Snakes are important predators of birds' nests, but the ecological factors that bring snakes and birds into contact are poorly known. One hypothesis is that snakes choose habitats based on abundance of alternative prey such as small mammals, with avian nest predation in those habitats arising coincidentally. At a study site in southern Illinois, we used radio telemetry to test the hypothesis that ratsnakes (Elaphe obsoleta) and racers (Coluber constrictor) preferentially use forest edges because small mammals are more abundant in edges. Diet analysis confirmed that both snake species preyed on birds and mammals. Although overall habitat use differed, both snake species preferentially used forest edges. Conversely, species of small mammals on which the snakes preyed were not more abundant (determined by live trapping) in edges, and seasonal variation in habitat use by snakes and mammalian prey abundance were not correlated. These results suggest that alternative hypotheses (e.g., abundance of nesting birds, thermoregulation) are more promising explanations for why these snakes preferentially use edges.

Breeding performance and offspring quality can be affected by temporal and spatial variation in ecological conditions. We analyzed the concomitant effects of timing of breeding, ambient temperature at different stages of the breeding cycle, and microhabitat conditions (presence or absence of livestock farming) on breeding success, morphology, and immunity of the offspring in an aerially insectivorous passerine bird, the barn swallow (Hirundo rustica). A seasonal decline in breeding success was observed, while maximum offspring phenotypic values of different characters were attained at different times in the breeding season, depending on presence or absence of livestock farming. In addition, environmental temperature at different stages of the breeding cycle (pre-incubation, incubation, and nestling stage) of individual pairs affected all the components of nestling phenotypic quality. The results of this correlational study suggest that parents are faced with complex decisions about optimal timing of breeding and microhabitat choice. In addition, different ecological factors seem to interact in influencing breeding success and offspring quality in the barn swallow. Natural and anthropogenic changes in ecological conditions, including ambient temperature and distribution of livestock, may therefore affect distribution and productivity of a population of a declining passerine species.

This study examines the relationships between water level changes and willow cover, one aspect of the controversy regarding ecosystem changes in the Peace–Athabasca Delta, northern Alberta, Canada. What water conditions are optimal for the delta's willows? Does flooding cause willow dieback? Are there species-specific differences in susceptibility, and what attributes of flooding are related to dieback? How has willow cover changed in recent decades? Vegetation cover and its temporal response to flooding and drying were investigated for five common willow species in relation to water levels. Willow cover declined over the period 1993–2001, but cover changes differed among species. Willow cover decreased the most on the wetter transects, while drier transects increased in willow cover. Salix bebbiana and S. discolor appeared to be the most susceptible to flood-induced dieback, and S. petiolaris the least susceptible, while S. planifolia (the dominant willow in the delta) and Salix exigua were intermediate in flood susceptibility. Willow dieback was correlated with water depth, duration of flooding, and time since flooding. Large, old willows were more tolerant of flooding than small, young willows. A pulse of willow establishment occurred during the early 1980s that coincided with a drying period in the delta, increased regional wildfire activity, a decline in river discharge, and a decline in the level of Lake Athabasca. An increase in willow cover followed and reached a peak ca 1993. Flooding in the mid- to late 1990s resulted in a decline in willow cover. Given current climatic trends towards desiccation, willows may continue to be a dominant feature of the delta's landscape.

Many stoloniferous plant species have the ability to exploit resource-rich patches via plastic growth responses. The most efficient responses are shortened spacers and increased branching frequency. Here we experimentally investigate the ability of the stoloniferous plant Linnaea borealis to respond to patches of increased light intensity and reduced water availability in natural systems. The significance of contrasts between patches was also investigated. A three-level factorial design was used, with light, water availability, and site as the factors. Increased light intensity was achieved through mowing of the surrounding vegetation, and reduced water availability was achieved by placing wooden ledges under the stolons. The treatments were applied at three subarctic sites that differ in light conditions. Branching frequency, number of new meristems, average internode length, leaf area, and dry weight production were studied 14 months after the manipulations. Increased light intensity increased branching frequencies; the strongest effects were obtained at the site with a closed canopy. Average internode length decreased 19% in response to increased light intensity. Root:shoot ratios decreased under increased light intensity and reduced water availability. A reduction in water availability alone did not affect any other investigated traits. We conclude that ramets of L. borealis are able to respond efficiently to small-scale variations in light intensity in natural systems, an ability of great importance for the performance of a prostrate species on shady forest floors.

