Competition Among Insects, Birds and Mammals for Conifer Steeds

Species of at least 5 orders of insects, 6 families of birds,and 2 orders of mammals, in various combinations, can exploitthe cones and seeds of most species of conifers. Lodgepole pineis the exception to this pattern of broad taxonomic diversityof seed predators in that only pine squirrels and coreid bugsattack its serotinous cones. The contrast between lodgepolepine and other conifers demonstrates that large intrinsic variationin the abundance of a resource fosters the evolution of a broadrange of taxonomic groups to exploit the resource. The diversegroups'are limited by different predators and alternate resourceswhen conifer seeds independently decrease in abundance.     

Mutualism, market effects and partner control

Intraspecific cooperation and interspecific mutualism often feature a marked asymmetry in the scope for exploitation. Cooperation may nevertheless persist despite one-sided opportunities for cheating, provided that the partner vulnerable to exploitation has sufficient control over the duration of interaction. The effectiveness of the threat of terminating an encounter, however, depends upon the ease with which both the potential victim and the potential exploiter can find replacement partners. Here, we extend a simple, game-theoretical model of this form of partner control to incorporate variation in the relative abundance of potential victims and exploiters, which leads to variation in the time required for individuals of each type to find a new partner. We show that such market effects have a dramatic influence on the stable level of exploitation (and consequent duration of interaction). As the relative abundance of victims decreases, they become less tolerant to exploitation, terminating encounters earlier (for a given level of exploitation), whereas exploiters behave in a more cooperative manner. As a result, the stable duration of interaction actually increases, despite the decreasing tolerance of the victims. Below a critical level of relative victim abundance, the model suggests that the cost of finding a replacement partner becomes so great that it does not pay to exploit at all.     

Water Stress, Osmolytes and Proteins

Organic osmolytes are small solutes used by cells of numerouswater-stressed organisms and tissues to maintain cell volume.All known osmolytes are amino acids and derivatives, polyolsand sugars, methylamines, and urea; unlike salt ions, most are"compatible," i.e., do not perturb macromolecules. In addition,some stabilize macromolecules and are "counteracting" towardsperturbants, e.g., methylamines can stabilize proteins and ligandbinding against perturbations by urea in elasmobranchs and mammaliankidney, and (our latest findings) high hydrostatic pressurein deep-sea animals. Methylamines appear to coordinate watermolecules tightly, resulting in osmolyte exclusion from hydrationlayers of peptide backbones. This makes unfolded protein conformationsentropically unfavorable (work of Timasheff, Galinski, Bolenand coworkers). These properties have led to proposed uses inbiotechnology, agriculture and medicine, including improvedbiochemical methods, in vitro rescue of misfolded proteins incystic fibrosis and prion diseases (work of Welch and others),and plants engineered for drought and salt tolerance. Theseproperties also explain some but not all of the considerablevariation in osmolyte composition among species with differentmetabolisms and habitats, and among and within mammalian tissuesin development.     

Diurnal and Seasonal Changes in Leaf Water Potential Components and Elastic Properties in Response to Water Stress in Apple Trees

Apple trees are very drought tolerant, having the capability to grow and carry on photosynthesis even at low water potentials. Much of the tolerance is due to the ability of apple leaves to maintain turgor potentials at levels conducive to growth and stomatal opening. Diurnally, leaf turgor is maintained through decreases in osmotic potentials (due to active solute accumulation), osmotic adjustment, or to concentration of solutes via tissue water loss. These two processes combined may decrease osmotic potentials by as much as 1.65 MPn during the day. Seasonally, osmotic potentials remain fairly constant, but leaf elasticity increases, allowing growth to continue and stomata to remain open us water and turgor potentials become progressively lower. Release of stored water from plant tissues to the transpiration stream is another means of preventing water potentials from reaching critical values for stomatal closure. A combination of a number of these physiological adaptations may account for much of the drought tolerance in apple trees.     

