There are several examples of sex-biased parasitism of invertebrate hosts. Sex biases in parasitism could be explained by differences between males and females either in exposure to or susceptibility to parasites. Our results show that for the common spreadwing damselfly, Lestes disjunctus, there was a female bias in mean intensity of parasitism by larval Arrenurus pollictus mites for newly emerged individuals sampled over emergence periods in both 2002 and 2003. This bias could not be explained by host body size and timing of emergence, factors thought to influence exposure of host larvae to larval mites. We suggest a novel explanation for sex-biased parasitism based on differences in developmental trajectories of larval male and female hosts, which should influence frequency of contact by larval mites. This explanation may help explain female-biased parasitism in other lestid damselflies, which should be exaggerated for early emerging species with compressed emergence periods. Further work is needed to test this novel explanation and determine whether it is applicable to other invertebrate host–parasite associations where parasites first come into contact with immature stages of hosts.

The role of wild rabbit in Retama monosperma seed dispersal was studied in a sand dune spit in coastal Lepe County, Huelva, Spain. Of 8301 randomly collected rabbit pellets, 2.4% contained R. monosperma seeds. The percentage of pellets containing seeds was significantly higher in summer and autumn. Germination of seeds from pellets (24%) was significantly higher than that of seeds extracted from fruits (13%). The R. monosperma seed shadow extended to all dune ridges surveyed in the active colonization zone; rabbit pellets with R. monosperma seeds were found in dune ridges not yet colonized by R. monosperma shrubs. Rabbit dispersal activities are one of the main causes of R. monosperma spread along El Rompido spit.

Stable isotopes can be used to evaluate trophic relationships, nutrient state, and temporal and spatial variation in diet, food webs, and behaviour both within and between species. Here we describe the development and application of models to predict habitat use of a common insectivorous bat (Eptesicus fuscus) based upon δ₁₃C and δ¹⁵N signatures of skin tissue. We used a 42-specimen sample collected from three well-characterized ecogeographic regions, disparate both in photosynthetic mechanism and fertilizer use, to generate the models. Significant univariate differences between these three sites in terms of δ¹³C (F_{2, 39} = 112.92, P < 0.0001) and δ¹⁵N (F_{2, 39} = 97.06, P < 0.0001), and multivariate significance of both variables (Wilk's λ = 0.032, F_{4, 76} = 87.02, P < 0.0001), made it possible to develop three predictive models using Fisher's linear discriminant functions: 1) a model predicting if bats forage in C₃ or mixed C₃/C₄ sites, 2) a model predicting if bats forage in agricultural areas, and 3) a combined model using both variables to predict specific habitat use. We present the results of model application to an independent dataset of 329 bats sampled from 10 states that included a broad range of δ¹³C (−26.53‰ δ₁₃C −17.20‰) and δ₁₅N (6.36‰ δ₁₅N 15.60‰) signatures. We validated the use of skin tissue samples (from wing membranes) in the model by comparing the sites used for model development across five tissue types, selecting skin samples for model development due to consistently low variance within this tissue type. Our results indicate non-specific habitat-use by big brown bats.

In pollination mutualisms, new resource patches are constantly being created as new flowering species come into bloom. These patch dynamics can have effects on the organization of multispecies pollination guilds, depending on the temporal and spatial variation of floral resources. The goal of this work was to investigate the temporal dynamics of the flower use by a hummingbird community in a highland temperate forest in central Mexico. The system was studied in terms of the seasonal changes in (1) specific composition and abundance of hummingbirds and their plants, (2) use of floral resources, and (3) nectar characteristics of hummingbird-pollinated plants. Eight plant species were visited by hummingbirds. These species differed in nectar and sugar characteristics and flowering intensity throughout the year. The highest floral abundance was recorded from May to October. Three plant species showed blooming peaks in this period; these species, Salvia elegans, Bouvardia ternifolia, and Penstemon roseus, were the most abundant and most visited flowering species over the year. Eight species of hummingbirds were recorded: two resident hummingbirds, three altitudinal migrants, and three winter visitors. Hummingbirds showed seasonal preferences and changes in foraging behaviour with respect to visitation rates and the use of floral resources among habitats. This work suggests that the availability and quality of resources can explain the temporal and spatial composition of the hummingbird community on a local scale.