Trees partition rainfall into throughfall and stemflow, resulting in a spatial distribution of nutrient and water fluxes reaching the soil centred on the trunks of trees. Stemflow fluxes of water and nutrients are then funneled preferentially belowground along tree roots and other preferential flow paths, bypassing much of the bulk soil. This double funneling leads to increased soil chemical, biological, and hydrological heterogeneity, which has been shown to persist for decades. In this paper, we review nutrient fluxes of stemflow water for a variety of tree species and climates. The amount of precipitation partitioned by trees to stemflow ranges over more than three orders of magnitude, accounting for 0.07–22% of incident rainfall in a range of precipitation regimes (600–7100 mm·y⁻¹). Stemflow fluxes of NO₃⁻ and K were found to be larger for species with greater stemflow partitioning, regardless of climate type. While stemflow volumes may increase in relation to increasing precipitation, stemflow nutrient concentrations tend to become more dilute. On an annual basis, however, it appears that plant canopy morphology is strongly related to stemflow fluxes for plant-mobile nutrients such as K (r² = 0.64) and NO₃⁻ (r² = 0.61). Root-induced preferential flow provides an additional feedback mechanism in nutrient cycling by which stemflow-derived nutrient fluxes are delivered to the rhizosphere.

Tree establishment is a potentially important factor affecting tree populations in alpine-treeline ecotones. Patterns of seedling establishment of Abies lasiocarpa, Pinus albicaulis, and Picea engelmannii were evaluated relative to neighbouring trees and herbs over two years and three treelines of the Rocky Mountains, USA. The greatest mortality rates were observed in seedlings that had just emerged from seed and were in their first year of growth and in seedlings that had the least amount of cover provided by trees or other landscape features that block exposure to the sky. Although herb cover promoted survivorship in microsites that were not near trees, no seedlings were detected at or above the upper limit of the treeline ecotone. Microsite tree cover was greatest for A. lasiocarpa and least for P. albicaulis seedlings, which matches predictions based on their relative photosynthetic tolerances to the bright sunlight and frequent frost that occur in exposed microsites. Interspecific differences in seedling requirements for neighbouring plant cover likely contribute to the apparent coexistence and possible interdependency of these conifers along a continuum of colonization and succession within treelines.

Tree-ring records of fires were used to examine the effects of inter-annual climatic variability on fire occurrence in forests dominated by the fire-adapted Araucaria araucana in the Andes of south-central Chile. Instrumental as well as tree-ring proxy records of climate indicate that low moisture availability is the main factor influencing fire occurrence. Years of widespread fire are strongly associated with warmer and drier summers. Years of extensive fire also tend to be favoured by one or two preceding years of dry climatic conditions. The El Niño–Southern Oscillation (ENSO) and its strong influence over large-scale climatic features is an important factor promoting fire activity. Years of high fire activity coincide with warm and dry summers following El Niño events. Fire in the Araucarian region is strongly related to inter-annual climatic variation associated primarily with the coupled effect of ENSO events and variations in the intensity and latitudinal position of the southeast Pacific anticyclone.

Because pollinator behaviour within plants affects the mating outcomes of animal-pollinated plants, plants may regulate the pollinator behaviour by controlling floral design and display to maximize pollination efficiency. We assessed how extent of the display size, inflorescence architecture, and nectar distribution within inflorescences alter bumblebee behaviour (number of flowers visited, time of stay, and nectar intake efficiency) under controlled conditions using artificial inflorescences. The effect of display size on bumblebee attraction was also measured. Eighteen types of artificial inflorescences, two display sizes (large and small) × three architectures (panicle, umbel, and raceme) × three nectar distributions (constant nectar content in every flower, gradual changes in nectar content, and random distribution of empty and rewarded flowers) were visited by bumblebees. The mean nectar amount per flower was set as 3 μL in 2003 and 1 μL in 2004. Although the inflorescence architecture had little effect on bumblebee behaviour, the random nectar distribution decreased the number of successive flower visits (both 3 μL and 1 μL) and staying time (only 3 μL) during a single visit and increased the foraging efficiency (both 3 μL and 1 μL). These features may contribute to decreases in the geitonogamous pollination and pollen loss during pollinator movements. Display size did not influence the attraction of bumblebees in this study. Our results suggest that nectar distribution within inflorescences can be a strategy for efficient pollination by manipulating pollinator behaviour.

