Effects of flowering plant's patch size on species composition of pollinator communities,foraging strategies,and resource partitioning in bumblebees (Hymenoptera: Apidae)

Summary In 4 common Middle-European mainly bumblebee-pollinated plant species (Impatiens glandulifera, Echium vulgare, Aconitum napellus, Symphytum officinale) the influence of patch size on species composition of the pollinator community was studied. Short-tongued species were most dominant in large patches, while small patches were frequented by middle- and long-tongued bumblebees. This phenomenon was extremely obvious in Symphytum officinale and Aconitum napellus, where short-tongued species had bitten a hole in nearly every flower of large patches. Long-tongued species were forced to small patches, where nectarrobbing occurred only exceptionally. In small patches visitationrate (Number of visits per flower per hour) was not lower but either equal or even higher then in large patches. Nectar measurements in Echium vulgare showed, that not only the mean quantity of nectar but also the variance was lower in small patches. As a result, the possible gain can be predicted much more precisely in a small patch than in a large one, and bumblebees have less difficulties in making the right foraging decisions. According to this, foraging strategies depend on patch size. This was confirmed by a computer simulation. The conclusion can be drawn, that many bumblebee species are able to share the same resource by using different patch sizes. Since large flower patches occur mainly in man-made habitats, the dominance of short-tongued species in many bumblebee communities studied by other authors may be unnatural.Supported by the Landesgraduiertenförderung Baden-Württemberg     

Parasitoid pressure and the radiation of a gallforming group (Cecidomyiidae: Asphondylia spp.) on creosote bush (Larrea tridentata)

Summary We tested the Enemy Impact Hypothesis, which predicts that communities of one tropic level are organized by the tropic level above. In the case of gallforming insect communities, the hypothesis predicts that gall morphology will diverge, minimizing the number of parasitoids shared among species. We used the monophyletic group of gallforming cecidomyiids (Asphondylia spp.) on creosote bush (Larrea tridentata) to test this hypothesis, predicting that species with thicker gall walls should exclude species of parasitoids with shorter ovipositors and have lower levels of parasitism. Of 17 parasitoid species reared from Asphondylia galls on creosote bush, 9 accounted for over 98% of parasitism. Seven of these 9 species had ovipositors long enough to penetrate 10 of 13 gall morphs measured. There was no significant relationship between gall wall thickness and number of associated parasitoid species (r ²=0.01, P>0.05, n=13). There was no relationship between gall wall thickness and types of parasitoid species colonizing galls: parasitoids with the shortest ovipositors colonized all types of gall morphs and were dominant members of the parasitoid assemblages in galls with the thickest walls. Ultimately, there were no significant differences in percent parasitism among Asphondylia species, regardless of gall wall thickness. We found no difference in numbers of associated parasitoids or percent parasitism in galls with different textures (e.g. hairy versus smooth), different locations on the plant or different phenologies. Our results suggest that enemy impact has not influenced the diversity of this gall community. Gall wall thickness, phenology, location on the plant and surface structure do not appear to influence the distribution of parasitoid species. Other explanations are offered to account for diversity in gall morphology among these species.     

Predator driven changes in community structure

Summary The zooplankton community of a small pond changed markedly with temporal variation in predation pressure. Long term changes in zooplankton community structure occurred following the replacement of planktivorous fish by phantom midge (Chaoborus americanus) larvae as the predominant predator of zooplankton. The interannual changes following the establishment of Chaoborus included the apparent or near extinction, of species ill adapted to the new predation pressure and the successful colonization of well adapted species. Seasonal changes in the species composition and size distribution of the zooplankton community correlate with temporal variation in predation intensity associated with temperature-activity patterns of the predator or changes in the stage structure of the predator population.     

The vectoring of cactophilic yeasts by Drosophila

Summary At two locations in the Sonoran Desert, yeasts were sampled from species of Drosophila, the flies' cactus hosts, and other neighboring sources of cactophilic yeasts to determine the relation between the yeasts vectored by the fly and the yeasts found in their breeding sites. D. mojavensis, D. nigrospiracula, and D. mettleri vectored yeast assemblages significantly more similar to the yeast species found on the rot from which the flies were collected than to the yeasts found on other rots from the flies host cactus or other rotting cactus at the same site. Rots with Drosophila had fewer yeast species than those without flies, suggesting that flies were associated with younger rots. Rots with flies and the Drosophila also had more yeast species with the capability to produce ethyl acetate than rots without flies. The results support the contention that cactophilic Drosophila feed on a subset of the yeasts available in an area, and may act to maintain differences among the yeast communities found on different species of cactus.     

