We investigated whether pelagic food web dynamics, expressed through a trophic triangle, could influence the potential success of whole-lake fertilization to enhance juvenile sockeye salmon growth. Muriel Lake (145 ha), located on Vancouver Island, was fertilized during 1984 with no apparent effect on juvenile sockeye growth. Unlike most sockeye nursery lakes, Muriel Lake contains a substantial population of the invertebrate zooplanktivore Neomysis mercedis. We hypothesized that competition for zooplankton prey between Neomysis and planktivorous fish (juvenile sockeye, threespine stickleback) could counteract beneficial effects of either natural or fertilizer-induced increases in food for fish. To test this, we assessed (1985–1986) biomass, production, and consumption of planktivorous mysids and fish and then used bioenergetics models to quantify potential competitive interactions. Our analysis suggested that N. mercedis consumed 7–8% of the zooplankton standing stock per day, while planktivorous fish consumed <1.0%·d⁻¹. Although mysids were the main consumers of zooplankton, mysids were also consumed by fish. Late in 1986, an increase in mysid consumption by large, 1 juvenile sockeye precipitated substantial declines in Neomysis biomass. Although this event came too late to reduce mysid competition with sockeye fry in Muriel Lake in 1986, it did highlight the potential importance of trophic triangles in pelagic food webs. We suggest that mysids may be held in check by juvenile sockeye when exogenous recruitment events result in high sockeye biomass. By contrast, recruitment failures and low sockeye biomass promote increases in Neomysis populations, which then control zooplankton communities such that sockeye gain little benefit from either natural or fertilizer-induced increases of zooplankton. For food web ecologists, the implication is that trophic triangles might produce alternate stable states that are mediated by external factors. For fisheries managers, the implication is that lakes containing mysids should only be fertilized when sockeye densities are high or mysid densities are low.

The number of different types of circulating leucocytes may provide information about the health state of birds in the wild. We counted the number and proportions of circulating leucocytes in blood smears of nestling pied flycatchers (Ficedula hypoleuca) shortly before fledging. We studied the relationship between these haematological measures and environmental factors like parasitism, body mass, hatching date, and brood size. The heterophils-to-lymphocytes ratio was higher in nestlings whose nests suffered from mite infestation and in lighter individuals, heterophils being the cells that responded preferentially to malnutrition. Recruited birds had lower lymphocyte and heterophil counts when nestlings than non-recruited ones. Our results show that heterophil count is a better predictor of local recruitment than other variables widely used for nestlings as survival predictors, like body mass or hatching date, supporting the hypothesis that low heterophil counts reflect a good individual health state in nestlings.

A good way to check hypotheses that explain the invasion of ecosystems by exotic plants is to compare congeneric alien and native species. To test the hypothesis that alien invaders grow faster than natives, the maximum relative growth rate and its components were compared in controlled growth conditions between four Senecio species, two aliens introduced from southern Africa (S. inaequidens and S. pterophorus) and two European natives (S. malacitanus and S. jacobaea). The four species colonize similar habitats, but the frequency and abundance of their populations and their distribution ranges differ. The two aliens showed a higher relative growth rate than the natives, and although there were differences between species for leaf area ratio, leaf dry matter content, and dry matter partition between stems, leaves, and roots, no clear pattern was detected to explain the differences in growth rates: several combinations of the components of the relative growth rate can give similar results. The higher relative growth rate of the alien species, combined with other ecological and life-history traits, may enhance their invasive capacity.

Spatial statistics were used in conjunction with standard plot experimentation to form an approach geared towards testing hypotheses about plant spatial interactions under different treatment conditions. Specifically, correlograms were used to quantify the changing patterns in the spatial distribution of plant size (total vegetative length) among pre-planted individuals within a sand box. A repeated-measures ANOVA procedure was then utilized to test for significant differences between the treatment effects as they evolved over time, where the experimental unit was the correlogram value obtained from within each sand box at a specific lag distance. Separate ANOVA tests were also executed for each spatial lag distance and temporal sampling period. In an example demonstrating both procedures, it was shown that a water-stress treatment provoked positive spatial autocorrelation to emerge up to 24 cm from Panicum amarum plants, while a normal water treatment resulted in no change in spatial structure. The difference between the two treatments, and the spatial correlograms of their total vegetative length values, were significant (P = 0.0007). The temporal divergence of the treatments, as spatial patterns evolved, was also significant (P < 0.0001). The approach discussed in this experiment is well suited for evaluating the statistical significance of treatment effects upon the spatial structure of interacting individuals, in both greenhouse and natural settings.