Scale of Physiological Processes Sensitivity to PEG-Induced Water Stress in Scots Pine Seedlings

The idea that water deficit strengthening induces concerted changes of plant physiological parameters is rather widespread. However, such changes are often difficult to identify due to challenges in establishments and maintenance of required water stress intensities using solid substrates. Therefore, we exposed Scots pine (Pinus sylvestris L.) seedlings to the range of water potentials from–0.15 to–1.5 MPa in PEG-water culture to identify the series of physiological parameters differently sensitive to water stress. We observed that even mild water stress (–0.15 MPa) inhibited root elongation, which could be one of the main pine seedlings vulnerabilities under drought. Active accumulation of osmolytes was already induced by mild water deficit and further increased with water stress severity. Plant fresh biomass growth sensitivity was more related to changes of relative water content (RWC) than to changes in tissue water content or dry weight accumulation. Plants were able to grow and accumulate dry weight down to–0.5 MPa, but lower medium water potentials (–1.0 and–1.5 MPa) suppressed growth and heavily damaged root cells, as judged from many-fold increase of Ca²⁺ content in roots. Chlorophyll a content was surprisingly sensitive to water stress, while carotenoids level was increased under severe stress conditions. In conclusion, the experimental system with stepwise water potential values allowed us to analyze the sensitivity scale of a range of P. sylvestris physiological processes to water stress. It was largely similar to those described earlier for other plant species, but its peculiarities were high sensitivity of root elongation, marked resistance of biomass growth to water deficit and well-developed ability to osmotic adjustment.     

Quantifying Relationships Among Phosphorus, Agriculture, and Lake Depth at an Inter-Regional Scale

To date, studies examining the impact of agriculture on freshwater systems have been spatially confined (that is, single drainage basin or regional level). Across regions, there are considerable differences in a number of factors, including geology, catchment morphometry, and hydrology that affect water quality. Given this heterogeneity, it is unknown whether agricultural activities have a pervasive impact on lake trophic state across large spatial scales. To address this issue, we tested whether the proportion of agricultural land in a catchment (% Agr) could explain a significant portion of the variation in lake water quality at a broad inter-regional scale. As shallow, productive systems have been shown to be particularly susceptible to eutrophication, we further investigated how lake mean depth modulates the relationship between % Agr and lake total phosphorus (TP) concentration. We applied both traditional meta-analytic techniques and more sophisticated linear mixed-effects models to a dataset of 358 temperate lakes that spanned an extensive spatial gradient (5°E to 73°W) to address these issues. With meta-analytical techniques we detected an across-study correlation between TP and % Agr of 0.53 (one-tailed P-value = 0.021). The across-study correlation coefficient between TP and mean depth was substantially lower (r = −0.38; P = 0.057). With linear mixed-effects modeling, we detected among-study variability, which arises from differences in pre-impact (background) lake trophic state and in the relationship between lake mean depth and lake TP. To our knowledge, this is the first quantitative synthesis that defines the influence of agriculture on lake water quality at such a broad spatial scale. Syntheses such as these are required to define the global relationship between agricultural land-use and water quality.     

Water Stress, CO2 and Climate Change

Climatic change may bring about increased aridity to large areasof Europe. Higher temperatures, larger water deficits and highlight stress are likely to occur in conjunction with elevatedatmospheric CO₂. This raises the question whether a high CO₂concentration in the atmosphere can compensate for the decreasein carbon gain in water-stressed plants. The processes whichdetermine dry matter production and the ways they are affectedby soil water deficits are discussed. It is now well establishedthat in most species and under most circumstances stomata arethe main limiting factor to carbon uptake under water deficit,the photosynthetic machinery being highly resistant to dehydration.However, when other stresses are superimposed, a decline inphotosynthetic capacity may be observed. In the short term,under drought conditions, the increase in CO₂ in the atmospheremay diminish the importance of stomatal limitation for carbonassimilation, inhibit photorespiration, stimulate carbon partitioningto soluble sugars and increase water-use efficiency. Some recentevidence seems to indicate that under conditions of high irradiance,plants growing at elevated CO₂ may develop protection towardsphotoinhibition, which might otherwise result in significantlosses in plant production under stress conditions. In the longerterm though, a negative acclimation of photosynthesis appearsto occur in many species, an explanation for which still needsto be clearly identified. Similarly, the effects of extendedexposure to elevated CO₂ under arid conditions are not known.Plant production is more closely related to the integral ofphotosynthesis over time and total foliage area than to theinstantaneous rates of the photosynthetic process. Water deficitsresult in a decrease in foliage area biomass and, therefore,in productivity. On the other hand, the increase in air temperaturemay result in more respiratory losses. However, experimentalas well as simulatory evidence suggests that doubling CO₂ concentrationin the air may improve carbon assimilation and compensate partiallyfor the negative effects of water stress even if we assume adown-regulation of the photosynthetic process as a result ofacclimation to elevated CO₂.     