The response of beetle and ant assemblages to fire (2–5 y old) and the dependence of that response on habitat type were analyzed. Using pitfall traps, beetles (Coleoptera) and ants (Formicidae) were sampled in replicated forest, scrub, and steppe areas including paired unburnt and burnt plots. A total of 176 species of beetles (8245 individuals) and 22 species of ants (115,056 individuals) were captured. Most beetle families (65%) were captured in the forest, while most ant species were most frequently captured in the steppe (45%). Beetle abundance was the same in unburnt and burnt plots in the three habitats, but ant abundance was higher in burnt than in unburnt plots. Fire usually decreased species richness of both taxa, but the strength of this effect depended on the habitat type. Coleoptera richness was lower in burnt than in unburnt plots in the forest and steppe but was the same in unburnt and burnt scrub plots. Ant richness was lower in burnt forest and scrub plots and was similar between burnt and unburnt steppe plots. For both taxa, species composition of the forest assemblages was different between unburnt and burnt forest plots (difference almost significant for ants) but not between unburnt and burnt steppe plots. Beetle species composition in the scrub was different between unburnt and burnt plots, but ant species composition was the same. Our results support the idea that the consequences of fire on native beetle and ant assemblages depend on the habitat type but also on the taxonomic group under analysis. Beetles and ants showed differences in their habitat preference and in the way that they use the habitats (e.g., preference for vegetated or bare soil patches) along the environmental gradient. These differences explain why beetles were in general more severely affected than ants, and why fire differentially affected both taxa in the scrub (habitat of intermediate complexity). A good knowledge of the habitat utilization by different groups at the local scale and a regional perspective (e.g., habitat preference along an environmental gradient) are necessary to fully understand the effect of disturbances on native arthropod assemblages.

The spatial pattern in the occurrence of two congeneric beetle species in the fruiting bodies of the wood-decaying fungus Fomitopsis pinicola was studied in an old-growth boreal forest in eastern Finland. The aim was to characterize the spatial pattern of the common Cis glabratus (Coleoptera: Ciidae) and the rare C. quadridens. A 25-ha study area was divided into 25- × 25-m quadrats (n = 400), and all the dead and dying fruiting bodies of F. pinicola (n = 737) were taken to a laboratory to collect the two species living inside. The quadrat-based count data were analyzed using the Index of Dispersion (I_d), the SADIE Index of Aggregation (I_a), and Moran's I spatial correlograms. The frequency distribution of the quadrat counts of F. pinicola deviated from a random, Poisson distribution towards significant aggregation (I_d). There was also a significant spatial autocorrelation (Moran's I) at short (≤ 100 m) distances. The SADIE methodology, however, showed that there was no overall spatial structure in the arrangement of the counts of F. pinicola fruiting bodies, i.e., the observed quadrat counts were randomly distributed in the 25-ha study area. Similarly, the counts of the trees occupied by the two beetle species occurred randomly among the quadrats with fruiting bodies, and the pattern was consistent at all lag distances. These results indicate that both species can readily utilize available resources in the study area, despite their spatial location.

In recent years, several researchers have noted a lack of sexual regeneration by several oak species in Mediterranean coppice woodlands of the Iberian Peninsula. The abandonment of the traditional use of oak coppice stands has resulted in an increase in tree cover and density, which may affect microhabitat availability and oak recruitment. Microenvironmental heterogeneity is known to determine the successful recruitment of some Mediterranean oak species, but few studies have evaluated how competitive effects of herbaceous vegetation constrain oak recruitment at small spatial scales. We indirectly assessed how the herbaceous understory interacts with Quercus faginea recruitment in a stand in northeast Spain by (1) establishing the vulnerability of seedling xylem to cavitation as related to the water potential in healthy seedlings located outside herb patches and unhealthy seedlings situated within these patches and (2) characterizing the small-scale spatial pattern of Q. faginea regeneration and herb cover as related to other environmental factors. We mapped all individuals located within a 20- × 20-m plot, which was subdivided into 400 quadrats of 1 m² each, to estimate litter depth, herb and shrub cover, and sunlight in the understory. Water potentials of Q. faginea individuals decreased in the following order: adults > sprouts > healthy seedlings > unhealthy seedlings. The summer soil-water content in the uppermost 40 cm of the soil was lower in microsites within dense herb patches than in those with low herb cover. Dense herb cover was positively associated with a greater density of unhealthy Q. faginea seedlings. Our results indicate that competition with the understory herbs for soil water should be regarded as one of the main factors controlling recruitment of Mediterranean oaks.