The relationship between incident light intensity and net ecosystem CO₂ exchange (NEE) was examined at a low-shrub bog located near Ottawa, Canada. Shrub height was 0.25 m and maximum leaf area index was 1.3 at the peatland. Light intensity was expressed as a clearness index (CI), where values approaching zero indicate heavy overcast conditions and values approaching unity represent cloudless conditions. Light saturation with respect to net CO₂ uptake at the canopy scale occurred at low CI, near 0.3. Contrary to reports from some forest ecosystems, in which peak NEE occurred with CI in the range 0.4–0.7, at the peatland there were no differences in NEE across all ranges of CI above 0.3. At the same time, CI in the range of 0.4–0.7 was infrequent and of short duration relative to clear skies and thick overcast conditions. Finally, we show that the use of half-hourly average radiation measurements to determine CI can lead to significant overestimation of the CI index in the range between 0.4–0.7.

The effects of site acidification on amphibian populations have been thoroughly addressed in the last decades. However, amphibians in naturally acidic environments, such as peatlands facing pressure from the peat mining industry, have received little attention. Through two field studies and an experiment, I assessed the use of bog habitats by the green frog (Rana clamitans melanota), a species sensitive to various forestry and peat mining disturbances. First, I compared the occurrence and breeding patterns of frogs in bog and upland ponds. I then evaluated frog movements between forest and bog habitats to determine whether they corresponded to breeding or postbreeding movements. Finally, I investigated, through a field experiment, the value of bogs as rehydrating areas for amphibians by offering living Sphagnum moss and two media associated with uplands (i.e., water with pH ca 6.5 and water-saturated soil) to acutely dehydrated frogs. Green frog reproduction at bog ponds was a rare event, and no net movements occurred between forest and bog habitats. However, acutely dehydrated frogs did not avoid Sphagnum. Results show that although green frogs rarely breed in bogs and do not move en masse between forest and bog habitats, they do not avoid bog substrates for rehydrating, despite their acidity. Thus, bogs offer viable summering habitat to amphibians, which highlights the value of these threatened environments in terrestrial amphibian ecology.

Carnivore home-range sizes vary profoundly between populations within a species. One commonly cited reason for this is variation in prey population density and environmental productivity. However, obtaining reliable measures of prey density in the field is both time and effort consuming. Therefore, a methodology that enabled scientists and managers to extrapolate home-range sizes across areas would be a valuable tool. So far, the potentials of different remote-sensing indices to represent environmental productivity have been poorly evaluated in this context. In this study, we have evaluated the utility of a readily available remote-sensing index, the Fraction of Photosynthetically Active Radiation absorbed by vegetation canopies (FPAR), to explain interpopulation variation in home-range size for 12 carnivore species. In multiple regression models, evaluated by the Bayesian Information Criterion (BIC), we found that the FPAR index added predictive power to the models for eight of the species. The explanatory power varied between 16% and 71% for the different species. We suggest that using remote-sensing indices such as FPAR to predict area specific home-range sizes for carnivores could potentially be a powerful tool, but that the methodology needs to be further developed in order to add more explanatory power for some species.

Deserts are open environments characterized by striking shifts in temperature and light regimes. We hypothesized that the abiotic environment mediates the interaction between an ant-dispersed plant, Datura wrightii (Solanaceae), ant mutualists, and rodent seed predators in the Sonoran Desert. Field experiments contrasting diurnal and nocturnal seed collection rates in the presence of ants only, vertebrates only, and in the presence of both groups indicated that 85%of seed collection by mammalian seed predators occurred at night (between 1900 and 0700). Seed collection by ants, in contrast, was similar between day and night, although seed collection decreased during very hot days and very bright nights. The total number of seeds collected by both groups foraging separately exceeded the number removed when both groups shared access to seed depots, suggesting that ants and rodents compete for seeds. However, D. wrightii plants dehisced 86%of their fruits between 0700 and 1900, increasing the likelihood of seed collection by ant mutualists rather than rodent seed predators. Dehiscence was sensitive to environmental cues: greenhouse plants kept at constant temperature and humidity dehisced 47%of their fruits between 0700 and 1900. Additional field experiments demonstrated that seed-collecting ants transported seeds considerable distances to their nests, microsites that can be rich in nutrients. The mean (±SE) seed dispersal distance was 6.1 ±0.5 m, the longest mean dispersal distance yet reported for an ant-dispersed seed.