Variation in the Nutritional Value of Primate Foods: Among Trees, Time Periods, and Areas

The study of nutritional ecology has proven to be useful for understanding many aspects of primate behavior and ecology and is a valuable tool in primate conservation. However, to date this approach has had limited application since chemical analyses of food items is very time-consuming and collections of perishable food material are often made in remote field locations. Such logistic difficulties have led to plant material being collected in a variety of fashions, and it is not known how variation in collection method might influence our understanding of the chemical basis of dietary selection. A standardization of collection methods is greatly needed to allow for direct comparison among studies. To develop an appropriate standardized method and to evaluate past research, it is necessary to understand along what dimensions plant chemistry varies. We evaluated variation in nutritional value—protein, fiber, digestibility, alkaloids, saponins, cyanogenic glycocides, and minerals—of leaf material from species eaten by red colobus (Piliocolobus tephrosceles) and black-and-white colobus (Colobus guereza) of Kibale National Park, Uganda. We consider variation at 3-levels: among trees, time periods, and areas. While there was considerable variation among species with respect to protein, digestibility, and saponins, there was also variation among individuals of the same species; in fact, individuals may vary by as much as 20%. The average coefficients of variation (CV) among individuals of the same species are 13.4 for protein, 12 for digestibility, and 43 for saponins, while the average CV among species are 35, 31.3, and 82.4, respectively. No species showed a variable response with respect to testing for the presence or absence of cyanogenic glycocides, while 2 of 11 species tested for alkaloids showed a variable response. Over 2 years there was evidence of variation among time periods in the chemical composition of the same food items. The protein-to-fiber ratio of mature leaves of the same species collected from 4 sites separated by 12 km within Kibale was also variable and in some cases the variation among sites was greater than the differences among species. For example, while Funtumia latifolia had little variation in protein-to-fiber ratio at 3 sites (0.44 at all sites), the remaining site was 28% greater. Because temporal variation is less than variation among individuals, it is likely more important to sample from multiple trees at a single point in time than to sample across time. However, the most accurate assessment of nutrient intake is obtained by collecting plant material from the specific trees selected for consumption.     

Cytokinins and Water Stress

It is almost impossible to find a single process in plant life that is not affected by both stress and hormones directly or indirectly. This minireview is focused on the interactions between water stress and cytokinins (CKs). The attention was paid mainly to changes in endogenous CK content and composition under water stress, involvement of CK in plant responses to water stress mainly in stomatal regulation of gas exchange, water relations of transgenic plants with elevated CK content, and possibilities to ameliorate the negative effects of water stress by application of exogenous CKs.     

Mutualism, hybrid inviability and speciation in a tropical ant-plant

Although biotic interactions are particularly intricate in the tropics, few studies have examined whether divergent adaptations to biotic interactions lead to speciation in tropical organisms. Ant-plant mutualisms are widespread in the tropics. Within Leonardoxa africana, two subspecies present contrasting defences against herbivores. Young leaves of subsp. africana are defended by mutualistic ants, whereas subsp. gracilicaulis satiates herbivores by synchronized leaf production. Subsp. africana possesses hollow internodes and many large foliar nectaries, housing and feeding ants. We detected no genetic introgression between the two subspecies in the contact zone between them. F1 hybrids were present. They were intermediate in phenotype, expressing reduced, nonfunctional but costly myrmecophilic traits. However, they suffered more herbivory than their parents. Hybrids remained small, failing to reach reproductive size, probably due to their maladapted defence phenotype. Hence, there could be a direct link between adaptation to mutualism and reproductive isolation: biotic interactions could be a driver of tropical diversity.     