In 1994, two ice storms (major freezing rain events) disturbed forests on east- and southeast-facing slopes of the southern Appalachian Mountains, Virginia, USA. This study investigates impacts of the disturbance on Quercus forests, the predominant vegetation type in the Appalachians. Forests were sampled at six sites on two neighbouring mountains during 1997 and 1998. The two storms killed a significant number of trees, reducing basal area and density by 28.7% and 29.2%, respectively, in the transects sampled for this study. The storms also removed an estimated 43.7% of the canopy cover. Uprooting of trees disturbed 3.5% of the soil surface, on average. The ice storms eliminated some minor tree species from the sampled transects, reducing species richness; however, evenness increased. Both abiotic and biotic factors (mountain, slope, tree size, and species) influenced patterns of tree damage and mortality. Differences among species in susceptibility to ice storm damage seem particularly important for forest dynamics. The dominant Quercus prinus, along with Acer rubrum and Carya species, exhibited low mortality, whereas Pinus rigida had exceptionally high mortality. More generally, mortality was low in shade-tolerant and moderately shade-tolerant species, and high in those intolerant of shade. Periodic ice storms may contribute to the successional replacement of Pinus rigida and other susceptible species by less susceptible species.

We investigated herbaceous richness and cover in relation to fire season and severity, and other variables, five growing seasons following prescribed fires. Data were collected from six stands consisting of three randomly applied treatments: no burn, spring burn, and fall burn. Fall burns had significantly more exotic/native annual/biennial (an/bi) species and greater cover of these species (6.5% exotic; 1.7% native) compared to spring and unburned areas. These patterns are likely related to indirect fire effects associated with fire severity and resource availability, rather than direct fire effects due to burn timing. CART models indicated that high native and exotic an/bi richness and cover were associated with overstory gaps and higher fire severity areas, conditions common to fall burns. Exotics may be more successful at exploiting these environments. No treatment differences were found for native perennials. Location was important for explaining native perennial patterns, but richness and cover were also positively associated with lower fire severity, greater tree cover, and coarse woody debris. Expectations for increased native perennial plant diversity and abundance following prescribed fires may not necessarily be met and exotic species spread may compromise other ecosystem attributes. Restoration in these forests presents a challenge as prescribed fires interact with present environmental conditions that are very different from historical ones.

Macrofossil and chronostratigraphic data of a peat monolith extracted from a palsa within a subarctic permafrost peatland were used to reconstruct changes in vegetation and trophic conditions of the study site during the last 4500 cal y BP. The vegetation cover developed from Carex marsh or wet meadow to poor fen, bog, and to palsa peatland communities. The ombrotrophication was favoured by late-Holocene cooling and by the high peat accumulation. Permafrost inception began about 1660–1630 calibrated y BP, at the end of the Sphagnum accumulation stage. A comparison of the late-Holocene development of two peatlands in the same region (Arlen-Pouliot & Bhiry, 2005, and this study) suggests that permafrost initiation was due to a cool, dry climate associated with the Little Ice Age. However, Sphagnum cover prevented the permafrost from melting.

We experimentally investigated the effects of plant quality and natural enemies on the abundance of different herbivore guilds on oak trees. Two oak species (Quercus laevis and Q. geminata) and four guilds of leaf herbivores (leaf miners, gall-formers, leaf-rollers, and chewers) were studied using a factorial design that manipulated predation/parasitism pressure and plant nutritional quality. Forty plants of each species were divided into four treatments: 1) control plants (nutrients and natural enemies unaltered); 2) nutrients added, natural enemies unaltered; 3) nutrients unaltered, natural enemies removed; and 4) nutrients added and natural enemies excluded. Fertilized plants exhibited significantly higher foliar nitrogen for both oak species, and tannins tended to increase over time and decrease with fertilization, but only for Q. geminata was this trend significant. Fertilized plants supported significantly higher densities of all herbivore guilds than did unfertilized plants, but exclusion of natural enemies did not significantly affect herbivore abundance for any guild studied. Our results demonstrate that all herbivores on oaks, regardless of guild type, respond more strongly to bottom-up effects such as host-plant quality than to top-down effects caused by natural enemies.