Two sites that burned in 1847 (H) and 1823 (I) in the mixedwood boreal forest in Québec were selected to follow aspen genetic and clonal diversity over time. At each site, three cohorts were identified by core dating, and about 30 trees per cohort were randomly selected to compare tree genotypes using four microsatellite loci. The first cohorts were of post-fire origin (large disturbance), while the second and third cohorts were promoted by gap disturbances. These gaps were created by the natural mortality of post-fire aspen trees and a spruce budworm outbreak that attacked the coniferous species. Expected heterozygosity ranged from 0.37 to 0.72 across cohorts and averaged 0.66 and 0.54 in H and I, respectively. More than 99% and 96%of the genetic variability existed within cohorts, respectively. Genotypic diversity was high in all cohorts, and most genets were unique. Only two clones suckered for three successive cohorts, indicating little selection for specific genets to dominate aspen stands with time. High genetic and clonal diversity changed slightly between post-fire and gap disturbance cohorts. Apical dominance might have favoured the suckering of genets that existed in the post-fire cohorts but that were later eliminated by natural mortality.

Using the holm oak (Quercus ilex) as a model system, we quantified the losses in the potential number of recruits from the flower to the sapling stage caused by abiotic factors, pre- and post-dispersal seed predators, and invertebrate and ungulate herbivores in one well-preserved forest plot (F) and in one dehesa (savanna-like) plot (D). Tree fecundity was an order of magnitude higher in D than in F. Predispersal (flower plus fruit) losses due to abiotic factors were much larger (71% in F and 90% in D) than those originated by biotic factors (29% in F and 10% in D). Post-dispersal predation of acorns under trees led to crop depletion in F, while predators did not deplete the acorn crop despite a much higher predation level in D. Surface acorns were invariably eaten, while most buried acorns survived to germination in both plots. Ninety-five percent of seedlings emerged under oaks in D and only 20% in F, the density of newly emerged seedlings being ten times greater in F. Emergence from experimentally sown acorns was very high on average (73%), while seedling survival to the 2-y sapling stage was primarily affected by water stress in the first summer, especially in the dehesa stand. A 75-fold difference in recruitment rates between populations was found (0.00150 in F and 0.00002 in D). This whole-cycle disparity was the result of differences in the conversion rate from viable fruits to newly emerged seedlings (two orders of magnitude larger in F). The inability to direct acorns to safe (shaded) sites by means of efficient dispersers appears to limit recruitment in open dehesa stands. This finding could explain the general lack of natural regeneration of dehesas as compared to the forests from which they developed.

In arid ecosystems, grazing-induced desertification can be either associated with an increase or a decrease in the spatial heterogeneity of plant communities. We explored the grazing-associated change in the spatial heterogeneity along a gradient of plot sizes (=grains) in a Patagonian steppe with two structural phases: scattered grasses and shrubs surrounded by grasses. We compared the biomass heterogeneity (estimated from locally calibrated digital photographs) of three sites: one ungrazed for the last 26 y and two moderately grazed. On each site, we analyzed three belt transects of 50 contiguous 20- ×20-cm plots and grouped the photographs into plots of increasing size. We used the p parameter (gamma distribution) to measure the global heterogeneity of the biomass and semivariograms to calculate internal heterogeneity and spatial dependence (estimated from the y-intercept and the fractal dimension of semivariograms, respectively). We found that the global and the internal heterogeneity were higher in the grazed sites than in the ungrazed one, possibly because grazing increases the contrast between the structural phases. However, the fractal dimension did not change with grazing. The difference between grazed and ungrazed sites did not disappear when grain increased, probably because the grazing-induced biomass increase of the shrub-dominated phase was higher than the biomass decrease of the scattered grasses phase.

We carried out a multi-taxa study to investigate the responses of species assemblages to stand and landscape variables in maritime pine plantation forests in the Landes de Gascogne (southwestern France). Breeding birds, carabid beetles, and ground-dwelling spiders were sampled simultaneously in a balanced set of 27 stands varying in tree species composition, age, and vertical structure. The composition and structure of surrounding landscapes were quantified within a 500-m radius around the stands. Species responses to a combination of 12 stand and landscape variables were estimated using multiple stepwise regression. Bird, carabid, and spider assemblages were related to the same explanatory variables using a set of co-inertia analyses. Tree height was the best predictive variable for all taxa at species richness, species assemblage, and individual species levels. Landscape variables were important as secondary factors, especially landscape fragmentation, spatial heterogeneity, and shape and spatial distribution of deciduous forest patches in the surrounding landscape. The occurrence of deciduous forest patches and an increasing landscape heterogeneity appeared to be beneficial to forest birds and spiders but not to carabids, probably because forest carabids are generally poor dispersers, more sensitive to habitat fragmentation.

