey stages are presented together rather than each alone. In another paper by Dicke et al. (1989) marked deviations between predicted and measured diet are reported when the predatory mite, Typhlodromus pyri Scheuten, was offered a choice between two prey species, i.e. apple rust mites and (larvae of) European red spider mites. The underlying causes are to be revealed by further research, the impetus of which is born out by use of the method proposed by Cock (1978) and extended in this paper. [ABSTRACT FROM AUTHOR]

This is the first in a series of papers on the growth, photosynthetic rate, water and nutrient relations, root distribution and mycorrhizal frequency of two Norway spruce forests at different stages of decline. One of the stands was composed of green trees only while the other included trees ranging in appearance from full green crowns to thin crowns with yellow needles. In this paper we compare the growth and carbohydrate relations of the two stands and examine relationships among growth variables in ten plots. The declining stand produced 65 percent of the wood per ground area compared with the stand in which all trees were green because its foliage produced less wood at any level of leaf area index. The difference in foliage efficiency between the sites could not be explained by differeneces in climate, competition or stand structure. The declining stand appeared to have lower carbon gain as indicated by a smaller increase in reserve carbohydrates before bud break, and weaker sinks for carbohydrates as indicated by less use of the stored carbohydrates than the healthy stand. Thus, growth reduction was probably related to factors which affect both photosynthesis and, even more, the sinks for carbohydrate. [ABSTRACT FROM AUTHOR]

Incubation of Heliothis zea (Boddie) eggs on foliage of Lycopersicon hirsutum f. glabratum C.H. Mull (accession PI 134417) results in neonates with elevated levels of tolerance to the toxic effects of PI 134417 foliage attributable to 2-tridecanone found in the glandular trichomes which abound on that foliage. The neonates from such eggs are also shown to have elevated levels of tolerance to the carbamate insecticide carbaryl. Incubation of eggs in an atmosphere containing 2-tridecanone similarly produced elevated levels of tolerance to 2-tridecanone among resulting neonates, indicating that 2-tridecanone is the likely inducing agent and that exposure to 2-tridecanone vapor, which is known to emanate from PI 134417 foliage, is sufficient for induction. Analysis of the cytochrome P-450 content in gut microsomes of fifth instar larvae indicated that exposure of larvae to 2-tridecanone in artificial diet or to PI 134417 foliage resulted in significantly elevated levels of cytochrome P-450 relative to larvae fed diet without 2-tridecanone or foliage of L. esculentum which contains no 2-tridecanone. In addition, removal of the glandular trichomes from PI 134417 foliage eliminated the ability of that foliage to induce elevated levels of cytochrome P-450. These results provide circumstantial evidence that cytochrome P-450 may be involved in the induced tolerance to xenobiotics among neonates from eggs exposed to 2-tridecanone or PI 134417 foliage. [ABSTRACT FROM AUTHOR]

Leaves of Betula were damaged artificially in April, June and August 1982. Palatability of damaged and adjacent undamaged leaves was assessed against controls in bioassays using the polyphagous Lepidoptera Spodoptera littoralis and Orgyia antiqua. Assessments were carried out at intervals from six hours to five months following each damage date. Palatability (relative proportions of leaves consumed) was significantly lower than controls in damaged and adjacent leaves after six hours and remained detectable for up to two months but this was less clear in the June-damaged samples and undetectable in the August group. Adjacent leaves were significantly affected whether distal or proximal to the damaged leaves. Marked and significant changes occurred in levels of soluble tannins in the damaged and adjacent leaves but the relationship between crude tannin levels and changed palatability was not simple cause and effect. No significant effects of damage on aphid ( Euceraphis punctipennis) reproduction could be detected among birches in a growth room experiment in which half the trees were artificially damaged and half were controls. The results are discussed in the light of earlier work on induced defence in birch and five areas of significant new information represented by the results in this paper are identified. [ABSTRACT FROM AUTHOR]

This paper represents an empirical study on the effect of different leaf orientations on the daily carbon gain and transpirational water loss of desert winter annuals. Laboratory physiological data on Malvastrum rotundifolium (Gray) and Lupinus arizonicus (Wats) were combined with energy budget concepts and field measurements of water relations and leaf movements to predict carbon gain patterns for horizontally oriented, diaheliotropic and paraheliotropic leaf movement types. The results showed contrasting patterns of carbon gain and water loss. L. arizonicus, which is capable of both dia- and paraheliotropic leaf movements, had the lowest rates of daily carbon gain and water loss. But these low rates resulted in the highest water use efficiencies under early season conditions and high water availability. M. rotundifolium, a diaheliotropic species, was predicted as having the highest rates of carbon gain and water loss on a daily basis over a wide range of environmental conditions and water availability. Despite possessing the highest rates for transpiration, its water use efficiency was higher in relation to other leaf movement types, under a variety of conditions. This result was extremely sensitive to soil water availability. The results were discussed in relation to the ecological ramifications of leaf movements in arid land annuals. [ABSTRACT FROM AUTHOR]