There is currently a great deal of interest in documenting habitat use of birds in the boreal forest, especially in response to disturbances such as harvesting and forest fires. Studies have shown that some avian species are much more common in recently burned areas, whereas others are absent. Despite this, we know little about the immediate effect of a large forest fire, and its considerable alteration of habitat, on birds during a single breeding season. In 1995, a 72,000-ha forest fire in our study area gave us the opportunity to assess the effects of fire on reproduction of American kestrels (Falco sparverius) in comparison to an adjacent control area. In addition, we also examined territory occupancy by kestrels in the burned and control areas 8 y following the fire. We found no immediate effect of the fire on body condition of adult kestrels, their ability to hatch eggs, or to fledge at least one young. Birds breeding in the area that burned, however, did fledge fewer offspring per nest, and these nestlings had reduced body mass compared to control nests, possibly because of an immediate reduction in available prey following the fire. Nonetheless, given the profound alteration of habitat following the fire, the fact than any birds remained to continue breeding is remarkable. Territory occupancy of the burned area in 2003 was similar to 1995, the year of the fire, but occupancy in the adjacent control area declined during this period. It therefore appears that the control area is becoming a less suitable habitat for kestrels as the forest continues to mature, whereas the burned area is still providing viable habitat for kestrels. We present evidence suggesting that some of the differences in occupancy between the control and burned area may be due to fewer nest predators in the burned area in 2003.

In sub-boreal forests of northeastern North America, late-successional stands often contain a fine-scale mixture of several coniferous and hardwood tree species, including an important proportion of relatively shade-intolerant species such as white birch (Betula papyrifera) and yellow birch (B. alleghaniensis). In order to better understand the recruitment of these species, we determined the age at 1 m height of each tree in sample plots located in three stand types representative of different site conditions in a small forest mosaic undisturbed by humans. Three recruitment pulses occurred synchronously in all stand types during the periods 1835–1860, 1910–1930, and 1970–1990, the last two being related to known spruce budworm (Choristoneura fumiferana) outbreaks. These pulsed recruitment episodes were identified for birch species, but also for more shade-tolerant species such as balsam fir (Abies balsamifera) and to a lesser degree white cedar (Thuja occidentalis). White cedar was the only species for which there was some recruitment during the 1940–1960 period, but this was restricted to only one stand type located on mesic sites. These results indicate that when examined over relatively long periods of time, resource availability after canopy disturbances is an important factor for the recruitment above 1 m height for most tree species. In sub-boreal forests, periodic and synchronous mortality of balsam fir can favour the emergence of multi-species, multi-cohort stands, especially where soil conditions are adequate for the recruitment of other tree species.

Well-documented shade adaptations include physiological and morphological traits of shoots and leaves. Belowground shade adaptations (roots or other buried structures) are rarely mentioned in the literature and have never been studied in tree seedlings. This study evaluates the functional role of adventitious roots developed by most balsam fir (Abies balsamea) seedlings under a closed canopy. Our goal was to describe the relationships between growth, biomass allocation, and adventitious root development in balsam fir seedlings (5–26 cm tall, 9 to 33 y old) excavated under a closed canopy (< 8% full sunlight). Seedling height, branch number and crown width increased linearly with stem diameter measured at ground level. The dry mass of stems and roots increased exponentially and tree vigour (photosynthetic/non-photosynthetic biomass ratio) decreased exponentially as stem diameter increased. Results indicate that shade tolerance is limited by size and concurrent radial increment at the stem base. Results also demonstrate that the proportion of adventitious roots and the number of annual growth units buried in humus (age of the belowground section) increased as seedlings aged. Therefore, the reverse taper phenomenon caused by the formation of adventitious roots simultaneously limits the radial expansion of the stem at ground level and the maximum size attainable by seedlings. Balsam fir's shade tolerance is attributable to root as well as shoot adaptations. The gradual burial of the stem in humus probably increases persistence in shade by limiting the plant's respiration costs.