Mutualism, parasitism and competition in the evolution of coviruses

Coviruses are viruses with the property that their genetic information is divided up among two or more different viral particles. I model the evolution of coviruses using information on both viral virulence and the interactions between viruses and molecules that parasitize them: satellite viruses, satellite RNAs and defective interfering viruses. The model ultimately, and inevitably contains within it single-species dynamics as well as mutualistic, parasitic, cooperative and competitive relationships. The model shows that coexistence between coviruses and the self-sufficient viruses that spawned them is unlikely, in the sense that the quantitative conditions for coexistence are not easy to satisfy I also describe an abrupt transition from mutualistic two-species to single-species dynamics, showing a new sense in which questions such as ''Is a lichen one species or two?'' can be given a definite answer.     

Competition between Exotic and Native Insects for Seed Resources in Trees of a Mediterranean Forest Ecosystem

The seeds of both cedar-of-Lebanon (Cedrus libani) and Cyprus cedar (Cedrus brevifolia) are attacked in their natural range by a specialised chalcid, Megastigmus schimitscheki. From 1995 to 1999, seeds were screened for insect damage in the main cedar plantations of southern France, as well as in the stands where cedar is mixed with firs (Abies spp.). X-rays were used to identify chalcid-infested seeds from which the insects were then reared. The surveys revealed the presence of M. schimitscheki in all the stands of Atlas cedar, Cedrus atlantica, planted at Mt Ventoux, southeastern France. The chalcid also infested seeds of an exotic fir, Abies pinsapo, planted in the same area. However, it has not yet reached the cedar plantations in southwestern France, where the seeds are colonised by a related exotic insect, Megastigmus pinsapinis, originating from North Africa. The latter species was common in cedar seeds at Mt Ventoux in the early 1990s but seems to have been supplanted by M. schimitscheki in the invasion zone. A native chalcid species, Megastigmus suspectus, was also shown to have shifted to a slight extent from a native fir, A. alba, onto cedar. The presence of three chalcid species competing for cedar seed resources may result in a substantial decline of the regeneration potential of that tree species. At Mt Ventoux, up to 92.6% of the cedar seeds were attacked, with 86.8% due to M. schimitscheki. The survey also revealed the widespread presence of another North American chalcid, Megastigmus rafni, in the fir stands of southern France. This revised version was published online in July 2006 with corrections to the Cover Date.     

Competition and Coexistence In a Multi-partner Mutualism: Interactions Between two Fungal Symbionts of the Mountain Pine Beetle In Beetle-attacked Trees

Despite overlap in niches, two fungal symbionts of the mountain pine beetle (Dendroctonus ponderosae), Grosmannia clavigera and Ophiostoma montium, appear to coexist with one another and their bark beetle host in the phloem of trees. We sampled the percent of phloem colonized by fungi four times over 1 year to investigate the nature of the interaction between these two fungi and to determine how changing conditions in the tree (e.g., moisture) affect the interaction. Both fungi colonized phloem at similar rates; however, G. clavigera colonized a disproportionately larger amount of phloem than O. montium considering their relative prevalence in the beetle population. High phloem moisture appeared to inhibit fungal growth shortly after beetle attack; however, by 1 year, low phloem moisture likely inhibited fungal growth and survival. There was no inverse relationship between the percent of phloem colonized by G. clavigera only and O. montium only, which would indicate competition between the species. However, the percent of phloem colonized by G. clavigera and O. montium together decreased after 1 year, while the percent of phloem from which no fungi were isolated increased. A reduction in living fungi in the phloem at this time may have significant impacts on both beetles and fungi. These results indicate that exploitation competition occurred after a year when the two fungi colonized the phloem together, but we found no evidence of strong interference competition. Each species also maintained an exclusive area, which may promote coexistence of species with similar resource use.