It is not clear why some species are able to naturalize and spread in a new region while so many other species are not. Several general properties have been reported for successful non-indigenous plant species (NIPS). These include presence of a lag time and population expansion following invasion, arrival from a similar climate, ability to self-fertilize, a short lifespan, clonal growth (if perennial), and production of small fruits. We examined these patterns in comparisons of all recorded abundant and rare NIPS in Ontario (n = 1153). We used cross-species and phylogenetic regressions to examine ecological patterns across present-day species and to determine whether evolutionary divergences in NIPS success have been correlated consistently with divergences in any of the life-history traits. We found a significant time lag in invader spread, with species arriving after 1952 being more likely to be rare. Successful invaders (i.e., abundant NIPS) were significantly over-represented among species originating in Europe and Eurasia. Successful invaders were significantly more likely to demonstrate clonal growth, to grow on variable soil moistures, and to have comparatively long flowering periods. While analyses such as this do not reveal causal mechanisms for the observed patterns, our correlative findings suggest important mechanisms for NIPS success that we discuss in light of the theoretical expectations of the attributes of successful plant invaders.

Rhododendron lapponicum is an ericaceous clonal dwarf shrub in which vegetative growth largely coincides with the formation of flowers and fruits. We used a flower-removal experiment and a descriptive data set to look for relationships and potential trade-offs between vegetative growth, i.e., branching, and fruit production in two contrasting subarctic populations (growing at low and high altitude). The two sites are similar in most respects and differ mainly in that the higher site has lower temperatures and a shorter growing season. We predicted that trade-offs between branching and fruit production would be less marked in a high-altitude population because at lower temperatures reproductive activities could be less resource dependent. At lower altitudes, where temperatures are higher, there would be competition for nutrients between fruiting and branching; hence, a trade-off between these two activities would be more obvious. In both populations about 80% of all new shoot modules were produced by reproductive shoot modules. We found evidence for a [linear] trade-off in the experimental study only, whereas the descriptive data set did not show any consistent indications of trade-offs between branch and fruit production. The high-altitude population showed several characteristics indicating that this population gave priority to vegetative growth over sexual reproduction. The data we present is a typical example of a case where co-variation in size and resource economy among modules masks potential trade-off patterns.

Alder plays an important role in the nitrogen (N) economy of boreal forests because of its high capacity for atmospheric N fixation. Range expansion and increased insect and/or pathogen attack are two potential consequences of climate change that may result in significant, albeit opposite, effects on these systems through influences on alder-mediated N inputs. This study contrasted the effects of weekly defoliation at different intensities on growth and N fixation in Alnus tenuifolia seedlings with recovery of these traits following a single but intensive defoliation event. Weekly removal of 15, 25, or 40% leaf area for 9 weeks reduced total plant weight by 7, 13, and 29%, respectively, and led to progressive increases in leaf weight ratio at the expense of shoot growth. Although maximum photosynthetic rates (P_max) were similar among treatments between defoliation events, increasing levels of defoliation led to progressive short-term declines in P_max immediately following treatments. Plants with 40% leaf removal had N fixation rates (48.3 ± 2.4 μmol N·g⁻¹·h⁻¹) that were 67% less than undefoliated plants (147.6 ± 8.9 μmol N·g⁻¹·h⁻¹), and even the lowest level of leaf removal led to significant reductions in fixation rates relative to controls. In the recovery experiment, N fixation rates in defoliated plants (158.4± 12.1 μmol N·g⁻¹·h⁻¹) were 40% less than control values (264.1 ± 18.3 μmol N·g ⁻¹·h⁻¹) 24 h following defoliation. After 28 d of regrowth, the total biomasses of defoliated and control plants were indistinguishable; however, N fixation rate in defoliated plants (39.2 ± 2.0 μmol N·g⁻¹·h⁻¹) remained 73% less than that of control plants, suggesting a strong competition between symbiont and host sinks for photosynthate.

The estimation of survival rates and the assessment of factors influencing variation in survival are essential to understanding population dynamics. However, in amphibians that alternate between an aquatic larval stage and a dispersing terrestrial stage, such understanding is limited due to the difficulty of estimating survival under field conditions. In this study, we obtained precise estimates of daily survival rates of tadpoles under field conditions using capture-mark-recapture (CMR) methods and assessed their temporal and spatial variation. Specifically, we assessed the effect of temperature, intra-specific density, and the presence of introduced bullfrogs (Rana catesbeiana) on the survival rate of Pacific treefrog (Pseudacris regilla) tadpoles in southern vancouver Island, British Columbia, Canada. Daily survival rates of tadpoles were relatively constant within a season and were also similar between years. Survival rates in different ponds varied from 95.4 to 87.9 %·d⁻¹. Among-pond differences in survival were best explained by the interaction of temperature and tadpole density. At low tadpole densities, survival increased with temperature, but at high densities, survival decreased with increasing temperature. It was not possible to detect the effect of introduced bullfrogs over the variation accounted for by differences in temperature and intra-specific density. As in terrestrial vertebrates, biotic and abiotic factors interacted strongly to determine survival rates in these tadpoles.