The convergent trajectories of bird communities along ecological successions in european forests

Summary There is much more variation in the composition of bird communities in the earlier open and semi-open seral stages of ecological successions in forested landscapes of Europe than later on in preforested and forested climactic stages. The demonstration of this trend is achieved from the study of four habitat gradients, two in the mediterranean region (Provence and Corsica) and two in central Europe (Burgundy, France and Poland). A multivariate analysis has been used to illustrate the dynamics of communities along these succession. Displays of the results in bivariate space as well as an illustration of the distributional profiles of some of the most characteristic species show that: i) there is a discrimination between the two mediterranean gradients and the two medioeuropean ones and ii) each succession starts with a very distinct set of species and then the four gradients regularly converge in the last climactic stage where there is almost no discrimination between communities. These results are discussed in the light of the history of European biotas during the Pleistocene. The reason why there is more variation in species composition in the earlier seral stages than in the later forested stages are discussed according to current theories on the role of habitat selection on speciation processes.     

Long-term vegetation change in the Juncus squarrosus grassland at Moor House,northern England

Data from the long-term experiment on sheep grazing versus zero sheep grazing (i.e. in an enclosure) on the composition of the Juncus squarrosus grassland at the Moor House NNR are presented. The data have been analysed to assess change in (1) the abundance of individual species, (2) higher plant and bryophyte communities, and (3) vegetation structure. The higher plant data suggested that both the enclosed-and grazed plots were changing in the same direction, but the enclosed plot was moving much faster. The general tendency was towards a blanket bog vegetation dominated by Eriophorum vaginatum and Calluna vulgaris. Juncus squarrosus has declined very quickly in the enclosed plot, but there has also been a slower decline in the grazed one. A significant increase in Calluna vulgaris has occurred only in the enclosed plot. In contrast, the bryophyte data showed completely different successional trajectories in the two treatment plots.The vegetation structure changed markedly after release from grazing, with a decrease in phytomass, but increase in necromass in the lowest height strata. There was no noticeable change in structure over the 18 year period in the grazed plot.Particular problems found in this study were that some species either fluctuated widely in response, or changed in a curvilinear manner.     

A new null hypothesis for measuring the degree of plant community organization

An attempt is made to derive a measure for the degree of plant community organization, which would not be based on species co-occurrence and co-abundance. The use of the independent random distribution hypothesis (IRDH) is suggested for this purpose. The hypothesis is expected to be valid, if no deterministic phytosociological-structure-generating mechanism is present. If structural variability is used as a statistic for testing the hypothesis, deviances from the conditions of IRDH (species distributions are independent from each other, environmental gradients are lacking) will be attributable either to species interactions (smaller structural variability than expected), or to environmental heterogeneity (greater structural variability than expected). Structural variability is evaluated as the variance of species diversity, the indexN=exp(H') is used for measuring diversity. The precise measure of the degree of community organizationW is computed as the shift between two empirical distributions:D ^* (VN) or Bootstrap distribution of variance of diversity in the community, andD ^o (VN) or the random community variability distribution, which is evaluated after simulating the IRDH conditions.A satisfactory interpretation can be given to the results of evaluatingW for 11 data sets of 10 relevés each.Abbreviation IRDH Independent random distribution hypothesis     

Regional influences on small-scale vegetational heterogeneity within grasslands in the Serengeti National Park,Tanzania

An index of vegetational heterogeneity in 16 grassland communities in the Serengeti National Park, Tanzania, was calculated using mean percent distance (MPD) among spatially separated quadrats. To determine whether regional factors such as annual precipitation, fire, soil sodicity, herbivore utilization, or percentage of species having clonal growth patterns influenced the amount of heterogeneity measured in individual grassland communities, the index was related to these factors by an interactively constructed multiple-regression model.Community heterogeneity varied in a curvilinear manner along a north-to-south transect; maximum values occurred in the center of the park with lower values near the northern and southern borders. Subsurface concentrations of Na⁺ and the presence or absence of mound-building termites were incorporated into the model, which explained 55% of the variability in grassland heterogeneity.Abbreviations PD= Percentage Distance - MPD= Mean Percentage Distance     

Response to grazing after nine years of cattle exlusion in a Flooding Pampa grassland,Argentina

This paper reports on changes induced by the introduction of cattle in a grassland that had remained ungrazed for 9 yr, in comparison with two adjacent grasslands: one that remained enclosed and one that has been continuously subject to grazing. Basal cover was measured on 25 interception lines, each 1 m long, three times during one year. The variables studied were: total cover, cover of grasses and dicots, cover of creeping grasses, floristic composition, and dissimilarity among sites. At the first sampling, 2 yr after cattle re-introduction, the newly grazed site was more similar to the ungrazed than to the grazed site. The newly grazed site had very low cover of dicots; the species of dicots present were different from those found in the continuously grazed area. Creeping grasses had higher cover in the newly grazed site than in the other sites, and continued to increase. At the last sampling, one year later, the newly grazed site had become more similar to the contiuously grazed site. Only after 5 yr of cattle grazing the exotic dicots that were dominant in the continuously grazed site, were recorded in the re-opened site. The absence of propagules of these species or the absence of safe sites may account for this delayed invasion.     

The gap theory in forest dynamics

Since the late 1970s, ecologists interested in forest dynamics have focused their attention to the responses of individuals, populations and communities to “gaps” which are openings created in the forest canopy. This review intended to introduce some collective knowledge on major subjects of the gap theory in forest dynamics, in relation to gap-disturbance regimes, tree regeneration behaviour and community structure.