Because the mat of dead grass stems and leaves, or litter, is a conspicuous feature of grasslands and has been shown to affect various plant processes, I performed a growth chamber experiment on the effect of both grass litter mass (density) and type (species mixes) on emergence of three common prairie grass species. I found that (1) switchgrass (Panicum virgatum) emergence decreased significantly at the highest litter density, but different types of litter had no effect; (2) little bluestem (Schizachyrium scoparium) emergence decreased significantly at the highest litter density and with those litter types that included switchgrass litter; and (3) indiangrass (Sorghastrum nutans) emergence increased significantly at medium density and decreased significantly under its own litter. The results show that the smallest-seeded species had reduced emergence through the litter mat and that amount of litter was more important than type of litter. In conclusion, the full range of litter effects occurred: (1) two species were inhibited by density while the other species was facilitated, and (2) one species was inhibited by litter from another species, one species was inhibited only by its own litter, and one species was unaffected by the species composition of the litter. Finally, the results indicate a potential for grass litter of different species to differentially influence regeneration of tall grass prairie species.

The influence of habitat structure and abiotic factors on winter bird distribution was studied at the within-habitat level in the montane Pyrenean oakwoods of central Spain. Abiotic factors associated with thermal stress were estimated based on altitude and solar radiation received by woodlands (calculated by the steepness and orientation of the terrain). This paper demonstrates the great importance of abiotic factors in influencing bird distribution. Several bird community parameters related to density and species richness decreased with altitude, while they increased with radiation incidence of oakwood plots (i.e., birds avoided northern orientations where solar radiation is minimal in winter). The most important habitat structure variables related to bird distribution were the density of young and mature oaks. A thick undergrowth of thin oaks negatively influenced total bird abundance and species richness and the number of species of the ground searchers guild. Conversely, oak maturity played a positive role on total bird density and species richness and on the number of species of tree canopy gleaners and trunk foragers. Bird density and species richness were better explained by tree regression models considering complex interactions between variables than by general linear regression analyses. To enhance winter survival and habitat suitability for birds, forest management in these mediterranean endemic oakwoods should preserve the most mature forests at lower altitudes exposed to the south.

Hybridization is frequent in Orchidaceae and may play an important role in their speciation. However, information about pollinators and their role in hybridization is sparse. In the Mediterranean region of France, confirmed pollinators observed on Orchis simia and O. anthropophora included only one species common to both, the beetle Cidnopus pilosus. This beetle thus appears responsible for hybridization. Cross-specific pollen transfer is bidirectional, but hand pollination showed higher fruit set after deposition of O. anthropophora pollen on the stigma of O. simia than in the opposite direction. Both locally and over a 1600-km² study area, the presence of hybrids was shown to be strongly predicted by the microhabitat preferences of this beetle. To my knowledge, this is a novel demonstration that the spatial distribution of a natural hybrid in plants can be predicted by the local occurrence of a pollinator species identified as effecting hybridization. I suggest that similar interaction could explain the distribution of hybrids in other orchid species pairs.

Sediment deposition and resuspension of particulate material are key factors in the functioning of coastal ecosystems. This paper reports the results of a study from February 2001 to June 2002 of a rocky sublittoral sponge community influenced by seasonal sand deposition. The largest impacts on the assemblages were a reduction in diversity, losses and substitution of species, and a shift from a relatively mature and stable community to a more unstable, less diverse community dominated by encrusting species more adapted to the local environmental conditions. These observations are consistent with studies that show that morphological diversity of sponge communities decreases when perturbation increases. Encrusting sponges such as Microciona sp. and Spirastrella decumbens were the most abundant and persistent throughout the study, followed by boring sponges of the genus Cliona. Massive species such as Haliclona caerulea, massive branching species such as Mycalesp., and cushion-shaped species such as Callyspongia californica were present only before deposition events. The general fluctuation in the patterns was consistent with the environmental conditions in this bay; the results suggest that the changes in the sponge assemblages may be due partly to great fluctuations in sediment deposition produced by a change in the direction of the dominant winds (from north to south) during the transition from the drought to the rainy season. This change provoked an increase in sedimentation/resuspension (up to 13 kg·m⁻²·d⁻¹ in May 2001) and water movement in the zone, which were key factors influencing the structure and composition of sponge assemblages.