La pluie pollinique actuelle et l'enregistrement pollinique tardiglaciaire et holocène de deux sites (Laghi dell'Orgials, 2130 m, exposition sud-ouest et Lago delle Fate, 2240 m, exposition est) localisés dans l'étage subalpin sur les deux versants du vallon de St Anna di Vinadio (Alpes Maritimes Italiennes) ont été étudiés. Les données recueillies ont permis de replacer l'évolution de la végétation de cet étage dans le cadre de son histoire régionale et de mettre en évidence les changements dus aux paramètres physiographiques et à l'influence humaine. Au Tardiglaciaire, les deux sites sont environnés de steppes à Artemisia et sont situés au-dessus de la limite des arbres formée par les pins. La présence de feuillus mésophiles (Tilia, Ulmus et Quercus caducifoliés) à basse altitude est perçue dès 12 000 BP environ. Avec le passage à l'Holocène, les steppes à armoises sont remplacées par des pelouses à Poaceae. Des différences importantes caractérisent les deux sites au début de l'Holocène : Betula connaît un développement important dès 9700 BP à Fate tandis qu'à l'Orgials la présence de Larix est enregistrée avant 8000 BP. Les pourcentages élevés de Pinus antérieurs à 9000 BP doivent être attribués à des apports lointains dans les deux sites. Après 9000 BP, une pinède subalpine se développe autour de Fate. Le développement de ces essences pionnières précède celui d'Abies dans l'étage montagnard, enregistré dans les deux sites autour de 7820 BP. À l'époque romaine (1875 BP), un déboisement important affectant les sapinières de l'étage montagnard et les pinèdes subalpines est responsable de l'ouverture du milieu. L'augmentation des Poaceae à cette époque reflète l'expansion des pâturages à Nardus stricta.

Ants frequently harvest seeds from faeces of frugivorous vertebrates. By transporting these seeds to nests, ants may influence seed dispersal success of plants. Seed removal by ants from vertebrate faeces is influenced by the seed species involved. Faeces from different vertebrates differ in chemical composition and physical characteristics. It remains unclear, however, whether the faeces itself affects the ant-seed interaction. In this study experimental seed-containing faecal portions were prepared using defecations of birds, marsupials, and monkeys and seeds of two aroids (Philodendron corcovadense and P. appendiculatum, Araceae) and one liana species (Schlegelia parviflora, Bignoniaceae). Faecal portions were arranged along a transect established in the understory of a lowland rainforest in southeast Brazil. For P. appendiculatum the probability of detection and the proportion of seeds removed were identical between marsupial and monkey faeces. For P. corcovadense and S. parviflora, the probability of detection was affected by seed species and, apparently, also by the interaction between seed species and type of faeces (P = 0.097), but not by the type of faeces itself (bird or monkey). Both factors (i.e., seed species and type of faeces) affected the proportion of seeds removed (faeces type was marginally significant; P = 0.08), whereas the interaction between them was not significant. The results indicate that seed species affects seed removal by ants, while the type of faeces probably interacts with seed traits to influence faeces detection.

Many ecological assemblages show a nested subset pattern of species distribution, i.e., common species occur in all assemblages, whereas rare species tend to occur in progressively more diverse assemblages. We examined the determinants of nestedness and assemblage composition of lake-dwelling snails and clams in a boreal landscape using nestedness calculator, rank correlation, and canonical correspondence analysis (CCA). Both snail and clam assemblages were highly nested, and the nested subset pattern correlated with an index of isolation and habitat suitability (mainly water chemistry) for snails and with habitat suitability for clams. Habitat suitability and isolation were themselves highly correlated, thus obscuring the detection of their relative importance to nestedness. Yet, it appears that nestedness in this molluscan fauna is due mainly to nested tolerance of abiotic factors. Partial CCA showed that isolation and habitat suitability were almost equally important correlates of species composition for snails, whereas lake area was the key factor related to clam assemblage composition, followed by habitat suitability. It thus appears that while the degree of nestedness in species composition of lake-dwelling clams and snails may be highly correlated with a single variable, the overall pattern of species composition among lakes requires multiple explanatory variables.